Smoke from massive wildfires in Canada and Minnesota envelops the Washington, D.C., skyline Friday morning, reducing visibility and casting a haze over the Washington Monument and Lincoln Memorial.

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One version of this story was originally published on January 10, 2025. It has been updated to reflect the wildfires currently bringing smoke to the US.

Interest in air filters and purifiers is increasing as wildfire smoke spreads, including questions about how to build a homemade air purifier.

On Friday, the air across much of the United States, from Minnesota to Virginia, was rated “very unhealthy” due to particles from large wildfires, according to federal air quality monitor AirNow.

The smoke is coming from wildfires in Ontario, Canada and northern Minnesota, where Duluth and other cities are enduring dangerous air at record levels, as Minnesota Public Radio reports.

On the AirNow map, a purple zone indicates “very unhealthy” air quality index values ​​of 201 to 300. That level triggers public health alerts, because “everyone can experience more serious health effects,” the Environmental Protection Agency says. Of particular concern are children, older adults, and people with heart or lung diseases.

Within the warning area, many communities are currently in an even more severe “hazardous” category, reflecting an AQI of at least 301, a level the EPA considers “extremely rare.”

A big concern, experts say, is the fine inhalable particles known as PM 2.5 (by 2.5 microns) that can lodge deep in our lungs.

And as researchers at the University of Washington point out, “smaller ultrafine particles (PM 0.1) can pass into the bloodstream and organs, including the brain.”

Here’s a quick guide to cleaner air:

Search filters by your MERV rating

Filters are key, whether you use them to upgrade your home’s HVAC system or put them in an air purifier or DIY air cleaner. A filter’s ability to remove particles and other matter from the air is rated based on Minimum Efficiency Reporting Values, or MERV. Filters with higher values ​​can trap smaller contaminants such as smoke, bacteria and viruses.

A MERV rating of 13 means the filter is efficient at trapping things 0.3 to 1 micron in size: “Bacteria, droplet nuclei (sneezes), most tobacco smoke, insecticide dust,” according to the California Air Resources Board, or CARB.

“Upgrading to a filter rated MERV 13 or higher may be especially important during periods of smoke to effectively remove fine particle pollution from smoke in indoor air,” the EPA says. He adds that most HVAC systems work well with MERV 13 filters, as long as the filter is replaced frequently, but if in doubt, consult an HVAC technician to be sure.