Everyone's different, of course.

Grew up in the SoCal region, back when. There were certainly days when the smog (or fire smoke) was hazy enough that it could be smelled. On those days, I skipped the "heavy lifting" type activities and tried to stay indoors as much as possible. But then, I've never had asthma or allergies, and generally have been able to ignore the sort of limitations on lung exposure most others have had to worry about.

That said, generally over ~100 AQI is when I really start paying attention. On a sub-1mi jaunt on a bicycle, I wouldn't have any trouble simply taking it easy and ensuring I get there without bad impacts on the lungs. Still, the AQI means the stuff's in the air, and any breathing is going to be inhaling the particulates and compounds. No way around that unless you have a respirator on.

I think the AQI system generally suggests that anything over 50 is where people should begin to worry, particularly those with "lung" challenges, those without solid cardiovascular, the aged, etc. So, yeah, generally speaking 150's one of those measurements that ought to concern most people.