Started in 1900, many parts of the Paris Metro date from well before the invention of air conditioning (climatisation), and even now it is not standard that all lines are cooled, with the result that in summer it can get pretty sticky down there.
But some lines are better than others and while the more modern lines have lovely cooled air circulating, some of the older lines feel like the inner circle of hell.
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What about the Metro lines?
As of 2026, the Île-de-France transport network still has some big disparities.
There are three types of ventilation on the lines: refrigerated ventilation (air conditioning), forced mechanical ventilation (outside air is drawn in and diffused inside) and natural ventilation (windows open).
According to Ile de France Mobilités, the coveted chilled air is circulated on six lines - 1, 2, 5, 9, 11 and 14 - which generally cross the city going east to west.
Line 4 is also equipped with this system, but only partially (31 trains), while some of the new trains on Line 10 also have it.
This system is not quite the same as air conditioning, which is very energy-consuming and expensive when underground, but still feels pretty good to passengers on a hot day.
Lines with forced mechanical ventilation are 4, 7, 7bis, 8 and 13 and lines with natural ventilation (no cooling method) are 3, 3bis, 6, 10 and 12.
What about other transport services?
Around 75 percent of buses are air-conditioned, in contrast to nearly 100 per
cent of trams, which are much more recent constructions than the Metro system.
As for the RER commuter trains, 93 percent of the RATP's fleet is either air-conditioned or served with mechanical ventilation. RER A and B have mechanical ventilation while lines C, D and E are air-conditioned.
Along the Transilien, almost all trains are air-conditioned.
So how hot does it get?
Extreme heat can put significant stress on transport infrastructure, requiring maintenance teams to respond quickly.
For the metro, being mostly underground helps limit temperature spikes. However, on above-ground sections and tram lines, high temperatures can cause roads and rails to deform.
French transport service company RATP monitors rail temperatures every day and has installed connected sensors that provide real-time temperature readings on above-ground sections of the network. In Paris, metro trains are required to reduce their speed whenever rail temperatures reach 57C.
What about the future?
All the new MF19 trains, which are being rolled out until 2034, will be equipped with chilled-air ventilation. All buses are expected to be equipped by 2035.
The RATP is also gradually rolling out water fountains. More than a hundred have been installed across the Île-de-France Mobilités network. The group has, furthermore, planned to launch “a climate change adaptation plan”. It should be ready “within about a year”, said Xavier Piechaczyk, CEO of the transport company.
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