People with health problems have been warned not to overdo things as high levels of air pollution are expected to sweep across the north-east of Northern Ireland.
The elderly and those with lung and heart conditions should 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 they may need to use their blue reliever inhalers more often as they could be prone to attacks today and over the next few days.
High levels of pollution are forecast for England and Wales. Those high levels will move north over much of coastal north-west England, to south-west Scotland and the north-east of Northern Ireland.
The elevated pollution levels have been caused by a combination of light south-easterly winds, the continental air flow and dust which has blown up from the Sahara desert.
Dr Helen Dacre, a meteorologist at the University of Reading, said the combination of factors have led to 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," she said.
"Saharan dust gets blown over to Britain several times a year – the current episode has been whipped up by a large windstorm in North Africa.
"This has all combined to create high concentrations of pollutants in the air."
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."