An episode of ozone pollution is forecast for the Paris region on Friday with just over a week to go before the Olympics opening ceremony, air monitoring agency Airparif said on Thursday.
The agency forecast "poor air quality" over most of the Ile-de-France region, with a high level of ozone, a harmful air pollutant when it is at ground level.
Harmful ozone is produced mainly when pollutants from road traffic and industrial activities react in sunlight.
The announcement comes eight days before the opening ceremony on July 26, which will take place along the river Seine in central Paris.
Along with concerns over water quality -- the Paris mayor Anne Hidalgo finally took a dip in the river on Wednesday to prove it was safe for events -- air quality has been a key talking point in the run-up to the Games.
Earlier this week, French charity Respire released a report into air quality problems at 112 sports fields around the French capital and found that the "vast majority" had air pollution above the maximum levels recommended by the World Health Organization.
The study used data from Airparif.
The effects of pollution on sports performance "are often hidden but very real. Even if they are small, they can be enough to change a podium or a score," Airparif said on its website.
As for a heatwave or a storm ruining the party on the day of the opening ceremony, the chances are low, according to the latest forecasts.
For the evening of July 26, when the ceremony takes place, Paris should enjoy a "fairly classic summer scenario", Cyrille Duchesne at weather channel La Chaine Meteo told AFP.
He was "quite optimistic" there would be a mixture of cloudy periods and clear skies.
The weather has been a nagging concern for the Games organisers, with the opening ceremony being held entirely outdoors, taking in many of the iconic sights of central Paris.
Sweltering heat is also unlikely, according to the latest forecasts, all of which is welcome news for the thousands of athletes, performers and 326,000 spectators expected to attend the extravaganza.