Stadjer

My guess is that the main reason is just density. Paris is very densily populated, but the whole metropolitan area Ile de France with it's about 15 million people, but it's just not dense enough to have people not travel by car like in NYC or Tokyo. Paris is as much a capital city as any city, so all roads lead to Paris, which means there's a lot of traffic going through the metropolitan area that has no business in Paris. City driving and parking as a 'contact sport' doesn't invite people to buy a new beautiful car, an old small car you can be happily indifferent about. There's also the shift to diesels in the last 15 years, which governments have made more attractive, it's good on CO2 but bad on other pollution and just like every European government they have ignored the pollution of the two-stroke mopeds for decades.

So basically there's just a lot of emissions from a lot of traffic coming together in a small area, that isn't very windy. I doubt it's the worst in the world though, they probably just want to enjoy fresh air together with fresh food more than others, they are a bit more sensitive to this quality of life than Londoners for example. I know people who left London because of the air.