Freezing fog has trapped pollution, causing levels to skyrocket and posing a health risk (Photo: Getty Images)

Cold, foggy weather led to an air pollution warning from London Mayor Sadiq Khan this weekend as levels skyrocketed – and the “pockets” of pollution are expected to continue.

According to the Defra website, levels in Greater London are currently “very high”.

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Emissions from cars and vans that burn fossil fuels are held back by the icy fog and cold weather – one of the reasons the mayor is expanding London’s controversial Ultra Low Emission Zone (ULEZ) to all London boroughs in August.

So what is an air pollution warning and what should you do to protect yourself and your loved ones?

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Here’s everything you need to know.

Check the current air pollution in your area

Air pollution alerts are sent when levels rise to levels hazardous to health.

Find out what the current level of air pollution is in your area, and many weather apps on your phone and other devices will tell you.

They are based on Defra’s daily air quality index, which is calculated from the levels of five pollutants, including nitrogen dioxide, sulfur dioxide, ozone and particles of various sizes.

Pollution is a threat to health and the planet (Image: Getty Images)

Air pollution alerts are issued by Defra when these levels exceed certain thresholds.

What should I do if air pollution increases and should I exercise?

Woman is preparing for a run

Training may need to be scaled back due to higher air pollution (Photo: Getty)

The Met Office insists that in the UK “no one should be afraid to go outside”. But the measures to be taken depend on your health.

There are precautions you can take, especially if you’re at risk – this includes adults and children with heart or lung problems.

When air pollution is low

The message to everyone is simple when levels are between one and three: “Enjoy your usual pursuits.”

It is possible that people who are sensitive to air pollution can experience symptoms even on “low” rated days.

These can include a dry cough, sore throat or sore throat and sore eyes – and cutting back on strenuous outdoor activities will help alleviate them.

When air pollution is moderate

Once the pollution increases to four to six, most people don’t have to do anything.

Anyone in a high-risk group should do this consider a reduction strenuous physical activity, especially outdoors, according to the Met Office.

When air pollution is high

Back of the car with exhaust pipe in a knot

Exhaust fumes are trapped by the freezing fog and cold weather (Credits: Getty Images)

When air pollution reaches levels seven through nine, anyone suffering from conditions such as sore eyes or sore throat and cough should consider a cut during strenuous outdoor activities, especially outdoors.

Those in the risk group should definitely reduce strenuous activity, and the elderly should be careful not to overexert.

When air pollution is very high

Once we reach level 10, everyone in the risk groups Reduce strenuous physical activity, especially if they have symptoms.

Those at higher risk and the elderly should do so the appearance heavy physical exertion.

If you have asthma, you may need to use your inhaler more often when air pollution levels are high.

Mid adult woman using inhaler

Asthma can flare up when air pollution is very high (Photo: Getty Images/Science Photo Libra)

What about athletes?

When ground-level ozone levels are high, some athletes may find they can’t perform as well as normal, and taking a deep breath can cause chest pain.

This usually happens in the summer, and while it’s not dangerous, it’s “wise” to limit physical activity while you have symptoms.