Put in a MERV 13 filter, or the highest rating your system can take, and make sure it fits snugly. Set the thermostat fan to "On" so air keeps moving through the filter, and close the fresh air intake. During heavy smoke, check the filter often and replace it when it looks dirty, sooner than the package says.
MERV tells you how much a filter catches
MERV stands for Minimum Efficiency Reporting Value. It rates how well a furnace or HVAC filter captures particles from 0.3 to 10 microns, using a test from ASHRAE, the heating and air conditioning engineers' society. The higher the number, the more it catches.
Smoke particles are small, so the column that matters most is the 0.3 to 1 micron range. Here's part of the EPA's table:
| MERV | Catches 0.3 to 1 micron | Catches 1 to 3 microns | Catches 3 to 10 microns |
|---|---|---|---|
| 1 to 4 | Not listed | Not listed | Under 20% |
| 8 | Not listed | 20% or more | 70% or more |
| 11 | 20% or more | 65% or more | 85% or more |
| 13 | 50% or more | 85% or more | 90% or more |
| 16 | 95% or more | 95% or more | 95% or more |
CARB says the thin, 1-inch fiberglass filters many homes use are usually MERV 1 to 4. They typically remove less than 20% of the particles that reach them.
Some brands use their own scales, like FPR or MPR. The EPA notes those belong to the companies, while MERV comes from a national consensus standard. On every scale, a higher number catches more.
Use MERV 13 or the best your system can take
The EPA's advice for smoke is a filter rated at least MERV 13, or as high as your system fan and filter slot can handle. MERV 13 is the first level that has to catch at least half of the smallest particles tested.
Fit matters too. The filter should sit snugly so air can't leak around the edges. Don't bend or crush one to make it fit. Filters come 1 to 5 inches thick in many sizes, so match the size printed on your old filter or the slot.
Ask whether your system can handle it
CARB notes that higher-efficiency filters can add airflow resistance. The EPA says most furnaces and HVAC systems can use a MERV 13 filter without equipment problems, as long as you replace it often. Systems usually aren't labeled with filter limits, so the EPA suggests asking an HVAC technician if you're unsure. A technician can also modify a system to take a better filter.
CARB says some high-efficiency filters have deeper or more pleats to reduce the airflow problem. Thicker filters, 2 to 5 inches deep, usually last longer, but CARB notes they only fit systems built or modified for them.
Your system only filters when the fan runs
The EPA says home HVAC systems usually run less than 25% of the time during heating and cooling seasons. When the fan is off, nothing goes through the filter.
During smoke, the EPA says to set the thermostat fan to "On" instead of "Auto" so it runs as much as possible. AirNow adds: use "Recirculate," and close the fresh air intake if your system has one.
There's a cost. The EPA says longer run times raise your electric bill and can make humidity harder to control in cooling season. For a smoke event, the EPA still says run it.
Change the filter more often during smoke
Normally, the EPA says manufacturers recommend a new filter every 60 to 90 days. During heavy smoke, keep spares on hand and plan to replace the filter more often than the manufacturer recommends. If old filters come out heavily soiled, change them more often.
Here's roughly what filters cost
These prices are from CARB's 2020 fact sheet, for disposable 1-inch filters.
| Filter | Typical rating | Price |
|---|---|---|
| Fiberglass | MERV 1 to 4 | $2 to $4 |
| Pleated, medium efficiency | MERV 5 to 8 | About $4 to $9 |
| Higher efficiency | MERV 9 to 12 | About $10 to $20 |
| Premium | MERV 13 | Up to about $25 |
Some systems need a different plan
Window and ductless units. The EPA says most window and wall-mounted "ductless" air conditioners have only enough filtering to keep the unit clean. They don't remove much fine smoke. Close the outdoor air damper on a window unit, and use a portable air cleaner in the room.
Evaporative (swamp) coolers. These pull in outdoor air. If you can reach the intakes safely, the EPA says to cover them completely with 4-inch MERV 13 filters, and expect to replace them often if wind or rain damages them. If you can't, use the cooler sparingly.
No central air. A portable air cleaner or a box fan filter handles one room. See our purifier guide and DIY guide.
What helps
Use MERV 13
Or the best your system can take.
Add a clean room
A purifier in the room you sleep in.
Box fan backup
When the furnace can't take a thicker filter.
Questions people ask
Is MERV 16 better than MERV 13 for smoke?
It catches more of the smallest particles: 95% or more against 50% or more. The EPA's advice is to use the highest rating your fan and filter slot can handle, so a technician can tell you whether your system can run a MERV 16.
Can a MERV 13 filter hurt my furnace?
The EPA says most systems can use MERV 13 without problems if you change it often. Frequent replacement is the EPA's condition, so don't stretch the schedule. If you're unsure, ask an HVAC technician.
Should I run my HVAC fan all the time?
During a smoke event, yes. The EPA says set the fan to "On" so air keeps passing through the filter. Expect a higher electric bill while you do.
Do I still need an air purifier if I have a MERV 13 filter?
It helps. The EPA says HVAC filters and portable air cleaners can be used together for more filtration. The HVAC filter covers the whole house, and a portable unit adds more filtering in the room where you spend the most time.