Warm, sunny weather across much of the UK is set to lead to the first summer smog, the Department for Environment, Food and Rural Affairs has warned.
High ozone levels are predicted for England and Wales over the weekend.
The high levels of ozone - which is formed when sunlight interacts with nitrogen dioxide and other pollutants - are set to persist until at least Monday, when Northern Ireland could also be affected.
Moderate levels of ozone have been recorded over the past few days for much of the UK.
Ground level ozone can affect some people's breathing. People with asthma are not necessarily more susceptible but if they are affected they can use their "reliever" inhaler, Defra said.
The Environment Department is urging the public to take precautions, including avoiding exercise outdoors in the afternoon to reduce exposure to ozone.
Defra also said people should avoid making unnecessary short car journeys where possible, as the pollutants which cause ground-level ozone come from sources including petrol and other fuels.
Much of the UK is set to experience warm temperatures and sunny weather over the next few days, although there will be scattered, sharp showers in some places, according to MeteoGroup UK, the Weather Division of the Press Association.