Indoor Air Quality During Wildfire Season: What California and Nevada Homeowners Need to Know

For homeowners in San Diego and North Las Vegas, wildfire season is no longer a regional event that happens elsewhere — it is an annual planning challenge. The 2020–2024 wildfire seasons each produced AQI readings above 150 (Unhealthy) across large portions of California and Nevada, and 2025 continued that pattern. Your HVAC system is your primary defense for indoor air quality, but only if it is configured correctly.

Why Wildfire Smoke Is Different From Normal Dust

Most residential HVAC filters are rated for particles 3–10 microns and larger — the visible dust, pet dander, and pollen that accumulate in the home. A standard MERV-8 filter stops the vast majority of these particles.

Wildfire smoke is fundamentally different. The combustion of wood, vegetation, and structures generates particles primarily in the PM2.5 range: 0.3–2.5 microns. These particles:

• Pass through MERV-8 and most MERV-11 filters with minimal resistance
• Penetrate deep into lung tissue (alveoli) rather than being trapped in the upper airway
• Carry carcinogenic compounds including polycyclic aromatic hydrocarbons (PAHs)
• Remain suspended in air for hours and travel hundreds of miles from the fire origin
• Are invisible to the naked eye — you cannot see that you are breathing them

The health consequences of extended PM2.5 exposure are well-documented: exacerbated asthma, cardiovascular stress, increased risk of respiratory infection, and for vulnerable populations (elderly, children under 5, people with heart or lung conditions), short-term hospitalizations.

Understanding AQI: What the Numbers Mean

What Your AC Can — and Cannot — Do

What Standard Central AC Does

A standard central air system takes air from inside your home, passes it over the evaporator coil (cooling it and removing humidity), then pushes it back through the supply ducts. It is a closed-loop recirculation system. It does not, by default, pull in outside air.

This means your AC can be your ally during wildfire events — it continuously passes indoor air through the filter, removing smoke particles each cycle. The key variable is whether your filter is rated to capture PM2.5 particles.

The Fresh Air Intake Exception

Some homes — particularly newer construction built to ASHRAE 62.2 ventilation standards — have a fresh air intake (also called a makeup air unit or ERV/HRV). This is a duct that intentionally pulls outdoor air into the return side of the HVAC system to meet minimum ventilation requirements.

During a wildfire event, a fresh air intake actively pumps smoke-laden outdoor air into your home. This is the opposite of what you want. If your home has a fresh air intake, locate the damper and close it during smoke events. If you are unsure whether your system has one, a technician can inspect during an HVAC maintenance visit.

What AC Cannot Do

Even a MERV-16 filter cannot capture 100% of PM2.5 particles. Air leaks into your home through electrical outlets, door gaps, window frames, and attic bypasses — smoke infiltrates around the system rather than through it. At AQI above 200, even a well-filtered, sealed home will see elevated indoor PM2.5 levels. This is why portable HEPA air purifiers serve as a supplement to HVAC filtration during the most severe events.

Filter Ratings Explained: What to Use During Smoke Events

Important caveat on MERV-16: Higher MERV ratings create more airflow resistance (static pressure). Not all residential HVAC systems are designed to operate with filters above MERV-13. Running a MERV-16 in a system with a weak blower motor can reduce airflow below the system's minimum, causing the evaporator coil to freeze or the compressor to overheat. Ask a technician whether your specific system can handle a MERV-16 before installing one.

How Often to Check Filters During Smoke Events

Normal filter change intervals (monthly for 1-inch pleated, every 3–6 months for 4-inch media filters) are based on typical household air quality. During active wildfire smoke:

• AQI 101–150: Check filter weekly; replace if visibly gray or brown
• AQI 151–200: Check every 3–4 days; a MERV-13 filter can saturate in under a week
• AQI 200+: Check daily; have spare filters on hand before the event begins

A clogged filter during a smoke event is worse than a clogged filter normally — it not only fails to capture particles, it also restricts airflow and can push your system into thermal lockout precisely when you need it most.

3 Things to Do Before Wildfire Season

1. Upgrade to MERV-13 or Higher

Before smoke season begins, replace your standard filter with a MERV-13 and keep a supply of 2–3 extras on hand. During a major smoke event, filter stock at home improvement stores sells out quickly. Ordering online with a week of lead time is the reliable approach.

2. Locate and Seal or Close the Fresh Air Intake Damper

Identify whether your home has a fresh air intake. If it does, locate the damper — typically a motorized or manual baffle in the duct where outdoor air enters the return plenum. Close it manually during smoke events, and reopen when AQI returns to below 100. If the damper is motorized and connected to the thermostat, ask a technician to show you how to disable the ventilation cycle during emergencies.

3. Schedule an HVAC Inspection Before Fire Season

A pre-season inspection confirms your system can handle the higher static pressure of a MERV-13 filter, identifies any duct leaks that would allow smoke infiltration around the filter, and ensures the blower motor is operating within spec. In San Diego and surrounding communities, scheduling this in late summer before the October–December peak fire months is ideal.

What NOT to Do During Smoke Events

• Do not run ventilation-only mode on your thermostat ("Fan On" without cooling). This still recirculates air, but some thermostats also activate the fresh air intake in ventilation mode.
• Do not open windows, even partially. Outdoor air with AQI above 150 is significantly worse than indoor air, even in a home with average filtration.
• Do not run a whole-house fan. Whole-house fans pull massive quantities of outdoor air into the home — during a smoke event, this is one of the worst things you can do for indoor air quality.
• Do not rely solely on your HVAC filter. Air infiltration around window frames, door seals, attic hatches, and electrical outlets bypasses your filter entirely. Weatherstripping and outlet foam gaskets are inexpensive and meaningful complements.

Wildfire Smoke vs. COVID/Flu: The Same Filters Help

There is a practical overlap between wildfire smoke protection and protection against airborne viral particles (SARS-CoV-2, influenza). COVID-19 particles are approximately 0.1 microns, smaller than PM2.5 smoke particles but captured by similar filter mechanisms (interception and diffusion). MERV-13 filters that reduce smoke PM2.5 infiltration also reduce airborne viral load in the recirculated air. This is not a substitute for ventilation and other measures, but MERV-13 upgrades are doubly beneficial in homes with vulnerable household members during both smoke and illness seasons.

After a Major Smoke Event: When to Replace Filters

After a significant smoke event — AQI above 150 for 3 or more consecutive days — replace your HVAC filter even if it does not look visually saturated. PM2.5 particles are too small to see on a filter, but a filter that has captured heavy smoke particle loads can off-gas volatile organic compounds (VOCs) captured from the smoke. Replace the filter and dispose of it in a sealed bag outdoors.

Also inspect the inside of the return air plenum (the metal box around your filter) for visible smoke residue. A light gray film on interior duct surfaces is normal after a major event; significant soot accumulation warrants a professional duct cleaning.

Portable HEPA Air Purifiers as a Supplement

For bedrooms, home offices, and rooms where vulnerable household members spend significant time, a portable HEPA air purifier provides meaningful supplemental protection beyond what your HVAC system can deliver. Look for units with an H13-rated HEPA filter (true HEPA) and a CADR rating appropriate for the room size. IQAir, Coway, Blueair, and Levoit are commonly recommended brands. A unit sized for a 300-square-foot bedroom running continuously during a smoke event can reduce indoor PM2.5 by 80–90% in that room.

Frequently Asked Questions: Indoor Air Quality During Wildfires

Does running my AC help during wildfire smoke in San Diego or Las Vegas?

Yes, but only if your filter is rated to capture smoke particles. A standard MERV-8 filter does little for PM2.5. Upgrade to MERV-13 before smoke season, run your AC continuously on cool or fan mode to maximize air passes through the filter, and keep all windows and doors closed. Ensure your fresh air intake is closed if you have one.

How do I know if my home has a fresh air intake?

Look for a round or rectangular duct connection entering the return air side of your air handler or plenum, separate from the main return duct. It often has a damper (a movable plate inside). Another sign is that your thermostat has a "ventilation" schedule setting — this usually controls a motorized fresh air damper. If you are not sure, an HVAC technician can identify it during an inspection. Homes built after 2012 in California and Nevada are more likely to have them due to building code ventilation requirements.

Can I install a HEPA filter in my existing HVAC system?

True HEPA filters create far more airflow restriction than most residential HVAC systems can handle. Installing a HEPA filter in a standard residential air handler typically reduces airflow enough to cause evaporator coil freezing and compressor damage. MERV-16 is the practical maximum for most systems. Use a standalone portable HEPA unit for rooms where highest-level filtration is needed.

How often should I change my filter after a wildfire event in California?

Replace your filter immediately after any smoke event where AQI exceeded 150 for more than two days, regardless of how the filter looks. During active smoke, check and replace as needed — weekly at AQI 101–150, every 3–4 days at AQI above 150. Stock two to three MERV-13 filters before the October–December fire season peak in Southern California.