Lots of people are asking where the haze over Mumbai is coming from, and nobody seems to have a good answer. Let me try. First, this isn’t surprising. October usually means an AQI of 110 to 140 in Mumbai, and that’s been true for most years since 2020, which is as far back as my data goes. See Fig 1. It gets worse in November and peaks in January. What you actually need to understand is where that high number comes from. US AQI is the air quality index used by the EPA, the US environmental agency. It looks at six pollutants. It turns each pollutant’s level into a score from 0 to 500 using a fixed table of cutoffs, and the highest of those scores becomes the AQI. The cutoffs are different for each pollutant, and they can change. The EPA made the PM2.5 cutoffs stricter in 2024. India has its own index, the National AQI from the CPCB, and its cutoffs are bullshit. PM2.5 at 60 µg/m³ is “Satisfactory” on the Indian scale and “Unhealthy” on the US scale. Whatever app you use, check which AQI it’s showing you. Now, why does Mumbai have haze, and why is the AQI so high? It’s PM2.5. Look at the orange line in Fig 2. PM2.5 is nasty stuff. The particles are fine enough to go deep into your lungs and fuck them up. It mostly comes from burning things, e.g., waste, vehicle exhaust, and industry. It’s mostly not from construction, which mainly produces the coarser PM10. Fig 3 shows PM10 falling and PM2.5 not falling. Last winter PM10 was the lowest it’s been, and the new construction dust rules probably helped with that. PM2.5 went up anyway. The haze you see isn’t a good guide. Fine particles do scatter light, and humid, cooler air makes the haze thicker. But you can have heavy haze without much pollution, and you can have high PM2.5 with a clear sky. Trust a monitor, not the view out your window. Sadly there’s no easy way to protect yourself from PM2.5. Your best bets are a well-fitted N95 mask outdoors and an air purifier at home. If someone is burning stuff near your place, that’s a big part of your problem, so shut the windows on that side. Oh, and by the way, this probably isn’t because Mumbai has more cars. Fig 4 plots the growth in vehicles over the last few years against PM2.5. More cars have given us an NO2 problem, but they don’t explain the swings in particulate matter. The main cause seems to be somewhere else. Find out where things are being burned.