Britain is on smog alert on Thursday as dirty air from the continent blows in to add to home-grown pollution, sparking health warnings for asthma sufferers and the elderly.
Wide swathes of England, Wales and Northern Ireland will see air pollution climb to moderate or high levels on Thursday, hitting nine out of 10 on the scale in parts of the north, the Department for Environment, Food and Rural Affairs (Defra) said.
A Defra spokeswoman said: "Winds bringing in pollution from the continent, combined with locally generated pollution and still weather conditions has led to some high pollution measurements across the UK."
Pollution levels are likely to remain moderate to high across Wales and southern England on Friday, before dropping to mainly low levels around the whole of the UK on Saturday, Defra said.
Kay Boycott, chief executive of Asthma UK, said: "Two-thirds of people with asthma find that air pollution makes their asthma worse, putting them at an increased risk of a potentially fatal asthma attack.
"When air pollution is high it's vital people with respiratory conditions including asthma check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution.
"People with asthma have told us that on days when air pollution levels are high they feel that they can't even leave the house for fear that it will trigger an attack."
A Met Office spokeswoman said high pressure and very little wind is causing the pollution patch to hang around.