**4.2 Air pollution risks**

 The around-the-clock operations of UOG production sites mean that people and communities in the vicinity may experience a continuous, albeit variable, exposure to airborne pollutants. In addition to infrequent but acute symptoms, they may thus suffer effects from cumulative exposure. The drilling, fracking and operation of UOG wells releases VOCs, from valves, pipes, condensate tanks, flowback and produced water tanks, and other infrastructure. Well maintenance operations such as offloading, additional fracking stages, etc. are often episodes of high air emissions of hydrocarbons, especially for natural gas wells. Residents have complained about odors and health symptoms such as headaches, nose bleeds, skin irritation, chronic fatigue, and neurological effects. A number of observational studies has shown associations between the occurrence of health symptoms and distance to the well, well density, and temporal coincidence with well-site activities [21–24]. Well completions, condensate storage tanks and compressors have been shown to release VOCs, including C2–C8 alkanes, aromatic hydrocarbons, methyl mercaptan, and carbon disulfide [4]. Also process-related is a study that found elevated concentrations of benzene, several aliphatic hydrocarbons in samples taken 130–500 feet from five well pads in Colorado during high-emission periods of uncontrolled flowback [4, 25]. The increased truck traffic also degrades local air quality through diesel exhaust, nitrogen oxides, dust, and other pollutants associated with diesel fuel combustion. Several studies of ambient air quality in densely populated areas with high UOG activity have shown that while the majority of wells produce emissions below regulatory standards and action levels, a few high-emitters can be responsible for the majority of emissions [26–28].

At the regional level, ozone, methane, benzene, and alkanes have been traced back to UOG production and installations, notably in Colorado's Front Range, the Denver-Julesburg Basin, the Niobrara Basin, the Uintah Basin, and the Upper Green River Basin [29–32]. Winter ozone levels in some of these regions have reached levels (149 ppb) exceeding the worst days of day-time ozone levels in Los Angeles, one of the most ozone-polluted cities in America. Emission inventories showed that 98–99% of the VOCs and 57–61% of NOx were attributable to unconventional oil and gas production [33]. Texas and Louisiana are also projecting increases in ground-level ozone concentrations of between 9 and 17 ppb above current concentrations for the low and high-emission scenarios, which may push some counties into non-attainment status of the federal ozone air quality standard (70 ppb).

Global effects of the growth in UOG arise from increases in methane emissions. Bottom-up and top-down studies have revealed higher methane levels in areas with UOG production, mostly natural gas shale plays, than under previous emission inventories released by the U.S. Environmental Protection Agency (EPA). According to the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), methane has a global warming potential that is 28 times that of carbon dioxide over a 100-year time horizon. Thus, while the transition of electric power generation from old, dirty coal-fired power plants to more efficient and cleaner natural gas plants is associated with regional air quality improvements, the climate benefits of UOG for shale gas remain somewhat disputed [34–37].
