Does my air conditioner filter out wildfire smoke? Indoor air quality questions, answered.

Dec 19, 2023

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Air conditioning, purifiers, and even box fans can help.

As outdoor air in the northeastern US hangs thick with wildfire smoke, indoor air is increasingly foul, too.

That's because the air quality inside buildings is a direct reflection of outdoor air quality, said Ian Cull, an environmental engineer and air quality expert based in Chicago. Few buildings (with the notable exceptions of some health care and laboratory settings) are hermetically sealed to prevent them from sharing any air with the outdoors. So people breathing air inside eventually end up breathing whatever's on the outside.

But not all buildings are created equal, and some HVAC (heating, ventilation, and air conditioning) systems do a better job of maximizing air quality than others.

I talked to Cull about how to make the best of what you have during wildfire season. Whether you have central, in-unit, or no AC — and whether you have little or lots of resources to commit to cleaning the air in your house — there are things you can do to improve the quality of the air you and yours breathe.

You can buy a small handheld or indoor air quality monitor, but really, the best indication of your indoor air quality is outdoor air quality, said Cull. "When it's really bad, yes, you can smell it," he said. "But even when the sight and smell goes away, there's going to be a period of time where outdoor air pollution is still exceeding standards."

It would be great if you could use some measurement of different components of indoor air pollution to help determine how clean and safe your indoor air is. (During a wildfire, the chief pollutant you’d want to measure would be soot particles — technically called "particulate matter." But at other moments, it's also important to minimize volatile organic compounds, or VOCs, which are compounds that can be toxic to human health.)

However, while there are tons of home-use air quality monitors on the market, they don't all work as well as advertised — and it can be really hard to determine which ones do. One 2019 study showed that home monitors made by Dylos and Purple performed almost as well as professional ones.

But for most people, Cull recommends using the AirNow app or site to determine the air quality in your region, and assuming your indoor air quality more or less approximates outdoor air quality.

Then, he said, work to prevent outdoor air from flowing inward — and clean the air that's inside.

The three most important strategies in reducing the inward flow of outdoor air are blocking its routes inward, turning off indoor exhaust systems, and minimizing ventilation.

1. Block routes for outside air to keep it from coming in — within reason. That means closing windows and doors, and, where it's relatively easy to do so, sealing visible crevices.

Cull said that caulking around drafty windows, or applying seasonal shrink-wrap window covering to prevent smoky air from getting in through unseen cracks, probably isn't going to yield much additional protection. "Air finds its way in," he said, and focusing your energy on other strategies like filtration (more on that below) is probably more effective.

But if you have a big gap to the outside that's easily sealable with duct tape, throw some on there, he said.

2. Keep exhaust fans off. Differences in pressure between the inside and the outside are what determine how much outside air flows into your space. Exhaust fans — like the ones in bathrooms and on hoods over kitchen ranges — are designed to suck air out of the room, which creates an incentive for outside air to flow in. That's normally a good thing, but in a smoky air situation, it's the opposite of what you want!

3. Switch from ventilation to recirculation on any and all HVAC units. When the outdoor air is clean, we generally want it to replace stale indoor air pretty frequently. But now, with the outdoor air quality so low, we want to keep it out. Instead, we want to just recirculate the better-quality air that's already inside our homes.

Doing that will look different depending on what your home uses for cooling the air — and on how much control you have over tweaking any central systems’ settings.

If you have central air, see if you can figure out how to close the fresh air intake ducts. These are generally openings on the outside of a building, often with a grill over them — and for someone not used to finding them, they can be hard to tell apart from other openings in the side of a building. Still, if you can find yours, closing it is a good first step.

If you have an in-unit ("window") air conditioner, you likely have the option of bringing in outdoor air (often symbolized with wavy one-way arrows) or recirculating indoor air (which often is indicated with arrows running in an oval or circle). When the air outside is bad, opt to recirculate.

No matter how well-sealed your windows and doors are, some outdoor air is making it inside — and with it, the particulate matter that makes smoke such a potent irritant to eyes, noses, and throats. Continuously running your indoor air past a filter, and using the air filter with the smallest holes your system can move air through, helps keep your air as clean as possible.

Again, this will look different depending on your home's HVAC system.

For people with central air conditioning, explore using a filter with a MERV (minimum efficiency reporting value) of 13 or higher (the range is 1 to 16). The higher a filter's MERV rating, the smaller the holes in the filter — and the better it is at catching soot particles of all sizes. However, an air conditioning unit works harder to move air through a very fine filter than it works to move air through a filter with big holes. Older units in particular might struggle with filters of higher MERV ratings. So if you detect a noticeable drop in airflow from your home's ducts after upgrading to a filter with finer holes, you might be stuck using a filter with bigger holes.

For people without central air conditioning who have forced-air heating (that is, it's heated by warm air coming out of vents, rather than by a radiator or hot pipe that circulates steam), the options are pretty similar. Thermostats usually have two settings — one controlling the heat (often with at least two options, including "heat" and "off"), and one controlling the fan (with options "on," "off," and "auto"). Normally, in the summertime, the heat is set to "off" and the fan is set to "auto." When the air gets smoky, you want to keep the heat off but turn the fan to auto. You also want to upgrade the filter in your furnace using the same considerations you would for an air conditioner.

If you’re using an in-unit ("window") air conditioner, or not using any air conditioner at all, your best bet is using a free-standing air purifier to clean the air in each room. An in-unit air conditioner doesn't typically have much filtration capacity, so as long as it's set to recirculate air, its effect on indoor air quality is neutral.

For those lucky folks who live in the best-case-scenario buildings — those with newer central air conditioning systems, with the option of upgrading their filters and closing their intake ducts — additional air filtration devices might not be necessary. An air quality monitor can help people in this situation decide whether they need more air purification help, said Cull. And some particularly high-risk people might reasonably choose to buy an air filtering unit just in case.

For those of us with more gaps in our HVAC systems’ capacity, the benefit of a standalone air purifier is more clear, said Cull. But it can be hard to tell the good from the bad or merely okay: The pandemic led to a flood of air filters and purifiers on the market, and some are better than others.

For the best objective insight on air purifier performance, Cull looks to the Association of Home Appliance Manufacturers’ Verifide site; Consumer Reports is also thorough, but paywalled, and the Environmental Protection Agency (EPA) also offers tips on choosing a product. He recommends putting one in each room and keeping it on full-time while air quality (which he assesses using the AirNow app or site) is low. (He also recommends avoiding ones with ionizers, which actually add pollutants like ozone to the air.)

Almost all air purifiers will remove the particulate matter that's the most prominent pollutant in wildfire smoke. However, some also remove volatile organic compounds (VOCs, toxic gases and compounds that can also lead to health consequences). Air purifiers whose filters include an activated carbon (or "charcoal") layer have this capacity.

During the pandemic, many people constructed their own, low-cost air filtration alternatives, most commonly Corsi-Rosenthal boxes. These contraptions involve four air filters and a box fan, but there's an even simpler alternative to them — a poorer-man's poor-man's air purifier, said Cull: taping a single MERV-13 (or higher) filter to the back of a box fan and placing the unit in the middle of the room. (You want the filter mounted on the upstream side of the fan, with the arrows on the filter — which indicate the direction of airflow through it — pointing toward the fan.) He recommends running this setup around the clock until air quality is back to the healthy range.

Certain common household activities — frying foods or cooking foods that smoke, using a gas stove, burning candles and incense, vacuuming (unless you use one with a HEPA filter), and smoking, for example — create byproducts that aren't great for our airways.

Under normal circumstances, when we’re maximizing ventilation by opening windows or allowing our HVAC systems to draw in outside air, these byproducts dissipate pretty easily. That happens less readily if you’re following any of the above suggestions.

For that reason, it's especially important to avoid doing these activities inside while you’re taking measures to maximize your indoor air quality. And it's always a good idea to avoid smoking.

You can get more tips on indoor air from the EPA's website.

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