Wildfire smoke events have become an annual reality for a growing share of the US population — not just in the western states where wildfires originate, but increasingly in the Midwest and East as smoke travels thousands of miles on upper-level winds. When Air Quality Index readings climb into the unhealthy range, the air inside your home matters more than the air outside — and your HVAC system is the primary tool you have to protect it.

Here is the complete guide to what your HVAC system can and cannot do during a wildfire smoke event, what changes to make right now, and what products actually work versus what does not.

The First Rule: Keep Outdoor Air Out

The single most effective thing you can do during a smoke event is reduce the amount of outdoor air entering your home. Wildfire smoke particles are very small (PM2.5 — particles smaller than 2.5 microns) and penetrate through gaps in building envelopes. Your HVAC system's first job during a smoke event is to recirculate and filter indoor air rather than drawing in smoke-laden outdoor air.

• Close all windows and doors. This is obvious but essential. Even small openings allow disproportionate smoke infiltration.

• Check your fresh air ventilation settings. If your home has a whole-house ventilation system — an Energy Recovery Ventilator (ERV), Heat Recovery Ventilator (HRV), or outdoor air intake — it should be turned off or minimised during smoke events. These systems are designed to bring outdoor air inside; during a smoke event, that is exactly what you do not want.

• Set your thermostat fan to AUTO, not ON. AUTO mode runs the blower only when heating or cooling — meaning air circulates through the filter only during active conditioning cycles. ON mode runs the blower continuously, cycling air through your filter constantly. During a smoke event, ON mode with a good filter increases the number of filtration passes the air takes — a net positive if you have a MERV-13 or higher filter installed.

The Filter Question — What Actually Works

Your HVAC filter is the primary defence against smoke particles — but not all filters are effective at capturing the tiny PM2.5 particles in wildfire smoke. Understanding which filters protect you and which do not is the most important HVAC decision you will make during a smoke event.

• MERV 13 or higher: This is the minimum filter rating that effectively captures PM2.5 wildfire smoke particles. A MERV 13 filter removes approximately 85 percent or more of particles in the 0.3 to 1.0 micron range — the size range that wildfire smoke occupies. If your system is compatible with MERV 13 (most modern systems are — check your manual or ask an HVAC contractor), install one before smoke events.

• MERV 8 to 12 (standard 'blue' or 'pleated' filters): These are not adequate for wildfire smoke. They capture larger particles effectively but allow most PM2.5 smoke particles through. During a smoke event, a MERV 8 filter gives you some filtration but not meaningful protection against the smallest, most health-relevant smoke particles.

• MERV 1 to 7 (fibreglass 'green' filters): These are essentially equipment protection filters — they protect the HVAC system from large debris but provide no meaningful protection against smoke particles. Do not rely on these during a smoke event.

MERV-13 rated HVAC filters are the minimum effective protection against wildfire smoke PM2.5 particles — capturing approximately 85%+ of particles in the health-critical 0.3 to 1.0 micron range that standard MERV-8 pleated filters allow to pass through. The EPA recommends MERV-13 or higher for wildfire smoke protection in homes with compatible HVAC systems.

What About Air Purifiers?

Portable HEPA air purifiers — the box units that sit in a room — are excellent supplements to your HVAC system during smoke events, particularly in the rooms where you spend the most time. A properly sized HEPA purifier (look for units covering at least the room's square footage at the CADR rating for smoke) can maintain very low particle concentrations in a single room even when whole-house filtration is imperfect.

The EPA recommends running portable air purifiers in the bedroom during sleep — when clean air matters most for health — even if the rest of the house has adequate whole-house filtration.

What Does Not Work

• UV air purifiers and ionisers in HVAC ducts: These technologies kill biologicals (bacteria, mould spores) but are not effective at removing smoke particles. Do not rely on in-duct UV or ionisation for wildfire smoke protection.

• Ozone generators: Ozone damages lungs and is itself a health hazard. Never run an ozone generator in an occupied space, and do not use them for smoke remediation.

• Leaving windows open at night 'because the temperature drops': Even if outdoor temperatures are comfortable at night, smoke particles remain in the air and will infiltrate your home. During a multi-day smoke event, keep the home closed.

Frequently Asked Questions

What HVAC filter protects against wildfire smoke?

MERV-13 or higher filters are the minimum effective protection against wildfire smoke PM2.5 particles. Check your HVAC system manual for maximum recommended filter MERV rating — most modern systems support MERV-13. Standard MERV-8 pleated filters and fibreglass MERV-1 to 7 filters do not provide meaningful wildfire smoke protection.

Should I run my HVAC during a wildfire smoke event?

Yes, with the right filter and the right settings. Install a MERV-13 or higher filter, close all windows and doors, turn off fresh air ventilation (ERV/HRV), and set the fan to ON mode to increase filtration passes. Avoid running the system in fresh air or economiser mode that draws outdoor air inside.

How do I know if my HVAC system is compatible with MERV-13 filters?

Check your system manual for maximum recommended MERV rating, or consult your HVAC contractor. Most modern residential systems support MERV-13. Older systems or those with narrow filter slots may have airflow restrictions at MERV-13 that reduce system efficiency — your contractor can advise on the best option for your specific system.