What do different air pollution alerts mean? Sadiq Khan issues London weather warning

London’s air quality has rapidly deteriorated in recent days amid freezing temperatures  (PA Archive)
London’s air quality has rapidly deteriorated in recent days amid freezing temperatures (PA Archive)

Following cold and wintry weather since Saturday, including freezing fog, London’s air quality has rapidly deteriorated.

This has led to the Met Office issuing a yellow warning for the capital. On Tuesday, Mayor of London Sadiq Khan issued a high air pollution alert for the city and urged Londoners to avoid unnecessary car journeys. Schools and boroughs will be notified of the alert and messages will be displayed across the Transport for London (TfL) network.

Mayor Khan said: “We know how dangerous toxic air is for Londoners — that’s why I’m doing everything in my power to tackle it. Following the latest forecast from Imperial College London, I am issuing a ‘high’ air pollution alert. We all need to be careful over the next few days.

“I’m urging Londoners to look after each other by choosing to walk, cycle or take public transport where possible, avoiding unnecessary car journeys, stopping engine idling and not burning garden waste, all of which contribute to high levels of pollution.

“This is particularly important in order to protect those who are more vulnerable to high pollution.”

So what do the different air pollution alerts mean? Here’s everything we know.

What do different air pollution alerts mean?

The Met Office divides ranks air pollution levels through four bands: low, moderate, high and very high.

The rankings depend on the ozone, nitrogen dioxide, sulphur dioxide and particulate matter levels.

When the air pollution levels are marked as “low”, life continues as normal.

To view this content, you'll need to update your privacy settings.
Please click here to do so.

When air quality is ranked as “moderate”, however, adults and children with lung problems, and adults with heart problems who experience symptoms are advised to decrease their strenuous physical activities.

When a “high” air pollution alert is issued, anyone who experiences discomfort is encouraged to reduce their activity, especially outdoors. The symptoms could be dry eyes, sore throat or coughs.

Those who have lung and/or heart problems and older individuals are advised to reduce physically taxing activities.

Meanwhile, asthma sufferers are encouraged to keep their reliever inhaler nearby, as they might need it more often than usual.

The most extreme air pollution warning is “very high”. It requires people to decrease their physically tiring activities and consider any symptoms the air pollution might be causing before spending time outdoors.

Older people and those with lung and heart issues are advised to avoid exhausting physical activities altogether.

Children may need to be kept away from school and discouraged from playing outside.

Foggy conditions make it harder for toxic fumes to dissipate (Pexels)
Foggy conditions make it harder for toxic fumes to dissipate (Pexels)

Why did the freezing fog cause high air pollution in London?

Freezing fog forms through exactly the same process that creates a normal fog. This involves land becoming colder overnight, radiating heat back into the atmosphere if there are clear skies.

This causes the air’s ability to hold moisture to plummet, allowing water vapour to become tiny droplets that eventually lead to fog.

However, when fog forms in temperatures that are below freezing level, water droplets stay liquid and become immensely cooled.

Foggy conditions such as these make it harder for toxic fumes to dissipate, causing high levels of air pollution, which are dangerous for our health.