The problem in Chicago and Illinois is the same as in Beijing. Cheap dirty coal power. Environmentalists in Chicago recently won a battle to shut down two coal plants that were in city limits and costing citizens hundreds of millions a year in health damage. But a lot remains to be done in the state.

Holy shit, as someone who worked a tiny bit on that campaign, never thought I would hear Fisk and Crawford mentioned on HN.

Neat! I didn't work on that campaign, but I worked at the Northwestern Environmental Advocacy Center (on the DePue Superfund site), and we had a close relationship with ELPC, which did one of the economic reports establishing how bad those plants were for Chicago.

Chicago might be something to do with the 4th holiday. The AQI maps for the city seem fine through last week until 7/3 and then a sharp spike on 7/4. Meanwhile: you can walk outside and still smell fireworks here. It was really bad last night.

The number for Chicago doesn't strike me as accurate. I'm in the area right now and it's a clear day. Plus, the levels for nearby areas in Chicago are far lower (80 or so) and the historical levels are much lower. Looks like a blip. That said, I'm sure Chicago is not the clearest city in general.

I live upwind of Chicago (by prevailing winds) in Minnesota, and we have read in our weather reports that forest fires in Canada have added a lot of particulate matter to our air. The sunsets here have been spectacularly red in recent days. I suppose checking those city-by-city reports of measured pollution for long-term trends and recent spikes might help figure out how unusual Chicago's current conditions are, as you thoughtfully suggest.

A quick Google and Wikipedia visit says that pm2.5 fine particulates of the kind reported as being extremely high in some parts of Chicago, are not visible except as a light haze over the city.

It appears to be true that you can have a bright mostly clear day and very high levels of fine particulate pollution.

This haze is part of what contributes to smog when combined with other more visible (course particulates) pollution. But these fine particulates don't appear to be the product of fireworks as suggested below, and are more likely to be the product of the coal power stations.

The measure for Chicago is probably skewed by one of the measuring stations being nearer to, or down-wind from, a coal power station.

Why do you assume that the sampling methodology and calibration is ridiculous?

The part of the city the reading was taken and time of day seems to play a huge factor.

For example, the previous posted said Paris was 24, Paris - where I am now, with the window open, slightly after midnight - is currently reporting 53.

Kunming, a city of 6+ million where I usually live in China's southwest, is a little higher at 68. (This counters the 'China is dirty' western media BS)

Sydney, where I grew up, is higher at 75. However, in another reading closer to the area of town where I used to live it's much lower at a mere 13.

So, check which part of the city you are looking at and what time of the day the reading was taken. Also, don't assume all of China is all dirty and horrible like the east coast megacities (though much of it may be, to varying extents).

Hm. I wonder about some of these results. That number for chicago is wild - that would be clearly visible, serious pollution. And my old haunt BKK, which I left specifically because of the awful air, 32!? The city where I would come home from work with a grey shirt that was white when I left is cleaner than zurich? From personal experience in both cities I assure you it is not.

It's anecdotal, I know, but I wonder about some of this data.

The Seattle data is heavily biased by for the fourth of July fireworks. Look at data before the fourth and across the city.