Air pollution levels remain high across parts of England and Wales but are expected to reduce later today.
The Department for Environment, Food and Rural Affairs (Defra) reports that more than 20 areas from northern England down to the South East are still gripped by high levels of pollution this morning.
But a fresh air mass arriving later today is expected to push the pollution eastwards over the North Sea, weather forecasters have said.
Ambulance services reported a rise in 999 calls yesterday as air quality plummeted across the UK
The London Ambulance Service recorded a 14% jump in emergency calls for help with breathing difficulties, asthma and heart problems, while the West Midlands Ambulance Service has also seen more people with breathing and heart trouble.
The capital experienced "very high" levels of pollution - the highest level recorded by Defra.
The smog-like conditions are being caused by a perfect storm of dust from the Sahara, emissions from the continent, low south-easterly winds and domestic pollution.
Tom Tobler, a forecaster for MeteoGroup, said: "The air quality will improve throughout the day.
"It will edge out to the east over the North Sea as some slightly fresher air mass comes in across the UK from the west."
Meanwhile David Cameron has been criticised by the European Commission's environment spokesman Joe Hennon for "misunderstanding" the air pollution problem.
The Prime Minister yesterday told the BBC: "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."
However Mr Hennon told the Guardian his comments were "more than disappointing" because air pollution is a long term problem.
"To say this is a temporary issue caused by Saharan dust shows a clear misunderstanding of the air pollution issue.
"It's clearly an issue you would expect any government to deal with if it's serious about protecting the health of the general public.
"It shows that the problem is not yet understood and one of the reasons we're taking legal action against the UK is that they've not met the targets they agreed to. If I was living in the UK then I would not be happy about that."
London mayor Boris Johnson told ITV London he was also unconcerned about the pollution levels.
He said: "I'm urging people just to have a little balance here. I cycled this morning and it seemed perfectly fine to me.
"I think we need to keep a little bit of a sense of proportion. I cycled perfectly happily around today. I understand asthmatics and people who are particularly vulnerable perhaps need to be cautious but there's no reason why people shouldn't go about their daily lives."
Onkar Sahota, health spokesman for the London Assembly Labour group, described the mayor's comments as "dangerously complacent" in the face of increased 999 calls.
West Midlands Ambulance Service, which covers Shropshire, Staffordshire, Warwickshire, Herefordshire and Worcestershire, confirmed it had experienced a noticeable spike in call-outs linked to breathing problems and chest pains.
A spokesman for the East of England Ambulance Service NHS Trust also said there had been a slight increase in 999 calls.
"Demand on Wednesday was up by 6% in comparison to recent Wednesdays," he said.
"But although we were busy first thing in the morning and then early evening, it is difficult to say the air pollution was a factor as the number of patients with breathing problems and associated sickness was not markedly high.
"It's really important that people with long-term conditions which could be exacerbated by the air quality take extra caution to help prevent their health getting worse."
Kay Boycott, chief executive of Asthma UK, said: "We know from previous research that two thirds of people with asthma find that air pollution makes their asthma worse.
"This new data demonstrates that 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.
"Asthma can be very serious, it takes the lives of three people every day so we want to do everything we can to help people minimise their risk of a potentially life threatening attack."
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.
Sotiris Vardoulakis, head of air pollution at Public Health England's (PHE) 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.
Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors, she said.
Some schools in London have banned pupils from outdoor playgrounds to reduce their exposure to the fog.
A spokeswoman for the Department for Education said it was up to individual schools to decide whether to keep pupils indoors.