On a bad day in LA you could barely see or breath, this pollution was anything but subtle.
20 years of legislation by the Californian government and in particular the California Air Resource Board, (CARB) encouraged the introduction of things like unleaded petrol, catalytic converters and of course hybrid and electric cars.
Now it's in the news again, like the fact that tens of thousands of diesel cars in London being a bad thing is a sudden shock to everyone.
Try cycling around London with an air filter face mask and have a look at the filter after a half hour pedal. It's properly black. I first noticed that in about 1979.
So, the sooner we get rid of all diesel taxis, busses, trucks, vans and cars the better.