Ozone Alert Day in Sacramento: What Your HVAC Can Do

On an ozone alert day, your HVAC filter does almost nothing for you. Ozone is a gas, and the pleated filter in your return catches particles, not gases. What your system can do is keep the house closed during the hours ozone runs highest and recirculate the air you already have. Timing is the whole game.

Know what the alert means first. AirNow rates an air quality index of 0 to 50 as good and 51 to 100 as moderate. Above 100, the air is unhealthy for sensitive groups, which includes children, older adults and anyone with asthma. An ozone alert means the forecast puts ozone past that line. Locally, the Sacramento Metropolitan Air Quality Management District announces these as Spare the Air days. They cluster on hot, still afternoons, and a 100°F October heat wave fits the pattern exactly.

Ground-level ozone forms when vehicle exhaust and other emissions react in strong sunlight and heat. It is not emitted directly. It cooks.

That is why it follows a daily cycle. Readings stay low in the morning, climb from late morning, peak from midday into early evening, and fall after sunset. The valley makes it worse. Surrounding hills and still air keep the afternoon's ozone from blowing out, and readings in the foothill suburbs such as Folsom often run higher than downtown because the air mass drifts east during the day.

Smoke and ozone are different problems. Smoke is mostly fine particles, and a good filter catches a lot of them. Ozone slips through every filter in your system. On a day with both, you need both strategies.

Your AC does not remove ozone, but it lets you avoid letting it in. Indoor ozone levels usually run lower than outdoor levels in a closed house, because ozone reacts with indoor surfaces and breaks down. The more outdoor air you pull in during the afternoon, the more you lose that advantage.

On an ozone alert day:

1. Close windows by late morning and keep them shut until the sun is down.
2. Run the AC on recirculation. Most central systems recirculate by default. If yours has a fresh-air intake, an economizer or a ventilator, set it to close or switch it off for the afternoon.
3. Leave the whole house fan off until after dark. Running it in the afternoon pulls the worst air of the day straight through your windows.
4. Pre-cool in the morning if your schedule allows it. A house cooled a few degrees before noon needs less from the AC through the peak.
5. Ventilate after sunset once readings drop, as long as there is no wildfire smoke in the air. Check the current reading on AirNow before you open up.

A filter with an activated carbon layer reduces some ozone, but only modestly. It is a reasonable upgrade for a household with asthma. It is not a substitute for keeping the house closed.

Never run an air cleaner that produces ozone. Some devices are sold as "ozone generators" or "activated oxygen" purifiers, and some ionizers release ozone as a byproduct. They add the exact pollutant the alert is warning you about.

The EPA is direct about this: at concentrations that do not exceed public health standards, ozone has little potential to remove indoor air contaminants. In other words, an ozone generator either does not clean the air or it puts out enough ozone to be a health problem itself.

California's Air Resources Board requires air cleaners sold in the state to be certified as emitting no more than 0.050 parts per million of ozone. Check for it before you buy, and be wary of older units or anything ordered from an out-of-state seller.

Our position: for ozone days, skip any purchase. A closed house, the AC on recirculation and a sensible daily schedule do more than any device you can plug in. Spend money on a portable HEPA cleaner only if smoke is the problem, because that is what HEPA is built to catch.

The most common ozone-day mistake happens in houses trying hardest to protect someone. Picture a household with a child who has asthma. They run a plug-in "ionic" purifier in the bedroom all summer, keep a ventilating fresh-air setting on for the "clean air," and open windows at 3 p.m. when the house feels stuffy. On a hot, still afternoon in a heat wave, every one of those steps raises indoor ozone instead of lowering it.

The fix costs nothing. Unplug the ionizer, close the fresh-air damper or switch off the ventilator for the afternoon, and move the window-opening to after sunset.

Call us if you are not sure whether your system pulls in outside air, if you have a fresh-air intake, economizer or ventilator and do not know how to close or schedule it, or if your AC cannot keep the house comfortable with the windows shut through a heat wave. Those are the situations where the system, not the schedule, is the limit.

PULSE HVAC serves homeowners across the Sacramento area. Call (916) 850-2221 or book online to have a technician check how your system handles outdoor air.

Sources

• AirNow — AQI Basics
• EPA — Ozone Generators That Are Sold as Air Cleaners