Residential Volatile Organic Compound (VOC) Measurements in the San Joaquin Valley: Findings from the SPHERE Study

California's San Joaquin Valley, a major agricultural region surrounded by mountain ranges, experiences severe air pollution due to its basin-like topography, which traps emissions. The San Joaquin Valley Pollution and Health Environmental Research (SPHERE) Study is a collaborative, community-based project led by UC Berkeley, UC Merced, the California Department of Public Health (CDPH), and local partners to characterize air pollution exposures in the region. To evaluate residential exposures to volatile organic compounds (VOCs), we performed 4-hour active air sampling in 16 homes in Fresno, CA between September and November 2023. A total of 23 indoor and 8 outdoor samples were collected and analyzed to quantify VOC concentrations. Twenty-three unique VOCs were measured. The nine most frequently detected indoor VOCs (detection frequency >50%) included toluene, para-xylene, 4-isopropyltoluene, styrene, ethylbenzene, 1,2,4-trimethylbenzene, ortho-xylene, meta-xylene, and benzene, six of which are BTEX compounds (benzene, toluene, ethylbenzene, and xylenes). Median (95th percentile) indoor BTEX concentrations were 0.26 (0.84) μg/m3 for benzene, 2.06 (5.06) μg/m3 for toluene, 0.56 (1.30) μg/m3 for ethylbenzene, and 2.10 (4.91) μg/m3 for total xylenes. Average indoor-to-outdoor (I/O) ratios exceeded 1 for all compounds except benzene (mean I/O ratios ranged from 0.70 to 20.4), indicating substantial indoor sources. Outdoor BTEX concentrations were inversely correlated with residential distance from State Route 99 (rho = -0.75; p<0.05) and positively correlated with ambient NO2 (rho = 0.88; p < 0.05), consistent with traffic-related emissions. Outdoor BTEX levels were negatively correlated with ozone, consistent with photochemical degradation processes that reduce BTEX concentrations as ozone forms. Real-time total VOC (TVOC) concentrations were measured using Atmotube Pro monitors, capturing temporal variability in indoor air quality. Overall, results indicate that indoor environments are the predominant source of VOC exposure in sampled homes, with concentrations frequently exceeding outdoor levels. Although measured levels were below federal health-based reference values, these findings highlight meaningful patterns of localized exposure in disadvantaged communities. Continued air quality monitoring, source attribution, and targeted interventions are warranted to reduce exposures among vulnerable populations.