Ever wondered what becomes of London's piles of Autumn leaves?

Their fiery hues are a nature photographer's delight. But once they drop, the sludgy mess left behind becomes a slippery tripping hazard on the capital's streets.

And unfortunately for green-thumbed Londoners , they won't be destined for the compost heap for a grim reason.

The fact is, central London is so polluted by heavy vehicle traffic, the Environmental Protection Agency warns the leaves can't be recycled.

Westminster City Council says the agency told it the leaf-sweepings in the built-up borough were full of heavy metals from pollution.

In central London, more than 1000 tonnes of leaves - the equivalent of 72 Big Ben bells - will be cleared from the borough of Westminster alone over the next six weeks.

But despite being tainted, they can still be put to an environmentally-friendly use.

The leaves are packed off to the council's street cleaning contractor Veolia’s SELCHP Energy Recovery Facility, where they are able to be used to generate heat and power for Westminster's homes.

LONDON, ENGLAND - NOVEMBER 13: People walk through autumn leaves during gusts of wind in Green Park on November 13, 2015 in London, England. (Photo by Chris Jackson/Getty Images)

Extra vehicles are being brought in to help the city's street cleaners clear away 50,000 bags of autumn leaves a week.

"Autumn makes our leafy streets even more scenic than usual, but when the leaves start to fall it does create a lot of work," Environment and city management leader Cllr Tim Mitchell said.

"Our on-street team will be branching out, sweeping away more than 1,000 tonnes over the coming weeks. Credit to them for their hard work, a small price to pay for having so much greenery in Westminster."

The leaf-sweepers will pair up to tackle the borough's leafiest neighbourhoods, with Hamilton Terrace and Elgin Avenue in Maida Vale posing the biggest challenges.

Veolia's Helder Branco said the season always proved a big task for the cleaners.

"In winter we cleared the snow, in summer we braved the heat and now in Autumn, we’ll have over 250 operatives busily clearing leaves, so that Londoners have clean and safe streets to travel on every day."