LONDON It has shrouded England's most famous monuments for days, prompted a rash of calls to the emergency services over health fears and even stopped Prime Minister David Cameron from taking his early morning jog.
A freak combination of weather conditions has left parts of the country covered in a smog haze made up of high levels of particles, including dust from the Sahara.
Pollution levels in the capital and southeast England were rated at a maximum level of 10 on Thursday, according to the Department for Environment, Food and Rural Affairs (Defra).
"The high level of air pollution this week is due to a combination of local emissions, light winds, pollution from the continent and dust blown over from the Sahara," a Defra spokesman said.
Pollution levels in Britain reach the highest level in particular places a few times a year, he said.
In March, air pollution levels in Paris hit such heights that transport chiefs enforced the most drastic traffic curbs in years, making public transport free in the French capital.
Landmarks like London's Big Ben and the huge "Angel of the North" sculpture in the city of Gateshead were shrouded in haze as frustrated tourists try to take pictures through the murk.
The pollution has been a particular bane for those with respiratory problems and health experts have been discouraging people from taking vigorous exercise outdoors.
Cameron was one who heeded the advice.
"I didn't go for my morning run this morning, I chose to do some work instead. You can feel it in the air," he told the BBC.
Asthma UK said an overnight poll showed there had been a surge in reported asthma attacks triggered by the high pollution levels.
Some 30 percent of respondents had had an asthma attack as a result of the pollution and 84 percent reported using their blue reliever inhaler more than usual, the charity said.
Chief Executive Kay Boycott said: "The current high levels of air pollution are having a significant impact on the health and quality of life of people with asthma and that they need to take urgent action to stay safe."
London's ambulance service said calls out for people with breathing problems were 14 percent higher than average on Wednesday.
London resident Barry Neild said he had asked for his daughter, who has asthma, to sit out a sponsored skipping event at her school in northwest London because the pollution had caused her breathing to become labored.
"It's obviously a concern for us to have her out and about," he said. "Normally she would cycle to school or get her scooter, which is quite an active journey for her but now she is walking very slowly."
Forecasters say the smoggy conditions are likely to last into the weekend.
(Reporting by Paul Sandle, Additional reporting by Lavinia De Luca, Editing by Angus MacSwan)