People have been advised not to jog or exercise outside, and warnings have been issued for adults and children suffering from heart or lung problems.
The smog, which is blowing over from the continent, could also cause sore eyes, coughing or sore throats, according to the Department for Environment, Farming and Rural Affairs, which is warning of moderate or high pollution levels until the weekend.
A Defra spokesman said: "Adults and children with heart or lung problems are at greater risk of symptoms.
"Follow your doctor's usual advice about exercising and managing your condition.
"It is possible that very sensitive individuals may experience health effects even on low air pollution days."
A Met Office spokesman added: "Some individuals, such as those with existing heart or lung conditions, may experience increased symptoms.”
Although London is one of the worst affected areas, eastern parts of the country such as East Anglia, and the south-west and Wales have the highest level of warning.
The main risk is from 'particulate matter' - small particles and chemicals thrown into the air from motor vehicles and fires. Yesterday the smog setttled over Paris, nearly obscuring some of the French capital's most iconic sites.
As well as health concerns, the smog could potentially ruin the highly anticipated solar eclipse, due to take place tomorrow morning.