Ozone, a Pollutant with Two Faces

Ozone (O₃) is an atmospheric gas whose molecule contains three oxygen atoms. In the troposphere, the lowest layer of the atmosphere, ozone is a pollutant that poses a major threat to human health and ecosystems. In contrast, in the stratosphere, between 10 and 50 km above the Earth's surface, ozone becomes an essential building block for life on Earth.

What is tropospheric ozone and how is it formed?

Tropospheric or ground-level ozone is an air pollutant that is highly damaging to human health, agricultural crops and ecosystems. It is also a greenhouse gas that contributes to global warming. Unlike primary pollutants, which are emitted directly into the atmosphere, ground-level ozone is a secondary pollutant formed by the interaction of sunlight with a number of precursor gases. These precursors are mainly volatile organic compounds (VOCs) and nitrogen oxides (NOx), which are mainly emitted by vehicles, fossil fuel power stations, oil refineries, agriculture and other industries.

But wasn't ozone "good"?

You could say that ozone (O₃) has two faces, because it behaves differently depending on where it is in the atmosphere. Colloquially, we speak of "good ozone" and "bad ozone" to distinguish between ozone concentrated in the stratosphere and ozone found in the troposphere. Stratospheric ozone reaches its highest concentration between 20 and 25 km altitude and forms the famous ozone layer. This layer plays a vital role in supporting life on Earth by absorbing ultraviolet radiation from the Sun. It is therefore the Earth's natural defence. The realisation that the ozone layer was being depleted by human activity led to the adoption of the Montreal Protocol and the implementation of measures to mitigate its depletion, which have proved successful. The ozone layer is now slowly recovering and is expected to be fully restored by 2066.

In addition to stratospheric ozone, there is also tropospheric ozone, which has a very negative impact on health. Worryingly, the results of a new study led by the Barcelona Institute for Global Health (ISGlobal) and the Barcelona Supercomputing Centre - Centro Nacional de Supercomputación (BSC-CNS) show an increase in ground-level ozone exposure in Europe.

How does tropospheric ozone affect our health?

Elevated levels of O₃ in the troposphere are associated with a number of adverse human health effects. It can cause coughing, irritation of the throat, neck and eyes, and dryness of the throat. It can also cause respiratory problems such as aggravation of asthma, chronic obstructive pulmonary disease, reduced lung function and respiratory infections, which in severe cases can lead to hospitalisation and death. Exposure to ground-level ozone is estimated to cause one million premature deaths each year. Children, the elderly, and people with respiratory or cardiovascular diseases are particularly vulnerable to the harmful effects of this pollutant.

What about climate and ecosystems?

Tropospheric ozone absorbs radiation and traps the heat in lower atmosphere, therefore also acting as a potent greenhouse gas, so its increased concentration in the atmosphere contributes to global warming. Another side effect of ozone is that it reduces the capacity of ecosystems to absorb CO₂, which has a negative impact on the climate. These effects are most pronounced in regions where tropospheric ozone precursors are emitted, and therefore affect the more industrialised regions of the world.

Ground-level ozone also damages vegetation. It penetrates plant leaves and reduces photosynthesis, slowing plant growth and increasing susceptibility to pests and diseases. In ecosystems, high levels of ozone can lead to loss of biodiversity and reduced forest growth. In agriculture, ozone can reduce crop yields.

How can tropospheric ozone be reduced?

Managing ozone is a complex challenge because of its secondary formation pathway from precursor gases. Addressing climate change, which affects ozone formation through increased solar radiation and rising temperatures, is critical for long-term ozone management and public health protection. In particular, strategies to prevent the formation of tropospheric ozone rely mainly on reducing emissions of precursor gases, especially from cars, power plants and other sources. The solution is therefore to adopt much more stringent measures to reduce air pollution and improve air quality, such as restricting traffic in our cities.