During wildfire events, outdoor PM2.5 in the LA basin can spike above 50 micrograms per cubic meter. On bad days, above 100. The EPA considers anything above 35 unhealthy for sensitive groups.
What happens inside your home during those events depends almost entirely on two things: how tight your building envelope is, and how well your filtration system works.
How Smoke Gets In
Wildfire smoke particles are small. Most are well under 2.5 microns, which is why PM2.5 is the relevant measurement. These particles infiltrate homes through every gap in the building envelope: door seals, window frames, utility penetrations, and the gap between your foundation and framing.
Older homes in the San Fernando Valley — particularly 1960s and 1970s ranch construction — have loose envelopes. During a smoke event, indoor PM2.5 in these homes can reach 60 to 80% of the outdoor reading within hours with the windows closed.
Newer construction and recently renovated homes are tighter. Indoor PM2.5 typically stays lower, but it still rises. No residential home is airtight. The question is how quickly outdoor concentrations infiltrate and how effectively your system removes what gets in.
What Filtration Actually Does During Smoke Events
A MERV 13 filter, properly installed and sealed, captures a meaningful percentage of smoke-sized particles. We have measured homes during moderate smoke events where outdoor PM2.5 was above 40 and indoor readings stayed under 15. The difference was filtration and filter fit.
The critical word there is "sealed." During smoke season, the consequences of filter frame gaps are amplified. A filter with bypass might keep indoor PM2.5 at 30 while the same filter, properly sealed, would keep it at 12. That gap is the difference between "unhealthy for sensitive groups" and "acceptable."
Running the HVAC fan continuously during smoke events — not just when the system is heating or cooling — forces more air through the filter per hour. This increases effective air changes and helps pull down indoor concentrations faster.
Portable HEPA Purifiers
Standalone HEPA purifiers are effective during smoke events, especially in bedrooms. A properly sized unit can reduce PM2.5 in a single room by 50% or more. The key is sizing: you need a unit rated for the square footage of the room, run on a high enough setting to turn the air over multiple times per hour.
During fire season, running a HEPA unit in each bedroom overnight is a reasonable practice for LA homeowners. The cost is modest relative to the exposure reduction.
The Recovery Curve
After a smoke event clears outdoors, indoor PM2.5 does not drop immediately. Particles settle on surfaces and re-suspend with foot traffic, HVAC cycling, and normal activity. In homes with adequate filtration, indoor PM2.5 returns to baseline within 12 to 24 hours after outdoor air clears. In homes with poor filtration or low ventilation, it can take 2 to 3 days.
Opening windows after the outdoor air clears accelerates recovery. This is one of the few times we recommend aggressive natural ventilation: flush the house with clean outdoor air once the smoke has passed.
Preparing Before Fire Season
The time to fix filtration is before the first smoke event, not during it. Check your filter fit now. Replace the filter with MERV 13 if you are using something lower. Run a test: set the fan to "on" for a few hours and see if you notice any gaps in the filter housing.
If you want to know how your home would perform during a smoke event, an IAQ assessment measures your current filtration effectiveness and identifies weak points. It takes 90 minutes and costs $195. Our free IAQ guide covers what PM2.5 readings mean and what thresholds to watch.