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Low-emission bus zones on some of London’s most polluted high streets have been praised for leading to an “overnight improvement” for toxic air levels.

No road-specific annual toxic air limits have yet been breached in the capital, which is an improvement after last year the threshold on one major street was broken in just five days.

London environment experts said this change is “largely” due to the roll out of new cleaner bus routes in the capital’s worst offending areas such as in Brixton Road, in south London, and Putney High Street, in west London.

Frank Kelly, environmental health professor at King’s College London (KCL), said London mayor Sadiq Khan’s scheme could have led to an "overnight" improvement.

He said: “If you remove the vehicles that create the problem, pollution just disappears. It’s as simple as that. The introduction of these eco-buses in places like Putney High Street would be a main reason why we have seen an improvement.”

It comes after last year London’s filthy air broke legal limits on annual levels of traffic fumes just 120 hours into the New Year.

Readings taken in Brixton Road five days into 2017 found levels of nitrogen dioxide (NO2) repeatedly breached the EU limit.

Under EU law, the average hourly level of NO2 must not exceed 200 micrograms per cubic metre more than 18 times in a year.

Mr Khan launched the first two additions of his planned network of low-emission bus zones in the capital last year on the worst two “offending” roads in Brixton and Putney.

The mayor said in December that the Putney route, which was the first to get the new buses in March, had seen a 90 per cent reduction in hourly breaches of toxic air levels.

Professor Kelly added that the capital has seen a “pretty clean” start to 2018 generally, in terms of pollution levels, which he said is reflected in NO2 data for this year.

But while the new buses appear to have had a major impact, Professor Kelly said the weather has also proved to be a contributing factor to lower levels of toxic air.

He said: “When we see the very worst of levels of pollution in London it is totally down to the weather. [Toxic air] travels from other countries like France and contributes to it here. It’s a global problem, not just London’s.”

It came as a new study on air quality in the UK revealed that three quarters of Britain’s worst pollution hotspots are in the capital, showing that Hyde Park Corner and Marylebone Road in central London have the most polluted postcodes in the country.

A newly-launched tool based on the data, created by air quality experts EarthSense and the BBC, allows people to check the pollution levels in their postcode.