The EPA's guidance is direct: "During smoky periods, upgrading to an HVAC filter rated MERV 13 or higher is recommended to effectively remove fine particle pollution from smoke in the indoor air." Their general home air-cleaner guidance reinforces the same baseline — "select a filter rated at least MERV 13 or as high as your system will accommodate."
Why MERV 13 specifically? MERV 13+ filters are required to show at least 50% removal efficiency on the smallest tested particle range (0.3–1.0 microns) — the size range wildfire smoke (PM2.5) falls into. Filters below MERV 13 catch dust and pollen well but let much more of that fine smoke particulate through.
Check your system first. Not every HVAC blower can handle a MERV 13 filter's added airflow resistance — check your system's manual before jumping up from a MERV 8 filter, especially in an older system.
During an active smoke event, a portable HEPA air purifier in the room you're in adds meaningfully more protection than a furnace filter upgrade alone — the furnace filter only cleans air that's actively circulating through the ducts.
Know your filter size? Look it up in the full FilterKey tool, or browse MERV 13 filters by brand.