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In last week’s column (July 2) we learned why it’s important to create a safe indoor air space during wildfire smoke season. This week, we’ll focus on practical advice for filtering smoke out of your indoor air.

If you have central air, you may be able to improve your indoor air quality by upgrading your system’s air filter. Air filters are rated using the Minimum Efficiency Reporting Value (MERV), with ratings ranging from 1-20. The higher the MERV rating, the more effective the filter will be at removing small particles. The minimum MERV rating for removing fine particulate in smoke is MERV 13. You don’t reach HEPA levels of particulate removal until MERV 17. However, a MERV 13 filter is effective at removing the fine particulate in smoke when air recirculates through the filter. The more times air passes through the MERV 13 filter, the cleaner it becomes. Note that as the MERV ratings increase, the cost of the filters also increases. Also, a higher MERV-rated filter will likely need to be changed more frequently due to the amount of material it accumulates.

Not all air handling systems can handle the added airflow resistance associated with higher MERV ratings, so you may want to have an HVAC technician check your system before upgrading your filter.

If you don’t have central air with a good filter, room air purifiers with HEPA filtration can dramatically improve indoor air quality. These standalone units use a fan to pull air through a true HEPA filter, which mechanically removes the fine particles in smoke. There are a lot of options on the market, and they all do pretty much the same thing with various bells and whistles.

If you choose to get a room air purifier, plan on keeping it in the room where you spend the most time. For example, it’s a good idea to run one in your bedroom while you sleep. Try to keep windows and doors to the room with the air purifier closed to allow the machine to recirculate air through its filter.

Questions to ask when you look at an air purifier:

1. Does this unit use a true HEPA filter?

There are “HEPA-like” filters on the market, which are not the same thing as true HEPA and won’t be as effective at removing the fine particulate in smoke.

2. How many square feet will it cover?

To see significant indoor air quality improvements, you want a machine that can recirculate your room’s air through its filter 2-3 times per hour.

3. Does this unit produce ozone?

Some air purifiers produce ozone, which is harmful to human health. Check to make sure your purifier has been approved by the California Air Resources Board (CARB) here: CARB only approves purifiers that do not produce harmful levels of ozone.

4. Is this unit Energy Star rated?

5. Is it noisy?

6. How effective is the unit at removing volatile organic chemicals (VOCs) in smoke?

There are multiple methods for removing VOCs, but the most common one you’ll see is an activated carbon prefilter. While activated carbon can remove VOCs, be aware that it can get saturated quickly, which limits its effectiveness. If you are concerned about the VOCs in smoke, change the prefilter frequently or invest in an air purifier with a robust activated carbon filter. Note that the hefty activated carbon filters can be quite pricey. Typically, the more fancy the VOC removal, the more expensive the machine (and the replacement filters). For some folks, removing VOCs will be worth the price.

Coming next week: Keeping cool during smoke season.

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Sarah Coefield is the air quality specialist for the Missoula City-County Health Department. 

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