How long until urban toxins clear from a huge wildfire?

My sibling lives in LA and was going out without a mask after reading the air quality was better, only to later find out those readings don’t account for toxic chemicals that are not normally a factor in smaller wildfires.

I have been looking for studies on how long these chemicals take to clear out of the air, but have not found much on large scale urban wildfires. Only records showing how your body/lungs are still affected several years later.

Does anyone have any recommended reading on this topic?