Poll: Are you worried about the health effects of toxic smog lingering over the UK?

Still air and high pressure mean the pollution is building up, getting trapped in the atmosphere and causing problems.

The current cloud is being caused by a combination of home-grown smog, mixed with air pollution from the continent, The Department for Environment, Food and Rural Affairs (Defra), said.

Experts have warned that the polluted air could cause fatal asthma attacks and have advised the elderly and those with health problems to be cautious.

Increased levels of toxic gases, such as nitrogen dioxide and sulphur dioxide, mean there is a lower concentration of oxygen in the air breathed, said Professor Liz Bentley, Chief Executive of the Royal Meteorological Society.

She added: "There are a mixture of gases that make up the current pollution that we have got."

Prof Bentley explained that some of the air pollution is being caused by industry in the UK, and that some of it is coming from the continent, as far as Russia.

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."

According to Defra, large parts of England will see air pollution climb to moderate levels today while parts of the West Midlands, Yorkshire and Humberside, the South West and Wales could climb to high or even reach eight 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."

She added that high levels are not seen very often, and that it is not unusual to see an increased level of pollution in the spring.

Met Office spokeswoman Nicky Maxey said high pressure and still air are causing the pollution patch to hang around.

However, she added that it was an "improving picture" with winds starting to blow in a north-easterly direction.