Ozone: both protective shield and irritant gas

There are two sides to ozone: the ozone layer (“stratospheric” ozone) provides the earth’s atmosphere with essential protection against UV radiation. Ground-level (“tropospheric”) ozone can cause health problems, though. This article explains how and when ozone is produced and how to prevent it impacting on health.

At a glance

  • Ozone is a gas that is present in various layers of the atmosphere.
  • Ground-level ozone is produced when solar radiation levels are high, mainly on hot, sunny days.
  • Ozone levels vary from region to region as ozone production depends on the weather and air pollutant levels.
  • The short-term effects on health include eye irritation, headaches and exacerbation of existing respiratory problems and allergies.
  • People who are especially sensitive to ozone or who have a respiratory disease and children in particular should protect themselves against high ozone levels.

Note: The information in this article cannot and should not replace a medical consultation and must not be used for self-diagnosis or treatment.

Blue sky obscured by white cloud with the sun shining behind it.

What is ozone?

Ozone (O3) is a colorless, toxic gas that is chemically similar to oxygen (O2). Unlike oxygen, direct contact with ozone can be harmful to human beings.

Ozone is divided into stratospheric and ground-level (or tropospheric) ozone. Whilst the ozone at ground level is harmful to health, the ozone ten kilometers above the earth (in the stratosphere) has a very important function: the ozone layer shields the earth and human beings from very high-energy harmful ultraviolet (UV) radiation from the sun. Without this protective shield solar radiation on the earth would be so strong that life would be impossible.

Certain pollutants, particularly hydrochlorofluorocarbons (HCFCs), can damage the ozone layer leaving an “ozone hole”. This means that more UV radiation reaches the earth, people get sunburnt more quickly and the risk of skin cancer increases. The increase in solar radiation can also cause damage to the eyes.

What causes ground-level ozone?

Ground-level ozone is a major component of “summer smog”. Ozone is produced by chemical reactions between pollutants such as oxides of nitrogen and volatile organic compounds in the presence of strong solar radiation (sunlight).

These “precursor pollutants” are mainly caused by human activities, particularly road traffic and the solvents in, for example, paint, lacquer, adhesives and cleaning products. However some of these volatile organic compounds (VOCs) are also emitted naturally.

So two conditions are required for ozone to be produced:

  • presence of precursor pollutants
  • intense sunshine

If these conditions are met, concentrations of ozone can occur in the lower atmosphere during prolonged periods of sunny weather.

When are ozone levels at their highest?

Ozone levels are generally highest from May to September. However they also vary based on the time of day: ozone production starts in the morning, reaching its maximum between 2.00 and 5.00 pm. It then falls overnight. Ozone levels are therefore lowest in the early hours of the morning.

Ozone levels are lowest in the early hours of the morning.

Where are ozone levels at their highest?

Contrary to what one might think, the highest ozone levels do not occur in cities where most of the precursor pollutants are produced, but tend to occur in suburban and rural areas. There are two reasons for this: firstly, many of the precursor pollutants are transported out of the cities by wind. Secondly the vehicle emissions which are mainly generated in cities also contain nitric oxide (NO). Some of the ozone produced from nitrogen dioxide (NO2) is partly broken down in the presence of nitric oxide (NO).

How do ozone levels vary over time?

Information and alert thresholds

(Hourly) peak ozone levels are measured as well as daily and annual averages. These are particularly relevant for health. To protect the health of particularly sensitive population groups, the public is informed when ozone levels rise above 180 micrograms per cubic meter, known as the information threshold.

Hourly averages of 240 micrograms per cubic meter or more are referred to as the alert threshold. The public is asked to exercise particular caution if this is reached. The alert threshold has only been reached in Germany on rare occasions so far.


Ozone concentrations are monitored at around 260 metering stations in Germany. Since 1990, there has been a decline in the number of times information and alert thresholds are reached compared with the pre-1990 period. The main reason for this is that the amount of ozone precursor pollutants in Germany has reduced since then.

Contrary to the positive trends for the information and alert thresholds, annual averages have increased somewhat since 1990. This is due to global background levels of precursor pollutants: although pollutants have decreased in Germany they are increasing globally.

How does climate change affect ozone production?

There are already clear signs of climate change in Germany in the form of hotter summers and the rise in average annual temperatures. Climate models predict that air pollution in the summer (“summer smog”) caused by ozone will increase. High air temperatures over several days tend to favor the production of ozone. The main reason for this is that chemical reactions speed up at higher temperatures. For example, more volatile organic compounds, potential precursors to ozone, are produced in hot weather.

Information and alert thresholds are increasingly being exceeded on hot days where the temperature reaches 30 degrees or more. In the 2003 summer heatwave, for example, this happened more frequently than in the years before or since.

The “ozone season” can also be extended if the weather stays warm and sunny into the autumn.

What impact does climate change have on health?

This video shows how some conditions are spreading as a consequence of climate change. Rising temperatures have consequences for people’s health.

This and other videos can also be found on YouTube

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So climate change does not only harm human health and wellbeing by heatwaves and periods of drought but also by increasing ozone levels.

How does ozone affect health?

High ozone levels can cause health problems such as streaming eyes, coughing or a sore throat, headaches, decline in lung function and reduced performance

Temporary effects

Since ozone is an irritant gas it can cause various forms of irritation. These include streaming eyes, respiratory problems such as coughing or a sore throat as well as headaches. The intensity of the irritation depends mainly on the period of time spent breathing air with high ozone levels. Breathing in more often and more deeply during severe physical exertion increases the amount of ozone in the lungs. This can impair lung function and restrict performance. These problems generally clear up within 48 hours.

Interesting fact: Individual responses to ozone can vary. It is estimated that 10 to 15% of the population have a sensitive reaction to ozone. It is not entirely clear why this is.

High ozone levels can harm people with pre-existing conditions such as asthma, COPD and pollen allergy in particular. The number of people referring themselves to hospital emergency departments for asthma-related problems increases when ozone levels are high.  There is also some evidence that the allergy potential (“allergenicity”) of many species of pollen is enhanced by ozone.

Long-term consequences

Inflammation and tissue damage may occur if large quantities of ozone enter the lung when ozone levels are high. The inflammation does not clear up completely. Although by itself it does not cause a respiratory system disorder it can make the lung more susceptible to, for example, allergens and pathogens. So an increase in ozone levels can increase the likelihood of asthma, allergies and infections. Babies and infants are particularly at risk as their lungs are still at the developmental stage. The amount of ozone they breathe in is also relatively higher due to their low body weight.

How can you protect yourself against high ozone levels?

Ozone levels in Germany have so far only reached harmful levels on rare occasions. However rising temperatures in the future may make an increase in ozone production more likely as the amount of volatile compounds emitted from solvents or the environment increases at high temperatures. So it is still essential to reduce production of ground-level ozone as much as possible.

Many people respond particularly sensitively to ozone and so there are a few things they need to consider even when ozone levels are only slightly elevated to protect their health and wellbeing. The same applies to people with respiratory disease, babies and infants. Everyone is advised to take the following measures when the alert threshold is reached, or earlier if possible:

  • Do physically demanding activities such as sports in the mornings when ozone levels are lowest.
  • Air rooms mainly in the mornings.
  • Spend as little time outdoors at midday and in the afternoon as this is when ozone levels are at their highest.

Where is information provided on current ozone levels?

The Federal Environmental Agency (Umweltbundesamt) website publishes daily air pollution levels (ozone, particulates and nitrogen dioxide) region by region.

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