Ambulance services have seen a rise in 999 calls as the UK continues to be gripped by high levels of pollution.
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 is currently experiencing "very high" levels of pollution - the highest level recorded by the Department for Environment, Food and Rural Affairs (Defra).
This morning, the South East also recorded "very high" levels of pollution although this has since dropped to a "high" recording. In total, 19 parts of the UK are currently experiencing high levels of pollution.
A perfect storm of dust from the Sahara, emissions from the continent, low south-easterly winds and domestic pollution has caused air quality to plummet across the UK and the smog-like conditions are not expected to clear until tomorrow.
The London Ambulance Service said it saw a 14% increase in emergency calls for patients with breathing problems yesterday, from an average of 200 normally to 227 calls.
Deputy medical director Fenella Wrigley said: "More people are calling us with breathing difficulties, asthma and heart problems.
"This is a busy time for us and I would urge Londoners to use us only in an emergency, and anyone with a minor condition should call NHS 111 or seek advice from their pharmacist or GP."
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.
Daily figures for calls reporting breathing or chest problems were largely level at around 460 on Friday, Saturday and Sunday last week.
But the corresponding figures for the first three days of this week were 547, 510 and 501.
A spokesman for the East of England Ambulance Service NHS Trust also said there had been a slight increase in 999 calls yesterday.
"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."
Prime Minister David Cameron gave up his regular morning jog because of the poor air quality.
He told BBC1's Breakfast: "It is unpleasant, and you can feel it in the air.
"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."
Meanwhile, London mayor Boris Johnson told ITV London he had been cycling around the city and "it seemed perfectly fine".
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.
An overnight poll of 532 asthmatics for Asthma UK found 30% have suffered an attack as a result of the pollution and 84% reported using their blue reliever inhaler more often than usual.
More than half had avoided going outside and 39% had sought advice about managing their asthma.
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."
Debby Waddell, lead nurse at Asthma UK, added: "We urge all people with asthma to keep a working blue reliever inhaler on them at all times and take their preventer inhalers as prescribed.
"We also suggest that people keep an eye on their symptoms and know what to do in an asthma attack.
"You know you're having an asthma attack if your reliever isn't helping, your symptoms are getting worse (cough, breathlessness, wheeze or tight chest) or you're too breathless or it's difficult to speak, eat or sleep. Children may complain of a tummy ache."
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.
Across much of England, moderate to high air pollution levels were measured yesterday, with level eight (high) in the South East and Eastern regions and level seven (high) in Greater London.
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.
Mike McKevitt, head of patient services at the British Lung Foundation, said: "It would be surprising if we didn't see an overall increase in the number of hospital admissions as a result of the pollution, certainly among people with respiratory condition such as asthma and chronic obstructive pulmonary disease (COPD)."
But he said calls to the charity's helpline were mainly from people seeking advice.
"Although there was an initial spike in calls to our helpline, these were largely from people seeking advice on how to minimise the impact of the pollution on their lung health, with far fewer calls coming from people actually experiencing symptoms.
"This hopefully suggests that people - especially those with pre-existing lung conditions - are taking appropriate precautions against the impact of the pollution, or managing the symptoms well enough themselves.
"That said, there is often a slight delay between exposure to pollutants and the onset of breathlessness, coughing and wheezing, so it is also possible that the ill effects of the pollution have yet to take hold in many people."