Asthma sufferers have been warned to stay indoors until Saturday to avoid toxic smog which has blown into Lancashire from the Continent.
Health experts have advised anyone with respiratory problems to take precautions while the air quality is so poor, especially in towns and cities.
The pollution arrived on Thursday from France and mixed with home-grown smog to leave a potentially lethal cloud hanging over parts of the UK, particularly the North West.
It is expected to remain at a moderate level on Friday and is not due to fully lift until the weekend.
Government specialists said people with lung and heart problems should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may need to use their inhaler more often. Older people should also reduce physical exertion.
Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.
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.Asthma UK chief executive Kay Boycott
It is the second consecutive year that spring smog has hit the UK.
Last year it led to emergency services reporting a rise in call outs to patients with respiratory issues, due to a combination of dust from the Sahara, pollution from the continent and light winds.
Asthma UK chief executive Kay Boycott 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.”