the environmental control of epidemic contagion

THE ENVIRONMENTAL CONTROL OF EPIDEMIC CONTAGION

I. AN EPIDEMIOLOGIC STUDY OF RADIANT DISINFECTION OF AIRIN DAY SCHOOLS

BY

W. F. WELLS,i M. W. WELLS 1 AND T. S. WILDER 2

(Received for publication July 14th, 1941)

EXPERIMENT I. THE GERMANTOWN FRIENDSSCHOOL

I. Plan of Experiments.1. Design of installations.2. Epidemiological techniques.3. Contagious diseases prior to the use of ultra-

violet lights

II. Results.A. The first experimental year: 1987-1938.

Susceptibility.Contagious diseases.

1 Laboratories for the Study of Air-borneInfection, supported by a grant from the Com-monwealth Fund to the University of Penn-sylvania School of Medicine.

2 Department of Pediatrics, University ofPennsylvania, and Physician to GermantownFriends School.

We wish to thank the many who have con-tributed to these studies. We are especiallyindebted to Prof. E. B. Wilson of the HarvardSchool of Public Health, whom we have beenprivileged to consult on the epidemiologicalproblems of the study; to Dr. Joseph Stokes,Jr., who has sponsored the study at the Ger-mantown Friends School and Dr. Alfred N.Richards who has sponsored the study in theSwarthmore schools. We are also deeply in-debted to Mr. Stanley R. Yarnall, Head Masterof the Germantown Friends School and Mr.Frank R. Morey, Superintendent of the Swarth-more Public Schools, from whom we have re-ceived most loyal support. Miss Anna Burk-hardt, school nurse of Swarthmore, has aided usmost efficiently in collection of records.

We wish also to thank the Hanovia Chemicaland Manufacturing Company who loaned theultra-violet lights and fixtures for the German-town study, the General Electric Company wholoaned the ultra-violet lights for the Swarth-more study, and the Wheeler Fixture Companywho loaned the fixtures for the Swarthmorestudy.

B. The second experimental year: 1938-1939.SusceptibilityContagious diseases

C. The third experimental year: 1939-1940.Susceptibility.Contagious diseases.

D. The fourth experimental year: 1940-1941.Susceptibility.Contagious diseases.

E. The frequency of colds before and after theuse of lights.

EXPERIMENT I I . THE SWARTHMORE PUBLICSCHOOLS

I. Plan of Experiments.1. Design of installations.2. Contagious diseases prior to the use of ultra-

violet lights.II . Results.A. The first experimental year: .1940-1941.

Susceptibility.Contagious diseases.

DISCUSSION

INTRODUCTION

The experiments described in thispaper were undertaken in the fall of1937, as a test of the hypothesis (1) thatthe confined atmospheres of otfr habita-tions constitute the vehicle for the epi-demic spread of contagion. Even thoughendemic incidence (person-to-person typeof spread) may be due to any one ofseveral modes of spread such as physicalcontact, direct hits by Fliigge droplets,or air, the phenomenon of epidemic con-tagion (the dynamic network of person-to-group infection) is indicative ofspread through the medium of a com-mon source: air. If epidemic respira-

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98 W. F. WELLS, M. W. WELLS AND T. S. WILDER

tory contagion can be controlled byradiant disinfection of air, then air mustbe such a vehicle of spread.

Improvements in ultraviolet lightsources and bacteriologic methods forthe study of sanitary ventilation havemade it practicable to eliminate micro-organisms from enclosed atmospheres atrates hitherto impossible. During coldweather, economy limits the number ofair changes per hour through mechanicaldisplacement to some 10 or 20 changesat most while, with radiant disinfection,it is possible to secure bacterial elimina-tion which is equivalent to 100 airchanges per hour, even with indirectirradiation and without erythemal ex-posure of the occupants.

When the experiments were begun theart of radiant disinfection of air wasless clearly understood than now, al-though irradiation had already been usedto prevent surgical infections (2, 3), andhospital cross-infections (4, 5). Suchinfections, however, inasmuch as theyoccur among "closed aggregations," i.e.,among persons dwelling continuouslywithin the same atmosphere, afford primafacie evidence of infection within a givenatmosphere. Control of contagion in"intermittent aggregations," on theother hand, necessitates the locating ofthe atmosphere within which infectionsare contracted, since obviously disinfec-tion of a confined atmosphere can pre-vent only those infections contractedwithin it.

While homes provide aggregations ofsusceptible children and while the studyof secondary attack rates of childhoodcontagions in families (where the factorof exposure can be assumed) has con-tributed much to an understanding ofthe communicability of these diseases, theuse of the family unit for study of theeffect of radiant disinfection of air onthe spread of contagion was not feasible.

Day-schools, however, bring togethergroups still largely susceptible to child-hood contagions, and irradiation of theiratmospheres seemed entirely practicable.

Experiment I. A study of radiant dis-infection of air in the German-

town Friends School

The choice of the Germantown FriendsSchool for the experiment has proved anexcellent one. It has afforded the obvi-ous advantage of providing an intelli-gent and cooperative group of educatorsand parents. Furthermore, the carefulrecords of all childhood contagions andof colds and other respiratory infections,which had been kept by the school physi-cian (T.S.W.) over the period of 5 yearspreceding the experimental session of1937-38, have provided a backgroundagainst which to evaluate the experi-mental findings.

The school includes a primary depart-ment (grades 1—4), an intermedite de-partment (grades 5-8), and a senior de-partment (grades 9-12); and each gradeis divided into two sections. The twosections of the primary grades have littlemore association with each other thanwith other classes in the department. Inthe intermediate and senior departments,however, the grades are divided into sec-tions with boys and girls in each section,but for some studies the grades are di-vided into a section of boys and a sec-tion of girls. The sections of the gradesin these departments cannot, therefore,be considered separately as is done inthe primary department.

It is an educational tenet of the schoolauthorities that children profit by someassociation with older and some withyounger children. Extracurricular ac-tivities (drama groups, sports, etc.)which bring together children of differ-ent grades are encouraged. This prac-

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RADIANT DISINFECTION OF AIR 99

tice is clearly reflected in the pattern ofspread of the contagious diseases.

Two kindergarten groups, for 4- and5-year-old children, have been disre-garded in this study because they arenot comparable with the regular schoolgroups, since they are each separatelyand more spaciously housed, do notjoin the extracurricular activities, donot use the common lunchroom or halls,have shorter hours, and are kept out ofdoors as much as the weather permits.

There is a school physician at theGermantown Friends School. He is inattendance for an hour or more in theearly morning and his private office isless than a square from the school.Prior to the experiment, efforts had beenmade to exclude from school childrenwith colds or in the prodromal stages ofcontagions, and no change was made inthis routine following the installation oflights. Nor could there have been anychange in the official Pennsylvania regu-lation that if a contagious disease oc-curred in a home, all susceptible childrenwere quarantined. (This regulation wasrepealed during July, 1941. )

In the fall of 1937, ultraviolet lightswere installed in one of the two homerooms of each of the 4 grades of theprimary department. The music room,library, nature room, halls, lunchroom,rest room and gymnasium were also ir-radiated. The control group shared allthe irradiated atmospheres except thoseof the home rooms, and even their homerooms ventilated to the irradiated hall-ways. Detailed study of the pattern ofspread of contagious disease during thefirst experimental year and during previ-ous years, however, led to the convictionthat much infection took place outsidethe home rooms but within the school, asindicated by the large number of intro-ductions into the classes, and by thesimultaneity of these introductions into

different classes. Moreover, the so-called"control" group of the primary depart-ment was actually a partially irradiatedgroup, since the pupils shared the ir-radiated common rooms, even thoughtheir home rooms were unirradiated.Prior, therefore, to the opening of schoolin the fall of 1938, the 4 home rooms ofthe control group were equipped withbactericidal lamps. In short, an effortwas made at that time to wall off theentire primary group from the rest ofthe school in so far as its air supply wasconcerned. This was not, of course, en-tirely possible. The irradiation of thelarge auditorium where the children at-tend "assembly" once a week, for ex-ample, presented technical difficultiesand would have added considerably tothe expense. (The consequence of omit-ting its irradiation, as revealed duringthe spring of 1940, could hardly havebeen foreseen.)

The upper classes, while they serve asa basis for comparison, are in no sense a"control." Even though attack ratesare computed among susceptible childrenonly, the "susceptible density" i.e., theinterspacing with immunes, varies toogreatly for the groups to be truly com-parable.

Design of installation

It seemed a reasonable assumption thatcontrol of epidemic contagion amongnormal aggregations would not requirethe amount of sanitary ventilation whichwas needed to protect open wounds orto isolate the sick in hospitals. Whilein the latter situations, infected dustmay be a common source of spread, thehypothesis upon which the experimentswere based is that the spread of epidemiccontagion is effected largely through thebreathing of infected droplet nucleifreshly expelled by carriers or by pa-tients in the prodromal stages of disease,

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and that these nuclei are especially vul-nerable to radiant energy (1).

Only indirect irradiation is used in theclassrooms of the Germantown FriendsSchool. Central radiant sources convertthe upper portion of the air into irra-diation chambers. Two crossed burn-ers 3 mounted in shallow aluminum pansare suspended approximately 4 feet be-low the center of the nonreflecting ceil-ing. Half the light radiates directlythrough a mean distance of about 5 feet,and the remaining half is reflected up-ward from the fixture with a loss of ap-proximately 50 per cent (figure 1).

There is no system of ventilation otherthan windows, and the lower portion ofthe room thus ventilates to the upperirradiated portion only by natural con-vection currents. An extensive studyhas been made of the sanitary ventilationof the Germantown Friends School andthe other schools of the experiment andwill be reported elsewhere (6). It canbe said here that the removal rates of testmicroorganisms are increased almosttenfold, by this system of indirectradiant disinfection, equivalent in sani-tary effect to ventilation rates of ap-proximately 50 air changes per hour.

Exposure of occupants to ultravioletradiation is determined by time and in-tensity. Only where exposure seemsnegligible has it been deemed permissibleto utilize direct radiation in the German-town Friends School, although this isthe most efficient method of air purifica-tion. The halls have been so treated be-cause the burners could be placed at aconsiderable height and the fixtures sotilted that persons receive direct raysonly at far ends of the hall. Moreoverthe children remain in the halls for shorttimes only in going back and forth toother rooms. The dining rooms, where

s Hanovia Safe-t-aire tubes in Hanovia fix-tures.

a stagger system of lunching insures thatno one group remain long, is also di-rectly irradiated, also at considerableheight.

Epidemiological techniques

For an epidemiological study theschool offered many advantages. Itspupils are drawn from a stable, homo-geneous group, and a majority of themattend the school continuously through-out the 12 grades. Sixty-four per centof the 1941 graduating class enteredeither the kindergarten or first grade.The immunity of the school, therefore,reflects the past incidence of contagiousdiseases in the school. The pupils, more-over, enjoy a certain social isolation,which limits the number of introductionsof contagious diseases resulting from out-side exposure, and which might there-fore be expected to simplify further thepatterns of spread. Among children ofthis social status, spread by physical con-tact might also be expected to be mini-mal.

Choice of an epidemiologic index ofspread of infection requires: that the in-fection be manifested as disease, ob-scured neither by the occurrence of car-riers nor by subclinical infections; thatthe disease be reliably recognizable clin-ically; that exposures to the infectionoutside the atmospheres under study shallnot be so numerous as to mask the pre-vention of spread within the atmosphere;and that artificial immunization shallnot so lower susceptible density as toprevent spread. Thus, measles, chickenpox and mumps were chosen as indicesin the school, other childhood contagionsseeming unsuited to the purpose. Ninetyeight per cent of the children have beenimmunized against diphtheria; 43 per •cent have received immunization againstwhooping cough, in addition to thosewho have had the disease; and scarlet

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fever would require periodic Dick teststo trace the spread of infection. Thecommon cold is discussed separately.

Very simple records have been kept:(a) Records of susceptibility. The

parents are requested to fill out, for eachchild, a report on the contagious diseasesfrom which he has suffered in the past,and the artificial immunizations he hasreceived. The aggregate of these rec-ords gives in readily available form, thenumber of susceptibles in each homeroom and the susceptible density, i.e.,the interspacing with immunes. Intables 2, 4, 6, 8, and 13, the "number ofsusceptibles" to measles, chicken pox andmumps, therefore, signifies the num-ber from whose parents was obtained ahistory of no previous attack.

(6) Records of contagious diseases.The date of onset for each contagiousdisease in the school is recorded. Theboard-of-health reports are not suffi-ciently reliable for the purposes of thestudy, and the date is checked with theparent. The diagnosis of the familyphysician is accepted, however.

(c) Records of class exposure. To-gether with the date of onset of each con-tagious disease there is recorded the dateof last school attendance prior to the ill-ness, the information coming from theteacher, and M'hether the case was con-tracted under quarantine from a priorease in the home. This careful checkon the interval between latest school at-tendance and onset of symptoms is im-portant in view of the Saturday-Sun-day interval in attendance, and the factthat Friday attendance by a child whobecomes ill on Sunday evening or Mon-day morning might be rated either as aprobable class exposure, as in the caseof measles, or not, as in the case ofchicken pox. A child who develops thedisease when under quarantine for amember of his family, or when absent for

other reason during the prodromalstages, obviously does not expose hisclass.

A distinction is made in this paperbetween an "introduction" of a con-tagious disease into a class, and a "classexposure." An introduction indicatesthe occurrence among the children ofone home room of a case not attributableto a prior case in that room and concern-ing Avhich information is not available asto whether the child's last attendance oc-curred so shortly before the onset ofsymptoms as to make it probable he wasinfective, in which case he caused a"class exposure."

In the years prior to the beginning ofthe experiment, no record was kept ofthe "class exposure" except in the in-stances where cases were contractedunder home quarantine. Through amisunderstanding, detailed records werelikewise not kept of class exposure dur-ing the first year of the experiment,1937-38, and it will be noted that in dis-cussing this year, the noncommital word"introduction" is used.

In this study the term "secondarycases" refers, unless otherwise qualified,to cases following, after an interval cor-responding to the incubation period of thedisease in question, a "class exposure,"that is, exposure in the home room. Theincubation periods given by the AmericanPublic Health Association (7) have beenadopted: measles, 10-12 days to thebeginning of symptoms, 13-15 days tothe rash; chicken pox, 14-21 days;mumps, 12-26 days. The term "sec-ondary," therefore, indicates only thepossibility that infection was incurredin the home room, and does not implythat any attempt was made to eliminateother sources of infection. It is obvi-ous that the probability of a case beinga "class secondary" decreases with in-crease in the number of introductions,

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Except iii the instances where outside ex-posure was to a case within the child'sown family, and where presumably theexposure was heavier than a class orschool exposure, and where the quaran-tine was a matter of definite record withthe school and the health department

the percentage of the susceptibles in agiven home room developing the diseasefollowing known class exposure; while theterm "attack rate among susceptibles,"if unqualified, refers to the percentageof recorded susceptibles in the variousdepartments of the school who, during

(i.e., a "home secondary"), no account the year in question, developed the dis-

FIGURE 1. Classroom, Germantown Friends School, central radiant sources.

has been taken of possible exposure out-side the school, though they are recordedwhen brought to our attention. Ourgreater familiarity with the teachers andstaff of the primary department causesus to hear more often of outside expos-ures such as sharing transportation,visiting in homes, attendance at parties,in these departments than in the upperunirradiated grades.

The term "secondary attack rateamong susceptibles" is used to refer to

ease. This takes no account of the"density of susceptibles," i.e., the per-centage of the total enrollment whichthese susceptibles form, the interspacingwith immunes, and the rates are there-fore not entirely comparable in the di£ferent departments.

Since irradiation of room atmospheresis directed primarily toward preventionof infection from infective nuclei ratherthan from direct hits by Fliigge drop-lets, it might seem that room charts

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TABLE 1

Contagious diseases, Germantown Friends School.Records for 6 years prior to installation of ultra-violet lights

Year

1932-331933-341934-351935-361936-37

Totalenrollment

596589579596612

Total, five years priorto irradiation

Cases, measles

Senior

37200

12

Inter-med.

2416140

45

Primary

2644

151

77

Cases, chicken-pox

Senior

o o

to

o

o2

Inter-med.

84

1631

32

Primary

174

583

14

96

Cases, mumps

Senior

011000

11

Inter-med.

025

100

26

Primary

014301

18

showing the desk locations of the chil-dren would be informative. This hasnot proved to be the case in the primarygrades of the schools studied, however,since desks, tables and chairs are mov-able, children move freely about therooms and arrange their chairs at pleas-ure.

Contagious disease record prior to theuse of ultraviolet lights

The incidence, during the 5-year pe-riod preceding the experimental intro-duction of radiant disinfection, of thediseases which were chosen as indices isshown in table 1. It will be seen that at

the commencement of the experiment, theschool had already been free from epi-demic spread of disease for 2 years.

A. The first experimental year: 1937-38

Susceptibility. During the fall of1937, ultraviolet lights in indirect fix-tures were installed in the home roomsof one section of each of the first 4grades, the plan being for the corre-sponding sections to remain as controls.All common classrooms, such as themusic room, nature room, art room, restroom and library were indirectly irradi-ated, and direct radiation was used inhallways and lunchroom. Table 2

TABLE 2

Susceptibility, Germantovm Friends School.Fall, 1937

Group

UnirradiatedSenior dept., Grades.9-12Interm. dept., Grades 5-8.. .Primary dept., Grades 1-4...

IrradiatedPrimary dept., Grades 1-4...

Num-ber

18818489

89

Noreport

971

6

Measles—susceptible

Number

214655

58

Per cent

11.726.062.5

69.9

Chicken-pox—susceptible

Number

303137

29

Per cent

16.817.542.0

34.9

Mumps—susceptible

Number

8713370

70

Per cent

48.675.179.5

84.3

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TABLE 3

Mumps, GermanUrwn Friends School, 1937-1938.Class introductions and cases possibly secondary to them

IrradiatedPrimary dept.

UnirradiatedPrimary dept.

UnirradiatedUpper classes

Grade,section

1,1

111,1

IV, 1

1,2

11,2

111,2

IV, 2

V

VI

VII

X

Susceptibles

Number

21

13

18

17

17

18

18

38

33

31

29

Per cent

100.0

68.4

75.0

80.9

73.9

90.0

75.0

84.4

71.7

70.5

60.4

Date, classintroduction

Jan. 4Apr. 18

Mch. 7Apr. 25

Jan. 31Apr. 4, 7

Feb. 18

Jan. 4Apr. 7

Jan. 31Apr. 26

Feb.18Apr. 19

Jan. 4Jan. 4Jan. 10Feb. 10Mch. 28

Mch. 2Apr. 11

Jan. 31

Feb. 8Apr. 19

Date, following cases

Apr. 22; May 16

Mch. 14; Apr. 5, 5, 5, 6; 18, 18, 18;May 9

May 16

May 4, 9

Apr. 13; May 2, 3

Feb. 14, 17; Mch. 3, 7, 14

shows the susceptibility, as denned underEpidemiological Techniques, of thegroups to the contagion indices in the fallof 1937.

Contagious diseases. Although thespring of 1938 was "measles year" inthe city of Philadelphia, the school re-mained free except for scattered cases.The year was marked, however, by anoutbreak of mumps (46 cases). As maybe seen from table 3, there occurred 7

introductions (class exposures not re-corded) into the irradiated and 7 intro-ductions into the unirradiated primaryclasses. Following these there were 2cases which were possible secondaries inthe irradiated; 12 in the unirradiatedrooms. In the upper unirradiatedclasses, there occurred 10 introductions,followed by 8 cases which were possiblysecondaries.

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B. The second experimental year:1938-39

As stated above, the large number ofintroductions into the classes and theirdates seemed to indicate interclass infec-tions (see table 3) and, therefore allclassrooms of the primary departmentwere irradiated prior to the session of1938-39.

Susceptibility. Table 4 shows thestate of susceptibility in the school at thebeginning of the year.

the third year of the experiment isshown in table 6.

Contagious diseases. Probably no con-tagious disease is more volatile thanchicken pox, and even those investiga-*tors who are reluctant to accept a gen-eral hypothesis of air-borne contagionconcede the probability that this diseaseis spread through the medium of air.During the school year of 1939-40, vari-ous classes of the primary departmentwere exposed to children in the pro-dromal (i.e., within 24 hours of the ap-

TABLB 4

Susceptibility, Germantovm Friends School.Fall, 19S8

Group

UnirradiatedSenior dept., Grades 9-12Interm. dept., Grades 5-8. . .


Num-ber

180199

171

Noreport

01

2


Number

2866

117

Per cent

15.533.3

69.2


Number

2540

83

Per cent

13.920.2

49.1

Mumps—susceptible

Number

101128

125

Per cent

56.164.7

74.0

Contagious diseases. The year 1938-39 was marked by another outbreak (33cases) of mumps which reached a peakduring November, and an isolated groupof 3 cases the following April. Thesecases are summarized in table 5. It willbe noticed that whereas the unirradiatedupper classes suffered 32 cases duringthe fall epidemic, the irradiated primarydepartment escaped with only one casewhich did not expose the class. Later inthe year, however, a missed case exposedthe same class on 3 successive days re-sulting in two secondary cases.

C. The third experimental year: 1939-40

Susceptibility. The state of suscepti-bility of the school at the beginning of

pearanee of symptoms) or early stages ofchicken pox 21 times within the irradi-ated atmospheres, as is shown in table 7.Only 7 secondary cases can be ascribedto these 21 exposures of classes in irradi-ated rooms; certainly not a record indi-cating facility of spread.

The volatility characteristic of chickenpox was, however, demonstrated in anexplosive outbreak following the expos-ure of one of these classes on 4 succes-sive days in an unirradiated atmosphere.It is customary for a class to be singledout each year to give a school play, andthis particular third-grade class wasrehearsed in the unirradiated audito-rium for about an hour on each of 4successive days with a missed case.

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TABLE 5

Mumps, Germantown Friends School, 1938-1989.Class exposures and secondary infections

IrradiatedPrimary dept


Grade

IV, 2

V

VI

VII

VIII

XI

XII

Susceptibles

Num-ber

14

31

35

33

29

28

21

Percent

66.7

63.3

70.0

64.7

60.4

58.3

44.7

Date ofonset

Nov. 28Apr. 10

Nov. 2Nov. 2Nov. 7Nov. 7Nov. 8Jan. 3Jan. 3

Oct. 3

Jan. 19

Nov. 7Nov. 7Nov. 7

Nov. 26

Dec. 2Jan. 17Jan. 26

Nov. 27Nov. 28

Class exposure

NoApr. 10,11,12(missed case)

YesYesNoNoYesNoNo

Yes

Yes

NoNoNo

No

YesNo (home sec.)

Yes

NoNo

Cases, secondary

Apr. 30, May 1

Nov. 28

Oct. 17, 18, 18,20, 20.24, 26

Cases, tertiary

Nov. 1, 7, 7, 7, 8

TABLE 6

Susceptibility, Germantovm Friends School.Fall, 1989

Group

UnirradiatedSenior dept., Grades 9-12Intenn. dept., Grades 5-8. . .


Num-ber

175199

169

Noreport

CO

CO

0


Number

2878

121

Per cent

21.739.8

71.6

Chicken-pox-susceptible

Number

2034

101

Per cent

11.617.7

59.8

Mumps—susceptible

Number

101104

127

Per cent

58.753.1

75.1

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RADIANT DISINFECTION OF AIE 107

Children from other classes were knownto have been in the auditorium at thesame time, but there was no record ofthose so exposed. When, on the eveningof May 9th, the day before the play, this

child, sole secondary to a previous classexposure on April 24th under the lights,was diagnosed, it was realized that hehad first shown vesicles on May 6th.Between the 14th and 20th day follow-

TABLE 7

Secondary cases of chicken-pox, primary department, following exposure in irradiated andunirradiated atmospheres, Germantown Friends School, 1939-1940

Class,section

Date, onsetof cases

Class lastexposed

Children exposed

Immune Susceptible

Secondary cases.Date of onset

11,2IV, 1IV, 111,21,1

11,1IV, 211,11,2

11,2

II, 1III, 111,21,2

111,211,2

IV, 2IV, 2

1,2

11,1111,2

1,2IV, 1

Dec.Jan.Jan.Feb.Feb.

Mch.Apr.Apr.Apr.Apr.

Apr.Apr.Apr.May-

MayMay

MayMayMay

MayMayJuneJune

9413225

114589

24*2426*20

2022

252526

26305*6

Exposures in irradiated

Dec. 8Jan. 3Jan. 12Feb. 1

Feb. 23 (exposure?)

Home sec, not exposedApr. 3Apr. 4Apr. 8Apr. 9

Apr. 23Apr. 24Apr. 26

May 20-24 (missed case)

Home sec, not exposedMay 22 and 23

(missed case)May 24May 24May 24

May 24May 30June 4June 5

atmospheres

91213105

—1592

11

106

123

.13

16

4

11115

14

13109

128

7141211

13171011

9

5

10

121198

00000

01 case, Apr. 24

01 case, Apr. 26

01 case, May 6

01 case, June 5

1 case, June 8f

0

0

2 cases, June 7 and 81000

Exposure in unirradiated atmospheres

111,1 May 6* May 6-9 (missed case) 16 15 cases, May 20, 21,21, 21,22, 22, 22, 22,23, 23, 23, 23, 24, 24,26 (also one teacher)

* These cases were themselves secondary to class exposure,t Last day of school, June 6th.

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l/n lft» W> » l W IM Ha # 4M 4/u 4/» »/» i/it I/it «M

Primary Classes, IRRADIATED ROOMS

-15%

l/M 1/1w n HOKIIINO

VM1

4/l4 4/11 4/U J/I

Upper Classes, UNIRRADIATED ROOMSFIGURE 2. Measles, 1941, Germantown Friends School. Weekly attaek rate among sus-

eeptibles in irradiated and unirradiated rooms, not including home secondary cases. Data intable 10.

ing the first day of known exposure(May 6th), 15 of the 16 children in theclass recorded as susceptible developedchicken pox in a typical static type epi-demic, a result indicative of intense com-mon-source exposure. Seven cases inother primary classes are so grouped intime as to make it probable that they toowere secondary to this case.

Among the unirradiated upper classesthere were 11 cases during the year. Ofthese, 4 children exposed their classes:the seventh grade (5 susceptibles, 19 im-munes) was exposed to two cases onMay 23rd; the eighth grade (2 suscepti-

bles, 24 immunes) was exposed May24th; the eleventh grade (3 susceptibles,34 immunes) was exposed to a missedcase May 22nd-24th. None of these 10exposed susceptibles developed the dis-ease.

D. The fourth experimental year,1940-41

The season 1940-41 has been markedby the largest epidemic of measles fromwhich the city of Philadelphia has eversuffered. As early as October an in-creased incidence was reported and anepidemic anticipated. During the 8

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months October 1940 through May 1941, cases) which the school has experienced25,463 cases were reported. The year during the 9 years for which records arehas brought to the Germantown Friends available.School also the largest epidemic (112 Susceptibility. Table 8 shows the

TABLE 8

Susceptibility, Germantown Friends School.Fall, 1940

Group

UnirradiatedSenior dept., Grades 9-12Interm. dept., Grades 5-8. . .


Num-ber

184202

149

Noreport*

64

6

Measles—susceptiblef

Number

4197

108

Per cent

23.048.9

75.5


Number

1641

63

Per cent

9.020.7

44.0

Mumps—susceptible

Number

111112

109

Per cent

62.456.5

86.1

* Five children in these groups who later developed measles are classed as susceptibles in tables9, 10, and 16.

t Four children who, in the fall, gave a history of having had measles later developed the disease.For simplicity, these children were classified in the tables as susceptible.

TABLE 9

Measles, 1941.Germantown Friends School

IrradiatedPrimarydepartment


Grade,section

1.11.2

11,111,2

111,1111,2IV, 1IV, 2

VVI

VII

VIIIIXX

XIXII

Susceptibles

Number

1010131417191713

2926

30

141413104

Per cent

76.971.476.5

100.073.982.673.956.5

60.455.3

56.6

26.928.028.922.710.3

Dates of cases*

No casesNo casesMch. 20, 26; Apr. 3Mch. 25Mch. 6, 7; Apr. 1, 5, 12; May 1Mch. 22, 23, 26; Apr. 3, 7Feb. 24; Mch. 10, 21, 21; Apr. 3Mch. 4, 19, 25, 28

Feb. 23; Mch. 6, 10, 10, 10, 16, 17, 22; Apr. 2, 3, 3, 29Feb. 21, 22, 28; Mch. 9, 20, 21, 24, 26, 28, 30; Apr.

3, 7, 8, 9Mch. 4, 4, 4, 5, 5, 7, 8, 8, 10, 11, 11, 19, 20, 21, 21, 21,

22, 22, 22, 23, 24, 24; May 5Feb. 21; Mch. 6, 6, 6, 7, 7, 7, 8, 8, 8, 18, 20Mch. 5, 8,9,10,10,12,18,18,19,19, 20,24,29; Apr. 3Mch. 7, 8, 10, 10, 16, 19, 20Mch. 18, 20, 21Mch. 18, 23; May 4

* Italicized figures indicate the date of a home secondary.

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110 W. F. "WELLS, M. W. WELLS AND T. S. WILDER

state of susceptibility at the beginningof the school year. Comparison of thesusceptibility to measles in the fall of1940 with that in the fall of 1937, thefirst year of the experiment and the lastepidemic year in the city of Philadelphia,shows that in each department an in-creased density of susceptible childrenhad been built up, the increase being: inthe senior department, from 11.7 to 23.0per cent of the total; in the intermediatedepartment, from 26.0 to 48.9 per cent;and in the primary department, from66.1 to 75.5 per cent.

Contagious diseases. On November19th, 1940, a child in the fourth gradeexposed his class, within the irradiatedatmosphere, to measles, developing a rashthe following day. No secondary casesfollowed in the class, although the child'syounger brother in the third grade de-veloped measles under quarantine; andon December 4th, a child in the secondgrade developed the disease, presumably

from an outside exposure since no con-nection between the cases could be dis-covered. No further cases followed these3 and, in spite of the epidemic surround-ing them, the school remained free untilFebruary 21st.

The epidemic began with the occur-rence of 4 eases in the intermediate de-partment during the week of February17th. Its course is shown in tables 9and 10 and the weekly attack rate amongsusceptibles (omitting home secondarycases, contracted under quarantine) ofthe irradiated and unirradiated atmos-pheres is plotted in figure 2.

It will be seen from tables 9, 10 and16 that in the upper unirradiated classes(i.e., the senior and intermediate de-partments), which contained 141 sus-ceptible children (37.2 per cent of thetotal), 88 cases occurred during the epi-demic, of which 10 were home second-aries, contracted under quarantine.These 88 cases represent an attack rate

TABLE 10

Measles, Germantown Friends School,Weekly attack rate among susceptibles

Week of

2/172/243/33/103/173/243/314/74/144/214/285/55/12

Unirradiated,

Suscep-tibles(Cases

de-ductedweekly)

141137136112997264595656565453

Totalcases

41

24132785300210

Senior and interm. depts.

Homesecond-aries,dev.

underquar.

0000422200000

Cases,not

incl.home

second-aries

41

24132363100210

Weeklyattackrate

amongsuscept.,not incl.

homesecond.

2.80.7

17.611.623.28.34.71.7003.61.90

Irradiated, Primary dept.

Suscep-tibles(Cases

de-ductedweekly)

1101101091061059994898787878686

Totalcases

0131655200100

Homesecond-aries,dev.

underquar.

00l0212200000

Cases,notincl.home

secbnd-aries

0121443000100

Weeklyattackrate

amongsuscept.,not incl.

homesecond.

00.91.80.93.84.03.20001.100

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among the 141 susceptibles of 62.4 percent; if the home secondary cases areexcluded, the remaining 78 cases rep-resent an attack rate among the 141susceptibles of 55.3 per cent. At theend of the epidemic there remained 53children (13.9 per cent of the total en-rollment) still recorded as susceptible.

In the irradiated primary department,

It will be seen in Table 10 and figure2 that the cases in the primary depart-ment, both those possibly contracted inthe school and those probably contractedunder home quarantine, followed theepidemic wave of the upper classes by a2-week interval and were apparently aninfiltration of infection from thoseclasses.

FIGURE 3. Classroom, Swaithmore public schools, slide wall fixtures.

on the other hand, there were at thebeginning of the epidemic 110 suscepti-ble children (75.9 per cent of the totalenrollment). There occurred 24 cases ofwhich 8 were home secondaries, an at-tack rate of 21.8 per cent of the 110susceptible children if the home second-aries are included, or of 14.5 per centif they are excluded. At the end of theepidemic there remained 86 susceptiblechildren, 59.3 per cent of the total.

The frequency of colds before and afterthe use of lights

In contrast to childhood contagions,the total numbers of colds after installa-tion of the lights has shown no detect-able reduction. The question arises: arecolds actually spread within the irradi-ated rooms, or can the results be in-terpreted to mean that children are sub-jected both within and without the school

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112 W. F. WELLS, M. \V. WELLS AND T. S. WILDER

to such repeated exposure that the in-cidence of the disease is determined bysusceptibility rather than by exposure?In the case of the common cold, wherelasting immunity is not conferred, whereseveral attacks per child per season mayoccur, and where adults as well as chil-dren harbor the virus, do home andrandom infections occur so frequentlyas to mask any reduction accomplishedin the school?

Analysis of the absences for colds be-fore and after installation of the lights

There has been no marked shift in thisdistribution since the lights were in-stalled, nor has there been any reduc-tion in the total number of colds. Intable 11, colds, before and after thelights were installed, are classified ac-cording to the first day of absence whichthey caused.

Experiment II. Swarthmore publicschools

Prior to the 1940-41 session, the ex-periment was extended to include the

TABLE 11

Incidence of colds by days of the week *Primary department, Gennantown Friends School, 1933-34—1939-40

Unirradiated rooms 1933-34—1937-38Number . . . .Per cent . . . . . . . .

Monday

87141.0

Tuesday

36817.3

Wednesday

29513.9

Thursday

28613.5

Friday

30213.7

Total

2122

Wed. Thurs. and Fri. 883 cases, 41.6 per cent.

Irradiated rooms 1937-38—1940-41NumberPer cent

71040.9

21512.4

173S

Wed. Thurs. and Fri. 736 cases, 42.3 per cent.

1 The colds in this table are classified according to the first day of absence which they caused.

carries conviction that the incidence isnot determined, at least in the German-town Friends School, by school exposure.If a cold is indeed an infection, and ifan incubation period of from 36 to 48hours is assumed, the absences beginningon Wednesdays, Thursdays, and Fridaysmight be expected to amount to morethan their proportionate share of theweekly total if school were the chiefsource of infection. As a matter of fact,however, absences beginning on thosedays constitute, both before and afterthe installation of the lights, just slightlyless than three-sevenths of the total.

public schools of Swarthmore, Pa. Thetown of Swarthmore (pop. 3,405) is asmall college community suburban toPhiladelphia. The two primary schoolsand high school constitute almost theentire school system of the community,there being no parochial school and onlyone small private school (a nursery de-partment and a college preparatory—no primary school grades) drawinglargely from outside the Borough ofSwarthmore.

These schools, while public, differ lessfrom the Germantown Friends Schoolthan would most public schools, as the

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RADIANT DISINFECTION OP AIE 113

J/w

Primary Classes, IRRADIATED ROOMS

V* Vn

Upper Classes, UNIRRADIATED ROOMSFIGURE 4. Measles, 1941, Swarthmore public schools. Weekly attack rate among sus-

ceptibles in irradiated and unirradiated rooms, not including home secondary cases. Data intable 15.

dominant influence of Swarthmore Col-lege results in the maintenance of stand-ards comparable to those of the German-town Friends School. Due to these highstandards there is a number of childrenwho live outside the school district whopay tuition to attend the schools, espe-cially the high school. The school popu-lation is less continuous than at German-town, and the immunity is less a reflec-tion of past contagion.

The school system includes a highschool (grades 7-12), and two primaryschools (kindergarten to sixth grade) :the College Avenue School, on the samegrounds and in close proximity to the

high school; and the Rutgers AvenueSchool, about a mile distant. The twoprimary schools were irradiated and thehigh school was left unirradiated. Theexperimental set up is, therefore, similarto the one at the Germantown FriendsSchool, in that there is an irradiatedlower group and an unirradiated uppergroup, with the difference, however, thatthe experimental group here includestwo grades (fifth and sixth) above thosein the Germantown group, and that theRutgers Avenue School is at some dis-tance from the unirradiated classes ofthe high school.

There is no school physician at Swarth-

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TABLE 12

Contagious diseases, Swarthmore public schools.Nine years prior to installation of ultra-violet lights

Year

1931-321932-331933-341934-35 .1935-361936-371937-381938-391939-40

Total, 9 years prior toirradiation

Measles

Primary

CoUege

46713010

14110

110

Rutgers

002000

5202

56

Highschool

013020

1000

16

Chicken-pox

Primary

College

158015

54600

125

Rutgers

131223

52200

93

Highschool

180014200

16

Mumps

Primary

CoUege

006020810

17

Rutgers

10

17020200

22

Highschool

001000

1711

20

Primary schools, grades Kindergarten to sixthHigh school, grades seventh to twelfth.

more, but there is a school nurse 3 daysa week. Naturally, exclusion from schoolfor contagious disease is not quite soprompt as at Germantown.

Design

(See figure 3)

The experiment at Swarthmore differsfrom that at Germantown in that onlythe "home rooms" are irradiated. Thedifference is, however, not so great asmight at first appear, for the pupilsspend relatively more time in the homerooms because the schools do not havespecial rooms for art, music, naturestudy, etc. The children of the variousclasses do, however, mingle with one an-other, and less often with the membersof other groups in the unirradiated hall-ways, lunchroom and gymnasium.

Radiant disinfection of the air of theprimary schools is accomplished withside-wall fixtures. Four 36-inch germi-cidal burners, mounted in suitable re-

flectors,4 radiate from a 7-foot levelthrough an approximately 5-foot distanceto 12-foot nonreflecting ceilings in roomsof some 7,000 cubic feet capacity, venti-lated by forced draft. Bacterial testsindicate the equivalent of approximately100 changes of air per hour.

Contagious disease record prior to theuse of ultraviolet lights

The incidence in the Swarthmoreschools of the contagious diseases chosenas indices is shown in table 12 for the9-year period prior to the installation ofthe lights.

A. The first experimental year:1940-41

Susceptibility. In table 13 is shownthe state of susceptibility of the pupilsto the contagious diseases at the begin-

* General Electric Company 36-inch fluores-cent type germieidal burners in Wheeler Com-pany fixtures.

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ning of the experiment. It will be noted,by comparing table 13 with table 8, thatthe susceptibility to measles was lowerat Swarthmore than at Germantown bothin the irradiated and in the unirradiatedgroups.

Contagious diseases. In the Swarth-more schools, as in the city of Philadel-phia and in the Germantown FriendsSchool, cases of measles reached a higher

eluded, the remaining 71 eases representan attack rate of 51.8 per cent of the137 susceptibles. At the end of the epi-demic there remained 62 children stillrecorded as susceptible (11.1 per centof the total enrollment).

In the irradiated College Avenue Pri-mary School class-rooms (located on thesame grounds as the high school) therewere at the beginning of the experiment

TABLE 13

Susceptibility, Swarthmore public schools.Fall, 19Jfi

Group

UnirradiatedHigh school (Grades 7-12)

IrradiatedCollege Avenue (Grades Kdg.-6)..Rutgers Avenue (Grades Kdg.-6)..

Number

535

174165

Measles—susceptible*

Number

137

10289

Per cent

25.6

58.653.9


Number

114

7481

Per cent

21.3

42.549.1

Mumps—susceptible

Number

291

148137

Per cent

54.4

85.183.0

* Thirteen children who, in the fall, gave a history of having had measles (and one who was notsure) later developed the disease. For simplicity, these children were classified in the tables assusceptible.

figure than in any year for which figuresare available, a total of 122 cases havingoccurred, 76 in the undirradiated highschool and 47 in the two irradiated pri-mary schools.

In table 14 the dates of the cases aregiven by the grades in which they oc-curred.

In table 15 it may be seen that in theunirradiated high school classrooms therewere, at the beginning of the epidemic,137 children susceptible to measles (25.6per cent of the total enrollment). Thereoccurred during the epidemic 75 cases ofthe disease, of which 4 were home second-aries contracted under quarantine.These 75 cases represent an attack rateamong the 137 susceptibles of 54.7 percent. If the 4 home secondaries are ex-

102 children susceptible to measles, or58.6 per cent of the total enrollment.There occurred during the epidemic 27cases of the disease, of which 11 werehome secondaries. These eases representan attack rate among susceptibles of26.5 per cent if home secondaries areincluded, 15.7 if they are omitted. Atthe end of the epidemic there remained75 children still recorded as susceptible(.43.1 per cent of the total enrollment).

In the Rutgers Avenue school therewere, at the beginning of the epidemic,89 susceptible children (53.9 per centof the total enrollment). There occurred20 cases of the disease, of which 12 werehome secondaries. The 20 cases repre-sent an attack rate among the 89 suscepti-bles of 22.5 per cent. If the home second-

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116 W. F. WKLLS, M. W. WELLS AND T. S. WILDER

50

45

40

35

30

25

20

15

10

5

0

— tprtmarytf school age 79y under school age M

-Secondary tVL

K SchooTA infected,t School^ Infected.

18 25 2 ? 16 25 50 6 13 20 27 4 II 18APD1L i i MAY JUNE JULY

FIGURE 5. I. Eenfrewshire—measles epidemie in an isolated village showing the weekly inei-dence of primary cases of school age, under school age and secondary cases.

35

30

* School X tnfeebd~t School*' fnfeefaL.

Primary of school age 140Vrvmgr trader school age 26T5 3 1M

5 J2 19 2 ? 2 9"l6*'23 I 8 15 2 2 2 9 S 12 1926 2 9 16 23 50 6 1520 27 4 1118 25 I 8 15JANY fTB i MAE I APD W JUNE « W f AU6.

FIQUEE 5. II . Renfrewshire—measles epidemic in a populous district showing the weekly inci-dence of primary cases of school age, under school age and secondary cases

Figure 5, I and I I are taken from Picken (9) and are reproduced by permission of theBoyal Society of Medicine.

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RADIANT DISINFECTION OP AIE 117

TABLE 14

Measles, 1941.

Swarthmore public schools

School

College Ave.(.Irradiated)

Rutgers Ave.(Irradiated)

High school(Unirradiated)

Grade

Kind.123456

Kind.123456

7

8

9

10

1112

Susceptible*

Number

12201412131615

1722118

13135

29

28

34

20

1410

Per cent

66.783.370.075.048.144.445.4

85.088.061.138.159.143.317.2

39.2

35.0

27.6

21.5

17.312.7

Dates of cases*

March 11, 22, 22; April 9April 15, 22March 21, 31; April 2, 3, 3, 4, SOApril 8; May 7April 23, 23, SOApril 6, 14, 15, 20, 27March 24; April 17, SO; May 5

March 5, 26March 19, 28, SO; April 2, 10, 27April 26April 13March 2, 10; April 2, 10, 10, 26; May 18March 18; April 2, 10No cases

Feb. 22; March 10, 11, 17, 19, 21, 22, 22, 22,23, 23, 24, 24, 24, 25, 26, 26; April 4, 13, 15,15,15

Feb. 21; March 19, 21, 21, 22, 22, 23, 24, 24,25; April 3, 10, 14, 15, 15, 19

March 6, 7, 7, 8, 15, 16, 17, 19, 21, 21, 22, 23,24, 27, 31

March 7,7,14,15,17,17,18,19,20, 20, 22, 23;April 17

March 17, 25, 28; April 4, 17March 24; April 4, 6, 18

* Italicized figures indicate the date of a home secondary.

aries are omitted the remaining 8 casesrepresent an attack rate of 9.0 per cent.There were, at the end of the epidemic,69 children still recorded as susceptible(40.2 per cent of the total).

DISCUSSION

During the 4-year study in the Ger-mantown Friends School and the 1-yearstudy in the Swarthmore public schools,there has been no epidemic spread ofcontagion among the highly susceptiblegroups of children of the primary schoolswithin irradiated atmospheres, although

epidemic spread has occurred among lesssusceptible groups of older children inthe departments of the schools whoseatmospheres were not irradiated.

During the first experimental year, 7introductions of mumps into the irradi-ated and 7 into the unirradiated class-rooms of the primary department atGermantown were followed by 2 and 12cases, respectively. During the secondyear, an epidemic of 32 cases of mumpsoccurred in the upper unirradiated class-rooms, with only one case appearing inthe irradiated primary classes. While

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mumps is a disease of lower incidencethan measles or chicken pox and sus-ceptibility, therefore, extends into theolder age groups, there is no selectiveaction for the older ages, the peak in-cidence in the United States being amongthe 7- and 8-year groups (8). In the5 years at Germantown and the 9 yearsat Swarthmore prior to irradiation, the

within the irradiated atmospheres re-sulted in only 7 secondary cases, butexposure of one class for a relativelyshort time on each of 4 successive daysin an unirradiated atmosphere with amissed case resulted in a static type epi-demic involving 15 of the 16 susceptiblesin the class. The susceptibility tochicken pox in the upper grades was so

TABLE 15

Measles, Swarthmore public schools, 1941.

Weekly attack rate among susceptibles

Unirradiated, High, school

nU

s f j

^ a S

Irradiated, College Avenue school Irradiated, Rutgers Avenue school

I!u 111

IIIII111

2/172/243/33/103/173/243/314/74/144/214/285/55/12

1371351351291239480747262626262

2066

291462

100000

2066

28145190000

1.504.44.7

22.814.96.31.4

12.50000

1021021021021019897908984807775

0001.03.01.05.21.13.42.4000

89898887868481787373707070

01.11.11.11.2002.601.4001.4

primary classes enjoyed no such exemp-tion. If, as some believe, the total in-cidence of mumps is lower than that ofmeasles or chicken pox because there arethose who are naturally resistant, suchpersons would be included among thesusceptibles in the tables and wouldgradually become an increasingly greaterproportion of these "susceptibles" asthis group became depleted in favor ofthe immune group.

During the third experimental year,21 exposures to chicken pox of classes

low that spread could not have been ex-pected.

The most convincing confirmation ofthe hypothesis comes from the results ofthe 1941 measles epidemics at German-town and Swarthmore. Reference totable 16 shows that the attack ratesamong susceptibles (not including homesecondaries) was: among the primaryirradiated classes, 14.5 per cent at Ger-mantown, 15.7 per cent at College Ave-nue, 9.0 per cent at Rutgers Avenue;among the upper unirradiated classes,

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55.3 per cent at Germantown, 51.8 percent at Swarthmore. In tables 10 and15 it will be seen that the time relation-ships of the epidemic are more indicativeof an infiltration from the upper schoolsinto the primary classes than of epidemicspread within the latter. Reference totable 16 also shows that in each of the 3primary schools, the susceptibility wasconsiderably higher at the end of theepidemic than it was in the upper classesat the beginning. Certainly the pat-

the eight upper classes, there were 38cases (9.7 per cent).5 At the German-town Friends School, 12.6 per cent ofthe pupils in the primary departmentwere attacked, and 23.2 per cent of thepupils in the upper classes.

Equally favorable results would notnecessarily follow radiant disinfection ofschools chosen at random. The preven-tion of air-borne infection among "in-termittent aggregations" necessitatesboth an effective means of control and

TABLE 16

Measles, 1941, Germantown and Swarthmore

Irradiated lowerGermantown grades 1-4Swarthmore grades Kdg.-6

College AvenueRutgers Avenue

Unirradiated upperGermantown grades 5 - 1 2 . . . .Swarthmore grades 7 - 1 2 . . . .

Pupils

145

174165

379535

Susceptibles- before epidemic1

Num-ber

110

10289

141137

Percent

75.9

58.653.9

37.225.6

Cases

24

2720

8875

Homesecond.

8

1112

104

Attack rateamong suacept.

Incl.homesecond.

21.8

26.522.5

62.454.7

Notincl.homesecond.

14.5

15.79.0

55.351.8

Susceptibles atend of epidemic

Num-ber

86

7569

5362

Percent

59.3

43.140.2

13.911.6

terns of the epidemics in the two schoolswere strikingly different from any previ-ous experience.

One might almost suspect a strain ofvirus with an affinity for older ages. The6-year group in Philadelphia, however,showed as usual the highest incidence.Likewise, at the Penn Charter School, aprivate day school for boys severalsquares distant from the GermantownFriends School, whose pupils are drawnfrom the same social class, the primaryclasses suffered most heavily. Among95 boys in the first 4 grades (suscepti-bility unknown), there were 47 cases(49.5 per cent) total; among 391 boys in

the employment of that means withinthe particular atmosphere in which theinfections actually were transferred.Picken (9) has previously emphasizedthat, so far as the administrative controlof measles is concerned, "any measuresfor this purpose must have regard to thechannels of infection. Children are in-fected in their homes or at school or atrandom"; and he has pointed out thatin rural areas the school plays a rela-tively greater role in the disseminationof infection than in urban communities.

5 These figures were kindly sent to us by Dr.T. C. Garrett, physician to the Penn CharterSchool.

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120 W. P. WELLS, M. W. WELLS AND T. S. WILDER

In figure 5 (charts I and II taken fromPicken), are shown two epidemics ofmeasles: in a rural area, measles spreadfirst in the school and was then carriedhome to the younger children, the curveof school cases and home cases follow-ing the same pattern with a time lagcorresponding to the incubation period;in the urban district, on the other hand,opportunity for exposure was not thuslimited to the home and the school,random infection playing a greater role.Picken comments: ' ' Clearly it would bedifficult to control an epidemic of thelatter type by efforts directed at theschool. Random infection plays toolarge a part. On the other hand, theepidemic traced in Chart I indicates thatcontrol through schools may be pos-sible."

Multiple exposures are even more ofan obstacle in the control of colds. Theinterlocking network of aggregations inwhich the children become infected withchildhood contagions is largely re-stricted to those consisting of children.The epidemiological pattern of colds is,however, more complex; lasting im-munity is not conferred. To the net-work of childhood aggregations in whichinfection may be spread must be addedthose shared with adults; and home ag-gregations especially become relatively

more important. Incidence in diseasesto which exposures are multiple withinand without the school becomes a meas-ure of susceptibility rather than expos-ure, and prevention will become manifestonly when the last source of infection iseliminated.

It should again be emphasized that theprimary objective of the experimentswas not to postpone childhood contagionsto high school ages (though this maybe desirable in the poorer districts oflarge cities, where mortality is highamong pre-school children to whom in-fection may be introduced by primaryschool children), but to test the hy-pothesis (1) that epidemic contagion isspread through the medium of confinedatmospheres and can be prevented byradiant disinfection of air. The resultsobtained in the experiments describedhere lend support to this hypothesis, andpoint to the immediate application ofmethods described to prevent the oc-currence of contagious diseases at un-favorable times, such as measles in mid-winter when the danger of complicatingstreptococcal infections is highest, andin unfavorable situations, such as armybarracks, where overcrowding aidsspread and where facilities for care maybe overtaxed by an avalanche of cases.

REFERENCES

1. Wells, W. F., and Wells, M. W.1936 Air-borne infection. J. A. M. A., 107: 1698 and 1805.

Wells, W. F.1940 Droplet nuclei as vehicles of contagion. Proc, Third Internat. Cong, for Micro-

biology, p. 266.Wells, W. F.

1940 Studies on air-borne infection. Science, 92: 457.2. Hart, D.

1936 Sterilization of the air in the operating room by special bactericidal radiantenergy. J. Thor. Surg., 6: 45.

3. Overholt, R. H., and Betts, E. H.1940 A comparative report on infections of thoracoplasty wounds. J. Thor. Surg., 9:

520.

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EADIANT DISINFECTION OF AIR 121

4. Wells, W. F.1939 Sanitary ventilation in wards. Heating and Ventilating, 36: 26.

5. delMundo, P., and McKhann, C. F.1941 Ultraviolet radiations in control of hospital infections. Am. J. Dis. Child., 61 :

213.6. Wells, W. F., and Green, B.

The environmental control of epidemic contagion. II . A baeteriologic study of radiantdisinfection of air in schools. Paper in preparation.

7. Com. Amer. Public Health Ass'n.1940 The control of communicable diseases. Pub. Health. Reports, Reprint No. 1697.

8. Collins, 8. D.1929 Age incidence of the common communicable diseases of children. U. S. Pub.

Health Reports, 44: 763 (see Figure 8).9. Picken, R. M. F.

1921 The epidemiology of measles in a rural and residential area. Proc. Boyal Soc.Med., 14: 75.

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the environmental control of epidemic contagion

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