Is smoke in your home? Here's how to make an air purifier from a box fan

Interest in air filters and purifiers is spiking in Southern California, including questions about how to build a homemade air purifier. Getting clean air is the latest challenge for residents who evacuated to safe areas, only to be forced to endure unhealthy smoky air.

On Friday, a large area in and around Los Angeles was listed as having unhealthy air due to particulate matter from large wildfires that devastated communities this week, according to the federal AirNow air quality monitor.
On AirNow' map, a "red zone" (signaling air quality index values from 151 to 200) stretched from Inglewood in the northwest to Long Beach and Sunset Beach in the southeast.
In the "unhealthy" range of 151 to 200, some people may experience health effects and members of sensitive groups — children and older adults or those with heart or lung disease — "may experience more serious health effects," the Environmental Protection Agency says.

A big worry, experts say, is fine inhalable particulate matter known as PM 2.5 (for 2.5 microns) that can lodge deep into our lungs.

And as researchers at the University of Washington note, "Smaller ultrafine particles (PM 0.1) can pass into the bloodstream and organs, including the brain."

Here's a quick guide to getting cleaner air:

Search filters by their MERV rating

Filters are key, whether you're using them to upgrade your home's HVAC system or put them in an air purifier or a DIY air cleaner. A filter's ability to pull particulates and other matter out of the air are rated by Minimum Efficiency Reporting Values, or MERV. Filters with higher values can catch smaller contaminants like smoke, bacteria and viruses.

A MERV rating of 13 means the filter is efficient at catching items from 0.3 to 1 micron in size: "Bacteria, droplet nuclei (sneeze), most tobacco smoke, insecticide dust," according to the California Air Resources Board, or CARB.

"Upgrading to a filter rated MERV 13 or higher can be especially important during smoky periods to effectively remove fine particle pollution from smoke in the indoor air," the EPA says. It adds that most HVAC systems work fine using MERV 13 filters, as long as the filter is replaced frequently — but if you're in doubt, ask an HVAC technician to be sure.

One thing to remember: If you're using your HVAC system to clean your home's air, it only does so when the unit is running. You can learn more about filter rating systems from LAist here.

An air quality map shows a large part of the Los Angeles metro area with "unhealthy" air, due to large wildfires fueled by strong winds.

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Air Now/ Screenshot by NPR

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Build an air cleaner at home

All you need is a box fan, good air filters and some duct tape, and you're in business.

NPR has previously posted a design and guide to building an air purifying cube known as a Corsi-Rosenthal Box, using four 20-inch MERV 13 filters, a 20-inch box fan and two 20-inch panels of cardboard.

A smaller and more affordable approach is to attach one MERV filter to the back of a box fan, an idea laid out in a website from the Montana group Climate Smart Missoula. The group notes that people using that option should use newer box fans, and be sure the motor is clean to reduce the risk of overheating.

The Centers for Disease Control and Prevention says these DIY purifiers can be effective for a temporary solution, especially if you don't have any other alternatives.

Use HEPA air purifiers and other methods

HEPA (for high-efficiency particulate air) filters are used in many air purifier machines. HEPA is roughly equivalent to a MERV 17 filter, the California Air Resources Board says.

"A well-sealed HEPA filter is more than 99.9% efficient for all sizes of particles," according to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, or ASHRAE.

If you plan to buy a portable air cleaner or purifier, the EPA says you should make sure it has a HEPA filter. To help your search, California has a table of state-certified air cleaners that lists more than 1,000 products by brand, model number, and type. The EPA also has a guide on matching your room size with the appropriate unit.

Whichever strategy you choose, experts say you should also make sure doors, windows and air ducts are well sealed, to keep out smoke and pollution. You should also drink plenty of water to help ease irritation — and wear a mask if you go outside, as NPR has reported.

More tips on staying healthy

Laist has a guide on how to navigate the unhealthy air here.

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