Wildfire Smoke and Indoor Air Quality in Marin and Sonoma County: What Your HVAC System Is and Isn’t Doing to Help
Wildfire Smoke and Indoor Air Quality in Marin and Sonoma County: What Your HVAC System Is and Isn’t Doing to Help
Quick Answer: Indoor air quality during wildfire events is determined by how well your home’s envelope, HVAC system, and filtration equipment intercept fine particulate matter (PM2.5) and toxic gases before they reach your living space. A standard 1-inch HVAC filter does not provide meaningful protection against wildfire smoke. Effective wildfire IAQ protection requires at minimum a MERV 13 filter in your air handler, a strategy for managing your HVAC system during smoke events, and ideally supplemental air purification with a true HEPA filtration stage. Ongaro & Sons is a licensed C-20 HVAC contractor based in Petaluma, CA, serving Marin, Sonoma, and Napa counties since 1932.
Residents throughout the North Bay don’t need to be told that fire season is real. The Tubbs Fire in 2017, the Kincade Fire in 2019, the Glass Fire in 2020, and a string of regional smoke events in subsequent years have made it a routine part of life here. Marin County carries its own wildfire risk: the Wildland-Urban Interface zones in the hills above Mill Valley, Tam Valley, Fairfax, and San Anselmo have been under evacuation warnings multiple times, and northern Marin around Novato has also seen PSPS events and elevated fire danger during high-wind conditions. Even when fires originate in Sonoma or Lake counties, Marin residents experience the same smoke. What’s less understood is what those smoke events are doing to the air inside your home, and whether your HVAC system is helping or making things worse.
For most homes in the North Bay, the honest answer is: it depends entirely on how your system is configured, and most homes are not configured well for smoke. This piece covers what wildfire smoke actually contains, how it gets into your home despite closed windows, what your HVAC system is and isn’t doing during a smoke event, and what a properly equipped home can do to protect indoor air quality during fire season.
What Is Actually in Wildfire Smoke?
Wildfire smoke is not the same as campfire smoke or the byproduct of burning dry wood in a controlled environment. It’s a complex mixture of combustion products determined by whatever the fire is burning, and in a residential wildfire, that includes homes, vehicles, landscaping, stored chemicals, building materials, plastics, and treated lumber.
The California Air Resources Board identifies the primary components of wildfire smoke as:
Fine particulate matter (PM2.5). Particles smaller than 2.5 microns in diameter. These are small enough to penetrate deep into the lungs, small enough to enter the bloodstream, and small enough to bypass most standard air filters. PM2.5 is the primary metric on the Air Quality Index (AQI) and the main driver of health impacts from wildfire smoke.
Carbon monoxide. A colorless, odorless gas produced by incomplete combustion. Carbon monoxide displaces oxygen in the bloodstream and can cause symptoms ranging from headaches to disorientation at elevated indoor concentrations.
Volatile organic compounds (VOCs). Wildfire smoke contains acrolein, benzene, formaldehyde, and polycyclic aromatic hydrocarbons, among others. Many of these are known carcinogens. Unlike particulate matter, VOCs are gases, not particles, and cannot be captured by particle filters alone.
Ozone. Wildfires contribute to ground-level ozone formation as smoke reacts with sunlight. Ozone is a respiratory irritant that aggravates asthma and lung conditions.
Heavy metals and ash. When structures burn, they release lead, arsenic, and other materials from building components, electronics, and finishes. These can be carried in smoke or settle as ash.
The practical implication for homeowners: a filter rated to capture PM2.5 addresses the most prevalent hazard, but wildfire smoke contains gaseous pollutants that require additional treatment, specifically activated carbon or chemical filtration, to reduce meaningfully.
How Does Smoke Get Inside Even with Windows Closed?
Closing windows during a smoke event is the right instinct. It’s also insufficient on its own.
Every home has a building envelope: the wall, ceiling, roof, and floor assemblies that separate conditioned interior space from the outdoors. No building envelope is perfectly airtight. Air infiltrates through:
- Gaps around window frames and doors
- Electrical outlets and switch plates on exterior walls
- Plumbing and wiring penetrations through wall and ceiling assemblies
- Attic bypasses and recessed light fixtures that open into attic space
- Crawl space and foundation vents that communicate with the interior
In a typical North Bay home, particularly older construction in Santa Rosa, Petaluma, and Healdsburg, and in Marin County towns like Fairfax, San Anselmo, and Mill Valley, built before modern air-sealing standards, the cumulative effect of these gaps can be significant. Researchers at Lawrence Berkeley National Laboratory have estimated that a typical existing California home exchanges its interior air volume with outdoor air multiple times per hour through infiltration alone, even with all windows and doors closed.
During a heavy smoke event, that infiltration brings smoke in continuously. The only ways to address it are to reduce infiltration through air-sealing and improved insulation, to create positive indoor pressure using your HVAC system so air is trying to escape rather than infiltrate, or to filter the air actively enough to clean it faster than infiltration can dirty it.
What Is Your HVAC System Doing During a Smoke Event?
Your central HVAC system can either help or hurt indoor air quality during a wildfire, depending on how it’s set up and how you operate it.
The positive pressure advantage. When your air conditioning or air handler is running, it circulates and pressurizes the interior of your home. A home running its HVAC system maintains slightly positive pressure relative to outdoors, which means air is moving outward through the envelope gaps rather than inward. This is a meaningful benefit during smoke events: a running HVAC system reduces smoke infiltration through cracks and gaps. This is one genuine reason to keep the system running during smoke events even if outdoor air quality is poor.
The fresh air intake problem. Many HVAC systems, particularly those with newer energy recovery ventilators (ERVs), heat recovery ventilators (HRVs), or whole-house ventilation setups, are designed to bring in a controlled amount of fresh outdoor air. During normal conditions, this is a feature that improves indoor air quality by diluting CO2, moisture, and odors. During a wildfire smoke event, it’s pulling smoke directly into your home. If your system has a fresh air intake or a whole-house ventilator, it should be closed or set to recirculate during heavy smoke events. Consult your system’s manual or ask your HVAC contractor where this control is located.
The recirculation mode. Most central HVAC thermostats and some portable units offer a recirculation or “recirculate” setting that bypasses fresh air intake and circulates only indoor air through the system. This is the correct operating mode during significant smoke events. The system continues to filter and condition the air inside the home without drawing additional smoke from outdoors.
What your filter is (and isn’t) doing. Even in recirculation mode, the protection your HVAC system provides depends entirely on the filter in the air handler. If that filter is a standard 1-inch fiberglass filter rated MERV 4 to 6, it is capturing large dust particles and protecting the equipment. It is not capturing PM2.5. The smoke is passing through it.
Why Don’t Standard HVAC Filters Protect Against Wildfire Smoke?
Standard 1-inch HVAC filters are designed to protect the equipment, not the occupants. A basic fiberglass filter rated MERV 4 captures roughly 20 percent of particles in the 3-to-10-micron range. Wildfire smoke is predominantly PM2.5, meaning 2.5 microns and smaller. A MERV 4 filter does essentially nothing to intercept it.
This is the most important misconception to correct: if your home has a forced-air HVAC system with a standard 1-inch filter and you’re running it during a smoke event, you’re circulating smoke through your home and not meaningfully cleaning it. The house may feel comfortable, but the air quality inside can approach outdoor levels during heavy smoke.
The EPA’s guidance on wildfire smoke recommends upgrading to higher efficiency filtration and using portable air purifiers with HEPA filtration during smoke events. The specific MERV rating matters significantly, and the gap between a MERV 8 and a MERV 13 filter in terms of smoke particle capture is not incremental. It’s the difference between partial and meaningful protection.
What MERV Rating Do You Need During Wildfire Smoke Events?
MERV (Minimum Efficiency Reporting Value) ratings run from 1 to 20. Higher numbers capture smaller particles more effectively. Here’s how they relate to wildfire smoke protection:
MERV 1 to 6: Designed to protect equipment from large particles. Provides no meaningful protection against wildfire smoke or PM2.5.
MERV 8: Captures a higher percentage of larger particles but still allows the majority of PM2.5 to pass through. Better than MERV 4 to 6 but not adequate for smoke protection during significant events.
MERV 11: Captures a meaningful percentage of PM2.5 and smaller particles. A practical upgrade for homes that want improved air quality without the potential airflow restriction of higher ratings. Appropriate for mild to moderate smoke days.
MERV 13: The threshold the EPA and ASHRAE recommend for meaningful PM2.5 capture in residential HVAC systems. Captures 75 percent or more of PM2.5-range particles. This is the minimum worthwhile target for wildfire season protection in a central air system.
MERV 16 and above: Hospital-grade filtration. Most residential air handlers cannot move enough air through filters this dense without significant airflow restriction, motor strain, and efficiency loss. Generally not appropriate for standard residential HVAC without a system specifically designed for it.
The practical recommendation for most North Bay homes, whether in Sonoma or Marin County: install a MERV 13 filter in your air handler if the system manufacturer’s specifications allow it. A qualified HVAC technician can confirm compatibility. If your system can’t handle MERV 13 without airflow issues, MERV 11 is a meaningful improvement over what most homes currently have.
One critical caveat: a higher-MERV filter loads up faster with particles and needs to be checked and replaced more frequently during smoke events. Check your filter every two to three weeks during heavy smoke seasons. A clogged filter reduces airflow regardless of its rating, and an airflow-starved system runs inefficiently and can damage the equipment.
What Whole-House Air Purification Options Actually Work?
For homeowners throughout Sonoma and Marin counties who want a more permanent and robust IAQ solution than a filter swap, several whole-house options are available. The distinction between technologies matters, because the market is crowded with products that make strong claims and deliver inconsistent results.
Media air cleaners / high-MERV media cabinets. These replace the standard 1-inch filter slot with a larger cabinet housing a thick, pleated media filter, often 4 to 5 inches deep. Because the media is thicker, it can achieve high MERV ratings (11 to 13) without the airflow restriction that a thin high-MERV filter causes. Media cabinets are effective, relatively inexpensive, and only require filter replacement once or twice a year depending on conditions. For many homes, a media cabinet upgrade is the highest-value IAQ improvement available.
Electronic air cleaners (electrostatic precipitators). These charge particles electrostatically so they adhere to collection plates, which require periodic cleaning. They can be effective for particulate capture but require maintenance discipline: dirty collection plates lose effectiveness quickly and some generate ozone as a byproduct. Ozone is a respiratory irritant that worsens the air quality problem they’re intended to solve.
HEPA filtration systems. True HEPA filters capture 99.97 percent of particles at 0.3 microns and larger, which covers PM2.5 comprehensively. Whole-house HEPA systems are available but require bypass ductwork configurations because standard residential air handlers cannot push air through HEPA media at normal flow rates. These are highly effective but more complex and expensive to install than media cabinet upgrades.
UV-C light systems. Ultraviolet germicidal irradiation (UVGI) systems installed in the air handler or ductwork are effective at inactivating airborne biological contaminants: bacteria, viruses, mold spores. They do not capture particles. A UV-C system does not improve smoke protection by itself, but it’s a legitimate addition to an overall IAQ strategy that already includes particle filtration.
Bipolar ionization and needlepoint ionization. These technologies release positive and negative ions into the airstream that are intended to cluster particles together, making them easier to capture, and neutralize some airborne contaminants. The scientific literature on effectiveness is mixed, and some units have been shown to generate ozone at levels that raise air quality concerns. If considering ionization, verify the product has third-party testing showing ozone emissions below 0.05 ppm.
For most homes in Sonoma and Marin counties, the highest-value IAQ investment for wildfire season is a media cabinet upgrade to MERV 13 combined with portable HEPA air purifiers in primary living spaces. That combination covers the particle problem effectively at a reasonable cost.
How Should You Use Portable Air Purifiers During Smoke Events?
Portable air purifiers are an important supplemental tool, particularly for bedrooms and other spaces where occupants spend significant time.
Look for units with a true HEPA filter (not “HEPA-type” or “HEPA-style,” which are marketing terms without a defined standard) and an activated carbon pre-filter. The activated carbon stage helps capture VOCs and some of the gaseous pollutants that HEPA filters don’t address. Look for units that carry the Energy Star certification, which ensures the energy efficiency claims have been independently verified.
Sizing matters. Portable air purifiers are rated by Clean Air Delivery Rate (CADR), which measures how quickly they filter a given volume of air. To properly protect a room, choose a unit with a CADR appropriate for the room’s square footage. The Association of Home Appliance Manufacturers (AHAM) recommends matching CADR to at least two-thirds of the room’s square footage for adequate air changes per hour.
During heavy smoke events, run portable purifiers continuously in occupied rooms. Keep bedroom doors closed with the purifier running through the night. For households where budget limits the number of units, prioritize the bedroom where the most vulnerable family members sleep.
The DIY Corsi-Rosenthal box, a cube of MERV 13 furnace filters attached to a standard box fan, has been validated by researchers at UC Davis and other institutions as a cost-effective supplemental air cleaner during smoke events. It’s not a replacement for proper filtration in your HVAC system, but it’s a legitimate option for adding filtration capacity quickly during an acute smoke event.
What About Ventilation Systems During Smoke Season?
Homes with energy recovery ventilators (ERVs) or heat recovery ventilators (HRVs) face a specific challenge during wildfire season. These systems are designed to bring in fresh outdoor air while recovering energy from the exhausted indoor air. During smoke events, they import smoke.
Most ERVs and HRVs have a bypass or closed position that stops fresh air intake while continuing to circulate indoor air. Locate this control before smoke season arrives, not during a smoke event when you’re searching for it with eyes irritated by smoke.
Some newer ERV/HRV systems include filtration on the incoming air stream, including HEPA or high-MERV pre-filtration. These can operate safely during smoke events with the filter in place and provide the benefit of controlled ventilation even during poor air quality periods. If your ventilation system doesn’t include this filtration, shutting off fresh air intake during smoke events is the right call.
For guidance on how your specific system should be configured during smoke events, an HVAC service call is the most reliable way to get an answer specific to your equipment.
What Lingers After the Smoke Clears?
The visible smoke goes away, but the indoor air quality problem doesn’t resolve immediately. Several things linger after a smoke event ends:
Settled particles in your ductwork. During a smoke event, PM2.5 and smoke particles circulate through your ducts along with the airstream. A meaningful portion settles in the duct lining, particularly in horizontal runs and low-velocity areas. When the system runs after the smoke event, it can re-suspend those particles into your living space. Professional duct cleaning after a significant smoke season removes settled contamination and is worth considering for homes that ran their systems during heavy smoke days with inadequate filtration.
Surface contamination. PM2.5 settles on surfaces throughout the home: furniture, carpet, bedding, and hard surfaces. Thorough cleaning after a smoke event, including HEPA-filtered vacuuming, reduces re-suspension of settled particles into the air.
VOC off-gassing from absorbed smoke. Porous materials, including upholstery, carpet, and drywall, can absorb VOCs from smoke and release them slowly over subsequent days. This is why homes can smell of smoke for days after outdoor air quality has returned to normal. Activated carbon air purifiers help address this phase of the problem. Ventilating the home with outdoor air when AQI conditions allow accelerates off-gassing.
A loaded filter. Your HVAC filter will be significantly loaded after a smoke event, which restricts airflow and reduces system efficiency. Replace it promptly once outdoor air quality returns to normal.
How Do You Build a Wildfire-Ready IAQ System for Your Sonoma or Marin County Home?
Rather than reacting to each smoke event, a growing number of homeowners throughout Sonoma and Marin counties are building an IAQ system designed to handle smoke season as a baseline condition. Here’s what that looks like in practice:
Baseline HVAC filtration upgrade. Replace the standard filter slot with a media cabinet housing a MERV 13 filter. Have your HVAC contractor confirm the system can handle the airflow requirement. Change the media filter at least twice per year: once before smoke season and once after.
Whole-house air purifier addition. A media cabinet addresses particulate capture. Adding a UV-C stage to the air handler addresses biological contamination. Some whole-house purification systems combine multiple stages in a single unit.
Locate and label your fresh air controls. Know where the fresh air intake damper is for your HVAC system and how to close it. Know where the ERV or HRV bypass control is. Label these clearly so anyone in the household can find them during an emergency.
HEPA portable purifiers in primary spaces. Two units, one for the primary living area and one for the master bedroom, provide meaningful supplemental protection when outdoor AQI is severe.
AirNow monitoring. The AirNow.gov website and app from the EPA and CARB provide real-time AQI data by ZIP code. Both Sonoma and Marin county monitoring stations give localized readings that can vary significantly between inland and coastal areas of each county. Knowing when AQI is above 100, 150, and 200, the thresholds for sensitive groups, unhealthy, and very unhealthy respectively, allows you to make informed decisions about ventilation, activity, and system operation.
Post-season duct maintenance. Schedule a duct inspection and cleaning assessment after each significant smoke season. For homes that experience repeated events with inadequate filtration, duct contamination accumulates over multiple seasons.
For a comprehensive assessment of your home’s current IAQ capability and what upgrades make sense for your specific equipment and construction, Ongaro & Sons offers indoor air quality services that cover filtration, purification, ventilation, and duct system condition.
Frequently Asked Questions
What is the best way to improve indoor air quality during wildfire smoke events? The most effective combination is running your HVAC system in recirculation mode with a MERV 13 filter installed, closing fresh air intakes, and running portable HEPA air purifiers in primary living spaces. Seal obvious gaps around doors and windows with towels or weatherstripping during acute events. Check and replace your HVAC filter more frequently during smoke season, as it loads up faster under high-smoke conditions.
Does running the air conditioner help with wildfire smoke? Yes, partially. Running your AC system keeps the home under slight positive pressure, which reduces smoke infiltration through gaps in the building envelope. It also continuously circulates indoor air through the HVAC filter. The benefit depends entirely on what filter is installed: a MERV 13 provides meaningful particle capture; a standard MERV 4 to 6 filter does not protect against PM2.5.
What MERV filter should I use for wildfire smoke? MERV 13 is the minimum rating recommended by the EPA and ASHRAE for meaningful PM2.5 capture in residential HVAC systems. Confirm your system’s airflow specifications allow MERV 13 before installing one. If your system cannot handle MERV 13 without airflow restriction, MERV 11 is a significant improvement over standard filters. Check and replace MERV 13 filters more frequently during smoke season.
Can wildfire smoke get inside a closed house? Yes. Even with windows and doors closed, air infiltrates through gaps in the building envelope: around window frames, through electrical outlets, at plumbing penetrations, through recessed lighting, and through attic bypasses. No home is airtight enough to fully exclude outdoor air infiltration. Running your HVAC system creates positive pressure that reduces but doesn’t eliminate this infiltration.
How does wildfire smoke affect indoor air quality differently than regular air pollution? Wildfire smoke contains higher concentrations of PM2.5 and a broader mix of toxic compounds than typical urban air pollution, including VOCs such as acrolein, benzene, and formaldehyde produced by burning structures and vehicles. Standard urban air pollution is primarily from vehicle exhaust and industrial emissions; wildfire smoke from structure fires contains additional heavy metals and combustion products from building materials.
Should I clean my air ducts after wildfire season? Yes, particularly if your home ran its HVAC system during heavy smoke events with inadequate filtration. PM2.5 and smoke particles settle in ductwork during smoke events and can be re-suspended into living spaces during subsequent HVAC operation. Duct cleaning after significant smoke exposure removes this contamination. At minimum, inspect duct condition and confirm no visible smoke residue before the next heating season.
What is the AQI threshold to close up your house and run filtration? The EPA’s AirNow categorizes AQI above 100 as unhealthy for sensitive groups, including children, the elderly, and anyone with respiratory or cardiovascular conditions. Above 150 is unhealthy for everyone. The standard guidance is to close up the home, stop outdoor activity, and run air purification at AQI levels above 100. At AQI above 150, all residents should limit outdoor exposure and maximize indoor air filtration.
Does an air purifier remove VOCs from wildfire smoke? A true HEPA filter does not capture gases or VOCs. An air purifier with an activated carbon stage in addition to HEPA filtration will capture a meaningful portion of VOCs, though the carbon media has a finite capacity and needs replacement. For significant VOC exposure, a purifier with at least a thick activated carbon filter stage is more effective than HEPA alone.
Improve Your Home’s Wildfire Season Air Quality in Sonoma and Marin Counties
Ongaro & Sons is a licensed C-20 HVAC contractor based in Petaluma, CA, serving Marin, Sonoma, and Napa counties since 1932. We are a NATE-certified, Diamond Certified® contractor. We assess home filtration, ventilation systems, and duct condition and recommend practical upgrades for homes throughout Marin and Sonoma counties that deal with fire season every year.
Whether you need a media cabinet upgrade, a whole-house air purification system, or a post-smoke-season duct assessment, our indoor air quality service team can tell you what your home actually needs rather than what’s easiest to sell. Schedule an assessment.