When it's really smoky outside, health officials recommend staying indoors. But what can you do about smoke inside your home? A HEPA air filtration system is the best solution, and necessary for people with certain health conditions, but they can be pricey. Here's a low-tech, low-cost idea to improve air quality inside your home.
If you have central air: Replace your standard furnace filter with a high-end microparticle filter and set the fan to recirculate continuously. Note that high-end filters are more restrictive than standard filter material and will reduce airflow. To compensate for this, you can use a deeper filter (2 or more inches deep instead of the typical 1" deep), if your system will accomodate it. Under normal conditions, a pleated furnace filter can last several months. In dusty or smoky conditions, they need to be replaced more frequently.
If you don't have central air: Get a box fan, some duct tape, and a high-end 20x20 furnace filter. Tape the filter to the front side (output side) of the fan, making sure to point the "air flow direction" arrow on the side of the filter toward the front of the fan. If there are any gaps around the edges where air could bypass the filter, seal them with duct tape. Run the fan continuously. To clean more air, use a 2" or 4" deep filter, if available, the deeper the better.
How to choose a filter:
There are several competing rating systems for furnace filters (MERV, MPR, FPR), which makes it hard to compare among brands. In all cases, filters rated with higher numbers are better at removing small particles, and they cost more. Smoke particles are much smaller than dust and pollen, so you'll want a high-end filter that specifically mentions smoke on the label. Typically, a minimum rating of MERV 11 / FPR 9 / MPR 1900 is needed to be effective at removing smoke. MERV 13 / FPR 10 / MPR 2200 is better. You should expect to pay $15-20 for this type of filter, versus $5 for a standard filter.
Microparticle furnace filters are available at most hardware stores and home-improvement centers. Extra-deep (2" or more) filters are less common, so you may need to order them online or buy through a local HVAC specialist.
A note on masks:
Dust masks are designed to stop large dust particles, not smoke. Even if the filter material can catch the small smoke particles, the mask must seal tightly around the nose and mouth or the smoke will simply bypass the filter. Don't rely on a dust mask or poorly fitting smoke mask for protection.
For more information about the health effects of wildfire smoke, and ways to protect yourself, read this article.