I’ve just returned to the UK after 5 days in SF. My perception is that it’s very difficult to judge the effects merely from the overall air quality figure produced from a number of different sources.
Taking it purely on a visibility level, on Saturday morning Alcatraz was clearly visible from the Fort Mason area, yet the readings were around 180 at the time. Parkrun, the free weekly 5k run at Crissy Fields, was cancelled as a result. On Sunday the readings at 7am were lower, perhaps around 150, yet Alcatraz wasn’t visible at all. Nonetheless, I was able to run a fairly easy paced 5 miles without difficulty.
I think it’s something that, perhaps, is quite localised and the overall figure isn’t much help. I spent one day mostly around SOMA and felt fine, but within minutes of getting within a few blocks of the Ferry Building I could immediately feel the effect on my chest and particularly throat.
Masks are a sensible precaution, particularly if you have any underlying health issues. I was careful, didn’t expose myself for long periods at a time in areas of the city that seemed subjectively worse, and didn’t feel I needed a mask. That seemed to be the view of most people, and actually the percentage of people wearing them is very small.
My advice, which has no professional basis whatsoever, is to go by what you feel from hour to hour - that served me better than any generic official numbers.