air pollution || some effects of air pollution

C H A P T E R 7

Some Effects of Air Pollution A L T H O U G H w e can all agree that air pollut ion is undesirable, and

that most o f its effects on health and d a m a g e to materials and

amenities are we l l known, the pattern o f our civilisation makes it

unavoidable . T h e purpose o f this chapter, is not to present the

terribly familiar catalogue o f misery and squalor but to d r aw

attention to mechanisms which illustrate that air pol lut ion p ro

duces its effects in m a n y compl ica ted ways . Some are understood,

others at present are not: the effects and the remedies are costly,

and therefore any advance in understanding wi l l make the

correction easier.

S M E L L S

Brickmaking and oil refining are t w o industries which make

nasty smells. Some o f these smells do not cause any direct d a m a g e ,

but smells are undesirable as such and are as much a nuisance as

noise, dirtiness, or corrosion. T h e y affect people 's frame o f mind .

Unfor tunately no instruments have been devised for measuring

the causes o f smells quanti tat ively, and evidence is ve ry difficult

to obtain. Smells are ve ry subjective, part ly because a person's

attitude to the cause o f a smell affects his v i e w o f the smell itself.

Employees in an industry associate smells wi th their source o f

income and soon ignore them; farmyard smells are tolerable in

their r ight context only , but any psychological accommoda t ion

to an inevi table smell is a hardening o f sensitivity and destructive

o f at least some aesthetic subtleties, and w e have no r ight to

impose smells on others on the grounds that i f they had the r ight

mental attitude to them, they wou ld not object .

124

SOME EFFECTS OF AIR P O L L U T I O N 125

T h e r e are also physiological mechanisms at work . T h e smell ing

organ becomes insensitive to smells to wh ich it is continuously

subjected. T h u s oil refinery employees b e c o m e physically in

capable o f detect ing some smells h o w e v e r much they m a y correct

any psychological adjustment that w o u l d be occurr ing a n y w a y .

F o r this reason it is useless to leave the detect ion o f smells in the

hands o f people w h o w o r k long hours near the source.

W h e n a nose has been rendered m o r e or less insensitive to a

smell b y continued strong doses o f the substance causing it, it

becomes comple te ly insensitive to the same chemical in much

smaller concentrations. But some smells become different w h e n

much di luted, and m a y even become m o r e object ionable so that

only at l o w concentrations d o they g i v e rise to complaints . I t

migh t therefore be falsely argued in defence o f a smell that it was

not found object ionable at higher concentrations nearer to the

source. T h e comple te causes o f these l o w concentrat ion effects are

not fully understood but one contr ibut ing factor is certainly the

comple te absence o f it in be tween whiffs.

T h e w i n d varies wi th height and there can be pockets o f slow-

m o v i n g air a m o n g buildings, wi th in trees, bushes, and tall crops,

wh ich retain traces o f a smell for long periods after a dense w h i f f

has passed by . Close to a source o f a smell, there m a y be no

moments at all dur ing wh ich the smelly substance is comple te ly

absent f rom the air w e breathe, so that our noses become insensi

t ive to it. A t greater distances, there are longer periods be tween

whiffs and all the pockets o f slow m o v i n g air become comple te ly

scoured out so that w e breathe fresh air, o r at least air free f rom

the smell in question, and w h e n another w h i f f arrives it is v e r y

noticeable, even though the concentrat ion is much less than the

lowest achieved closer to the source whe re it is not detectable .

T h e whiffs seem much m o r e object ionable too , w h e n they are

less frequent. Noises at unpredictable w i d e l y spaced intervals are

more disturbing than much louder but m o r e or less continuous

or regular ones; and so it is wi th smells. T h e impor tan t point here

is that smells are not di luted into the atmosphere in a s imple

manner . O f course, the concentrat ion o f pollutant in bodies o f

126 AIR P O L L U T I O N

air containing it gradual ly decreases but they are as sharp edged

as ever .

T h e outl ine o f a smoke p lume or cloud is usually sharp to beg in

wi th and only seems to become m o r e diffuse because o f the

decreasing difference be tween inside and outside. Nevertheless ,

w e should still speak o f whiffs because only after m a n y hours in

the air is a pollutant so diffused that it is no longer possible to

define the boundary o f the air containing it. A l e w seconds after

emission, whiffs o f smoke or smell are clear cut, and a few minutes

later, the sudden arr ival o f a smelly w h i f f three miles d o w n w i n d

can be equal ly sudden and v e r y object ionable . W e m a y refer to

F ig . 6 (page 32) to see that the further w e are from a source, the

less frequent is the passage o f the p lume ove r us and the l o w e r is

the concentrat ion w h e n it does arr ive , but in the geomet r ica l

sense, the arr ival is similar. I f an instrument w e r e to be devised

to detect smells at large distances, it w o u l d be impor tan t not to

take a longer term average further from the source as w o u l d seem

appropr ia te for other reasons, but to measure the rapidi ty w i th

which the m a x i m u m cpncentrations in the whiffs arr ive . T h e

whiffs p robab ly seem to pass over m o r e quickly than they actually

d o because the nose is most sensitive to rapid increases in con

centration and soon fails to detect a uniform level .

H E A L T H A N D O T H E R A M E N I T I E S :

S O M E M I S C O N C E P T I O N S

Conten tment is an impor tant contr ibutor to g o o d health, and

therefore, any disturbance b y smells, noise, dirtiness, or even

appearances, impairs health. W e shall not discuss any medica l

aspects o f the p r o b l e m here but mere ly ment ion some causes o f

error and misguided effort.

Undoub ted ly , the direct harmful effects o f smoking on the

health o f the general public are far greater than any due to air

pol lut ion. O n l y in special circumstances does pol lut ion d o c o m

parable d a m a g e . F o r example , special legislation has been

necessary to protect workers in certain industries, and this is

general ly so g o o d that the greatest danger is p robab ly to peop le


l iv ing close by w h o are not so protected. A n y o n e interested in the

nation's health wi l l perform the most r eward ing service b y get t ing

rid o f the evils o f smoking.

I n almost all cases o f outdoor air pol lut ion, there are very g o o d

other reasons for get t ing rid o f it, and the case on health grounds

is not usually as compel l ing .

O n e o f the most c o m m o n difficulties is that in public and

pr ivate discussion, pol lut ion is attr ibuted to the w r o n g source or

an effect is incorrectly attr ibuted to the pollutant. Fo r example ,

brickworks at a distance o f ten miles have been b lamed for acid

smuts such as are sometimes produced from the stacks o f oil-fired

boilers. These smuts had burnt holes in pa in twork and fabrics. I f

they had c o m e from the distant brickworks, their concentrat ion

at smaller distances w o u l d have been so great as to make life

intolerable there. T h e smuts w e r e observed at the same t ime as

the we l l -known brickworks smells because they came from the

same direction, and i f this kind o f ev idence is adduced, a bad case

is made , and a well-refuted, i l l-founded compla in t serves only to

strengthen the position o f the polluter.

Complain ts o f 'sulphur fumes' are often misguided. A public

campa ign was conducted against the p o w e r station whose

ch imney plumes are shown in Plate 5.6 largely on account o f the

whiteness o f the p lume.

I t was incorrectly held responsible for much pol lut ion from

other sources because it could always be seen to be emit t ing at the

t ime when fumes were detected. T h e C i t y o f L i v e r p o o l at that

t ime contained many most disgraceful sources o f pol lut ion, the

worst o f which w e r e domestic chimneys and ships. T h e r e was a

permanent smelly haze o f domestic smoke over some o f the older

housing areas, which reddened the sun and dir t ied every object

to the touch; yet the propagandists compla ined that the occasional

shadow o f the p lume was cutting of f much needed sunlight.

T h e total emission o f sulphur was quoted to show that the

p o w e r station was a major source o f pol lut ion in the city. State

ments o f total emissions are to be found in m a n y documents,

official and propagandist , as i f the information could be useful on


its o w n . O n e could argue that knowledge o f the total emissions is

a necessary starting point in discussions about pol lut ion but this

is absurd unless w e have a certain means o f deducing, f rom this

knowledge , wha t concentrations o f pollut ion wi l l occur at g round

level or in other places where it is important . I n fact, w e d o not

have such means, and our theories a lways have constants in them

wh ich have to be deduced by comparison wi th observations. I f

the theories w e r e universally true so that from one set o f observa

tions, w e could evaluate the constants and deduce wha t wou ld

happen on other occasions, the data on total emissions could be

used, but this is not the case. T h e topography and the weather

are so var iable that w e real ly have to study each case on its o w n ,

and i f measurements cannot be m a d e at the place o f interest, a

special study m a y be required in a v e r y closely analogous situa

tion. T o p rov ide the public wi th data o f total emissions left in r a w

state, as part o f a discussion o f a serious p rob lem, is d a m a g i n g to

progress. I t m a y even hide some small, but insidious and d a m a g

ing source in a co lumn label led ' O t h e r s " .

T h e question whether sulphur emissions on their o w n are

d a m a g i n g to health is an open one, but it seems probab le that

they are not. T h e w h i f f from the night-watchman's brazier is so

much m o r e intense than any other w h i f f o f S O 2 rece ived by the

public that i f its effects we re harmful, the braziers w o u l d have

been long ago condemned .

Traff ic exhaust is object ionable. Because o f a longer experience

o f petrol engine fumes, the public finds diesel fumes m o r e un

pleasant. Diesel engines need not produce black smoke, but as

eve ry cyclist knows, their exhaust is often ve ry unpleasant. N e v e r

theless, the main argument against them m a y not be on grounds

o f d a m a g e to health. T h e r e is p robab ly a serious effect due to

carbon m o n o x i d e produced b y petrol engines in streets wi th dense

traffic or road tunnels. But G O cannot be smelled, and headaches

and fractiousness it produces m a y be laid at other doors. A n

investigation in L o n d o n bus garages showed that the health o f

employees in diesel bus garages was much better than in electric

trolley-bus garages, and this was almost certainly because on ly in

the latter was smoking permit ted.


T h e r e is serious difficulty in using informat ion about complaints

either to condemn or to whi tewash a source o f pol lu t ion . I n v i e w

o f the complex i ty o f the effects o f di lut ion on smells a l ready

ment ioned and other mechanisms o f wh ich observers m a y be

ignorant , any authori tat ive suggestion tends to be taken as a

general truth and this makes witnesses qui te unrel iable. Recen t ly ,

I asked residents in a street close to an incinerator whether it was

a cause o f nasty odours, and people thereupon at tr ibuted to the

public incinerator smells c o m i n g f rom a local e lectroplat ing

works which was not ment ioned in the conversation. Clear ly , the

wish to be helpful had some influence on their spontaneous

replies, and it is much less interesting to have no information to

impar t .

T h e average ci t izen is too busy to bother to make official c o m

plaints about large industries, and does not wish to ha rm his

neighbour ly relations by compla in ing about smelly chimneys or

bonfires. R e c o r d e d complaints are no measure o f the magni tude

o f a nuisance, but more representative o f a chance combina t ion

o f the standards o f the complainers and a large number o f other

factors such as vested interests, psychotic conditions, press

campaigns , leading questions, and prejudices.

T h e out look w o u l d be black, in v i e w o f all these and other un

certainties, i f w e d id not really k n o w wha t the serious sources o f

pol lut ion w e r e . T h e uncertainties enter the argument w h e n

pressures i r relevant to the facts o f the pol lut ion and its effects

begin to be exerted. These pressures are, o f course, not necessarily

disreputable, because the wea l th o f the communi ty depends v e r y

much on industry and transport wh ich at present have to pol lute

the air. Progress is achieved by being aware o f the mechanisms

and processes by which pol lut ion harms us so that w e steadily

el iminate the known worst sources and app ly ever m o r e stringent

standards to new sources.

B L A C K E N I N G O F S U R F A C E S

I t is natural to regard the blackening o f a solid surface exposed

to the air as be ing due to the straightforward deposit ion o f sticky


PLATE 7 .1 . Thermal precipitation on to a cold wall and ceiling above a hot water pipe. The deposition occurs from the hottest air, and this is where the pipe is closest to the wall, or where some unevenness or obstruction to smooth flow stirs warm air from the convection current

to the surface

substances such as domestic smoke on the surface. But m o r e

careful observation shows that m a n y compl ica ted mechanisms are

i n v o l v e d , and some o f these are scarcely understood.

T h e r m a l precipi ta t ion (P la te 7.1) is a cause o f much indoor

blackening o f walls and ceilings. T h e r e are t w o distinct mechanisms

i n v o l v e d : first, the deposit ion occurs f rom w a r m air on to a co ld

surface. Consequent ly it is found a b o v e central heat ing radiators

and hot pipes. I t also occurs on ceilings, but often the deposit ion

there is so uniform as to be impercep t ib le ; h o w e v e r on some

ceilings the non-uniform thermal conduct iv i ty o f the ce i l ing is

shown b y the faint outl ining o f the rafters and hidden nail heads

in the pattern o f the deposit ion. I t is prevented, or at least reduced

by ensuring that the hot air is wel l m ixed wi th r o o m air before it

makes contact w i th the wa l l .


PLATE 7.2. The north face of an octagonal tower on Exeter Cathedral is completely black while the adjacent faces remain relatively clean. The stonework is also blackened under eaves while the upward facing parts of the window stonework appear washed clean. Similar patchy

blackening can be found on many buildings, old and new

T h e second mechanism which causes increased deposit ion is

local turbulence in the airstream. Th i s brings w a r m air m o r e

quickly to the surface and therefore increases the temperature

gradient close to the surface. T h e turbulence also has the effect o f

carrying particles to the surface, and i f they are sticky they

remain on it. T h i s last mechanism operates in the backs o f our

noses where the air w e breathe passes through a na r row passage

and emerges wi th considerable turbulence into a w i d e r naza l

passage in which dust particles are deposi ted on to a surface

covered wi th mucous. T h e same kind o f deposit ion due to turbu

lence in nar row air passages takes p lace around w i n d o w and door

frames, especially where they are not tight-fitting.


PLATE 7.3. The flying buttresses on Lincoln Chapter House show blackening in the 'shadows' of the columns at their outer ends. Also the south-east-facing wall is less blackened than the east- or south-facing ones, and this appears to be connected with the frequency of

sunshine, the south-east face being much less shaded These pictures were taken on an overcast day so that there were no

sharp shadows


PLATE 7.4. Stone figures on Somerset House, London, where the upper surfaces are washed clean and the lower surfaces carry an accumulation or growth of black pollution. Similar disfigurements were common in the very soluble Headington stone used in Oxford

before it was recently replaced

T h e effect o f the airf low in de te rmining the locat ion o f stone

blackening is often clearly seen on the corners o f buildings

(P la te 7 .2) . T h e r e is p robab ly not a single cause for this but

almost certainly the separation o f the airf low at corners plays a

major part. T h e blackening is not usually a simple deposi t ion,

but is often organic and caused b y the absorption o f S O g f rom

the air. T h e r e is a var ie ty o f chemical reactions and other

mechanisms which induce or inhibi t b lackening o f bui lding stone

or brick, a m o n g which are the fo l lowing :

( i ) the effect o f w i n d in dry ing out the surface after rain.

( i i ) the effect o f w i n d in producing a differential we t t ing o f the

surface. W a t e r m a y be an agent for c leaning the surface i f it is

sufficiently plentiful or for causing the blackening i f it remains

on the surface (P la te 7 .4) .


PLATE 7.5. Dust deposition on a figure in terracotta (non porous) on the Royal College of Science, London. This figure is protected from most rain by an overhanging balcony, and shows how darkening would occur under the effects of deposition alone. There is intense blackening in the places where no direct sunshine penetrates which

may be due to a growth which is killed by sunshine

( i i i ) the possibility o f frequently being ve ry we t , thus intense

blackening often occu.rs where water f rom an over f low p ipe or

leaky gutter often impinges on a wa l l , but where the stone is

soluble it m a y be cleaner in such places.

( i v ) the effect o f shadows in de lay ing the d ry ing o f a surface by

sunshine. Shadowed areas such as under w i n d o w sills or north-

facing walls often show m o r e blackening (Plates 7.2, 7 .3) .

( v ) the effect o f direct sunshine in bleaching or prevent ing the

g rowth o f organisms.

( v i ) the chemical nature o f the surface, and in particular its

solubility in a weak solution o f the pollutant.

( v i i ) the physical nature o f the stone surface, for example its

p o w e r to hold water for long periods after wet t ing (P la te 7 .6) .

( v i i i ) the presence o f a (metal l ic?) catalyst for the chemical

processes which cause blackening. Thus blackening often occurs


PLATE 7.6. Stonework on the south porch of Liverpool Cathedral which is built in red sandstone. The mortar between the stones seems to be sufficiently alkaline to prevent black growth: the stone surfaces vary greatly in some attribute, probably porosity, and have not been blackened uniformly. This stone is very susceptible to blackening where water runs down it, and this is detectable in this scene where rain running off the smooth mortar has caused a strip of blackening

on the stone below

be low an iron bolt head or nail in a wa l l , notably on red brick

surfaces. Th i s m a y be connected wi th the falling o f rain d o w n the

wal l at the protuberance, but appearances suggest a chemical

effect.

( i x ) the accumulat ion o f black g rowth on the under side o f

stonework: this m a y be a mere deposit ion o f mater ia l washed

from the soluble parts, or a g rowth b y chemica l reaction in the

presence o f water wh ich is favoured in shaded parts o f the

stonework.

( x ) direct deposition o f black particles undoubtedly takes p lace

on to solid surfaces. But this can scarcely be the cause o f blacken

ing o f vert ical walls, and general ly on ly occurs whe re the surface

is not exposed to rain (P la te 7.5) wh ich w o u l d wash o f f the

deposit ion.


E X P A N S I O N A N D C O R R O S I O N

M u c h corrosion is caused b y natural pol lut ion. P la te 7.7 shows

one o f the effects o f a irborne salt particles. Bulges and weaken ing

are produced in overhead p o w e r cables which contain strands o f

a luminium and o f steel. A n electrolytic eflfect produces m o r e rap id

deteriorat ion o f the conductor cables than o f the earth wires in

general , but in coastal and bad ly pol luted industrial areas the

earth wires seem to suflfer equal ly severely. T h e same is true o f

the stunting o f trees and other vege ta t ion—it is worse at the coast

than in industrial areas, w i th a few localised exceptions.

T h e expansion due to react ion be tween a substance and an air

pollutant often causes the flaking o f stonework. M a n y cathedrals

suflfer from disfigurement o f statues or decorated masonry because

o f this. Evident ly the S O 2 in the air combines wi th carbonates o f

i ron in some stones to form compounds, in the presence o f water ,

o f larger vo lume .

A n interesting deve lopment in architecture is the decora t ion

o f large areas o f wa l l o f box-l ike buildings wi th some relief. A

major reason for this is that streaks o f blackening often cause

unpleasant disfigurement o f large uniform surfaces and the break

ing up o f the surface so that this disfigurement is not dominan t , is

a t tempted. Of ten , however , the decorat ion and rel ief are them

selves disfigured in ways which are unpredictable because the

mechanisms and reactions which cause disfigurement are scarcely

understood.

Often circular columns have one side blackened, and the edge

of the blackening m a y be the l ine o f separation o f the ai r f low or

the edge o f the rain deposit ion w h e n the w i n d is in the most

c o m m o n direct ion for rain. Sometimes it is the edge o f the reg ion

not subject to sunshine. W i t h a fuller knowledge o f the causes o f

disfigurement by blackening it migh t be prevented . Var ious

possibilities include the spraying o f the stonework after washing

wi th silicones or other non-wet table substances or wi th a solution

o f a chemical poisonous to the g rowth (this has obvious dangers) .

SOME EFFECTS OF A I R P O L L U T I O N 137

PLATE 7.7. Examples of power cable corrosion from the coast of Cornwall. The upper picture shows a bulge due to the expansion of material in chemical reaction and the lower picture aluminium conducting stands taken from a point l i miles from the sea where the effect was much less than close to coast. The four samples (left to right) show the outer and inner surfaces of an outer strand and the

outer and inner surfaces of an inner strand

air pollution || some effects of air pollution

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