Exposure to air pollutants in English homes

BRE has conducted a national representative survey of air pollutants in 876 homes in England, designed to increase knowledge of baseline pollutant levels and factors associated with high concentrations. Homes were monitored for carbon monoxide (CO), nitrogen dioxide (NO(2)), formaldehyde and volatile organic compounds (VOCs). In the majority of the homes, concentrations of the measured pollutants were low. However, some homes have concentrations that would suggest a need for precautionary mitigation. Those factors that are most likely to lead to exposures of concern in homes are identified as gas cooking (for CO and NO(2)), the use of unflued appliances for heating (for CO and NO(2)), emissions from materials in new homes (for total VOC (TVOC) and formaldehyde), and painting and decorating, with a significant increase in risk suspected to exist where there is not a place to store materials away from the living space (for TVOC). It is noteworthy that seasonal effects on CO and NO(2) were largely due to indoor sources. This would need to be considered when interpreting time series studies of the effect of outdoor air pollution on health. It is also of some significance that the critical factors are related much more to sources than to ventilation: source control is therefore, as would be expected, the most appropriate approach to reducing the risk of hazardous exposure to air pollutants in homes.     

From health-based to technology-based standards for hazardous air pollutants.

The Clean Air Act Amendments of 1990 represent a major shift in regulatory emphasis for toxic air pollutants. Recognizing the immediate public health benefits that can be gained from the application of currently available and affordable control technologies, Congress has abandoned its insistence that health risks should be the only consideration in establishing emissions standards. Previously excluded concerns about economic costs and technological feasibility will now determine the initial level of pollution control required of toxic air pollution sources. In exchange for reducing the stringency of emissions limits, the newly amended act extends the scope of regulation by listing 189 toxic substances that must be controlled over the next decade. This exchange of regulatory depth for breadth occurs against a history of reluctance by the Environmental Protection Agency to implement the original health-protective language of the Clean Air Act. It mirrors earlier compromises under the Clean Water Act and the Occupational Safety and Health Act.     

Modeling population exposures to outdoor sources of hazardous air pollutants

Accurate assessment of human exposures is an important part of environmental health effects research. However, most air pollution epidemiology studies rely upon imperfect surrogates of personal exposures, such as information based on available central-site outdoor concentration monitoring or modeling data. In this paper, we examine the limitations of using outdoor concentration predictions instead of modeled personal exposures for over 30 gaseous and particulate hazardous air pollutants (HAPs) in the US. The analysis uses the results from an air quality dispersion model (the ASPEN or Assessment System for Population Exposure Nationwide model) and an inhalation exposure model (the HAPEM or Hazardous Air Pollutant Exposure Model, Version 5), applied by the US. Environmental protection Agency during the 1999 National Air Toxic Assessment (NATA) in the US. Our results show that the total predicted chronic exposure concentrations of outdoor HAPs from all sources are lower than the modeled ambient concentrations by about 20% on average for most gaseous HAPs and by about 60% on average for most particulate HAPs (mainly, due to the exclusion of indoor sources from our modeling analysis and lower infiltration of particles indoors). On the other hand, the HAPEM/ASPEN concentration ratio averages for onroad mobile source exposures were found to be greater than 1 (around 1.20) for most mobile-source related HAPs (e.g. 1, 3-butadiene, acetaldehyde, benzene, formaldehyde) reflecting the importance of near-roadway and commuting environments on personal exposures to HAPs. The distribution of the ratios of personal to ambient concentrations was found to be skewed for a number of the VOCs and reactive HAPs associated with major source emissions, indicating the importance of personal mobility factors. We conclude that the increase in personal exposures from the corresponding predicted ambient levels tends to occur near locations where there are either major emission sources of HAPs or when individuals are exposed to either on- or nonroad sources of HAPs during their daily activities. These findings underscore the importance of applying exposure-modeling methods, which incorporate information on time-activity, commuting, and exposure factors data, for the purposes of assigning exposures in air pollution health studies.     

The exposure of cyclists,car drivers and pedestrians to traffic-related air pollutants

Volunteers provided with personal air sampling (PAS) equipment covered concurrently, by car or bicycle, various selected routes. These comprised two inner city routes in Amsterdam (ICR 1 and 2) as well as a route including a tunnel on a busy highway (TR) and a rural route just south of Amsterdam (RR). A third inner city route, a busy narrow street, was subsequently also selected, and covered by bicycle or walking (ICR 3). Each run lasted about 1 h; the sampling time on the TR route was approximately 30min. The sampling periods in January and May lasted 2 weeks with four sampling days per week. In August only ICR 3 was covered, this sampling period lasted 2 days. CO, NO₂, benzene, toluene and xylenes were measured in the personal air samples. A monitoring vehicle covered the routes concurrently and measured CO, NO₂ and pm₁₀ (semi) continuously. Lead and PAH content in pm₁₀ was determined. The ventilation of the volunteers was measured while they were using a car or a bicycle. The route and the type of transport influenced (P < 0.001) the concentrations of CO, benzene, toluene and xylenes. The daily average temperature was positively associated with the exposure of car drivers and cyclists to most compounds measured. A volunteer exhaled on average 2.3 times more air as a cyclist than as a car driver. Despite the much higher concentrations in the personal air samples of car drivers, the uptake of CO, benzene, toluene and xylenes of cyclists sometimes approached that of the car drivers. The uptake of NO₂ of cyclists was clearly higher than that of car drivers.     

Health assessment of hazardous air pollutants The Netherlands.

In The Netherlands, risk assessment of air pollutants goes back to 1980. From 1985 onwards the risk assessment of chemicals has crystallized in a systematic multi-media approach taking into account air, water, soil, and food. Criteria Documents support the formulation of environmental quality standards. Until now, 23 documents were prepared. Some toxicological aspects of the risk assessment for man in these documents are discussed with emphasis on carcinogenicity. As examples the main aspects of the risk assessments of benzene and trichloroethylene are described. A list of proposed health based standards for air pollutants is added.     

Exposure to nitrogen oxides and other air pollutants from automobiles.

We have conducted an epidemiological study to investigate the association between exposure to automobile exhaust and respiratory health. Three zones were selected in Tokyo according to expected exposure levels. Zone A is within 20 meters from the roadside of major roads with heavy traffic. Zone B is 20-150 meters away from the roadside of the same road. Zone C is the residential district in a suburb of Tokyo. Zone A, which was nearest to the roads, had the highest mean levels of personal exposure to nitrogen dioxide for housewives, followed by Zone B and Zone C. Ambient levels of nitrogen oxides, and mass concentrations, polycyclic aromatic hydrocarbons and mutagenicity of suspended particulate matter were higher at the roadside within Zone A. A cross-sectional study of respiratory symptoms and repeated examinations of pulmonary function were also performed in each zone. These results suggest that exposure to automobile exhaust may be associated with respiratory symptoms. Nevertheless, pulmonary function didn't show consistent differences over all examinations. We should have further analyses about a decline of pulmonary function with age.     

Ranking cancer risks of organic hazardous air pollutants in the United States

BACKGROUND: In this study we compared cancer risks from organic hazardous air pollutants (HAPs) based on total personal exposure summed across different microenvironments and exposure pathways. METHODS: We developed distributions of personal exposure concentrations using field monitoring and modeling data for inhalation and, where relevant, ingestion pathways. We calculated risks for a nonoccupationally exposed and nonsmoking population using U.S. Environmental Protection Agency (EPA) and California Office of Environmental Health and Hazard Assessment (OEHHA) unit risks. We determined the contribution to risk from indoor versus outdoor sources using indoor/outdoor ratios for gaseous compounds and the infiltration factor for particle-bound compounds. RESULTS: With OEHHA's unit risks, the highest ranking compounds based on the population median are 1,3-butadiene, formaldehyde, benzene, and dioxin, with risks on the order of 10(-4)-10(-5). The highest risk compounds with the U.S. EPA unit risks were dioxin, benzene, formaldehyde, and chloroform, with risks on a similar order of magnitude. Although indoor exposures are responsible for nearly 70% of risk using OEHHA's unit risks, when infiltration is accounted for, inhalation of outdoor sources contributed 50% to total risk, on average. Additionally, 15% of risk resulted from exposures through food, mainly due to dioxin. CONCLUSIONS: Most of the polycyclic aromatic hydrocarbon, benzene, acetaldehyde, and 1,3-butadiene risk came from outdoor sources, whereas indoor sources were primarily responsible for chloroform, formaldehyde, and naphthalene risks. The infiltration of outdoor pollution into buildings, emissions from indoor sources, and uptake through food are all important to consider in reducing overall personal risk to HAPs.     

Inhalation of hazardous air pollutants from environmental tobacco smoke in US residences

In the United States, 48 million adults smoke 3.5-5 x 10(11) cigarettes/year. Many cigarettes are smoked in private residences, causing regular environmental tobacco smoke (ETS) exposure to roughly 31 million nonsmokers (11% of the US population), including 16 million juveniles. (Upper bound estimates are 53 million exposed nonsmokers including 28 million juveniles.) ETS contains many chemical species whose industrial emissions are regulated by the US federal government as hazardous air pollutants (HAPs). In this paper, average daily residential exposures to and intakes of 16 HAPs in ETS are estimated for US nonsmokers who live with smokers. The evaluation is based on material-balance modeling; utilizes published data on smoking habits, demographics, and housing; and incorporates newly reported exposure-relevant emission factors. The ratio of estimated average exposure concentrations to reference concentrations is close to or greater than one for acrolein, acetaldehyde, 1,3-butadiene, and formaldehyde, indicating potential for concern regarding noncancer health effects from chronic exposures. In addition, lifetime cancer risks from residential ETS exposure are estimated to be substantial ( approximately 2-500 per million) for each of five known or probable human carcinogens: acetaldehyde, formaldehyde, benzene, acrylonitrile, and 1,3-butadiene. Cumulative population intakes from residential ETS are compared for six key compounds against ambient sources of exposure. ETS is found to be a dominant source of environmental inhalation intake for acrylonitrile and 1,3-butadiene. It is an important cause of intake for acetaldehyde, acrolein, and formaldehyde, and a significant contributor to intake for benzene.     

Estimating cancer risk from outdoor concentrations of hazardous air pollutants in 1990

A public health concern regarding hazardous air pollutants (HAPs) is their potential to cause cancer. It has been difficult to assess potential cancer risks from HAPs, due primarily to lack of ambient concentration data for the general population. The Environmental Protection Agency's Cumulative Exposure Project modeled 1990 outdoor concentrations of HAPs across the United States, which were combined with inhalation unit risk estimates to estimate the potential increase in excess cancer risk for individual carcinogenic HAPs. These were summed to provide an estimate of cancer risk from multiple HAPs. The analysis estimates a median excess cancer risk of 18 lifetime cancer cases per 100,000 people for all HAP concentrations. About 75% of estimated cancer risk was attributable to exposure to polycyclic organic matter, 1,3-butadiene, formaldehyde, benzene, and chromium. Consideration of some specific uncertainties, including underestimation of ambient concentrations, combining upper 95% confidence bound potency estimates, and changes to potency estimates, found that cancer risk may be underestimated by 15% or overestimated by 40-50%. Other unanalyzed uncertainties could make these under- or overestimates larger. This analysis used 1990 estimates of concentrations and can be used to track progress toward reducing cancer risk to the general population.     

2017年广州市空气污染物对急救人数的影响分析

目的 了解空气污染物浓度对广州市居民的急救就诊影响情况,为有效提高空气污染相关疾病的早期预防和控制能力提供科学依据。方法 采用时间序列的半参数广义相加模型（GAM）,控制每日急救人次的长期趋势、气象因素、“星期几效应”等混杂因素后,分析广州市2017年1月1日至12月31日大气中逐日的SO₂、NO₂、CO、PM_2.5、PM₁₀、O_3-1h、O_3-8h浓度与居民急救人数的关系。结果 2017年广州市空气中主要污染物为CO,全年空气中PM_2.5、PM₁₀、NO₂、CO浓度12月最高,6月最低;O₃（O_3-1h、O_3-8h）8月份浓度最高,1月最低;SO₂12月最高,1月最低。2017年日均急救人数为405.08例,呼吸系统疾病日均急救人数为26.57例,循环系统疾病日均急救人数38.44例。GAM模型分析结果显示,SO₂、NO₂、CO浓度每上升10个单位,当天及滞后3 d内日均急救总人数相应增加。SO₂浓度升高的当天及滞后第3 天时对呼吸系统疾病急救人数影响最大,每上升10个单位,呼吸系统疾病急救人数分别增加8.01%、9.62%。逐日空气污染物浓度对循环系统疾病急救人数的影响无统计学意义。结论 广州市空气污染物对居民健康存在急性影响,尤其对呼吸系统产生影响,应做好污染物控制,在中、重污染天气下做好敏感人群的健康防护。     

Occupational exposure to hazardous airborne pollutants: effects of air mixing and source location

The presence of airborne pollutants in indoor environments has been associated with occupants' discomfort and/or adverse health effects. This study investigates occupational exposure in relation to indoor air mixing and source location relative to a human body. Experimental and computational methods were used to provide information about the pollutant distribution in the vicinity of the human body for different levels of room air mixing. Study results show that the often used assumption of uniform pollutant distribution in an occupied space is not always appropriate for estimation of inhalation exposure. Results also indicate that an occupant may experience very high acute exposure to airborne pollutants when little air mixing exists in a space and the pollutant source is in the vicinity of the occupant. The buoyancy-driven flow induced by the convective heat transfer from an occupant's body can transport pollutants in the occupant's vicinity to the breathing zone. Specific study results reveal that a source located in the occupant's front chest region makes a relatively large contribution to the breathing zone concentration compared with the other sources in the vicinity of the human body. With the source position in this region, exposure can be nine times greater than that calculated with the uniform mixing assumption. The buoyancy-driven convective plume around a body seems to have a significant influence on pollutant transport and human exposure, especially in the absence of room air mixing.     

Evaluating the health significance of hazardous air pollutants using monitoring data

OBJECTIVE: Though many contaminants are released into the atmosphere, in the US only six air pollutants-ozone, particulate matter, sulfur dioxide, carbon monoxide, nitrogen dioxide, and lead-are closely monitored and carefully assessed for health significance. Other pollutants, even if highly toxic, are neither widely monitored nor routinely assessed at the national level. The goal of this study was to analyze the availability of information needed to characterize the health significance of hazardous air pollutants, focusing on urban areas in California. METHODS: The authors compared different approaches to identifying which contaminants should be considered hazardous air pollutants of potential health concern; reviewed the availability of toxicity values for these pollutants; and analyzed the usefulness of air monitoring data from California agencies for determining populations risks, by comparing method detection limits with health benchmarks. RESULTS: Approaches to identifying air contaminants of possible health concern differ. Toxicity values are not available for many hazardous air pollutants, including those identified in the Clean Air Act. In California, monitoring data are available for many, though not all, pollutants of concern. Monitoring methods for several pollutants do not have adequate sensitivity to detect all relevant concentrations. CONCLUSION: The information necessary to fully assess the health significance of hazardous air pollutants is not currently available.     

Exposure of Paris taxi drivers to automobile air pollutants within their vehicles

OBJECTIVES—To study the exposure of Parisian taxi drivers to automobile air pollutants during their professional activity.
METHODS—A cross sectional study was carried out from 27 January to 27 March 1997, with measurements performed in the vehicles of 29 randomly selected drivers. Carbon monoxide (CO) content was measured over an 8 hour period by a CO portable monitor. The fine suspended particles were measured according to the black smoke index (BS), with a flow controlled portable pump provided with a cellulose filter. The nitrogen oxides, NO and NO₂ were measured with a passive sampler.
RESULTS—These drivers are exposed during their professional activity to relatively high concentrations of pollutants (mean, median (SD) 3.8, 2 (1.7) ppm for CO, 168, 164 (53) µg/m³ for BS, 625, 598 (224) µg/m³ for NO, and 139, 131 (43) µg/m³ for NO₂.) For CO the concentrations were clearly lower than the threshold values recommended by the World Health Organisation. The situation is less satisfactory for the other pollutants, especially for the BS index. All concentrations of pollutants recorded were noticeably higher than concentrations in air recorded by the ambient Parisian air monitoring network and were close to, or slightly exceeded, the concentrations measured at the fixed stations close to automobile traffic. Pollutant concentrations were also influenced greatly by weather conditions.
CONCLUSION—This first French study conducted in taxi drivers shows that they are highly exposed to automobile pollutants. The results would justify a medical follow up of this occupational group.


Keywords: taxi drivers; exposure assessment     

Smoking and Exposure to Occupational Hazards in 8,304 Workers in Guangzhou, China

This cross-sectional study aimed to describe the pattern ofsmoking in relation to occupational hazard exposure in a workingpopulation in Guangzhou, China. In 1994, data on smoking andoccupational hazard exposure from occupational health recordsof 8,304 subjects aged 35 years or older from 47 randomly selectedfactories were studied. About 49% of the men and 55% of thewomen were exposed to dust, chemicals or other hazards. Theprevalence of smoking was 56.1% in men and in women. The prevalenceof smoking in men was higher in those who were younger, withprimary education or who were workers. In women, those who wereolder, with primary education or in management jobs had highersmoking prevalence. In men and women, subjects who were exposedto occupational hazards had higher smoking prevalence: the highest(71.6%) was found in male workers exposed to dust. Urgent tobaccocontrol measures are needed to prevent the epidemic of smoking-relatedand occupation-related diseases in the workplace in China.     

Exposure to carcinogenic air pollutants among policemen working close to traffic in an urban area

OBJECTIVE: This study evaluates individual airborne exposure to gaseous and particulate carcinogenic pollutants in a group of policemen working close to traffic in the center of Grenoble, France. METHODS: Sixty-two personal active air samples were collected during the workshifts of eight policemen in summer and in winter during the occurrence of the thermal inversion phenomenon. Seventeen stationary air samples were monitored in the policemen's work area during the same period with the same sampling devices as used for the personal samples. The respirable particle concentration was determined using the gravimetric method. The concentrations of polycyclic aromatic hydrocarbons and aldehydes were measured with high-performance liquid chromatography with fluorimetric or ultraviolet detection. The benzene-toluene-xylene levels were determined with gas chromatography with flame ionization detection. RESULTS: The median concentration of the personal samples for respirable particles was 55.5 microg/m3 in the summer and 124 microg/m3 in the winter. The corresponding values were 0.10 and 0.28 ng/m3 for benzo(a)pyrene, 14 and 21 microg/m3 for formaldehyde, and 10.5 and 23.5 microg/m3 for benzene. The median personal concentrations were equal to or higher than the corresponding stationary levels for both seasons. Compared with the concentration of aldehydes, the concentrations of respirable particles, polycyclic aromatic hydrocarbons, and benzene-toluene-xylene appeared to show higher individual variability. CONCLUSIONS: The occupational exposure of policemen does not exceed any currently applicable occupational or medical exposure limits. Individual particulate levels should preferably be monitored in Grenoble in winter to avoid underestimations.     

Human Exposure to PBDEs Via House Dust Ingestion in Guangzhou,South China

Numerous studies have demonstrated the important role played by indoor dust in human exposure to polybrominated diphenyl ethers (PBDEs), particularly in children, who spend most of their time indoors. Few data have been available for PBDEs in house dust in China. In this study, dust samples were collected randomly from 46 houses in Guangzhou, South China, for the first time to investigate levels and pattern profiles of PBDEs and to estimate human exposure to PBDEs for adults and toddlers (6 months to 2 years old). Using gas chromatography–mass spectrometry in selected ion monitoring mode, congeners—including BDE28, 47, 66, 85, 99, 100, 153, 154, 183, and 209—were determined. The results show that BDE209 is the predominant congener in house dust, comprising 90.7% to 99.8% (mean 97.4%) of the total concentration. Of ∑tri-hepta BDEs, BDE47, 99, and 183 were the most abundant congeners, with mean contributions of 23.3%, 25.2%, and 21.9%, respectively. The intake of ∑tri-hepta BDEs via dust ingestion was 0.16–3.82 ng/d for adults and 2.1–7.64 ng/d for toddlers, accounting for 0.65% to 13.6% and 18.6% to 45.5%, respectively, of the total (based on data in our previous work) (Chen et al. 2008). With respect to intake of BDE209, the values increased to 11 to 264 ng/d for adults and to 145 to 527 ng/d for toddlers. Risk assessment indicated that PBDEs may possibly affect the health of toddlers in Guangzhou, South China. Further studies are needed to investigate the bioavailability and metabolism of PBDEs in humans to improve risk evaluation.     

Autism spectrum disorders in relation to distribution of hazardous air pollutants in the san francisco bay area

OBJECTIVE: To explore possible associations between autism spectrum disorders (ASD) and environmental exposures, we linked the California autism surveillance system to estimated hazardous air pollutant (HAP) concentrations compiled by the U.S. Environmental Protection Agency. METHODS: Subjects included 284 children with ASD and 657 controls, born in 1994 in the San Francisco Bay area. We assigned exposure level by census tract of birth residence for 19 chemicals we identified as potential neurotoxicants, developmental toxicants, and/or endocrine disruptors from the 1996 HAPs database. Because concentrations of many of these were highly correlated, we combined the chemicals into mechanistic and structural groups, calculating summary index scores. We calculated ASD risk in the upper quartiles of these group scores or individual chemical concentrations compared with below the median, adjusting for demographic factors. RESULTS: The adjusted odds ratios (AORs) were elevated by 50% in the top quartile of chlorinated solvents and heavy metals [95% confidence intervals (CIs) , 1.1-2.1], but not for aromatic solvents. Adjusting for these three groups simultaneously led to decreased risks for the solvents and increased risk for metals (AORs for metals: fourth quartile = 1.7 ; 95% CI, 1.0-3.0 ; third quartile = 1.95 ; 95% CI, 1.2-3.1) . The individual compounds that contributed most to these associations included mercury, cadmium, nickel, trichloroethylene, and vinyl chloride. CONCLUSIONS: Our results suggest a potential association between autism and estimated metal concentrations, and possibly solvents, in ambient air around the birth residence, requiring confirmation and more refined exposure assessment in future studies.     

Geographic risk modeling of childhood cancer relative to county-level crops,hazardous air pollutants and population density characteristics in Texas

Background

Childhood cancer has been linked to a variety of environmental factors, including agricultural activities, industrial pollutants and population mixing, but etiologic studies have often been inconclusive or inconsistent when considering specific cancer types. More specific exposure assessments are needed. It would be helpful to optimize future studies to incorporate knowledge of high-risk locations or geographic risk patterns. The objective of this study was to evaluate potential geographic risk patterns in Texas accounting for the possibility that multiple cancers may have similar geographic risks patterns.

Methods

A spatio-temporal risk modeling approach was used, whereby 19 childhood cancer types were modeled as potentially correlated within county-years. The standard morbidity ratios were modeled as functions of intensive crop production, intensive release of hazardous air pollutants, population density, and rapid population growth.

Results

There was supportive evidence for elevated risks for germ cell tumors and "other" gliomas in areas of intense cropping and for hepatic tumors in areas of intense release of hazardous air pollutants. The risk for Hodgkin lymphoma appeared to be reduced in areas of rapidly growing population. Elevated spatial risks included four cancer histotypes, "other" leukemias, Central Nervous System (CNS) embryonal tumors, CNS other gliomas and hepatic tumors with greater than 95% likelihood of elevated risks in at least one county.

Conclusion

The Bayesian implementation of the Multivariate Conditional Autoregressive model provided a flexible approach to the spatial modeling of multiple childhood cancer histotypes. The current study identified geographic factors supporting more focused studies of germ cell tumors and "other" gliomas in areas of intense cropping, hepatic cancer near Hazardous Air Pollutant (HAP) release facilities and specific locations with increased risks for CNS embryonal tumors and for "other" leukemias. Further study should be performed to evaluate potentially lower risk for Hodgkin lymphoma and malignant bone tumors in counties with rapidly growing population.

     

Evaluation of the agreement between modeled and monitored ambient hazardous air pollutants in California

Elevated breast cancer incidence rates in urban areas have led to speculation regarding the potential role of air pollution. In order to inform the exposure assessment for a subsequent breast cancer study, we evaluated agreement between modeled and monitored hazardous air pollutants (HAPs). Modeled annual ambient concentrations of HAPs in California came from the US Environmental Protection Agency’s National Air Toxics Assessment database for 1996, 1999, 2002, and 2005 and corresponding monitored data from the California Air Resources Board’s air quality monitoring program. We selected 12 compounds of interest for our study and focused on evaluating agreement between modeled and monitored data, and of temporal trends. Modeled data generally underestimated the monitored data, especially in 1996. For most compounds agreement between modeled and monitored concentrations improved over time. We concluded that 2002 and 2005 modeled data agree best with monitored data and are the most appropriate years for direct use in our subsequent epidemiologic analysis.     

Childhood cancer incidence rates and hazardous air pollutants in California: an exploratory analysis

Hazardous air pollutants (HAPs) are compounds shown to cause cancer or other adverse health effects. We analyzed population-based childhood cancer incidence rates in California (USA) from 1988 to 1994, by HAP exposure scores, for all California census tracts. For each census tract, we calculated exposure scores by combining cancer potency factors with outdoor HAP concentrations modeled by the U.S. Environmental Protection Agency. We evaluated the relationship between childhood cancer rates and exposure scores for 25 potentially carcinogenic HAPs emitted from mobile, area, and point sources and from all sources combined. Our study period saw 7,143 newly diagnosed cancer cases in California; of these, 6,989 (97.8%) could be assigned to census tracts and included in our analysis. Using Poisson regression, we estimated rate ratios (RRs) adjusted for age, race/ethnicity, and sex. We found little evidence for elevated cancer RRs for all sites or for gliomas among children living in high-ranking combined-source exposure areas. We found elevated RRs and a significant trend with increasing exposure level for childhood leukemia in tracts ranked highest for exposure to the combined group of 25 HAPs (RR = 1.21; 95% confidence interval, 1.03, 1.42) and in tracts ranked highest for point-source HAP exposure (RR = 1.32; 95% confidence interval, 1.11, 1.57). Our findings suggest an association between increased childhood leukemia rates and high HAP exposure, but studies involving more comprehensive exposure assessment and individual-level exposure data will be important for elucidating this relationship.