High levels of polluting particles known as PM10s and ozone are anticipated until Sunday across England and Wales.
The Department for Environment, Food and Rural Affairs (Defra) urged people to take ‘sensible precautions’.
It warned that those suffering from lung diseases including asthma and particularly the elderly ‘should be aware that their symptoms might worsen’.
Defra added: ‘They may need to consider modifying their treatment as they usually do when symptoms increase; consulting their doctor if this is not effective.’
The department advises the public to avoid exercise outdoors in the afternoon to reduce exposure to ozone. People should also avoid making unnecessary short car journeys where possible can reduce the formation of ozone.
Defra will provide regular updates on levels of pollutants and ozone on its website or by calling its free phone helpline on 0800 556677.
This number also offers health advice to those who may be particularly sensitive to air pollution.