@sueinmn It all depends on what you are masking for. N95 and KN95 masks don't protect one from ozone, because it's a gas. So, if the ozone AQI is high (like maybe 100 at the time you check), then maybe one should stay indoors.

And, if you are dealing with PM2.5 particle pollution, it matters whether it's wildfire smoke or not. If it's wildfire smoke, then the same AQI number for PM2.5 is much more dangerous than for other particles. So, you have to adjust whether you should stay inside based on what kind of PM2.5 pollution it is.

I realized that riding the train on a high ozone day isn't a great option, since the commuter trains, buses and subways all bring outside air into their ventilation systems (in addition to it coming inside everytime the doors open).