London Marathon strives to reduce its carbon footprint ahead of 43rd edition

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 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 rubbish. The London event has previously generated as much as seven tons of rubbish and four tons 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 significant."

Smith, a member of the IOC Athletes' Commission, keeps a shoe box of his most important medals under his bed, but he has thrown out the rest.

"Hopefully I'm not being a hypocrite when I say that I have not kept the majority of them, which is not ideal, and I think in hindsight it would have been better if the option (to accept a medal) wasn't there."

Smith suggested giving runners the choice of opting in to receive a medal.

Chapman said that would not sit well with participants, however.

"There's been a lot of research done across the mass participation sector as to what people value in terms of what they get at the end of an event, and the medals are the thing that people are most fond of," she said.

About 2,500 runners did however opt out of receiving the traditional race T-shirt as part of London's Trees not Tees programme which gives participants the option to have a tree planted instead.

Among London's other initiatives: Water bottles and finisher bags are recyclable, leftover food will be donated to a food waste charity, and most of the official vehicles are electric.

Since bottles with water can't be recycled, London has a "drink, drain, drop" campaign, to encourage runners to drain their bottles before tossing them.

The CRS, who have certified 211 global events since 2008, will measure the impact of Sunday's race based on numerous factors. One of the positive factors, for example, is the huge amount of money raised for charity. Last year's marathon raised more than 58 million pounds.

The ultimate goal is achieving CRS's highest "Evergreen" certification. Only four global events have earned that status in the past year.

"We've always taken a balanced approach between environmental, social and economic responsibility for mass participation events ... and the legacy that's left behind and the economic impact," said CRS board member Kevin Phelan.