Although acute adverse respiratory effects have been established for EPA criteria air pollutants such as ozone, there is little information on respiratory effects from air pollutants such as volatile organic compounds (VOCs) from outdoor toxic emission sources. We evaluated acute effects of air toxics in school children with asthma and characterized VOC exposures using subject time-activity reports, breath sample GC-MS, and personal, indoor home and outdoor stationary site VOC samplers. We recruited 26 Hispanics, ages 10-16, living in the Huntington Park, East Los Angeles County, an area flanked by major freeways and trucking routes. Subjects filled out symptom diaries and performed peak expiratory flow (PEF) lung maneuvers twice daily, Nov. 1999 to Jan. 2000. Two subjects dropped out, 4 had invalid diary/PEF data, and 1 had no breath samples. Central site measurements were made for VOCs and criteria air pollutant gases daily. On asthma episode and baseline symptom-free days, subjects collected samples of their exhaled breath in evacuated canisters; we analyzed these for 1,1-dichloroethane, benzene, carbon tetrachloride, chloroform, styrene, tetrachloroethylene, toluene, m,p-xylene, o-dichlorobenzene, o-xylene, and p-dichlorobenzene. Personal and indoor home VOC passive samples were collected in four subjects on 34 days. The ratios of breath VOC/indoor VOC were less than 1. Personal exposures were correlated with indoor exposures, but did not correlate with outdoor measurements for most VOCs. Only outdoor benzene, styrene and m,p-xylene on the previous two days appeared to be correlated with current day breath levels. Breath VOCs showed greater within- than between-individual variance. We found positive associations between asthma symptoms and breath concentrations of benzene (93 person-days), but not other VOCs. However, significant adverse effects of ambient VOCs on asthma symptoms (938 person-days) were found for benzene, 1,3-butadiene, ethylbenzene, styrene, tetrachloroethylene, toluene, m,p-xylene, o-xylene, acetone, acetaldehyde and formaldehyde. Symptoms were also positively associated with NO2, SO2 and O3. In a subset of days with particle data available, symptoms were associated with organic and elemental carbon, which notably confounded effects of PM10. Deficits in PEF in relation to pollutant increases were largely not statistically significant. This study has provided valuable insight regarding the measurement methods needed to assess personal VOC exposures and doses in children. Our findings are compatible with the view that many of these pollutants may be markers for a causal mixture of combustion-related pollutants in areas with high traffic density. Results suggest more work is needed on potentially causal air toxics in the pollutant mix from both traffic and industrial sources.