Swathes of south west Hertfordshire are suffering from very high or high levels of air pollution in the still, cold weather.

The county is 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.

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Defra warns that many places across the UK will see moderate or locally high air pollution levels during Monday as the weather conditions of high pressure and light winds continue.

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.

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.