Environ. Sci. Technol. 1994, 28, 915-923
Evaluation of the Harvard Ozone Passive Sampler on Human Subjects Indoors L.-J. Sally Llu, Melvin P. Olson 111, George A. Allen, and Petros Koutrakls'
Harvard University, School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 021 15 Wllliam F. McDonneli and Timothy R. Gerrlty U.S. Environmental Protection Agency, Medical Research Building C, University of North Carolina, Chapel Hill, North Carolina 27514
Our study examined the performance of the Harvard ozone passive sampler, providing comprehensive evaluation results and investigation of ozone behavior around the human body through a series of chamber and indoor field tests. The results demonstrated excellent agreement between the passive and continuous measurements for both personal and indoor monitoring in an environmental exposure chamber. An ozone concentration gradient around the human body was found, most likely due to depletion and dilution effects. When the passive sampler was attached to a polystyrene backing plate and worn on the chest, ozone depletion on clothing was significantly reduced. Results from additional indoor field tests support those from the chamber study. The effective collection rate for personal samplers with backing placed on the chest was comparable for both chamber and field conditions, while the effective collection rate for microenvironmental samplers was lower in the field.
Introduction Accurate and precise ozone (03) concentration measurements are critical for the assessment of exposure in O3epidemiologic studies. An 0 3 personal passive sampler is important in that it can be used in large-scalefield studies and, thus, will improve our ability to measure 0 3 concentrations and to understand its characteristics for either pollution research or public health protection. The passive nature of the personal monitor allows samples to be collected simply and with relatively low cost. A personal passive monitor can be worn conveniently by the participant near the breathing zone or can be placed in various microenvironments at the breathing level. The current national ambient 0 3 standard (0.12 ppm) is based on the effects due to acute exposures of 1 h as measured by continuous monitors. However, exposures to O3for up to 8 h at less than 0.12 ppm have been shown to result in progressive and significant changes in respiratory function in exercising individuals, suggesting that the current national ambient air quality standard may not sufficiently protect public health (1-3). Our ability to determine the dose-response relationship for O3 may be greatly enhanced through the use of the integrated passive monitor due to its simplicity and low cost. The ozone passive sampler developed by Koutrakis et al. ( 4 ) can function as a personal monitor, an outdoor stationary monitor, or an indoor monitor. The limit of detection of this sampler is 25 ppb ( 4 ) , and the capacity is 2500 ppb for 8 h (4). The performance of the sampler
has been tested under a variety of laboratory conditions and has been found to be independent of temperature between 0 and 40 "C,relative humidity (RH) between 10 and 80%, wind speed between 25 and 295 cm/s (sampler under a wind cover), ultraviolet (UV) radiation (4), and other atmospheric oxidant interferences including NO2 and peroxyacetyl nitrate (5). In a field study conducted by Liu et al. (6),good concordance between the outdoor ozone concentrations measured by the passive sampler and a collocated UV monitor was observed. However, due to the high reactivity of ozone and the complex fluid dynamics around the human body, these tests may not be sufficient to determine the ozone passive sampler's reliability when worn by human subjects. Preliminary tests of the passive sampler conducted at the U.S. Environmental Protection Agency (EPA) suggested that a number of factors must be considered when the sampler is used to measure personal exposures. In these tests, passive samplers were suspended within an environmental exposure chamber and were worn by subjects in the chamber. Ozone concentrations were also measured by two continuous monitors. The agreement between the indoor samplers was high, as was that between the personal samplers. However, concentrations measured by indoor and personal passive samplers were 8 and 28 % , respectively, lower than those measured by the chamber's continuous monitors. Results from these tests suggested the following hypotheses: (1) that 0 3 reactions with clothing result in a depletion effect; (2) that human activities cause airflow to be blocked around the passive sampler resulting in a movement effect; and (3) that human expiratory air flow near the chest or titration by some endogenous species such as NO found in exhaled air (7) result in a dilution effect. To test these hypotheses, we conducted two chamber studies and one indoor field study. The first chamber study was to verify all these hypotheses. In this study, the passive sampler was tested both as a personal monitor and as an indoor monitor under typical indoor ozone concentration and wind speed conditions. In the second chamber study, continuous monitors were used to examine whether the depletion, movement, and dilution effects create ozone concentration gradients around the human body. The third study, conducted in an indoor residential environment, tested the hypothesis that the performance of the passive sampler in a natural indoor setting is equivalent to its performance in the chamber.
Methods
correspondence to this author; e-mail address: Petrosa hsph.harvard.edu.us.
Ozone Monitoring. The first and second studies were conducted in one of the environmental exposure chambers of the U.S. EPA Clinical Exposure Facility (Chapel Hill,
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Environ. Scl. Technol., Vol. 28, No. 5, 1994 915
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NC). Different ozone concentrations were generated in the stainless steel chamber, which was maintained at 25 OC room temperature and 40% RH. Ozone, produced by passing filtered air under ultraviolet (UV) lights, passed into the chamber from ducts in the ceiling and passed out through uniformly distributed holes in the floor. A detailed description of the chamber and the ozone-generating, delivery, monitoring, and control systems has been previously published (€49). The chamber was comparable to a typical living room (4 X 6 X 3.2 m) and allowed the participants to conduct normal activities, including walking, reading, typing, and watching TV. Two chemiluminescence (CL) (Bendix Model 8802) monitors, three UV (Thermo-Environmental Model 49, Franklin, MA) monitors, and numerous passive samplers were used to measure ozone in the chamber. A common manifold inlet for the CL monitors was located 7 f t above the floor of the chamber. Measurements from CL monitors (hereafter referred to as chamber continuous measurements) were used as the reference value for microenvironmental ozone concentrations. Three UV monitors were used as personal monitors by attaching a 25-ft-longTeflon sample line (1/16-in. i.d.) from each UV monitor to the subject’s eyeglass frame. These UV measurements were considered as the reference values for the true ozone concentration in the breathing zone. In the ozone concentration gradient study, the inlet of the sample line was placed in different locations to measure the ozone gradient around the body. The Harvard ozone passive sampler, which was used as both an indoor and a personal sampler, is composed of a badge clip and a barrel-shaped body. At each end of the body, there is a glass fiber filter supported by two metal screens and fixed by a diffusion barrier endcap. The filters are coated with sodium nitrite and potassium carbonate (4). The size of the sampler is 2 (diameter) X 3 (width) X 4.5 cm (length), and the weight is 7 g. The nominal collection rate of the sampler is 18.2 cm3/min (4). Wind Speed Measuring. A hot-wire anemometer (Kurz Series 1440) was used to check wind speed in both the chamber and field studies. The accuracy of the instrument is &2%, and the sensitivity and readability is &3 cm/s. Study Design. H u m a n Exposure Chamber Study. The first chamber study, conducted in January 1993,was composed of two sets of 4 days, with the same five subjects participating in each set of days. For the first 4 days, ozone concentration in the chamber averaged 81,40,121, and 60 ppb, respectively, with an average airflow of 5000 ft3/min (cfm, or 2.36 m3/s), In the second set, ozone concentration averaged 61, 121, 41, and 81 ppb, respectively, with an average airflow of 8000 cfm (or 3.78 m3/s). The four levels of ozone concentrations were chosen to cover the range of ozone concentrations most likely experienced in indoor environments. The wind speeds corresponding to 5000 and 8000 cfm were 10 and 15 cm/s, respectively, and were determined by dividing the average airflow rate in the chamber by the cross-sectional area of the chamber (4 X 6 m). These speeds correspond to the face velocities that a personal sampler might be exposed to in an indoor environment (10,II). All passive samplers were exposed for 7-8 h. On each day, five human subjects participated in the study, each wearing four passive samplers. One sampler was placed approximately 10 cm in front of the nose by 916
Environ. Scl. Technot., Vol. 28, No. 5, 1994
being clipped at the center of an arched Teflon tubing with both ends attached to the frame of eyeglasses. One sampler was clipped on the shoulder. Two more samplers were attached on the chest, with one clipped directly to the shirt and the other clipped to a Teflon ring connected to a 10 X 10 cm polystyrene backing plate, which was clipped to the shirt. The distance between the chest and the passive sampler on the backing plate was about 5 cm. Three continuous UV monitors also were used to measure ozone concentration near the nose of three of the five subjects. The inlet of the sample line for each continuous UV monitor was located about 10 cm above and in front of the nose and 10 cm from the nose passive sampler. During the study, four subjects were usually reading or watching movies in a more open area of the chamber, while the fifth one was usually typing or reading in a corner of the chamber. Two female mannequins (or dummy subjects) with hair were utilized as a control group to determine the effects of expiratory air flow and movement. Dummies with the head held upright were selected so that the airflow would not be obstructed by the head or hair. Each dummy subject was dressed with a cotton T-shirt. Two passive samplers were placed on the chest of each dummy, one sampler with plastic backing and the other without. One dummy was located in the area where most of the subjects were, while the other was placed in the back corner near the fifth subject. On each day, two chamber continuous CL monitors were used to measure chamber ozone concentration. Four pairs of passive samplers were suspended by wires, approximately 6 f t above the floor. These passive samplers (hereafter referred to as microenvironmental samplers) were located slightly above the subjects’ height (6 ft). Two pairs of samplers were located within 2 ft of the manifold inlet of the chamber CL monitors (hereafter referred to as sites 1 and 2 microenvironmental samplers), and one pair of samplers was located in the center of the chamber (site 3) where most of the subjects’ activities occurred. One pair of samplers was placed at the back corner of the chamber (site 4). To examine wind direction effects, for eachpair of the sites 1and 2 microenvironmental samplers, one sampler was oriented horizontally so that the wind was blowing over the surface of the two sampler endcaps, while the other was oriented vertically. The sites 3 and 4 microenvironmental samplers were all oriented horizontally. Ozone Concentration Gradient Study. In the second study, the three continuous UV monitors, as described in the human exposure chamber study, were utilized to examine the ozone concentration gradient around the body of a single human subject. These continuous monitors were carefully calibrated against the same calibrator and were operated concurrently. All these tests were performed while ozone concentrations averaged 81 ppb. Due to the limited number of measurements, results from these tests are exploratory and are not intended to lead to definitive conclusions. Field Study. Although the previous chamber study conditions simulated those in the real world. They may still be dissimilar from those that would be encountered in the field. The vertical wind pattern (from ceiling toward floor) was of particular concern, and it was unclear how this wind pattern would affect study findings. Therefore, we conducted a field study to verify the chamber study
results. This study was performed in the living room of a residential house in Boston, MA, for 4 days in July 1993. Three windows in the living room were kept open, and a fan was operated in front of one window throughout the monitoring period. Ozone was generated in the room using a high-voltage ionizing ozone generator (Thermo-Environmental) for three of the sampling days to ensure sufficient ozone. During the study, two pairs of indoor microenvironmental passive samplers were collocated with a continuous UV monitor (Thermo-Environmental Model 49). One pair of the passive samplers was clipped on a “sampling tree”, which included a small fan to maintain a constant wind speed of about 25 cm/s. The other pair of samplers was clipped on an identical sampling tree without a fan, approximately 20 cm away from the other sampling tree. Three pairs of passive samplers were worn by a human subject, who sat near the inlet of the continuous monitor. One pair of the samplers was attached directly to the shirt, while another pair was clipped to a backing plate attached to the shirt. The last pair was attached to Teflon tubes extending from a backing plate so that the samplers were 10 cm away from the shirt. The exposure duration for all passive monitoring ranged between 7 and 8 h. Statistical Analysis. SAS (12) software was used for all data management and statistical analyses. The data capture rate was 98% of all possible passive samples. Simple linear regression analysis was performed for comparisons of different types of personal measurements and continuous measurements. Variations in passive sampler measurements at different locations were examined using mixed-effect analysis of variance (ANOVA) models. The particular model used is referred to as a split-plot model in the statistical literature (13). For example, the effects of intersubject variation and wind speed are the whole plot factors, and ozone concentration and its interactions are the subplot factors of the splitplot model. An observation, o i j k , is a sample collected from a specific location on subject sk ( k = 1, ...,5 ) under ozone concentration, Cj 0‘ = 1, ..., 4), and wind speed Wi (i = 1, 2). The Oijk’s are normalized to obtain Yijk by dividing each measurement by the average nose continuous (UV) concentration of the day. This ratio, Yijk, is then used as the dependent variable in the ANOVA models. Assuming that all Yijk’Shave a common mean p,an ANOVA model for Y i j k can be expressed as the sum of the common mean p, wind speed Wi, ozone concentration Cj, subject wind by concentration Wi*Cj, within wind speed sk(Wi), and an error term, Ejk( Wi):
sk(wi)
where Wi, cj, and Wi*Cj are fixed effects, and Ejk( Wi)are random effects and are assumed to be normally distributed with a common mean of zero and variances of gS2and ue2,respectively. Paired two-sample t-tests were used to examine ozone depletion and dilution effects, comparing samples on the chest of human and dummy subjects, with and without backing. The mixed-effect model also was used to examine factors affecting variation of the microenvironmental measurements and samples collected from dummy subjects. The effective collection rate was calculated and compared for samplers at different locations both in the chamber and in the field studies.
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Quality Assurance and Quality Control. The continuous UV monitors used in the chamber were calibrated twice a day for span checks at 0 and 120 ppb of ozone against a UV calibrator (Thermo-Environmental Model 49 PS),while the chamber continuous CL monitors were independently dry-calibrated against a UV ozone monitor (Model 1003AH;Dasibi, Glendale, CA). For UV continuous measurements, ozone concentration readings were corrected using the slopes calculated from the span checks each day. The continuous UV monitors also were compared with the chamber CL monitors twice a day by collocating the inlet of the sample line with the chamber monitors manifold inlet. All the calibrations and comparisons in the chamber were performed with the 25-ft sample line in place at both the beginning and the end of each day when subjects were not present. Figure 1 demonstrates results from the morning and evening comparisons of the UV and chamber CL measurements when subjects were not present. Note that the figure is overlaid with a 45O line, which has a slope of 1.0. When the UV continuous measurements are regressed on the chamber continuous measurements, the R2 is 0.995. The slope of the regression line is 0.98 f 0.01 and the intercept is within experimental error. The 2 76 difference between the UV and chamber CL measurements may result from either measurement error or systematic bias of the CL monitors due to relative humidity (14). In spite of this small error, measurements from the continuous UV and chamber CL monitors are considered to be in excellent agreement. For each day, three passive field blanks were used to account for possible handling errors. These field blanks were prepared and handled identically to the other samplers, except that the exposure time was less than 1 min. In addition, 10 coated filters were kept intact as laboratory blanks. The field blanks averaged 45 f 10 ppb for an equivalent exposure time of 8 h. This corresponds to a limit of detection of 240 ppbah (or 30 ppb for an 8-h sample), which is consistent with those found in our previous studies (4,6). The laboratory blanks averaged 38 f 3 ppb and are comparable to the field blanks (twoEnviron. Scl. Technol., Vol. 28. No. 5, 1994 917
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sample t-test, p = 0.06). The mean field blank value was used for correction of sample values. Results and Discussion
Human Exposure Chamber Study. Validation of Continuous Monitors. During the first monitoring study, the sample line of the three UV monitors was worn by the subjects to obtain "nose continuous measurements". When the nose continuous measurements are regressed on the chamber continuous measurements, the R2 is 0.99 (Figure 2, overlaid with a 45O line). The intercept of the regression line is negligible, and the slope is 0.97 f 0.02 (or 0.95 f 0.01 after forcing the intercept to zero), which together imply that ozone concentrations at the nose was slightly lower than those measured at the manifold inlet (7 f t from the floor). This 3% difference may be due to the instrument error described earlier and to ozone scrubbing by people either from depletion on surfaces or from breath dilution. Nevertheless, the nose and chamber continuous monitors were in good agreement. In the following analyses, nose continuous measurements were used as the reference value for ozone concentrations in the breathing zone, while chamber continuous measurements were used as the reference value for ozone concentrations in the chamber microenvironment. Validation of Personal Passive Sampler. For each different location on human subjects (i.e., nose, shoulder, chest with plastic backing, and chest without plastic backing), passive sampler measurements were regressed on nose continuous measurements as shown by Figure 3ad. The highest correlation between passive and nose continuous measurements was obtained for the samplers attached to the Teflon tubing on eyeglass frames (Le.,nose samplers) (R2 = 0.96, Figure 3a). Those placed on the shoulder showed the most variability (R2 = 0.82, Figure 3b). Intercepts of the regression models were then forced to zero, and the resulting slopes are shown on the plots, The slope of 1.02 f 0.02 for nose passive samplers indicated no significant ozone loss around the samplers when compared to the reference value (Figure 3a). Samplers 918
Environ. Sci. Technoi., Voi. 28, No. 5, 1994
placed on the chest without plastic backing (Figure 3c) measured about 40% less ozone than the nose continuous monitors. Both shoulder (Figure 3b) and chest with plastic backing samplers (Figure 3d) underestimated ozone concentrations by about 10%. Results indicated that the passive sampler with proper placement performed very well as a personal monitor, especially when placed near the nose. Furthermore, if the 10% ozone loss was homogeneous among subjects, making adjusted measurements possible, samplers on the chest with backing may be adequate for use in field studies. We therefore examined the intersubject variation ((r,2) of the passive sampler measurements, which reflects the comparability of the measurements among different subjects. If a large intersubject variation exists after controlling for true ozone concentration and wind speed, it would imply that the performance of the personal sampler varied from subject to subject under identical conditions. In this case, improvements to the sampler would be necessary to reduce intersubject variation in order to estimate true individual exposure. The mixed-effect ANOVA model results for samples collected from different locations are shown in Table 1. Recall that the model-dependent variable was the ratio of the passive sampler measurement to the nose (UV) continuous measurement, the latter of which is considered to be the reference value. The intersubject variation in this ratio was not significant for samples at any locations. The effect of concentration (Cj) was significant, indicating that the passive/nose continuous ratio increases toward 1.0 with ozone concentration. One explanation may be that a greater ozone gradient was produced around a human body at a lower ozone concentration so that the passive sampler measures a much lower ozone concentration. When the difference between the passive and continuous measurements was used as the dependent variable, the concentration dependence effect became marginally significant for samplers on the chest with backing (Fvalue = 2.19, p value = 0.12) and on the shoulder (Fvalue = 2 . 8 1 , ~value = 0.06). The wind speed effect was not significant at any location except for the shoulder. For the nose samplers, the interaction term Wi*Cj was significant but was more a reflection of the ozone concentration effect rather than that of a true interaction. Ozone Depletion on Clothes. Since ozone is highly reactive, it is likely that ozone reacts with clothing surrounding the passive sampler, producing an ozone concentration gradient. Evidence supporting this theory was provided from the different ozone measurements on the chest with and without a backing plate. The mean difference between passive measurements on the human chest with and without backing is 19 f 11 ppb (paired t-test, p < 0.01), while the mean difference for the measurements on dummy subjects is 9 f 8 ppb (paired t-test, p < 0.01). Results from both paired t-testsindicated that the backing plate protected against loss of ozone on clothes and thus resulted in measured concentrations closer to the reference values. Figure 4a,b further compares human and dummy chest samples, with and without backing, respectively. For the samples with backing (Figure 4a), the mean difference between human and dummy measurements is 23 f 11 ppb (paired t-test,p < 0.01). For samplers without backing (Figure 4b), the mean difference increased to 33 f 15 ppb
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10 cm/s, while is a measurement under wind speed of 15 Cm/s. Note that the figure is overlaid with a 45' line. (a) Nose passive vs nose continuous measurements. (b) Shoulder passive vs nose continuous measurements. (c) Chest passive wlthout backing vs nose continuous measurements. (d) Chest passive with backing vs nose continuous measurements.
(paired t-test, p < 0.01). The lower measurements on human chest samplers may result from human expiratory air flow that dilutes ozone concentrations near the chest, especially when the head is inclined forward. The greater difference for human vs dummy samples without backing, Le., 33 vs 23 ppb, was most likely due to the dilution effect and movements. Movements of the upper human body may cause folding of the clothing near samplers without backing, thus obstructing the airflow and reducing the ozone measurements. In addition, the airflow pattern in the chamber, which was vertical from the ceiling toward the floor and laminar, may contribute to the lower concentrations measured near the human chest. It was observed from the anemometer measurements that airflow near the chest of a human subject was usually lower than that of a dummy subject,
An ANOVA model was applied for the samples collected from dummy subjects to test for possible factors affecting the measurement variation, of which the backing plate and wind speed effects were of primary concern. For the model, the independent variables included backing status (B,with or without), wind speed (W, 10 or 15 cm/s), ozone concentration (C,40,60,80, or 120 ppb), dummy subject (S,dummy 1 or 2)) and interaction terms, W*C, S*W , W*B,S*C,and S* W*C. The dependent variable was the ratio of the dummy passive measurement to the chamber continuous measurement. Results (Table 2) indicated that the backing effect was the only significant effect (p < 0.01). Neither wind speed nor ozone concentration effects were significant, indicating that the variation of the passive measurements on dummies $id not depend on wind speed or ozone concentration. Envlron. Scl. Technol., Vol. 28, No. 5, lSB4
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Table 1. Results from Mixed-Effect ANOVA Models for Personal Passive Samplers at Different Locations of the BodyP
location of sampler chest with backing chest without backing
F effects value
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mean value ratio RMSE p
0.31 0.59 0.83 3.28 0.04 1.72 0.15 2.57 0.08 0.09 0.83 0.56 4.92 0.01 1.84 0.12 0.67 0.58 0.36 0.57 0.97 13.61
