London Marathon strives to reduce its carbon footprint

Some 50,000 runners will race in the 43rd London Marathon on Sunday and then head home with participant medals around their necks and silver foil blankets around their tired shoulders. And they will leave a large carbon footprint in their wake.

Like any mass participation event, climate concerns are a growing part of marathon planning, and this year London Marathon Events have teamed up with the Council for Responsible Sport (CRS) to measure the social and environmental impact of the race.

"I'd say absolutely, (sustainability) is something that has become much more important, whether it's the participants, to sponsors, to partners," Kate Chapman, the London Marathon's sustainability adviser, told Reuters.

Travel has the biggest environmental impact on large marathons, and so a 26 pound ($32) carbon levy to help offset greenhouse gas emissions is part of the entry fee for international participants.

But marathons also leave a huge trail of trash. The London event has previously generated as much as seven tonnes of rubbish and four tonnes of recycling.

Neither the bib numbers, which are weather-resistant and contain timing chips, nor participants' medals are recyclable.

Oluseyi Smith, who competed for Canada in both the 2012 Summer Olympics in athletics and the 2018 Winter Olympics in the bobsleigh, has made sustainability in sport his career since he quit competition and became an engineer.

Smith is the founder of Racing To Zero, an environmental consultancy. In a video on its website Smith sits with dozens of medals and race bibs at his feet. "Look at all this stuff," he tells the camera. "As rewarding as it was to win these, the impact to a sport event's carbon footprint from the procurement, from the stuff we get, can be