High air pollution warnings have been issued for parts of England, including in the South East - Brighton, Eastbourne and Hastings.
There is also a very high alert on Friday along eastern coastal areas from Skegness, Lincolnshire, to Great Yarmouth - and down to Clacton-on-Sea, Essex, and also the Isle of Sheppey in Kent.
The high alert stretches from London to Ipswich and Cambridge, and further north to Nottingham and also to Hull, according to the Department for Environment, Food and Rural Affairs (DEFRA).
Leicester, Northampton, and Manchester are also among the places affected.
The warning also covers areas along the south coast, including Brighton, Eastbourne, and Hastings.
In areas where there is very high air pollution, the public is advised to reduce physical activity, especially outdoors.
People with lung problems, adults with heart problems and older people "should avoid strenuous physical activity", according to DEFRA.
Where a high alert applies, anyone experiencing discomfort such as sore eyes, a cough or sore threat should consider reducing activity, particularly outdoors.
People with lung problems and adults with heart problems should "reduce strenuous physical exertion", especially outdoors and if they experience symptoms.
Older people should also reduce physical activity.
People with asthma may find they need to use their reliever inhaler more often where there is high pollution.
'Avoid unnecessary car trips'
Mayor of London Sadiq Khan said:
"Toxic air is dangerous, particularly for those with heart and lung problems."
He told residents:
"Please look after each other by walking, cycling, avoiding unnecessary car trips, stopping engine idling & not burning wood or garden waste."
It comes after a high air pollution warning was issued for London for the first time since August 2020, according to City Hall.
As of Thursday evening there was still very high pollution in Greater London and high pollution in five regions: East, South East, East Midlands, West Midlands, Yorkshire and Humberside.
This is likely to be unwelcome news for hay fever sufferers too.
Almost two thirds (64%) of them believe pollution is the biggest factor that affects their allergies, according to a survey by Kleenex.
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