Cumulative Human Health Risk Assessment of Regional Ozone and Volatile Organic Compounds from Unconventional Oil and Gas Sites in Colorado's Front Range.

BACKGROUND Most unconventional oil and gas (UOG) extraction in Colorado occurs within the Denver Metro/North Front Range (DMNFR) ozone Non-attainment Area (NAA). Previous UOG human health risk assessments do not consider cumulative risk from both volatile organic compounds (VOCs) and criteria air pollutants (CAPs) like ozone. METHODS We conducted a cumulative human health risk assessment (CHHRA) utilizing regulatory-grade 1- and 8-hour ozone measurements from a DMNFR United States Environmental Protection Agency (EPA) monitoring station and weekly and short-term (15 second - 1 minute) VOC air quality monitoring (AQM) data collected between 2018-2023 from ten near-pad air monitoring sites, three community sites, and one background site. Acute and chronic non-cancer hazard indices (HIs) for multiple health endpoints, as well as cancer risks, were calculated during well drilling, well completions, and production activity periods and compared between sites. VOC concentrations were compared between operations that used a petroleum-based drilling fluid versus a synthetic drill fluid. Differences in weekly chemical concentrations between sites, UOG phases, and drill muds were analyzed using repeated measures analysis of variance (ANOVA) with post-hoc pairwise comparisons with Bonferroni adjustment. RESULTS Acute HIs of VOCs and ozone ranged from 1.34 x 10-4 to 31.33 at the 95th percentile concentrations at all sites. One of the three community monitoring sites, Anthem, exceeded EPA thresholds for respiratory, immunological, and developmental endpoints during production and for the immunological endpoint during well completions. At the near-pad sites, acute hazards exceeded EPA thresholds during well completions for immunological, respiratory, reproductive, developmental, and neurological endpoints. Neurological and immunological HIs were above thresholds for the drilling phase at near-pad sites. Chronic HIs ranged from 8.43 x 10-4 to 0.47 at 95th percentile concentrations and, therefore, were below the HI threshold for all near-pad and community sites for all health endpoints. Cancer risks ranging from 209 to 335 in a million at 95th percentile concentrations were above EPA's thresholds for all sites, including the background site which is more heavily influenced by Denver-metro traffic emissions. CONCLUSIONS Our results suggest that for communities located near UOG well pads in the DMNFR ozone NAA, acute health risks persist after the implementation of best management practices to reduce emissions. Greater protection to public health could be afforded by establishing policies that require drilling and well completions to be conducted outside of summer ozone season. Further research is needed to address potential health risks from the use of synthetic drilling fluid. https://doi.org/10.1289/EHP16272.