seasonal and yearly patterns of indoor nitrogen dioxide levels: data from albuquerque, new mexico

Indoor Air 1994, 4: 8-22 Printed in Denmark . all rights reserved

Copyright Q Munksgaard 1994

Indoor Air ISSN 0905-6947

Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide Levels: Data from Albuquerque, New Mexico

Margo Schwab', 2, Aidan McDermott2, John D. Spengle8, Jonathan M. Samet3 and William E. Lambert3 Harvard School of Public Health, Boston, MA

Abstract There are fez0 data sets appoprute for characterizing the indoor concentrations of air pollutants over the h g term. A n understanding of the variability in indoor pollutant levels is particu- lurly relevant to the design of epidemiologic investigations: mis- classifiation of exposure due to the inaccuracy of exposure estimates tends to weaken the association of exposure with health outcome. This paper uses a series of indoor NO2 measurements collected at two-week intervals over 18-month periods between 1988 and I991 to describe the seasonal andyear-to-year variability in indoor NO,. The data show that there can be large year-to-year differemes in both the sample distribution of indoor NO2 as well as the household average. For homes with gas ranges with continuously-burning pilot lights, the average bedroom NO2 concentration was 25% higher in the winter of 1990-1991 than in the winter of 1989-1990 but only 4% higher during the winter of 1988-1989 than during the winter of 1989- 1990. The winter-to-winter correlations within homes ranged from a low of 0.53 to a high of 0.88. The year-to-year differences in mean indoor concentrations were not related to temperature patterns. Occupant behaviors that influence air exchange rate andlor source use are hypothesized to be the major determinant of the observed pattern. Exposure data collected during a single year should be cautiously extrapolated to other years. However, in Albuquerque homes, the data suggest that the yem-to-year variability in household NO2 levels will not have a strong impact on classiJjzng exposure into broad categories.

KEY WORDS Nitrogen dioxide, Human exposure, Indoor air, Seasonal pollution patterns, Yearly variation, Passive monitoring.

Manuscript received: 27 October 1992 Accepted for publication: 9 June 1993 ' Now at ManTech Environmental Technology, 2 Triangle Drive,

Box 12313, Research Triangle Park, NC 27709, Fax: 919-541- 4958; Voice: 919-541-2245 Department of Environmental Health, Harvard School of Pub- lic Health, Boston, MA, USA University of New Mexico Medical Center, Albuquerque, NM, USA

Introduction Concern has been raised about the potential respiratory health effects of exposure to nitrogen dioxide (NO2) in indoor and outdoor environments, particu- larly for young children (Samet et al., 1987; Samet and Utell, 1990). The residential environment has been the focus of recent studies on patterns of exposure and health effects because people spend most of their time indoors where combustion emissions from household appliances can raise indoor levels well above outdoor concentrations (NAS, 1981). Through the use of source inventories and inexpen- sive passive sampling devices, these investigations have provided abundant information on the range and levels of typical exposures, the relationship between indoor and personal exposures to NO2, and the contributions of household sources to indoor NOz (Goldstein et al., 1979; Dockery et al., 1981; Spengler et al., 1983; Clausing et al., 1986; Quacken- boss et al., 1986; Noy et al., 1986; Wilson et al., 1986; Ryan et al., 1988a,b; Ryan et al., 1990; Spengler et al., 1990). However, because protocols for most field studies have involved monitoring only once within a given season, little is known about the accuracy of such limited measurements for representing long- term household NO2 levels. An understanding of the variability in indoor pollutant levels is particu- larly relevant to the design of epidemiologic investigations; in epidemiologic studies, misclassification of exposure due to the inaccuracy of exposure estimates tends to weaken the association of exposure with health outcome. A few recent investigations have addressed the variability in NO, measurements, showing that within a household, indoor concentrations tend to remain relatively stable over a single winter season (Brunekreef et al., 1986; Lam- bert et al., 1992). A related concern for epidemiologic studies is the stability in the year-to-year and

Schwab et al.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels 9

season-to-season distributions of indoor NO, concentrations and the factors that influence the temporal profile of NO2 concentrations. This stability is of importance for determining the appropriate number and timing of samples and for weighing the need for prospective measurements. There have been, however, few data sets appropriate for characterizing the indoor concentrations of air pollutants over the long term.

In a prospective cohort study designed to test the hypothesis that exposure to NO, increases the inci- dence and severity of respiratory infections in infants (Samet et al., 1992), we collected indoor NO, measurements at two-week intervals over an 18- month period (Lambert et al., 1993). An important component of this investigation was to quantify the contributions of NO, sources to variations in indoor contaminant levels. The staggered starting time of the participants, from 1988 through 1991, yielded over three years of data. The analysis presented in this paper used the resulting temporal series of measurements to describe the seasonal and year-to- year variability in indoor NO,.

Method Sample Collection The overall design of this investigation of respiratory illness and NO, exposure is described in detail elsewhere (Samet et al., 1992). Briefly, participants were recruited at birth from Albuquerque, NM area hospitals. The principal selection criteria were that the infant be healthy at birth and from a nonsmok- ing household of English speakers, and that the par- ents intended to care for the child at home during the 18-month study period. The sampling frame was stratified by type of cooking range fuel: 25% electric to 75% natural gas.

The protocol used for measuring NO, was based on the results of pilot studies (Harlos et al., 1987; Marbury et al., 1988). Two-week integrated samples of NO, were collected by placing passive diffusion samplers or “Palmes tubes” (Palmes et al., 1976) inside the infant’s home and at 11 outdoor locations. The details of the sampler placement, laboratory analysis methods, and quality assurance practices are described by Lambert et al. (1993). The current paper focuses on the measurements made in infants’ bedrooms. In homes with gas ranges, bedroom NO, measurements were made in consecutive two-week periods for the 18-month study period. In homes with electric ranges, NO2 measurements were made

in alternate two-week periods. A small number of four-week samples were also collected at the beginning of the study.

During the initial home visit a technician showed the mother where to place the tubes; subsequent ex- changes of the sampler tubes were accomplished by the mother with prompting during scheduled semi- monthly telephone calls from the field technician. During this call, information was also obtained on the number of minutes that the stove and oven were used during the previous 24 hours.

Analysis Data Set The analysis presented here has been restricted to the bedroom samples collected between April 1,1988 and April 1, 1991. Data screening steps included re- moving the samples with missing or erroneous information (4% of the available samples) and remov- ing those with durations less than one week or greater than six weeks, and tubes opened twice (2%). Among the remaining tubes, the median duration of exposure was 15 days. Finally, samples that recorded concentrations less than the limit of detection of 1 ppb (Boleij et al., 1986) were set to 1 ppb (4%).

For each household, each day for which data were available was assigned a NO, value. If a tube was open for 15 days, each day during that monitoring cycle was assigned the value associated with that integrated sample. In the event that two Palmes tubes were exposed on the same day (e.g. on the day during which the sampler was changed), the NO2 level collected during the earlier two-week cycle was assigned to the day. For electric range homes, the four-week samples collected during the early months of the study were used to fill in the gaps when two-week samples were not available. Each day for which data were not collected or did not meet the quality assurance standards described above was assigned a missing value.

Most previous studies have shown that range type is the strongest determinant of indoor NO2 levels (Goldstein et al., 1979; Spengler et al., 1983; Quack- enboss et al., 1986; Wilson et al., 1986; Ryan et al., 1988a,b; Ryan et al., 1990; Spengler et al., 1990); thus the sample was initially stratified into three groups: electric ranges, gas ranges with continuously burning pilot lights, and gas ranges with manually-lit or electronic-ignition pilot lights. For brevity, the latter group is referred to as “gas ranges without pilot lights”.

A household was only enrolled in the study for a maximum of 18 of the 36 months covered by this

10 Schwab et 0 1 . : Seasonal and Yearlv Patterns of Indoor Nitroaen Dioxide levels

data set. Thus within any given season for a given year, the mix of homes under observation differed from those contributing data from other seasons. Since analysis presented in this paper focused on the house rather than on the child, each time a partici- pant changed residences the data were treated as a separate observationhouse (5% of the participants).

To explore the seasonal variation in NO2 that has been found in other locations (Spengler et al., 1983; Wilson et al., 1986; Ryan et al., 1988a,b), the data set was stratified into winter and summer periods. For the purposes of this analysis, winter was defined as November 1 through March 15, cooler months when daily mean temperatures averaged 35-40 degrees Fahrenheit, and summer as May 1 though Septem- ber 30, warmer months when temperatures averaged 65 to 80 degrees (National Oceanic and Atmo- spheric Administration, 1988-1992). This definition of “winter” includes 80% of the average year’s heating degree days based on the 30-year climatological records (NOAA, 1990); the “summer” period includes only 15% of the average year’s heating degree days. The weeks between the designated winter and summer periods were excluded from the seasonal analysis. A tube was assigned to the season associated with 75% or more of its exposed time.

Results Housing Characteristics The data set used in this analysis included 1,269 houses, 45% of which had gas ranges with continuously-burning pilot lights, 30% had gas ranges without pilot lights, and the remainder had electric ranges. Gas range homes with pilots had an average of 23 bedroom measurements per home, covering an average of 381 days (standard deviation = 186 days, 95” percentile = 564 days, Yh percentile = 43 days) throughout an l8-month observation period. The distribution of coverage was similar for gas range homes without pilot lights. Homes with electric ranges were sampled in alternate two-week periods, producing an average of 12 tubes per home, covering an average of 378 days (standard deviation = 168 days, 9Yh percentile = 554 days, Yh percentile = 47 days).

The distribution of housing unit characteristics represented in the data set changes by day and by season based upon the homes for which data were available, i.e., missing data and new homes entering the study or homes completing the data-collection protocol influence the composition of the sample. In

general, however, systematic recruiting yielded a sample that was fairly homogeneous over time with respect to housing unit characteristics. As reported by Lambert et al. (1993), most of the participants lived in single family detached units (73%) having one floor (79%) and constructed of wood (77%). In 88% of the units, the water heaters were fueled by gas; the primary heating system was natural gas in 93% of the units. Central forced air furnaces were used in 78% of the units and floor or wall furnaces in ll%. Most of the homes were equipped with evaporative coolers (90%) rather than refrigerating air conditioners. Homes with ranges with continuously-burning pilot lights were more than twice as likely as other range types to heat with floor or wall furnaces, and floor or wall furnaces were more often associated with older homes. Kerosene space heaters were used in only 1% of the homes; most of these homes had electric ranges. Mobile home units and multi-family units were frequently equipped with ranges with continuously-burning pilot lights (88% and 60%, respectively).

Seasonal and Year-to-Year Patterns Figure 1 shows the temporal pattern of NO2 levels across the entire study period. Each point on the plot represents the daily average, based upon all households that provided data on that day. Several patterns are evident from this plot. First, the duration of the period of elevated NO2 varied from year to year. Second, the extent to which the indoor NO2 was elevated during the winter varies from year to year. Third, there were year-to-year variations in the magnitude of differences in concentrations among range-type groups. Table 1 shows that the within- home average NO2 for those with electric ranges averaged 7 ppb during all seasons and years with a standard deviation of 5-8 ppb. During the warmer months of 1989 and 1990, homes with continuously- burning pilot lights averaged 14 ppb (standard deviation (sd) = 10 ppb); levels in gas range homes without pilot lights averaged 9 ppb (sd = 6 ppb). During the cooler months of 1988-89 and 1989-90, average NO2 concentrations in gas range homes were twice as high as during the warmer months, and the coef- ficient of variation was 100%. But the distribution of NO2 levels was surprisingly similar during the summer of 1988 and the winter of 1990-91 for homes with gas ranges.

However, this comparison does not acknowledge that the membership of the sample changed across the three-year period and the patterns documented

Schwab et al.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide Levels 1 1

40 7 35

30

25

20

15

10

5 -

n P P .

.

-

-

-

p. Y

6 z

Fig. 1 Temporal distribution of daily bedroom NO2, averaged across all homes by range pilot light type.

iJ 1 LGas Homes

I ' Electric Homes

I I I I I I I I I I I l l Apr. Jul. Oct. Jan. Apr. Jut. Oct. Jan, Apr. Jut. Oct. Jan. Mar. '88 '88 '88 '89 '89 '89 '89 '90 '90 '90 '90 '91 '91

0 ' I

above could be created by differences in the characteristics of homes represented in each season. There- fore, additional analyses were performed restricting the data set to homes providing measurements in a consecutive pair of winters or summers. For each pair of seasons, for each range type, we then calculated the distribution of indoor NO2 for homes providing at least four measurements and the same number of measurements in each yearheason. Table 2 shows the same magnitude of the between-season differences observed in Table 1 and Figure 1. For homes with gas ranges, paired t-tests show that there were statistically significant differences in the concentrations recorded in gas range homes between the summer of 1988 versus the summer of 1989 and between the winter of 1989/90 versus the winter of 1990/91, but not between winter 1988/89 versus winter 1989/90 or between summer 1989 and summer 1990. Thus, these results indicate that average household concentrations do vary from year to year within a given season.

Temperature Observed year-to-year differences in indoor NO2 concentrations may have been the result of fluctuations in temperature. For instance, unusually cool weather would likely be associated with increased use of supplemental heating sources, such as unvented space heaters and gas ranges. On the other hand, unusually cool summers would likely reduce the fre-

quency with which windows were opened and de- crease use of evaporative coolers, thereby reducing air exchange rates. Yet Figure 2, which shows the distribution of average daily temperatures across each season, does not suggest strong differences in temperature among the three winters or summers. On the other hand, median temperatures were substantially different between the summer of 1988 and the winter of 1990-91 (79 degrees F versus 41 degrees F, respectively) periods during which NOz levels were not significantly different. Regression analysis confirms the conclusion that seasonal temperature estimates are not a good predictor of the seasonal average indoor NO2. The average temperature during the time period for which tubes in a given household were exposed explained less than 5% of the variability in the average indoor NO2 measured during that period (p > 0.05).

Ambient NO2 Levels We then explored the hypothesis that the observed pattern of yearly differences in indoor NO2 were a function of variability in NOz ambient levels. As documented in Lambert et al. (1992), the outdoor NO2 levels measured at 11 sites averaged 15 ppb during the winter months and 10 ppb during the summer months. Average ambient concentrations were higher than the average bedroom concentrations in electric range homes. This finding is probably attribut- able to the reaction of NO2 with surfaces as outdoor

12 Schwab et 01.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels

Table 1 Bedroom NO? (in ppb) by season/year and range type

Season Gas with Gas without Electric pilots pilots

Summer 88 Tubes* N** Mean Std. Dev Median

Winter 88-89 Tubes N Mean Std Dev Median

Summer 89 Tubes N Mean Std Dev Median


Summer 90 Tubes N Mean Std Dev Median


704 149

20.32 11.60 18.10

1453 254

28.58 26.33 22.89

2341 341

15.25 8.31

13.66

2247 358

27.72 24.81 22.01

1926 284

14.80 9.57

13.29

1001 157

20.87 13.02 17.33

474 93

11.70 6.16

10.84

851 144

19.76 23.30 15.33

1468 209 9.03 4.92 8.07

1416 240

16.92 16.61 13.88

1645 222 9.24 4.54 8.44

994 155

15.01 12.84 12.42

217 67

7.97 5.1 1 6.39

41 5 127

7.85 11.77 5.75

662 183

7.35 5.69 6.1 5

578 183

7.30 5.01 6.27

591 163

6.95 3.87 5.97

321 92

6.29 4.32

5.593

* Number o f tubes across all homes in that seasodyear ** Number of homes for the seasodyear; basis of statistics.

air moves into homes and the tendency for homes with electric ranges to be located in areas with lower ambient NO2 levels. In Figure 3 the box plots of the distribution of the two-week average ambient levels available during a given season (usually ten measurements at each of the sites) show only slight differences in the distribution of ambient concentrations across the three years. Note, however, that the 2 ppb difference in median ambient levels between the summer of 1988 and the other summers is reflec- ted in homes characterized by both electric ranges and gas ranges without pilot lights (Table 2 shows

Table 2 Paired-season comparison of bedroom NO*, accounting for repeated measures*

Pair of Seasons Gas with Gas without Electric pilots (ppb) pilots (ppb) (ppb)

Summer 88 N=301++ N=214+2 N=47

Summer 89 Mean (Std Dev) 21.5(19.0) 10.4(6.6) 8.3( 12.4)

Mean (Std Dev) 16.0(11.7) 8.4(5.1) 6.6(4.2)

Winter 88-89 N = 541 N=214 N=52

Winter 89-90 Mean (Std Dev) 28.7(33.5) 14.1(8.8) 6.7(4.5)

Mean (Std Dev) 27.6(32.2) 14.9(9.8) 6.3(4.6)

Summer 89 N = 495 N=357 N=42

Summer 90 Mean (Std Dev) 14.9(9.0) 9.1(6.5) 6.4(3.0)

Mean (Std Dev) 15.2(9.9) 9.7(6.5) 5.8(3.1)

Winter 89-90 N=354+ N=336 N=37

Winter 90-91

* Homes providing at least three measurements and the same number o f measurements in the pair of consecutive seasons

+ Difference in means based upon paired t-test is statistically significantatp < .05

+ + Difference in means based upon paired t-test is statistically significantatp < .001

Mean (Std Dev) 26.7 (22.7) 17.1(18.4) 7.5(8.2)

Mean(Std Dev) 21.1(13.3) 15.2(13.9) 6.3(4.6)

that summer 1988 averages 2 ppb higher than summer 1989). But for homes with gas ranges with pilot lights, indoor levels are actually higher in the summer of 1989. This pattern suggests that in homes with strong sources of NO2, indoor sources domi- nate the outdoor contribution, a finding documented in previous studies (Dockery et al., 1981; Spen- gler et al., 1983; Quackenboss et al., 1986; Ryan et al., 1988a,b). Unfortunately, however, the limited spatial coverage of the outdoor measurements pre- cluded further statistical analysis of the relationship between indoor and outdoor variability with this data set.

Household Characteristics The next step was to explore the effects of key household variables on the above documented seasonal and year-to-year variability in indoor NO2 levels. After accounting for the range pilot lights, past studies have shown that use of the range for heat, unvented gas or kerosene space heaters, and vented heaters with faulty flues, backdrafting, or leaky combustion chambers are likely to be the principal additional indoor sources of indoor NO2 (Wilson et al., 1986; Ryan et al., 1988b). The latter problems are more frequently associated with older floor and wall

Schwab et 0 1 . : Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels 13

Fig. 2 Box plots of the distribution of daily average temperature (F) by year and season.

90

80

70

60

50

40

30

20

40

3 30 Q Q v

0" = 20

5 .cI

- a E a 10

Fig. 3 Box plots of the distribution of ambient NO2 by year and season, based on two-week average measurements collected at 11 sites.

0

95th Percentlle 90th Percentlie Ih 75th Percentile

25th Percentile 10th Percentlle 5th Percentile

Summer Winter Summer Winter Summer Winter '88 '88189 '89 '89190 '90 '90191

95th Percentile 90th Percentile 75th Percentile

25th Percentile 10th Percentile

I I I I I I

90 89 108 98 103 87 Summer Winter Summer Winter Summer Winter

'88 '88189 '89 '89190 '90 '90191

furnaces. In addition, all of these variables tend to covary with housing unit age; it is thus likely that households will have more than one of these sources.

Between 10-12% of the mothers in homes with gas ranges with pilot lights represented in a given year/ season reported that they currently use the range for heat (asked only once). The exception is the winter of 1990-1991, when only 7% of the participants in

homes with pilot lights reported use of the range for heat. Among those with gas ranges without pilot lights, about 4-6% of the households represented during a given yearheason reported using the range for heat. Figures 4a and 4b compare the temporal variability in indoor NO2 between subsamples that did and did not report using the range for heat. Re- ported use of the range for heating is associated with higher levels of NOz throughout the year. In ad-

14 Schwab et al.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide Levels

120 1 100

20

OC I I Fig. 4a Temporal distribution of I I I I I I I I I I

Apr. JuI. Oct. Jan. Apr. JUl. Oct. Jan. APr- Jd- Oct. Jan. daily bedroom NO, for partici-

their range; homes with gas ranges without continuously

'88 '88 '88 '89 '89 '89 '89 '90 '90 '90 '90 '91 pants who reported heating with

Date Oven Used for Heating ++++++ Yes - NO burning pilot lights.

I I I I I I I I I I I 1 Fig. 4b Temporal distribution of Apr. Jul. Oct. Jan. Apr. Jul. Oct. Jan. Apr. Jul. Oct. Jan. daily bedroom NO2 for partici- '88 '88 '88 '89 '89 '89 '89 '90 '90 '90 '90 '91 pants who reported heating with

their range; homes with gas ranges with continuously burning pilot lights.

Oven Used for Heating ++++++ Yes Date

- NO

dition, among those households with continuously- burning range pilot lights, winter NOz levels averaged about 22 ppb higher for those who reported heating with gas ranges during 1988-1989 and 1989- 1990 but only about 8 ppb higher during 1990-1991. Among households without continuously-burning

range pilot lights, winter averages were 38 ppb higher during the winter of 1988-1989 for those who reported using the range for heat, but only 15 ppb higher during the subsequent two winters. During the summers, households that reported using the range for heat had average NOz levels that were

Schwab et 0 1 . : Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide Levels 15

Fig. 5 Temporal distribution of monthly median NO? for 1989 by source type: baseline is electric ranges without other sources.

Fig. 6 Temporal distribution of monthly median NO2 for 1989 by source type: baseline is gas ranges without continuously burning pilot lights and no other sources.

20 I- 1

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Month 0 Electrlc rdnges wlthout other sources (n I 119). W Electrlc ranges wlth gas or kerosene space heaters (n I 6). + Electric ranges wlth floor or wall furnace (n I 11). 7 Gas ranges (no pllot Ilght) but wlthout other sources (n = 248) A Gas ranges wlth pilot llghts but wlthout other sources (n = 3m.

14 -

8 -

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Month 0 Gas (no pllot light) ranges but wlthout other sources (n = 248). W Gas (no pilot Ilght) ranges with gas or kerosenespace heaters (n I 6). + Gas (no pllot Ilght) ranges wlth floor or wall furnace (n I 16). A Gas ranges wlth pilot lights but without other sources (n = 377).

about 3-6 ppb higher than the remainder of the sample. This pattern suggests that there is something different about these homes; perhaps the higher summer levels reflect occasional use of the range for heat on unusually cool mornings, covariations with older leaky appliances, or the likelihood that these were smaller units with reduced airflow.

After excluding those who reported using their range for heat, the sample was divided into mutually

exclusive categories to ascertain the increase in seasonal variability associated with additional sources. Figures 5-7 show the monthly median bedroom NOz for homes providing measurements in each source category across 1989. In Figure 5 the baseline category is households without any sources (i.e. electric ranges, no gas or kerosene space heaters, and no floor/wall furnaces) the second category consists of households with electric ranges and gas or kerosene

0 9

Month

I I I I I I I I I I I:

0 Gas ranges with pilot llghts but without other sources (n I 377). Gas ranges with pilot ilght and gas or kerosene space heaters (n I 6). + Gas ranges with pilot lights and floor or wall furance (n = 44).

space heaters). The third category consists of households with electric ranges with floor or wall furnaces. The fourth category is homes in which the only source is gas ranges without pilot lights (i.e. does not include those with kerosene space heaters or floor or wall furnaces). The last source-use category is for homes in which the only source is gas ranges with pilot lights (i.e. does not include those with kerosene space heaters or floor or wall furnaces). Figure 6 displays the additive contribution of additional sources to the base case of homes with gas ranges without pilot lights but not other sources reported. The next category is gas ranges without pilots and a floor or wall furnace, followed by gas ranges without pilots and a gas or kerosene space heater, followed by gas ranges with continuously- burning pilots but not other sources. In Figure 7 the base is homes with gas ranges that have continuously-burning pilots. Figure 7 illustrates the effect on indoor NO, levels of adding a floor or wall furnace and the effect of adding a kerosene or gas space heater.

These figures demonstrate that additional indoor sources can lead to increases in both average indoor NO2 levels and seasonal variability. For instance, the presence of a floor or wall furnace increases both the baseline concentration and the seasonal variability in indoor NO, to the levels observed in homes with stronger range sources (i.e. electric range homes with floor or wall furnaces have patterns similar to homes with gas ranges without pilots and those with

monthly median NO2 for 1989 by source type: baseline is gas ranges with continuously burning pilot lights and no other sources.

both gas ranges without pilot lights and floor or wall furnaces show patterns similar to homes with continuously burning range pilot lights). Homes with gas or kerosene space heaters show steep, month-to- month fluctuations that may reflect intermittent use of the heater or the small number of homes with these sources.

Range Usage We speculate that range usage varies with family size, season, and the use of other cooking appliances such as microwave ovens. Over 85% of the households owned microwaves at the beginning of the study; increased saturation of microwaves was not documented over the course of the study (Lambert et al., 1993). Another possible explanation for the seasonal and year-to-year fluctuations in indoor NO, displayed above is variations in range use. From the data collected on a household’s duration of range use on one day during each two-week period, we calculated a seasonallyear average range use for each home. These data provide an indication of typical patterns rather than a documentation of the actual patterns. Nonetheless, such a variable should be use- ful for identifying trends in range usage that would affect year-to-year trends in indoor NO,. Table 3 shows the average minutes of range use reported during each seasodyear. The median use of the range was about 10-15 minutedday longer during the winter than during the summer for all range types. There does not appear to be a difference in range use

Schwab et 0 1 . : Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels 17

Table 3 Range usage (average minutedday) by seasodyear and range type

Season Gas with Gas without Electric uilots pilots

Summer 88 N** Mean Std Dev Median

Winter 88-89 N Mean Std Dev Median

Summer 89 N Mean Std Dev Median


Summer 90 N Mean Std Dev Median


142 51.1 43.9 43.2

238 58.8 57.7 56.8

316 38.9 28.9 32.8

345 55.4 52.1 45.0

284 36.1 28.9 30.09

157 54.0 42.2 41.2

85 46.6 35.1 42.5

147 51.1 37.2 41.9

207 40.0 25.8 33.2

240 52.8 37.7 45.0

222 40.6 29.4 33.3

155 51.5 35.9 44.7

70 51.2 39.1 41.7

118 47.3 32.6 42.2

164 40.0 25.8 37.5

165 53.3 37.9 45.8

147 39.4 26.1 35.0

86 55.6 40.6 41.4

** Number of homes with data for the seasodyear

across the three winters studied (e.g., medians = 46 midday, 45 midday and 41 midday for homes with gas ranges with continuously-burning pilot lights). But for all range types, range use during the summer of 1988 averaged longer than the other summers, showing a distribution more similar to those displayed in the winters. This trend is suggestive of an explanation for the elevated concentrations observed during the summer of 1988. To test the signif- icance of these trends, we generated regression models for homes providing data in pairs of seasons. The dependent variable was the difference in household average NO2 between a pair of seasons and the inde- pendent variable was the household’s difference in average range use between the seasons. To test the robustness of the result, similar models were generated using the relative difference in concentrations and range use (i.e., the difference as a proportion of the average). The coefficients of determination asso-

ciated with the models were never above 0.06 and were rarely statistically significant at p < .05. Thus, the statistical analysis of these data do not support our more qualitative observations that differences in range use explain the variation in a household’s seasonal average NO2 level across either consecutive winters or summer/winter seasons.

Prediction of Estimated NO2 In order to understand the implications of the observed year-to-year variability in the distribution of both seasonal and yearly levels of indoor NO2 levels, we examined the extent to which values recorded in one season or year aid in estimating exposure during subsequent periods. Figures 8a-c illustrate the relationship between household average NOz levels recorded in a pair of similar (i.e., based on t-statistics above) winters, a pair of similar summers, and a pair of consecutive wintedsummer seasons. The plots are for the full sample (Le., households with all range and heating system types); Table 4 gives the Pearson’s correlation coefficients between all pairs of winter and summer seasons, disaggregated by range type and heating system (i.e. floor or wall furnace versus central heating system).

All three plots show substantial scatter; some homes differ by a factor of two between consecutive summers or consecutive winters. Homes with values near the limit of detection during one season but with concentrations greater that 10 ppb the following season, however, may be homes in which the protocol was not followed (i.e., tubes never uncap- ped) during one season. The results of linear regression analysis show that 66% of the variation in the household average indoor NO2 levels recorded during the winter of 1988 is explained by variations in concentrations recorded in the winter of 1989; the intercept is 3.8 ppb (3 homes with average concentrations greater than 100 ppb were removed from this analysis; inclusion of these points brings the rZ to 0.81). Only 48% of the variability in concentrations recorded in summer 1989 is explained by concentrations recorded in summer 1990 (intercept = 3.0 ppb). Figure 8c shows that 38% of the Variability in concentrations during the summer of 1989 is explained by the variability in concentrations recorded during the following winter (intercept = -0.3). Ex- amination of Figures 8a-c show that in spite of the scatter displayed, only 5-10% of the households appear to be misclassified from year to year among broadly defined exposure groups (e.g., 0-19 ppb 20- 40 ppb, and > 40 ppb).

18 Schwab et al.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels

'"I 80

c 40}

2ot 1I . 1

' 4.7- * . 0 CI I I I I . I I I

0 10 20 30 40 50 60 70 80 90

Winter 1988-89 Fig. 8a Household average bedroom NO?: Linear regression between winter 1988/89 and winter 567.4107 Estimate SE T Prob, IT1 RSquare 0.6602 F

Bedroom NO, 0.7368 0.0309 23.82 0.0001 RootMSE 7.6504 N Intercept 3.8006 0.7319 5.193 0.0001 Ad/ RSqr 0.6591 Prob>F O*OOO1 1989/90 with 95% confidence in-

294 terva I.

Summer 1989 Fig. 8b Household average bedroom NO?: Linear regression be-

Estimate SE T Prob> hl RSquare 0.4821 F 2a2.g405 tween summer 1989 and sum- Intercept 2.9764 0.6083 4.893 0.0001 AdjRSqr 0.4804 Prob>F 0.0001 mer 1990 with 95% confidence Bedroom NO, 0.7321 0.0435 16.821 0.0001 RootMSE 5.6971 N 306 interval.

Table 4 shows that, with a few exceptions, the correlations tend to be higher for those with stronger range sources (i.e. higher correlations for gas ranges with pilot lights than for gas without pilots than for

electric). Because the sample sizes are small for households with gas ranges without pilot lights but with floor or wall furnaces, trends are more difficult to discern. In general, however, correlations are

Schwab et 01.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels 19

Fig. 8c Household average bed- Summer 1989 room N02: Linear regression between summer 1989 and winter 1989/90 with 95% confidence interval.

Estlmate SE T Probz IT1 RSquere 0.3766 F 329.7926

Intercept -0.2679 1.3866 10.193 0.8469 AdJRSqr 0.3754 ProbzF O.OOO1 BedroomNO, 1.7781 0.0979 18.16 O.OOO1 RootMSE 18.0773 N 548

stronger from winter-to-winter for those with floor or wall furnaces than without but weaker from summer-to-summer for those with floor or wall furnaces than without. Finally, given the yearly trends documented above, it is not surprising that the winter- to-winter correlation is stronger between 1988 and

1989 than between 1989 and 1990 for homes with gas ranges with pilot lights. However, for homes without pilot lights (gas or electric), the correlation is stronger between 1988 and 1989 than between 1989 and 1990.

Table 4 Pearson’s correlation coefficients between pairs of seasons

Range type Gas with pilots Gas without pilots Electric Furnace type Central Floor or wall Central Floor or wall Central Floor or wall Pair of seasons

Summer 88 0.82** 0.46 0.70** 0.33 Summer 89 n=95 n=14 n=62 n=5 Summer 89 0.56**n 0.60** 0.86** 0.55 Summer 90 n=115 n=28 n=87 n = 4 Winter 88/89 0.88** 0.98** 0.63** 0.96* Winter 89/90 n = 125 n=22 n=72 n=5 Winter 89/90 0.53** 0.61 * 0.92** 0.74* Winter 90/91 n=89 n=16 n=96 n = l l Summer 88 0.60** 0.37 0.53** 0.12 Winter 88/89 n = l l l n=18 n=73 n = 7 Summer 89 0.50** 0.47* 0.75** 0.84* Winter 89/90 n=211 n=45 n=152 n = 8 Summer 90 0.53** 0.61* 0.76** 0.79* Winter 90/91 n = 123 n=24 n=142 n=13 Summer 88 0.49** 0.57 0.35* 0.33 Winter 89/90 n=54 n=10 n=32 n = 4 Summer 89 0.45** 0.61 + 0.82** 0.97 Winter 90/91 n=36 n=13 n=36 n = 3

*p<.Ol;**pc.O05; + p<.05

0.59** n=51 0.28 + n=60 0.38* n=66 0.81** n=43 0.44** n=54 0.36** n=114 0.45** n=78 -0.00 n=21 0.28 n=15

0.91* n = 6 -0.32 n=5 0.96** n = 7 0.80 n = 4 0.36 n = 6 -0.91 n = 9 0.78 n = 4 1 .o n = 2

n = l -_

20 Schwab et at.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels

Discussion Summary The primary goal of this paper was to describe the factors that may explain variability in seasonal indoor NO, levels. Our data, collected across a three- year period, show seasonal and source contribution patterns that are similar to those in previously reported studies. In Albuquerque homes, winter NO, concentrations are substantially higher than summer concentrations, especially in homes with gas ranges and/or heat with floor or wall furnaces. This seasonal pattern in indoor NO2 levels may be due to increased use of gas heaters and in cooking time during the cooler months, increased ambient levels during the winter, and/or seasonal patterns in air exchange rates. The operation of evaporative coolers during the summer probably increases exchange rates whereas the tightening of houses during winter to conserve heat reduces rates.

We also found that there can be large year-to-year differences in both the sample distribution of indoor NO2 as well as the household average. For homes with gas ranges with continuously-burning pilots, the bedroom NO2 values differed by an average of 5.5 ppb between one pair of winters but only 1 ppb between a different pair of consecutive winters. These differences are not solely a reflection of uni- formly higher NO2 levels. The between-year correlations also differed; not only were concentrations higher during 1989 than 1990, but the ranking of the homes differed from winter to winter. For those with gas ranges with pilot lights, winter-to-winter correlations ranged from a low of 0.53 to a high of 0.88. On the other hand, few of the households exhi- bited winter-to-winter differences that were greater than 20 ppb. The low year-to-year correlations differ somewhat from the findings of Houthuijs et al. (1990). They reported high year-to-year agreement in one-week average NO, measurements collected during two consecutive winters in 56 homes in the Netherlands; the distribution of average personal exposures only differed by 2 ppb and the test-retest correlation was 0.89 for bedroom values. The con- trast between the findings may reflect the differences in factors affecting source strength or air exchange rates.

We explored a variety of potential causes for the year-to-year differences. There was no change in the monitor placement, quality assurance, or sample analysis protocol among the sampling periods. Nei- ther outdoor NO, concentrations nor ambient tem-

perature showed large year-to-year differences. The lack of temporal variation across homes without sources (i.e., electric range homes without gas appliances), however, provides evidence that the year- to-year trends were not a consequence of fluctuations in ambient levels. The observed patterns, therefore, are hypothesized to be caused by occupant behaviors that influenced air exchange rates and/or source use. The difference in the extent of temporal variability between groups of homes with different source factors supports this hypothesis. But our analysis did not show a strong relationship between range use and household NO, levels. It may be that the variable used (24-hour recall once every two weeks) was not sensitive enough to discern the range use activities that were driving the NO2 patterns.

limitations The collection of several measurements in many homes over multiple seasons and years has provided an opportunity to study the potential for exposure misclassification associated with more limited monitoring efforts. These same features, however, raise difficult statistical issues, including repeated measures and a changing mix of homes across the study period. We attempted to address these problems by using the within-home average concentration for a given season and including only homes represented in a pair of seasons. While information was lost by averaging, the use of household averages based on 3 to 6 measurements per season increased the accuracy of the seasonal mean estimates (Lambert et al. 1992). Although we were unable to establish relationships between indoor NO2 levels, temperature, and ambient NO2 concentrations, these factors may influence indoor levels in other areas where the climate is more extreme or ambient NO, concentrations are higher.

Implications Epidemiologic investigations typically use either source description or limited exposure measurements to characterize exposure to NO2. In the ab- sence of information on the year-to-year stability of indoor NO, levels, it has been assumed that measurements collected in one year are the same as those collected in another year. Our finding, that the distribution of indoor NOz can differ significantly from year to year, suggests that measurements from a single year should be extrapolated cautiously to other years. Differing patterns of source use and am-

Schwab et al.: Seasonal and Yearly Patterns of Indoor Nitrogen Dioxide levels 21

bient NO2 levels may contribute to this variation. For some purposes, e.g., long-term health studies, multiple years of data collection may be needed.

Specifically, the results presented above under- score the potential for exposure misclassification in epidemiologic analyses when relying on one-time measurements. Previous analyses have demonstrated the implications of the choice of exposure indicators on the outcome of epidemiologic studies (Shy et al., 1978; Gladen and Rogan, 1979; Ozkaynak et al., 1986). Brunekreef et al. (1987) have shown that variability in NOz exposure measurements can lead to bias in regression coefficients, reducing the power of a study to detect a significant association. On the other hand, Lebret (1990) has shown that random error in exposure indicators can also lead to an over- estimate of effects. As discussed by Lambert et al. (1992), misclassification can be reduced through re- peat measures and consideration of the sample timing. The analysis presented in the current paper provides additional support for the need for prospective designs for studying health/exposure relationships, in which exposure measurements are closely matched in time with the health measurement of interest.

On the other hand, the results presented above suggest that the year-to-year variability in household NO2 levels will not have a strong impact on classifying exposure into broad cohorts, as is often done in modeling the community distribution of exposure. Further, the analysis shows that the presence of indoor sources not only influences the baseline indoor NOZ level, but also the extent of year-to-year variability. Thus, if the potential for yearly variability is built into indoor-source parameters, models should be able to predict successfully the community distribution of exposure and the proportion of the population with exposures over a designated level of concern.

Acknowledgements The authors wish to thank the families of Albuquer- que who participated in the residential monitoring program. The research reported was conducted under contract to the Health Effects Institute (HEI), an organization funded by the U.S. Environmental Protection Agency (EPA) (assistance agreement X812059), automobile manufacturers, and the Gas Research Institute (GRI). The contents of this paper do not reflect the views of HEI, nor do they reflect the policies of the EPA, automobile manufacturers,

or GRI. The comments of the anonymous reviewers and those of Gerry Alkand helped improve this paper.

References Boleij, J.S.M., Lebret, E., Hoek, E, Noy, D. and Brunekreef, D.

(1986) “The use of Palmes diffusion tubes for measuring NO2 in homes”, Atmospheric Environment, 20,597-600.

Brunekreef, B., Boleij, J.S.M., Hoek, E, Lebret, E. and Noy, D. (1986) “Variation of indoor nitrogen dioxide concentrations over a one-year period”, Environment International, 12, 279-282.

Brunekreef, B., Noy, D. and Clausing, l? (1987) “Variability of exposure measurements in environmental epidemiology”, Ameri- canJournal of Epuiernwbgy, 125,892-898.

Clausing, I?, Mak, J.K., Spengler, J.D. and Letz,R. (1986) “Per- sonal NO2 exposures of high school students”, Environment In-

Dockery, D.W., Spengler, J.D., Reed, M.E and Ware, J. (1981) “Re- lationships among personal, indoor and outdoor NO2 measurements”, Environment International, 5,101-107.

Gladen, B. and Rogan, W.J. (1979) “Misclassification and the design of environmental studies”, American Journal of Epdemio-

Goldstein, B.D., Melia, R.J.W., Chinn, S., Florey, C. du V, Clark, D. and John, H.H. (1979) “The relationship between respiratory illness in primary school children and the use of gas for cooking: I1 - Factors affecting nitrogen dioxide levels in the home”, IntemationalJournal of Epidemiology, 8,339-345.

Harlos, D., Marbury, M., Samet, J. and Spengler, J. (198711 “Rela- ting indoor NO2 levels to infants personal exposures”, Atmo- spheric Environment, 21,369-376.

Houthuijs, D., Dijkstra, L., Brunekreef, B. and Boleij, J.S.M. (1990) “Reproducibility of personal exposure estimates for nitrogen dioxide over a two year period”, Atmospheric Environ-

Lambert, W.E, Samet, J.M, Stidley, C.A. and Spengler, J.D (1992) “Classification of residential exposure to nitrogen dioxide”, At- mospheric Environment, 26A, 2185-2192.

Lambert, W.E., Samet, J.M., Hunt, W.C., Skipper, B.J., Schwab, M. and Spengler, J.D. (1993) “Exposures to nitrogen dioxide”, Part I1 of Nitrogen Dwxide and Respiratoy Illness in Infants, Cambridge, MA, Health Effects Institute (Research Report No. 58).

Lebret, E. (1990) “Error in exposure measures”, Toxicology and In- dustrial Health, 6,147-156.

Marbury, M.C., Harlos, D.E, Samet, J.M. and Spengler, J.D. (1988) “Indoor residential NO2 concentrations in Albuquerque, New Mexico”, Journal of the Air Pollution Control Association,

National Academy of Sciences (NAS) (1981) Indoor Pollutants, Washington, D.C., National Academy Press.

National Oceanic and Atmospheric Administration (1988-1992) Local Climatological Data, Ashville, NC, National Climate Data Center.

Noy, D., Willers, H. Winkes, A. et al. (1986) “Estimating human exposure to nitrogen dioxide: Results from a personal monitoring study among housewives”, Environment International, 12,

Ozkaynak, H., Ryan, EB., Spengler, J.D. and Laird, N.M. (1986) “Bias due to misclassification of personal exposures in epidemiologic studies of indoor and outdoor air pollution”, Environ- ment International, 12,389-393.

Palmes, E.D. Gunnison, AX, Dimattio, J. and Tomcyzk, C. (1976) “Personal sample for nitrogen dioxide”, American Indus- trial Hygzene AssociationJournal, 37,570-577.

Quackenboss, J., Spengler, J.D., Kanaek, M.S. Leu, R. and h f @ ,

ternational, 12,413-417.

logy, 109,607-616.

m t , 24A, 435-437.

38,392-398.

407-4ll.

22 Schwab et 01.: Seasonal and Yearlv Patterns of Indoor Nitroaen Dioxide levels

C.P (1986) “Personal exposure to NOt: Relationship to indoor/ outdoor air quality and activity patterns”, Environment, Science and Technology, 20,775-783.

Ryan, EB., Soczek, M.L., Spengler, J.D. and Billick, I.H. (1988a) “The Boston residential NO, characterization study: I. Preli- minary evaluation of survey methodology”, Journal of the Air Pollution Control Associutiun, 38,22-27.

Ryan, EB., Soczek, M.L., Treitman, R.D., Spengler, J.D. and Bil- lick, I.H. (1988b) “The Boston residential NO2 characterization study: 11. Survey methodology and population concentration estimates”, Atmospheric Environment, 22,2ll5-2125.

Ryan, EB., Spengler, J.D., Schwab, M. and Soczek, M.L. (1990). ‘‘Personal monitoring for nitrogen dioxide: I. The Boston personal monitoring study”. In: hceedings of the First I-1 SympoSium on Total Expsure Assessment Methadobgy: A New HmL zon, Pittsburgh, PA, Air and Waste Management Association.

Samet, J.M., Marbury, M.C. and Spengler, J.D. (1987) “Health effects and sources of indoor air pollution: Part I, American Re- view of Respiratory Diseases, l36,1486-1506.

Samet, J.M., Lambert, W.E., Skipper, B.J., Cushing, A.H., McLaren, L.C., Schwab, M. and Spengler, J.D. (1992) “A study of respiratory illnesses in infants and NO2 exposure”, Archives of Environmental Health, 47,57-63.

Samet, J.M. and Utell, M.J. (1990) “The risk of nitrogen dioxide: What have we learned from epidemiological and clinical studies?”, Toxicology and Industrial Health, 6,247-262.

Shy, C.M., Kleinbaum, D.G. and Morgenstern, H. (1978) “The effect of misclassification of exposure status in epidemiological studies of air pollution and health effects”, Bulktin of the New Ymk Academy of Medicine, 54,ll55-ll65.

Spengler, J.D. Du@, C.E Letz, R., Tibbitts, T.W. and Ferris, Jr. B.G. (1983) “Nitrogen dioxide inside and outside 137 homes and implications for ambient air quality standards and health effects research”, Environment, Science and Technology, 17, 164- 168.

Spengler, J.D., Ryan, EB., Schwab, M., Billick, I.H., Colome, S.D. and Becker, E. (1990). “An overview of the Los Angeles personal monitoring study”. In: Proceedings of the First Znter- national Symposium on Total Exposure Assessment Methodology: A New Horizon, Pittsburgh, PA, Air and Waste Management As- sociation.

Wilson, A.L., Colome, S.D., Baker, EE. and Becker, E.W. (1986) Residential Indoor Air Quality Charaterization Study of Nitrogen Dioxide, Phase Z, Volume 2, Final report, Engineerinfie- search, Los Angeles, Southern California Gas Company.

seasonal and yearly patterns of indoor nitrogen dioxide levels: data from albuquerque, new mexico

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