Swathes of the UK are suffering from very high or high levels of air pollution in the still, cold weather.
Northern Ireland, London, the South East and Eastern regions are experiencing very high levels of pollutants known as particulate matter, or PMs, which come from sources such as traffic emissions, in particular diesel engines.
With the pollutants failing to disperse in the still, settled conditions, all other regions of England except for the North East are suffering from high levels of air pollution.
South Wales is also experiencing high levels of pollution, the latest data from the Environment Department's (Defra) UK Air website show.
Defra warns that many places across the UK, except Scotland, will see moderate or locally high air pollution levels during Monday as the weather conditions of high pressure and light winds continue.
But some very high levels are also expected, mainly across south east England and also for some urban areas in Northern Ireland, and central and eastern England.
With the high pressure and still conditions continuing, air pollution is also expected to be a problem across much of the UK on Tuesday.
In very high pollution conditions, people are advised to limit exercise outside, while those with lung and heart problems and older people should avoid strenuous activity.
Where there is high air pollution, adults and children with lung problems and adults with heart problems, as well as older people, should reduce the amount of strenuous exercise they do.
High and very high levels of pollution can cause people with asthma to need to use their inhaler more.
In London, one of the areas worst hit by the conditions, Mayor Sadiq Khan has issued pollution alerts at Tube stations, bus stops and roadsides.
The Green Party's Baroness Jones accused the Government of not doing enough to warn people elsewhere in the country of the issue.
And she said: "When air pollution episodes are capable of triggering an extra 300 deaths as well as hundreds of emergency admissions to hospitals around the country, I think that we have to consider emergency measures to discourage driving, encourage a switch away from diesel and promote less polluting alternatives."
Air pollution from sources including factories and vehicles is linked to the early deaths of around 40,000 people a year in the UK, and causes problems such as heart and lung diseases and asthma.
Responding to advice that people should reduce outside activity because of the pollution, Friends of the Earth London campaigner Sophie Neuburg said: "It's outrageous that children, who have done nothing to cause the problem, need to be kept indoors when air pollution is bad.
"Instead, the mayor should introduce emergency traffic restrictions to reduce air pollution quickly and make our air safer.
"We know traffic is one of the biggest problems for air pollution and diesel is the worst of all.
"This is why Sadiq Khan must follow the lead of Paris, Athens and Madrid and commit to banning dirty diesel from London by 2025."