Four factors causing the high levels of pollution

The BT Tower can just be seen amid the London smog. Photo: ITV News

Pollution levels in London are 'high' today according to the Government and the Met Office but what is causing the increase?

Here are four factors contributing to the rise:

Saharan dust

Storms in North Africa a few days ago whipped up huge amounts of dust and sand from the Sahara into the atmosphere. The finest particles have been transported by a south-south-easterly wind towards the UK.

European pollution

The air mass moving in our direction has been sitting over the near continent recently. The pollutants from major European cities are trapped in the air

Home grown pollution

On top of the pollution blowing over from the continent is the pollution we create here in London.

These pollutants include tiny particles from emissions and brake dust (PM2.5 and PM10) and nitrogen dioxide produced largely by vehicles. They are estimated to cause at least 4,000 premature deaths in London each year.

High pressure

The mix of Saharan dust, European and home grown pollution are all being trapped by our current weather pattern. High pressure acts a bit like a lid trapping in the pollution.

When will it get better?

The worst levels of air pollution are expected today.

Things should gradually improve tomorrow afternoon (dropping down to 'moderate') as we see a change in direction of air flow drawing in fresher air from the Atlantic rather than the near continent. By Friday levels should have dropped back to 'low'.

We could however continue to see the effects of Saharan dust as more storms are expected in North Africa today and tomorrow.