People in parts of England should be braced for “very high” levels of air pollution on Wednesday, the government has said.
Environment department Defra said high levels recorded on Tuesday in eastern England and the Midlands would spread.
The pollution – a mix of local and European emissions and dust from the Sahara – is set to move across southern England, the Midlands and East Anglia.
Those with lung or heart disease are urged to avoid strong exercise outside.
Defra has a 10-point scale for measuring air quality – with 1 meaning there is a “low” risk of air pollution and 10 warning of “very high” levels.
Levels hit the maximum of 10 in north-west Norfolk on Tuesday.
Readings for Wednesday are expected to reach 8 or 9 in much of southern England, East Anglia and the Midlands, and possibly 10 along England’s southern coast.
Advice on the department’s website states that for high levels of pollution: “Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.
“People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.”
It adds that “anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors”.
Andrew Sibley, specialist forecaster for the Met Office, said: “Conditions are going to get worse again as we go through Wednesday and the rest of the week.”
He said that a south-easterly wind would bring with it yet more pollution from Europe, and further compound the situation in the UK.