Millions of people with health problems and the elderly have been warned that the UK will continue to be plagued by record levels of air pollution.
A perfect storm of dust from the Sahara, pollution from the Continent, low south-easterly winds and domestic pollution has caused air quality to plummet.
And the smog-like conditions are not expected to clear until Friday, according to experts.
Meanwhile those with lung and heart conditions have been told to continue 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 said.
Asthmatics have been warned of the need to use their blue reliever inhalers more often as they could be prone to attacks over the next few days. Around two-thirds of the 3.6 million people with asthma find that air pollution makes their asthma worse.
The advice, from Public Health England (PHE), Asthma UK and the Department for Environment, Food and Rural Affairs (Defra), comes after a warning that parts of England are experiencing the highest level of air pollution ever recorded by Defra.
Defra ranks air pollution from one to 10, with one being the lowest and 10 the highest.
Across most of England, moderate to high air pollution levels have been forecast, with level 10 - "very high" - forecast in parts of East Anglia and the East Midlands.
Tomorrow, high levels of pollution are forecast for East Anglia, the Midlands, Lincolnshire, eastern parts of Wales, through the Wirral and the north coast of Wales. High levels will move north over much of coastal north-west England, to south-west Scotland and the north-east of Northern Ireland.
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."
Kay Boycott, chief executive of Asthma UK, said: "The two-thirds of people with asthma who find that air pollution makes their asthma worse will be at an increased risk of an attack following the alarming Defra warning of high pollution levels around the country.
"Asthma UK warns the 3.6 million people at increased risk to be sure they always have a working blue reliever inhaler on them and take their preventer inhalers as prescribed."
Frank Kelly, professor of environmental health at King's College London, said: "Whether home-produced or arriving from the continent, the tiny particles we take into our bodies with each breath cause immediate problems for some individuals such as those with asthma and contribute to longer term problems for most of us in the form of heart disease and stroke.
"For those who are sensitive to air pollution, it's important they are provided with accurate forecasts of when air quality will deteriorate so they can plan their activities to reduce exposure, perhaps by taking different routes to work or school or avoiding strenuous exercise on those days."
Paul Cosford, director of health protection at PHE, told BBC Radio 4's Today programme that the air pollution was a "serious issue" but should be kept in perspective.
"It's a small number of days of very high air pollution levels," he said. "The pollution will go down towards the end of the week."
Dr Helen Dacre, a meteorologist at the University of Reading, said high air pollution could cause "unpleasant and dangerous effects on health", both long and short term.
She said: "Toxic gases, such as nitrogen dioxide and ozone, as well as fine dust particles in the air blown in from the Sahara and from burning fossil fuels, all contribute to cause problems for people with heart, lung and breathing problems, such as asthma.
"The problem is likely to be particularly bad today because weather conditions have conspired to create a 'perfect storm' for air pollution.
"British car drivers and heavy industry create bad enough smog on their own, but the weather is also importing pollution from the industrialised urban parts of Europe, which is blowing across Britain.
"Saharan dust gets blown over to Britain several times a year - the current episode has been whipped up by a large wind storm in North Africa. This has all combined to create high concentrations of pollutants in the air."
Dr Keith Prowse, honorary medical adviser to the British Lung Foundation, said: "Heavy air pollution of the kind we're seeing in several places across the UK at the moment can have a significant impact on people with pre-existing respiratory conditions such as COPD (chronic obstructive pulmonary disease) and asthma, worsening symptoms such as coughing and breathlessness.
"When levels of air pollution are high, people with these conditions, or anyone else who finds themselves coughing or wheezing in times of high pollution, should avoid strenuous exercise outdoors and are better off trying to exercise away from pollution hotspots, such as busy roads or during rush hour."
Ian Colbeck, professor of environmental science at the University of Essex, said: "This pollution episode comes just a week after the World Health Organisation estimated that seven million premature deaths annually are linked to air pollution.
"It is now the biggest single environmental health risk. In the past, respiratory diseases (such as chronic obstructive pulmonary disease) were thought to be the main killers but it now emerges that heart disease and strokes account for up to 80% of deaths."