I was in the south bay from 1985-2003, and the air quality seemed decent. But every time when I biked up Mt Hamilton and looked down into the valley, it was like looking into a bowl of thin soup. Talking about it with a friend who grew up there (he was born about 1960) he said the air quality used to be absolutely atrocious.

Although it is easy to lose hope about so many things heading the wrong direction, air quality is something which has improved substantially, and of course it would be great for it to improve even more.