Runners and spectators at this year's Cardiff Half Marathon are being urged to be more environmentally friendly.
A record 27,500 people have signed up for the event with almost twice that number expected to line the streets.
However that has raised concern over the "significant environmental impact" of the race after a survey found 70% of runners arrive by car.
Among the measures taken by race organisers are smaller water bottles and medals made of recycled zinc.
As well as the health and wellbeing benefits for so many participants, the event provides an economic boost worth more than £2m in 2018 to the Welsh capital.
But Run 4 Wales said it recognised the need to "alleviate some of the negative impacts" of what has become the biggest mass-participation event in Wales.
This includes cutting the use of plastic, reducing waste, avoiding unsustainable materials and encouraging different travel choices.
Last year's race saw a 49% decrease in CO2 travel-related emissions and a recycle rate of 96%.
"Half marathons have become the fastest growing race in the UK, and attract tens of thousands of runners and spectators," said Dr Andrea Collins, of Cardiff University.
"They can have economic benefits for cities, but their environmental impact is becoming increasingly hard to ignore and organisers need to act now if they are to continue staging them in the future.
"Solutions such as those taken will enable organisers, runners and spectators to take responsibility to reduce the environmental impact of the Cardiff Half Marathon and leave a positive legacy."
Those travelling to the event are being encouraged to use public transport, car share or consider low carbon options such as walking or cycling.
NextBike are offering free and discounted bike hire during the event weekend in Cardiff.
Run 4 Wales said it remained "committed to creating a more sustainable race" by also cutting down on waste with an action plan aimed at reaching 100% recycling rate.
Among the measures already taken are:
- Using recycled non-woven materials for branding and signage
- Medals made from recycled zinc
- Smaller water bottles that are 100% recyclable
- Goodie bags made from 100% compostable corn starch bags
- More waste drop zones along race route
- Using recycled paper for all print advertising
Chief executive Matt Newman said: "The increased volume of footfall in the city has brought with it an unavoidable increase in carbon emissions and other harmful environmental factors.
"But we are now battling back to ensure a greener future for the event.
"We are seeing improvements year on year as the public awareness of these environmental issues increases, and we continue our mission to make the Cardiff Half the most sustainable event it can be."
Live coverage of the Cardiff Half Marathon is on BBC One Wales from 09:30 and BBC Two network at 09:45. Highlights will be on BBC Two Wales at 22:00 and on the BBC iPlayer.