Smog surrounds the Canary Wharf financial district in London April 3, 2014. A dust cloud from the Sahara has covered south east England, raising pollution levels to the highest level in London, preventing people taking exercise outdoors, including the Prime Minister, David Cameron, who said he had skipped his morning jog.[Photo/Agencies]

Pollution from Europe and dust swirling in from the Sahara desert created a "perfect storm" of smog in Britain on Wednesday, prompting authorities to warn people with heart or lung conditions to reduce tough physical exercise outdoors.

The nation's environment ministry has a 10-point scale for measuring air quality, ranging from 1 for a "low" risk of air pollution, to 10, which signifies "very high" levels.

Most of the country was rated at 5 on Wednesday morning, meaning moderate, but forecasters said pollution levels could reach 8 or 9 later.

The smog was caused by pollution from Britain and industrialized areas of the continent trapped in place because of light winds mixing with dust blown up by a storm in the Sahara. The highest levels of smog were forecast for Wales and southeast and central England.

Many motorists across England awoke this week to find cars covered in a film of red dust left by overnight rain.

Paul Cosford of Public Health England told the BBC that people with heart or respiratory problems should reduce the amount of strenuous exercise outdoors over the next few days.

Helen Dacre, a meteorologist at the University of Reading, said an unusual combination of factors conspired to create a "perfect storm for air pollution".

"Toxic gases, such as nitrogen dioxide and ozone, as well as fine dust particles in the air blown in from the Sahara and from burning fossil fuels, all contribute to cause problems for people with heart, lung and breathing problems, such as asthma," she said.

Despite efforts to make industry and vehicles cleaner, air pollution remains a major problem in Britain and across Europe.

Last month, Paris took the drastic step of banning half the city's cars from the roads for a day after a week in which pollution levels exceeded those in smoggy cities including Beijing and New Delhi.

Air pollution is the world's single biggest environmental health risk, responsible for about one in eight deaths, the World Health Organization said last week.

It kills about 7 million people worldwide every year, with more than half of the fatalities due to fumes from indoor stoves and the rest from outdoor pollutants, the WHO said.

AP-AFP