High air pollution levels across England prompted the government to issue health warnings on Wednesday.
The pollution, a mix of local and European emissions and dust from the Sahara, was forecast to be highest in southeast England, central England and Wales.
Asthmatics were warned to keep their inhalers handy as they could be prone to attacks, while the elderly and people with lung or heart disease were urged to avoid strenuous exercise outside.
DEFRA, the environment ministry, has a 10-point scale for measuring air quality, ranging from one for a "low" risk of air pollution to 10, which signifies "very high" levels.
Most of the country was rated as five on Wednesday morning, meaning moderate, but forecasters said pollution levels could reach eight or nine later in the day.
Although skies above London were turned hazy by the smog, authorities said there were no plans to restrict car use, as Parisian authorities did last month when the French capital was hit by air pollution.
Doctor Helen Dacre, a meteorologist at the University of Reading, said: "Saharan dust gets blown over to Britain several times a year.
"The current episode has been whipped up by a large wind storm in North Africa. This has all combined to create high concentrations of pollutants in the air."
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