About Ozone

Ozone is a secondary pollutant formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight. The precursors to ozone are produced primarily by human activities, including industrial processes and the burning of fossil fuels. Some volatile organic compounds also come from natural sources.

The ozone that forms near ground level is a harmful pollutant. This is different from the protective “ozone layer” that forms in the upper atmosphere as a result of natural processes.

Long-term exposure to ground-level ozone is linked with chronic obstructive pulmonary disease (COPD), a progressive and debilitating disease that makes it harder to breathe.

Why it Matters

Ozone has serious health impacts, accounting for 489,000 deaths globally. Because COPD typically takes many years to develop, older adults — and countries with aging populations — suffer the highest burden.

Ozone pollution is also connected with climate change. Warmer air causes ozone to form faster. In some instances, this means that extreme heat, especially in urban environments, can result in more poor air quality episodes. Ozone is also a greenhouse gas and contributes to warmer temperatures on a global scale. 

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Ozone Trends

Globally, studies suggest that ozone levels today are 30% –70% higher than they were 100 years ago. This is because of two trends: emissions of ozone precursors have risen, and at the same time, rising global temperatures have sped up the chemical reactions that form ozone.

Countries with the highest average ozone exposures in 2020 were in the Middle East (Qatar, Bahrain, Kuwait, Saudi Arabia, and Iraq), South Asia (Nepal, India, Bangladesh, and Pakistan), and East Asia (Republic of Korea). In 2021, 79% of the countries already meet the WHO Air Quality Guideline for peak season ozone (60 µg/m3). Unlike some other pollutants, there is no consistent relationship between ozone exposures and a country’s level of sociodemographic development, in part because ozone is often transported far from its sources, affecting multiple countries. Regions like South Asia and East, West and South Africa have increasingly high exposure to ozone.

The overall number of ozone-related COPD deaths increased by nearly 25% over the past decade. Regional trends vary widely, reflecting regional differences in ozone exposures and COPD mortality rates, as well as underlying health and population characteristics. Together, India and China account for 70% of the total global mortality burden due to their combination of high ozone exposures, high COPD death rates, and large populations. Interestingly, the United States has one of the highest ozone-attributable  global disease burdens among high-income countries.