[jacobolus]

Vehicle emissions (and other air pollution) are a problem all over the world. The USA and especially California now have reasonably tight emissions control standards, but Los Angeles used to have horrid smog, to the point children would be told not to go outside to play on the worst days. Now after public outrage followed by well enforced regulatory changes, it’s much better.

In general, a relatively small percentage of older vehicles produce a huge proportion of auto emissions. If governments can enforce regular emissions tests and pull the offending vehicles off the road, it makes an enormous difference to air quality.