When outdoor air quality plummets due to events like wildfire smoke or high smog days, the air fills with fine particulate matter, most notably PM2.5, which poses a serious health risk. These microscopic particles can easily infiltrate homes, making it a priority to maintain a safe indoor environment while keeping your living space cool. Running your air conditioning unit is generally advisable during these times, as keeping windows and doors sealed is a strong defense against outdoor pollutants. However, using your AC requires specific adjustments to ensure you are filtering indoor air effectively and not inadvertently pulling in polluted outside air.
Understanding How AC Systems Interact with Outdoor Air
Central heating, ventilation, and air conditioning (HVAC) systems are primarily designed to cool and recirculate the air already inside your home. The outside unit, or condenser, removes heat from the indoor air and releases it outside. It does not typically pull in large amounts of outside air for circulation. While most residential central systems recycle indoor air, some may have a fresh air intake damper intended for ventilation, which becomes a liability during air quality events.
Window-mounted air conditioning units function similarly by cooling and dehumidifying the indoor air. However, many models have a vent or damper that can be opened to draw in outdoor air. Using a window unit with this vent open during a smoke event will directly introduce pollutants into your home. Portable air conditioners that use an exhaust hose to vent hot air outside can also create negative pressure, which may pull smoky outdoor air in through small cracks and gaps elsewhere in the home.
Critical AC Settings for Air Quality Events
The most direct action you can take to protect your indoor air is to ensure your AC system is operating in a closed loop, maximizing recirculation. This requires locating and closing any fresh air intake or outside air vent your system may have, effectively sealing the indoor environment. For central systems, this is often a damper control, and for window units, it is typically a lever or switch labeled “vent” or “outside air” that should be set to the closed position.
Operating your AC in this recirculation mode ensures the air is cooled using only the air already inside the house, which then cycles repeatedly through the system’s filter. Another beneficial setting is to run the system’s fan continuously, even when the compressor is not actively cooling. This constant operation pushes the indoor air through the filter more frequently throughout the day, increasing the total number of times the air is cleaned. This continuous filtration process helps gradually reduce the concentration of fine particulate matter that may have seeped into the home.
The Role of Filtration in Indoor Air Safety
The effectiveness of your AC system in cleaning the air depends largely on the filter you use, which is rated using the Minimum Efficiency Reporting Value (MERV) scale. Standard fiberglass filters typically have a low MERV rating (1 to 4), sufficient only for trapping large particles like lint and dust. To combat the fine particulate matter (PM2.5) found in wildfire smoke, a filter with a MERV rating of 13 or higher is highly recommended by the U.S. Environmental Protection Agency (EPA).
A MERV 13 filter is designed to capture a significant percentage of particles in the 0.3 to 1.0 micron range, which includes the bulk of hazardous PM2.5. While higher-rated filters offer better filtration, they are also denser and can restrict airflow. Before installing a MERV 13 or higher filter, confirm your specific HVAC system can handle the increased resistance without causing damage.
During prolonged air quality events, filters accumulate pollutants more quickly, so they should be checked and replaced more often than the typical schedule, potentially monthly or bi-weekly. For an extra layer of protection, supplement your AC system with standalone High-Efficiency Particulate Air (HEPA) purifiers in the rooms you use most. HEPA filters are a gold standard, capturing 99.97% of particles as small as 0.3 microns, making them highly effective against smoke and other ultrafine pollutants.