Leeds has been placed on a health alert because a potentially dangerous cloud of air pollution is blowing over the county.
Experts said the smog could cause fatal asthma attacks and have 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 home-grown smog to create the problem.
Air pollution in urban areas of West Yorkshire is currently rated as “high”, especially near busy roads.
Officials warned this could trigger health difficulties for those with respiratory illnesses and the elderly, although it does not pose a health threat for the general population.
Large parts of England will see air pollution climb to moderate levels today while parts of the north will see it hit nine out of 10.
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.
“Levels are expected to return to low by Friday.”
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 few winds is causing the pollution patch to hang around.