Tropospheric ozone

Powerful greenhouse gas and air pollutant that is harmful to human health, agricultural crops, and ecosystems

Tropospheric (or ground-level) ozone is a short-lived climate pollutant that remains in the atmosphere for only hours to weeks. 

It does not have any direct emissions sources, rather it is a compound formed by the interaction of sunlight with volatile organic compounds (VOCs) – including methane – and nitrogen oxides (NOX) emitted largely by human activities.  

Strategies to prevent the formation of tropospheric ozone are primarily based on methane reductions and cutting the levels of atmospheric pollution arising from cars, power plants and other sources.

Top facts


Tropospheric ozone has an atmospheric lifetime ranging from a few hours to a few weeks in polluted urban regions.

Greenhouse gas

Ozone absorbs radiation, acting as a strong greenhouse gas and altering evaporation, cloud formation, and atmospheric circulation.

$11-18 billion 

Estimated global crop production losses owing to ozone total 79–121 million tonnes, worth US$ 11–18 billion annually.

1 million

Long-term exposure to ozone air pollution is linked to 1 million premature deaths per year due to respiratory diseases.

Types of ozone

Ozone (O3) is a gas that exists in two layers of the atmosphere: the stratosphere (upper layer) and the troposphere (ground level up to 10km).

In the stratosphere, ozone protects life on Earth from the sun’s ultraviolet radiation. In the troposphere, it is a powerful greenhouse gas and air pollutant that harms human health and ecosystems. It is also a major component of smog.


In the troposphere, ozone is the product of the atmospheric reaction of a number of precursor pollutants, which have both natural and man-made sources.

Precursor pollutants created by human activities include volatile organic compounds and nitrogen oxides, which are largely emitted by cars and other vehicles, fossil fuel power plants, oil refineries, the agriculture sector and a number of other industries.

Vehicles emit some of the pollutants that form tropospheric ozone. High ozone levels are most likely to occur in urban areas and on hot summer days.

Emissions trends

While air quality has improved in high-income countries, it has generally deteriorated in most low- and middle-income countries, due to large-scale urbanisation and industrialisation.

Since 1995, free tropospheric ozone has increased by between 2-12 % per decade depending on the region of the world. The largest increase in tropospheric ozone is found over developing regions (East Asia, the Persian Gulf, India, northern South America, the Gulf of Guinea, and Malaysia/Indonesia). The trend is weaker in Europe and North America. 

Trends in seasonal average population-weighted ozone concentrations (ppb) for the most populated countries and regions
2017. Health Effects Institute.


Though it is short-lived in the atmosphere, tropospheric ozone has multiple negative impacts on humans, plants, and the climate.

Breathing ground-level ozone can result in a number of health effects, including respiratory illnesses.

Health Impacts

Air pollution exposure has a massive toll on human health worldwide. Exposure to air pollution is estimated to cause millions of deaths and lost years of healthy life annually – on a par with other health risks such as unhealthy diet and tobacco smoking.  

Tropospheric ozone is a major component of smog. It can worsen bronchitis and emphysema, trigger asthma, and permanently damage lung tissue.

Tropospheric ozone exposure is responsible for an estimated one million premature deaths each year. Children, the elderly, and people with lung or cardiovascular diseases are particularly at risk of the adverse health impacts of ozone.

The World Health Organisation has established clear air quality guidelines which offer global guidance on thresholds and limits for key air pollutants – including ozone and PM2.5 – which pose health risks. The most recent guidelines drastically reduced what are considered acceptable levels of air pollution. Unfortunately, nearly all of the world’s populations (99%) currently breathe unsafe air. 

Climate Impacts

Ozone absorbs radiation and consequently acts as a strong greenhouse gas. Tropospheric ozone affects the climate beyond increased warming, having impacts on evaporation rates, cloud formation, precipitation levels, and atmospheric circulation. These impacts mainly occur within the regions where tropospheric ozone precursors are emitted, and so disproportionally affect the Northern Hemisphere.

Agriculture and Ecosystem Impacts

Tropospheric ozone is a highly reactive oxidant that significantly reduces crop productivity as well as the uptake of atmospheric carbon by vegetation. Its effects on plants include impeded growth and seed production, reduced functional leaf area and accelerated ageing. 

Studies have shown that many species of plants are sensitive to ozone, including agricultural crops, grassland species and tree species. These effects damage important ecosystem services provided by plants, including food security, carbon sequestration, timber production, and protection against soil erosion, avalanches and flooding. 

Economic Impacts

Reduced methane and tropospheric ozone returns economic benefits through gains in productivity and environmental services. The Global Methane Assessment has estimated that every one million tonnes of methane reduced avoids:

  • Approximately 1 430 annual ozone-attributed premature deaths per year
  • 4 000 asthma-related accident and emergency department visits and 90 hospitalisations per year
  • Losses of 145 000 tonnes of wheat, soybeans, maize and rice  due to ozone exposure every year
  • Annual losses of roughly 400 million hours of work due to extreme heat


Strategies to prevent the formation of tropospheric ozone are primarily based on reducing one of the main precursors, methane.

Methane’s short atmospheric lifetime means taking action now can quickly reduce atmospheric concentrations and result in reductions in warming rates and ozone pollution, in this and subsequent decades.

Methane’s short atmospheric lifetime means taking action now can quickly reduce atmospheric concentrations and result in similarly rapid reductions in climate forcing and ozone pollution.

The Climate & Clean Air Coalition (CCAC) supports implementation of methane control measures that, if globally implemented by 2030, would reduce global emissions by as much as 40%. Several of these measures could be implemented with net savings, delivering important health and economic benefits from reducing tropospheric ozone.

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