Over in Paris, authorities announced a few days in December where people could use public transport for free after a spike in air pollution.
At the time, the Airparif agency said the French capital was suffering its worst and most prolonged winter pollution in at least 10 years, so Paris mayor Anne Hidalgo took the action. Vehicles with odd-numbered and even-numbered registration plates were also banned from the roads on alternate days.
Air pollution has also been a significant problem for London. A recent report by the mayor showed every London borough recording illegally high levels of air pollution in the past two years.
Five days into the year, London had already broken air pollution limits set for the whole of 2017.
And while Khan said improving the capital's air quality is "one of my highest priorities as mayor", he has said there are "no plans to introduce free travel days".
"TfL modellers estimate that one day's free travel would cost up to £12m in lost revenue," Khan told Green Party London Assembly member Caroline Russell, in response to her question about whether the mayor had looked into the cost of free public transport on days when air pollution was particularly high.
Khan has laid out a range of other measures to tackle the issue, including bringing in an emission surcharge and introducing an ultra-low emission zone in 2019.
TfL has also committed to converting 4,000 buses to the latest Euro VI emissions standard by 2020 to reduce emissions and brought in licencing requirements for private hire vehicles and taxis to update the fleets to zero emission capable vehicles.
Earlier this month, the mayor launched the capital's first low emission bus zone in one of the most polluted areas of London: Putney High Street. The zone will now be serviced by cleaner buses in a drive to slash nitrogen oxide emissions. 11 other zones will be brought in by 2020.
"These initiatives all directly affect air quality and will halve NOx emissions from road transport in central London by 2020, and reduce them by 40 per cent in inner London and 30 per cent in outer London," Khan said. "I believe this is the right way to drive pollution down and make London a better place to live and work."