What I’ve Learned Here in the Epicenter of Very Unhealthy Air Recently in Wisconsin

Wildfire smoke from Canadian fires has settled over Wisconsin and much of the eastern US recently, driven by heat and drought conditions. The smoke carries fine particles smaller than 2.5 microns, small enough to get deep into lungs. A similar problem shows up in some western valleys during winter, when an "inversion" traps car and wood-stove pollution close to the ground instead of letting it rise and disperse.

For home air, most HVAC systems run MERV 8 filters (MERV rates how well a filter traps particles). If your system has a 4-inch filter slot, you can step up to a MERV 11 filter, which catches roughly 85 percent of particles in the 3.0 to 10.0-micron range and also grabs a good share of smaller ones. A MERV 13 filter removes more of the smallest particles but can strain your HVAC's fan motor, so a stand-alone HEPA air filter may be a better route if you want stronger filtration without stressing the system. For homes without central air, a portable window or standalone air conditioner is an option, and some models include filtration.

On masks: surgical masks only filter 20 to 30 percent of smoke particles, not much protection. N95 masks, left over from the pandemic, filter at least 95 percent of airborne particles down to 0.3 microns, including smoke, and are worth digging out on bad air days. Commercial buildings built to ASHRAE standards are required to use MERV 13 filtration or better, a useful benchmark if you're comparing your own home's setup.