Is it safe to run AC during wildfires and smog and what filters to use
Wildfire smoke, smog, and your AC: what you need to know
When the Air Quality Index (AQI) spikes from wildfires or summer smog, your first instinct is to shut windows and turn the AC on. That can be the right move, but only with the correct settings and filtration. This guide explains when it is safe to run your air conditioner, which filters actually capture smoke, and how to reduce indoor exposure to PM2.5 and odors without risking your equipment or safety.
Is it safe to run AC during wildfires or heavy smog
Yes, you can usually run central AC safely if you:
- Keep the system in recirculate mode (no outdoor air).
- Close windows and doors and minimize exhaust fans that pull outside air in.
- Use a high-quality pleated filter, ideally MERV 13, if your system can handle it.
- Monitor for smoke odor indoors; if it increases, reassess settings and filtration.
Do not run evaporative coolers (swamp coolers) during smoke events, because they draw outside air by design. Window and portable AC units can be leaky; see special guidance below.
Which filters actually work on smoke
MERV ratings explained
- MERV 8: Captures larger dust, not great for smoke.
- MERV 11: Better for smaller particles, some smoke reduction.
- MERV 13: Target level for wildfire smoke; improves capture of PM2.5 and finer particles.
- MERV 14–16: Higher capture efficiency, but static pressure can exceed many residential blowers.
Not every air handler can support MERV 13 or higher without testing static pressure and airflow. If your blower is marginal, a too-restrictive filter can reduce cooling, cause coil icing, and increase energy use. When in doubt, use the highest MERV your system safely supports and supplement with portable True HEPA air purifiers in occupied rooms.
HEPA and carbon: what they do
- True HEPA (H13 or H14) captures very fine particles including PM2.5. Most central HVAC systems are not designed for in-duct HEPA because of pressure drop; HEPA is excellent in standalone portable units.
- Activated carbon or charcoal prefilters reduce some odors and certain gases common in smoke and smog. They do not remove carbon monoxide. Keep CO detectors installed and tested.
System settings for smoke days
- Set the thermostat fan to On for steady filtration if the filter is clean and properly sized. If the filter is loading fast, use Auto to avoid overworking the blower.
- Disable or close any outdoor air intake, ERV, or HRV during severe smoke. Many systems have a fresh air damper or motorized intake—make sure it is shut.
- Seal gaps around doors and windows, and reduce use of kitchen and bath exhaust fans to avoid drawing smoky air indoors.
- Create a clean room: choose one or two rooms, keep doors closed, and run a portable True HEPA purifier continuously. Aim for at least 4–5 air changes per hour in that space.
Central AC vs. ductless vs. window and portable AC
Central AC and heat pumps
With a proper MERV 13 pleated filter, central systems can significantly cut indoor PM2.5 during wildfire smoke. Change filters more frequently during events—sometimes weekly. If you notice reduced airflow, icing on the coil, or rising energy bills, the filter may be too restrictive or clogged.
Ductless mini-splits
Most mini-splits have coarse washable screens that do not capture smoke. Keep them in recirculation with doors and windows closed, and add portable True HEPA units in the rooms you occupy. Some models support add-on filtration kits; verify pressure drop and warranty implications before modifying.
Window and portable AC
Single-hose portable units and many window ACs can pull in outside air and can leak. If you must use them:
- Set to recirculate if the unit has a vent lever—close it fully.
- Seal around the install kit with foam and tape.
- Pair the room with a HEPA purifier and consider a carbon prefilter to reduce odor.
Smog vs. wildfire: different pollutants
Wildfires primarily raise PM2.5 and VOCs from smoke; smog can increase ozone and other gases. MERV 13 helps with particles. For odors and some gases, use activated carbon filters in portable purifiers. Central systems can include carbon media in specialized filter cabinets, but this adds resistance—get a professional evaluation to avoid airflow and warranty issues.
Maintenance and filter schedule
- Start the season with a new, properly sized pleated filter; keep spares on hand.
- During heavy smoke, inspect filters weekly and replace when loaded.
- Check door sweeps and weatherstripping to reduce infiltration.
- Verify condensate drains and coils are clean; restricted airflow worsens smoke infiltration and can lead to icing.
Safety, compliance, and insurance notes
DIY modifications to intake dampers, filter racks, or ductwork can create unsafe static pressure, equipment damage, and code violations. Many insurance policies and local codes require that HVAC alterations be performed and documented by qualified technicians, especially for commercial sites and large residential systems. If your building has dedicated outdoor air systems, ERVs, or complex controls, turning off or reconfiguring equipment without documentation can risk warranty and insurance coverage.
Our approach to clean, healthy indoor air
#1 AC Guys is a fourth-generation family team serving Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County. We focus on residential and commercial projects, especially large and complex systems, with a mission of healthy air at home for a healthy life. Our engineers bring 80+ years of engineering experience and factory training with Fujitsu (Japan), Mitsubishi Electric (Thailand), Midea (China), Gree (China), and Haier (China). That depth matters when balancing filtration, airflow, and comfort during wildfire and smog events.
Quick checklist for smoke days
- Close windows and doors; minimize exhaust fans.
- Set HVAC to recirculate; shut fresh air intakes, ERV, and HRV.
- Install the highest MERV filter your system safely supports, ideally MERV 13.
- Run portable True HEPA purifiers in occupied rooms; add carbon for odors.
- Create a clean room and keep it closed.
- Replace filters frequently; watch for airflow and coil icing.
- Use CO detectors; don’t rely on carbon filters for CO.
- Avoid swamp coolers during smoke; seal window or portable ACs.
Service area and typical scenarios
We routinely address smoke and smog strategies across Los Angeles County and nearby areas, including cities such as Los Angeles, Long Beach, Pasadena, Glendale, Burbank, Santa Monica, Anaheim, Irvine, Santa Ana, Huntington Beach, Fullerton, Ventura, Oxnard, Thousand Oaks, Simi Valley, Camarillo, Riverside (western), Corona, Norco, and Jurupa Valley. Building types range from single-family homes and ADUs to mid-rise offices, labs, and retail. Each case balances filtration, fan capacity, and indoor pressure to keep PM2.5 down while protecting equipment.
Bottom line
Run your AC on recirculation with the best filter your system can handle, aim for MERV 13 where feasible, and supplement with portable True HEPA purification. Use carbon for odors, not for CO. Shut outdoor air intakes during severe smoke, create a clean room, and change filters often. For systems with complex ventilation, a qualified review helps you stay safe, maintain comfort, and meet code and insurance requirements.
Yasmine is currently an Associate Professor of Psychology at Mount Saint Mary College where she teaches a wide array of courses in the Psychology department. She is a Fulbright Scholar spent a year working at the Medical Decision Making Center at Ono Academic College in Israel.
Yet, as many higher education professionals can surely attest to, I have also witnessed the other challenge in group decision making. In academia, engaging in critical dissent is encouraged (reviewed by Jetten & Hornsey, 2014), and while this is a fine attribute, practically,
