When smoke from wildfires or other events blankets your region, the desire for cool air conflicts directly with the need for clean air. The answer to whether you should run your air conditioning is generally yes, but only with specific precautions to protect your indoor air quality. Smoke is primarily composed of fine particulate matter, known as PM2.5. Keeping windows and doors sealed against the outdoor air is the first defense against this pollutant. Your home’s cooling system can become an ally, provided you adjust its operation and enhance its filtration capabilities.
Operating Your Central AC System
If your home uses a ducted, central air conditioning system, it is designed to recirculate and cool the air already inside your home. The most important operational adjustment is to ensure your system is not introducing smoky outside air. If your HVAC unit has a dedicated fresh air intake, you must locate and close the damper or turn off the ventilation mode immediately.
The next step involves modifying the thermostat fan setting from “Auto” to “On.” The “Auto” setting only runs the fan when the AC is actively cooling. Switching to “On” forces the blower to run continuously, even when the cooling cycle is complete.
This constant operation ensures that indoor air is cycled through the system’s air filter much more frequently. Continuously pulling air through the filter maximizes the removal of fine smoke particles that seep into the home. While this setting uses more electricity and may require more frequent filter changes, the benefit of improved air purification during a smoke event outweighs the cost.
The Critical Role of Air Filtration
The effectiveness of your central AC system as an air purifier hinges entirely on the quality of the filter installed. Standard fiberglass filters (MERV 1–4) are designed only to protect the HVAC equipment from large debris and are ineffective against the microscopic PM2.5 particles found in smoke.
To effectively capture smoke, you need to upgrade to a filter with a Minimum Efficiency Reporting Value (MERV) of 13 or higher. MERV 13 is the minimum rating recommended by the U.S. Environmental Protection Agency (EPA) for removing fine particles, capturing up to 95% of particles in the 0.3 to 1.0 micron range. Before installing a MERV 13 or higher filter, check your HVAC system manual or consult a professional, as the denser material can restrict airflow.
Reduced airflow can strain the blower motor, potentially causing the unit to overheat or break down. If your system handles the upgrade, you must monitor the filter closely during a smoke event. Heavy smoke loads clog high-efficiency filters much faster than normal dust, meaning replacement may be needed every 30 to 60 days to maintain optimal airflow and filtration performance.
Addressing Window Units and Portable ACs
Non-centralized cooling units require different precautions because they interact directly with the outside environment. Most window AC units cool indoor air while using outdoor air to cool the condenser coil. Although the airflows are separate, the unit’s seal to the window is a common point of smoke entry.
Many window units feature a vent or damper control that allows the unit to intentionally draw in outside air for ventilation. Ensure this vent is set to the “closed” or “recirculate” position to prevent the direct intake of smoky air. You should also use weather stripping or foam to seal any gaps between the unit casing and the window frame.
Portable air conditioners are categorized as either single-hose or dual-hose models, and their operation differs significantly in smoky conditions. Single-hose units draw cooled indoor air over the condenser and exhaust it outside. This process creates negative air pressure inside the room, which sucks in unfiltered, smoky air from outside through available gaps. Dual-hose units are safer because they use one hose to draw outside air for cooling the condenser and a second hose to exhaust that air back out, maintaining a more balanced indoor pressure.
Maintaining Indoor Air Quality Without AC
When your AC system is insufficient for the smoke level or you rely on non-ducted cooling, dedicated air purification becomes necessary. Portable, standalone air purifiers equipped with High-Efficiency Particulate Air (HEPA) filters are the most effective supplemental solution. A true HEPA filter is certified to remove 99.97% of particles that are 0.3 microns in size, making it highly effective against smoke particles.
You should select a purifier with a Clean Air Delivery Rate (CADR) appropriate for the room size, running it continuously in frequently used spaces. Many purifiers also include an activated carbon layer, which absorbs the gaseous pollutants and odors associated with smoke. To keep your home cooler while minimizing smoke infiltration, avoid activities that generate heat indoors, such as high-heat cooking. Drawing blinds and curtains during the day also helps reduce solar heat gain, lessening the demand on your cooling system.