Tropospheric Ozone: A Critical Review of the Literature on Emissions, Exposure, and Health Effects

Tropospheric ozone is a significant air pollutant with severe adverse effects on human health. The complex dynamics of ozone formation, distribution, and health impacts underscore the need for a comprehensive understanding of this pollutant. Despite well-documented health risks, including an estimated 423,100 deaths annually due to ozone exposure, millions of people in major countries continue to be exposed to unhealthy levels. Notably, the epidemiological evidence linking long-term ozone exposure to health outcomes is limited compared to short-term exposure studies, leaving some findings incomplete. Regulatory standards vary globally, with the implementation of the World Health Organization recommendation for an 8-h average limit of 50 ppb to protect public health remaining heterogeneous, leading to significant disparities in adoption across countries, and often significantly higher. Emissions from diesel and gasoline vehicles are major sources of VOCs and NOx in urban areas, and their reduction is a key strategy. Additionally, climate change may exacerbate ozone pollution through increased natural precursor emissions, leading to higher ground-level ozone in polluted regions, like the eastern US, southern Europe, and parts of Asia. Addressing tropospheric ozone effectively requires an integrated approach that considers both natural and anthropogenic sources to reduce concentrations and mitigate health impacts.