Started in 1900, many parts of the Paris Metro date from well before the invention of air conditioning, 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 second circle of hell.
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The are three types of ventilation on the lines – natural ventilation via grates at street level, mechanical ventilation with what are essentially fans and ventilation with chilled air.
The coveted chilled air is circulated on just four lines 1,2,5 and 9, which generally cross the city going east to west. 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.
The middle level of ventilation with a fan is found on lines 4, 7, 8, 13 and 14 while the lines with no cooling method at all are lines 3, 3a, 6, 10, 11 and 12.
This is generally bad news for people who live in the southern part of the city, which is only served by fan cooled or not cooled lines.
If you're looking for a chilled transport method your best bet is the trams. The modern tram system is entirely air conditioned apart from the T1, which will be cooled from 2022. However the tram system tends to serve the suburbs and outskirts of the city, so it is not much help for people needing to travel in the city centre.
And if you're taking a bus don't even bother looking for air con – 94 percent of the city's fleet has no air conditioning.
On the suburban RER train service it's a bit hit and miss, the more modern trains have it while the older ones don't but they all run on all lines so there's no way to know which will be cool and which will be roasting hot until you get on.
This summer there is the added challenge of wearing masks – which are compulsory on all public transport (as well as as indoor public spaces) at risk of a €135 fine.
While acknowledging that masks can be uncomfortable in hot weather, health authorities have urged people to continue to wear them to slow the spread of Covid-19, which has seen a spike in numbers in recent days.