"I can't trust air I can't see", was my favorite line of my grandfathers, who lived in LA for the past 60 years. He always mentioned how incredibly clear and clean the air is now compared to a few decades ago (where I live in Northern California now, even the Bay Area seems polluted and smoggy most days).
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;
