A view of London's skyline, Thursday, April 3, 2014.
Schools closed their playgrounds and ambulance staff reported a surge in breathing problems as smog blanketed parts of England for a second day on Thursday.
Environmentalists criticized Prime Minister David Cameron after he described the haze - a mix of local pollution, European emissions and Sahara sand - as "a naturally occurring weather phenomenon."
As air pollution hit the top level of 10 in London and the southeast Thursday, Green Party European lawmaker Keith Taylor said Cameron's "flippant response to this invisible killer is utterly disgraceful."
Some schools kept children inside, while the London Ambulance Service said it was seeing a higher-than-normal number of calls from people with breathing difficulties, asthma and heart problems.
Forecasters said the pollution should be swept away Friday by fresh winds from the Atlantic.
The World Health Organization says air pollution kills about 7 million people a year globally, with more than half of the deaths due to fumes from indoor stoves.
Gary Fuller, an air-quality expert at King's College London, said Britain's recent smog had caught public attention partly because it could be seen. Motorists awoke earlier this week to find their cars covered in red dust from a storm in the Sahara desert.
"Normally air pollution is sort of invisible," he said.
Fuller said air pollution in London and other European cities had improved little over the past decade, even though new cars are cleaner than older ones. He said that had been offset by an increase in the number of more polluting diesel vehicles.
"'Dieselization' is much discussed in air-pollution terms," Fuller said. "It is not having a favorable effect."