These days when I get behind a classic car that has a carburetor, I close the vents. Just can't stand the smell of the unburned fuel they put out. Which is odd because I grew up with them (my first car was a '75 Impala with the malaise-era small-block V-8). These days there are many more cars on the road and individually they're far cleaner than any 70's car was. So much cleaner that collectively they're cleaner than the population of cars from back then too.

I expect the same is true of many "industrial" cities in developed countries...

I live in Kawasaki, just outside of Tokyo, which is famous for its industry. The air now is lovely and clean, there are essentially no pollution issues at all, but 50 years ago, things were apparently very, very, different... In old pictures, it's like a black miasma hanging over everything.... oO;

I posted a link to this in another post, but here's a chart showing how bad the pollution in London during the great smog of 1952 was: http://www.st-andrews.ac.uk/~dib2/atmos/london.jpeg

Notice that SO2 and smoke particles are measured in units of milligrams per m^3 (today such measurements are in micrograms per m^3). To put Beijing's air in the context of this graph, the smoke particles in Beijing on a bad air day will go up to 0.2 to 0.8 mg/m^3 and SO2 will spike up to 0.3 for short periods. Compared to the Great Smog of 1952, Beijing's air isn't nearly as bad, but it's still incredibly bad. The Great Smog killed 0.03% of the population of London per day. Beijing's air regularly gets to around 1/4 the amount of smoke in the air as during the Great Smog.

A friend of mine actually left because of the pollution. It's thought provoking that it could get that bad.

I left Shanghai because of the pollution... And Beijing is even worth...

They passed a variety of clean air acts to prevent burning coal within certain boundaries and raise chimney heights.

http://www2.epa.gov/aboutepa/londons-historic-pea-soupers

http://www.air-quality.org.uk/03.php

There are still problems with NO2. http://www.bbc.co.uk/news/science-environment-32512152

Similar things to what was done in America. Lots of regulations. The "Great Smog" of 1952, which resulted in several thousand deaths in a matter of days, spurred action, resulting in the Clean Air Act of 1956 and other legislation. It banned many of the more polluting activities which had been common at the time, use of low grade coal and wood in home heating, use of unfiltered diesel engines on transportation in the city, use of coal in power stations near to the city. Factories and power plants were also moved farther away from the city.

Here's an interesting little writeup that has some really fascinating graphs on London pollution: http://www.st-andrews.ac.uk/~dib2/atmos/urban.html

Among other things, a switch to gas-fired central heating (with the discovery of gas in the North Sea) and a mandate to use "smokeless fuel" in the remaining hearths. Smokeless fuel is a sort of briquette made of ground and reconstituted coal that's had most of the sulphuric impurities washed and cooked out of it, along with a blend of other things like sawdust and molasses.