Parts of England were placed on a health alert Thursday by the government's environment agency because of a cloud of air pollution sweeping across the country.
The north of the England was the worst affected, with air pollution expected to hit nine out of 10 on the Met Office's pollution scale.
Commuters in the north headed to work on Thursday with high buildings shrouded in a smog.
Experts said the smog could cause fatal asthma attacks and warned the elderly and those with health problems to be cautious.
The Department for Environment, Food and Rural Affairs (Defra), said air pollution was blowing in from Europe and mixing with local particulate matter to create the smog problem. However, levels were expected to fall later in the day with moderate or low levels predicted for Friday.
A Met Office spokeswoman said high pressure and very little wind is causing the pollution patch to hang around.