At what AQI level do you mask outside?

When PM2.5 in a city does not come from wildfires or fireworks, it mostly comes from: vehicle exhaust, industrial burning, heating buildings, construction dust, and chemical reactions in the air.

@esgred Even stating home is not necessarily the answer, as most homes are very permeable, and there is a limit to how much ozone is removed by even the best air purifiers. I use 2-stage filters - activated carbon plus HEPA, changed out frequently, but have no way to measure how effectively they remove ozone.
Does anyone know a reasonably priced, simple way to accurately measure ozone levels in the home?
My pulmonologist recommends lowering ones activity level in times of high ozone to limit heavy breathing.

@sueinmn I have an indoor monitor but it’s only measures VOC’s not ozone. I see Ring now has one if you have other compatible devices and heard good things about Govee products when I was looking for something to track the air quality. Mine doesn’t have a screen so I can see if the indicator changes color then look at the app for the levels.

@sueinmn Staying indoors on high ozone days, is better than going outside, and when you open a door, the ozone can come inside. Both N95 or KN95 respirator masks offer zero protection against ozone gas.

If one is concerned about ozone coming into the home, one can set up a clean room.

1. Choose the Right Room:
a. Pick a room large enough to hold everyone comfortably.
b. A bedroom with an attached bathroom is ideal.
c. Avoid rooms with fireplaces, large open spaces, or exterior doors.

2. Seal the Space:
a. Close all windows and doors tightly
b. Use tape or damp towels to seal gaps under doors.Keep windows locked to compress the seals.
c. Turn off kitchen and bathroom exhaust fans to avoid drawing in outdoor air.

3. Clean the Air:
a. Run a high-efficiency particulate air (HEPA) purifier continuously.
b. Ensure the purifier is sized correctly for that room's square footage.
c. Important for Ozone: To filter ozone gas, your purifier must include a heavy activated carbon filter. Standard HEPA filters only stop particles, not gases.

4. Adjust the Climate:
a. Use an air conditioner on "recirculate" mode if it is hot.
b. Ensure the AC fresh-air intake vent is closed.
c. Avoid using swamp coolers, which bring in outside air.

5. Keep the Room Clean:
a. Do not smoke, burn candles, or use gas stoves.
b. Avoid vacuuming, as it kicks up settled dust.
c. Wet-mop or wipe surfaces with a damp cloth if needed.

@esgred Staying in a tightly sealed a room like you describe for extended periosd, with or without an air purifier, is not healthy. It should only be done in an emergency (eg wildfire smoke and you cannot leave). Carbon dioxide and other gaseous pollutants that are not removed by an air filter will build up as well as moisture. . Regular external air exchange is absolutely necessary exchange is necessary, the more people and pets in the room, the more frequent the turnover required.