Quick answer: When TCEQ calls an Ozone Action Day or smoke pushes Austin’s Air Quality Index into orange or worse, keep outside air outside: close windows, doors and any fresh-air intake, and let your AC recirculate indoor air through the best filter your system can handle. When it drops back to green or yellow, air the house out.
What is an Ozone Action Day in Austin?
It’s a heads-up from the Texas Commission on Environmental Quality (TCEQ) that tomorrow’s weather favors high ground-level ozone, the main ingredient in summer smog. TCEQ issues these forecasts daily during ozone season, roughly March through November in Austin, for several Texas metro areas. Austin is one of them.
Ground-level ozone doesn’t come straight out of a tailpipe. It cooks up from other pollutants in sunlight, and it thrives under clear skies and light wind, which describes a lot of Austin afternoons. The EPA notes that ozone usually peaks in the afternoon, when sunlight is strongest, which is why the standard advice is to move outdoor workouts and yard work to early morning.
You can sign up for TCEQ’s forecasts and Ozone Action Day alerts by email or text on the TCEQ website.
What do the AQI numbers and colors mean?
The Air Quality Index is the EPA’s 0 to 500 scale for how polluted the outdoor air is now or is forecast to be; the higher the number, the dirtier the air. AirNow, the federal site run with state agencies, shows it by ZIP code, and the EPA splits it into six color bands:
- 0 to 50, Green: Good
- 51 to 100, Yellow: Moderate
- 101 to 150, Orange: Unhealthy for Sensitive Groups
- 151 to 200, Red: Unhealthy
- 201 to 300, Purple: Very Unhealthy
- 301 and up, Maroon: Hazardous
An AQI of 100 lines up roughly with the national air quality standard. In Austin, two pollutants usually drive the number: ozone in the warm months, and fine particles called PM2.5 when smoke or dust is in the air. For smoke, the EPA points people to the Fire and Smoke Map at fire.airnow.gov, which shows sensor readings closer to real time.
Where does smoke in Austin come from?
Mostly from fires somewhere else. TCEQ’s forecast discussions have tracked smoke from spring agricultural burning in Mexico and Central America drifting north over the Gulf into Texas, and NASA satellite images have caught that plume over the state. Add the occasional big wildfire elsewhere and a local grass fire now and then, and you get hazy days when the sky looks like weak tea.
Is the air inside my house better on a bad air day?
Usually yes for ozone, not necessarily for smoke. The two behave differently indoors, so treat them as two separate problems.
Ozone is a gas, and it tends to fade indoors
The EPA notes that ozone levels run higher outdoors than indoors. Ozone is reactive, so when it drifts inside it reacts with walls, furniture and other surfaces and gets used up. A closed-up, air-conditioned house starts with an advantage.
HVAC filters catch particles, and ozone is a gas, so a better filter does little for it. Shut windows do much more.
Smoke is mostly particles, and filters can catch them
Smoke is the opposite story. Fine particles slip in through every gap in the house and float around until something catches them. Your HVAC filter can be that something, which is why filtration matters far more on a smoky day than on an ozone day.
How do I keep bad outdoor air out of my house?
Close up the house and let the AC do the breathing. Several steps from the EPA’s wildfire guidance fit Austin homes well:
- Keep windows and doors closed, and use the AC and ceiling fans to stay cool. In Central Texas heat, don’t trade smoke for overheating.
- Close the fresh-air intake, if your system has one, or set the system to recirculate. Some newer Austin homes have a small outside-air duct tied into the return (the big grille and duct that pull air back to the AC); ask your HVAC tech where yours is and how to close it.
- Hold off on anything that adds particles indoors. The EPA’s list includes frying or broiling, candles or incense, gas stoves, aerosol sprays, smoking and vaping, and vacuuming without a HEPA filter.
- Keep bathroom and kitchen exhaust fan runs short. They pull air out, and replacement air comes in through whatever cracks the house has.
That last point surprises people. A house works like a balloon with a slow leak: push air out in one place and outside air finds its way in somewhere else, often through the attic, the garage or gaps around light fixtures.
What AC filter is best for wildfire smoke?
The EPA recommends a MERV 13 filter (MERV rates how fine a filter catches, and higher catches smaller particles), or the highest rating your system’s fan and filter slot can handle. Two practical notes from the field:
- Fit matters as much as rating. A MERV 13 filter with a gap around the edge lets smoky air walk right around it, and the EPA specifically says the filter should fit snugly.
- Not every system can handle a MERV 13. Many builder-grade Austin systems have undersized returns, and a denser filter can choke airflow. If the filter bows or whistles, or the system seems to struggle after the switch, have someone check the static pressure (how hard the fan is straining to push air) before you keep going.
Portable air cleaners can help too. The EPA suggests choosing one sized for the room that produces little or no ozone, and setting up one “clean room,” often a bedroom, where the air gets the most help.
Should I set my AC fan to ON or AUTO when it’s smoky?
ON, while the smoke lasts. The EPA suggests setting the thermostat fan to ON instead of AUTO so air keeps moving through the filter even when the AC isn’t cooling. On AUTO, the fan runs only during cooling cycles, and the filter sits idle the rest of the time.
When the smoke clears, switch back to AUTO. In a humid Austin summer, a nonstop fan can blow moisture from the wet coil back into the house between cycles, and indoor humidity creeps up. On an ozone-only day, AUTO is fine, since the filter does little for ozone anyway.
When should I open the windows again?
Once the AQI drops back to green or yellow. Then the EPA recommends airing out the house by opening windows or the fresh-air intake to flush out smoke that built up inside. Then check two things:
- Check the filter. After a smoky stretch it may need changing early, and the EPA notes that filters may need replacing more often than the box says during heavy smoke.
- Return the fresh-air intake to its normal setting if you closed it.
One Austin wrinkle: opening windows on a muggy morning lets in a lot of moisture, so pick a drier day when you can.
What if someone at home is sensitive to bad air?
Ask their doctor. Health questions about alert days belong with someone who knows the person, not with an HVAC guide. Your job on the house side is simpler: keep the bad air out, filter what gets in, and open up once it passes.
What should I do next?
- Sign up for TCEQ’s Ozone Action Day alerts by email or text.
- Find your fresh-air intake, if you have one, and learn how to close it.
- Keep a snug-fitting spare filter on hand for smoke days. Swapping it is a safe DIY job.
- Book a Gold Eagle visit and ask for a static pressure check if you’re not sure your system can take a MERV 13.
Can’t find a fresh-air intake, or not sure your system can handle a MERV 13 filter? A Gold Eagle technician can check your filtration and ventilation setup on your next regular visit; see our indoor air quality services.
For more on the filter side, see our guides on how often to change your AC filter and running your fan on ON or AUTO.