London was put on alert for a public health emergency today over toxic air pollution.

Sadiq Khan, the London Mayor, said air pollution could rise to 'moderate' in more than half of the capital's 32 boroughs as he issued the warnings for the next three days.

Central London will be worst hit, with the warnings rising to 'high' in Westminster and the Square Mile tomorrow.

London Mayor Sadiq Khan, pictured this morning, issued the warnings and said air pollution could rise to 'moderate' in more than half of the capital's 32 boroughs

Airtext forecasts issued by City Hall warned that London's busiest main roads will experience 'high' pollution on all three days.

During 'high' alerts people experiencing symptoms such as sore eyes, a cough or sore throat should reduce activity - particularly if they are outdoors.

The 'high' alert is the second highest warning. The smog could remain until Sunday, the Environment Department said.

Official public health advice says older people should also reduce physical exertion and people with asthma may find they need to use their reliever inhaler more often.

During 'moderate' alerts adults and children with lung and heart problems who experience symptoms should consider reducing physical activity.

Airtext forecasts issued by City Hall warned that London's busiest main roads will experience 'high' pollution on all three days

Mr Khan said: 'London's dirty air is a public health emergency.

'We will continue to use all the technology at our disposal to inform Londoners about levels of air pollution in their neighbourhoods.'

Friends of the Earth said it was time Mr Khan banned diesel vehicles from London.

Campaigner Sophie Neuburg said: 'It's a scandal that millions of people, in one of the richest cities in the world, are regularly forced to breath toxic air.

WHAT CREATES SMOG? ‘Smog’ describes a mix of smoke and fog. Most of the smog created today is photochemical. This means it’s produced when the sun reacts with nitrogen oxides and volatile organic compounds in the atmosphere. Car exhausts and power plants produce nitrogen oxides, and volatile organic compounds are released into the atmosphere by paints, gasoline and cleaning solvents. When sunlight hits these chemicals, airborne particles are formed, which creates smog. Overall , air pollution is expected to kill more than 6.5 million people a year worldwide by 2050. Advertisement

'The mayor is right to alert people to the threat this pollution poses to their health, and has already taken some steps to tackle the issue, but far more needs to be done.

'Sadiq Khan must follow the lead of Paris, Athens and Madrid and commit to banning dirty diesel from London by 2025.

'Air pollution is a huge public health threat – it's time to stop tinkering under the bonnet and give transport policy a complete overhaul.'

London has had a long history of suffering from poor air quality, with problems recorded as far back as the 1200s.

Air quality worsened in the 1600s but the worst air-pollution event in the history of the UK came in December 1952, when a mysterious fog swept through the city.

It blanketed the capital in a dense layer of pollutants that killed thousands of people and animals and made it difficult to breathe for days.

It gripped newspaper headlines for days and engulfed Sir Winston Churchill's government in crisis.

Visibility was reduced to just three feet in some areas and causing transportation to shut down.

The smog was caused by coal burning. The cold weather preceding the Great Smog of London led Londoners to burn more coal than usual to keep warm.

But the relatively low-grade, sulphurous make-up of the coal increased the amount of sulphur dioxide in the smoke.

The public health disaster led to a number of changes in practices and regulations, including the Clean Air Act 1956.