Prime Minister David Cameron gave up his regular morning jog because of the toxic smog that has descended on the capital.
London's braver runners and cyclists have in some cases donned masks to protect them from the exceptional levels of pollution, fuelled by the dust from Sahara storms.
Cameron told BBC1's Breakfast: "It is unpleasant, and you can feel it in the air.
The PM, usually a keen morning jogger, did not go out this morning (file photo)
"The advice I would give to people is listen very carefully to what the Met Office is saying about the weather. Public Health England's website, you can look at that.
"Or just go to gov.uk, which is the Government's website, which will give you the latest in terms of medical advice.
"I didn't go for my morning run this morning. I chose to do some work instead. You can feel it.
"But it's a naturally occurring weather phenomenon. It sounds extraordinary, Saharan dust, but that is what it is."
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Those with lung and heart conditions have been told to avoid strenuous activity outdoors while people suffering symptoms of pollution - including sore eyes, coughs and sore throats - should cut down the amount they do outside, health experts have said.
Some schools in London have banned pupils from outdoor playgrounds to reduce their exposure to the fog.
Sotiris Vardoulakis, head of air pollution at PHE's centre for radiation, chemical and environmental hazards, said most people will not be affected by short-term peaks in air pollution.
But some groups, such as those with existing heart or lung conditions, may experience increased symptoms.
"On occasions where levels are high, adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms," he said.
"Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors."