Ozone exposure: health effects, who is most at risk and what to do

Ozone is a gas made of three oxygen atoms. High in the atmosphere it forms a protective layer, but at ground level it is a pollutant. Asthma + Lung UK counts ozone among the four particularly damaging types of air pollution, with particulate matter, nitrogen dioxide and sulphur dioxide.

Unlike many pollutants, ground-level ozone is not emitted directly. It forms when sunlight reacts with other pollutants, which is why levels follow the weather and time of day rather than traffic alone. In the workplace, ozone can be generated by some welding processes, so exposure can also be occupational.

Asthma + Lung UK says there are higher levels of ozone in the spring and summer and in the afternoon, and lower levels in winter and in the mornings. It adds that ozone levels are often higher in the countryside than in the town, and its advice on lowering your risk makes the same point about countryside ozone.

In practice this means that a hot, still afternoon is when people who are sensitive to ozone are most likely to notice it, even in a rural place that seems free of pollution.

According to Asthma + Lung UK, high ozone levels can reduce your lung capacity, the amount of air your lungs can hold, and make it uncomfortable to breathe. More people with lung conditions are admitted to hospital when ozone levels are high, and there is a greater risk of getting pneumonia and bronchitis.

The Department for Environment, Food and Rural Affairs (Defra) health advice for air pollution generally lists a range of short-term effects that can start within hours or several days after exposure.

• Coughing and wheeze.

• Chest tightness and pain.

• Difficulty breathing, or being more breathless doing activity outdoors.

• Eye, nose and throat irritation.

• Worsening of asthma symptoms and COPD symptoms.

• Worsening of heart-related symptoms such as palpitations.

Defra advice identifies adults and children with heart or lung conditions, and older people, as the groups more likely to be sensitive. Asthma + Lung UK adds that children are more at risk than adults because their lungs are still developing and they breathe faster, which means they take in more polluted air, and that if you are pregnant pollution can affect your unborn baby.

Some people experience symptoms even when levels are low. Defra advises people with lung or heart conditions to follow the usual advice from their doctor or nurse and their agreed management plan, for example an asthma action plan, and to ask for a plan if they do not have one.

The HSE describes ozone as one of the gases in welding fume. It says ozone in particular can cause dryness of the throat, coughing or tightness in the chest when TIG welding stainless steels and aluminium, that these effects tend to be short-lived, and that extreme exposure to ozone can cause fluid on the lungs.

Employers are required to assess and control exposure to hazardous substances under COSHH. The HSE's preferred approach is capture at source using local exhaust ventilation, with respiratory protective equipment where needed. If symptoms such as cough and tight chest begin or worsen during working hours and ease away from work, mention that pattern to a GP or occupational health service.

Defra's Daily Air Quality Index (DAQI) describes pollution as low, moderate, high or very high, with health advice for each band. For most people, short-term exposure to low or moderate levels is not an issue and they should continue usual outdoor activities. At high and very high levels, it advises trying to reduce your exposure, especially if you have symptoms, and adapting physically demanding outdoor activities if you have a lung or heart condition or are older.

It advises speaking to your doctor or nurse if you have new symptoms, your symptoms get worse or they do not get better after a week. People with asthma who use a reliever inhaler should carry it with them and use it as soon as symptoms start, as Asthma + Lung UK advises. Local forecasts are available from the Check air quality service run by Defra.

Because afternoon is when ozone is highest, one practical approach for sensitive people is to plan strenuous outdoor activity for the morning on warm days, in line with Asthma + Lung UK's description of when levels peak.