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There isn't a clear ranking because air pollution varies quite a bit depending on the season and how it's measured, but cities like La Paz, Lima, and Mexico City usually rank among the worst when it comes to air quality. La Paz in particular has that brutal problem because it's at a really high altitude and because cold air traps pollution, while Lima suffers because of its coastal geography that doesn't disperse pollutants well. That said, the data varies a ton from year to year depending on climate factors and the measures they implement.
La Paz is still one of the most fucked by altitude and geography that doesn't let air circulate, but Lima and Mexico City compete hard depending on the time of year.
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