Wildfires are more common now than ever before. According to a study performed by the University of East Anglia, wildfire season increased by 27% between 1979 and 2019, which is equivalent to two more weeks. While the overall area consumed by wildfires has decreased over this period, specific regions like the northwestern United States saw a major increase in acreage consumed by fire.

Since global temperatures are predicted to continue to rise, more areas will be at risk from wildfires. In addition to the risk of fire damage, wildfires spread dangerous smoke well beyond the area of the fire. Certain weather conditions, most notably high winds, can carry wildfire smoke across county and state lines. In fact, if a fire is powerful enough to launch its smoke into the stratosphere, that smoke can travel thousands of miles. Smoke from the 2021 Bootleg Fire in Oregon reached the eastern United States, settling over Philadelphia and New York City.

As we adjust to this new normal, it’s important to understand how we can protect ourselves from wildfires and wildfire smoke. This blog will explain how smoke enters your home, and answer questions like “Do air conditioners filter wildfire smoke?” In addition, we’ll cover different types of air conditioners, explaining how portable ACs might not filter smoke while window unit ACs do filter smoke. As always, in a crisis situation you should pay attention to local emergency services and follow the directions of the authorities. And remember, because of how well smoke travels, you should be familiar with wildfire smoke safety even if you don’t live in an area with a high incidence of wildfires.

Why is Wildfire Smoke Dangerous?

We know that wildfire smoke spreads far beyond the fire itself, but why is it dangerous? Understanding the dangers of wildfire smoke is part of understanding whether AC units do or don’t filter smoke.

Smoke is made up of gasses and fine particles produced from the burning of organic materials, like wood. According to the Environmental Protection Agency, the greatest risk from smoke comes from the fine particle component. These fine particles—which you can think of like a microscopic version of ash—enter your eyes and respiratory system. They can cause irritation to your eyes and lungs, and prolonged exposure can cause serious illnesses like bronchitis. If you have a chronic health condition affecting your hearts or lungs, the particles will aggravate your pre-existing condition.

As our high school health classes taught us, smoking is bad for you. Part of the problem with factory cigarettes is the additional chemicals beyond tobacco which have been added to the cigarette. Even if wildfire smoke was composed entirely of organic particles from plant matter and wood, it would still be harmful. But wildfires don’t discriminate. They’ll burn anything in their path, adding plastics and other harmful compounds to their smoke. Even if you are a healthy person without respiratory or pulmonary issues, inhaling wildfire smoke is a serious risk to your well-being.

Other specific groups at-risk from smoke inhalation are young children, the elderly, and pregnant people. Especially for those who are pregnant, smoke inhalation can cause complications in the pregnancy which can result in long-term health issues for both the child and the parent.

How Wildfire Smoke Gets In

As we’ve covered in separate blogs, the laws of thermodynamics dictate that hot air will always naturally move towards cool air. Smoke isn’t hot air—it’s tiny particles of matter that were burned to fuel the fire. However, smoke rises because the heat from the fire heats the air which carries the smoke. This is all to say that, if your home is cooler than the outdoor air, hot air will attempt to carry its smoke into your home.

According to the Environmental Protection Agency, Wildfire smoke has three ways of entering your home. They are:

Natural Ventilation: Smoke entering through open windows or doors
Mechanical Ventilation: Smoke entering through devices like kitchen fans or bathroom vents, or through heating, ventilation and air conditioning systems that intake outside air
Infiltration: Smoke entering through small cracks in exterior walls or leaks in windows or doors

Preventing Smoke from Entering Your Home

To prevent Natural Ventilation, keep doors and windows shut when there is a smoke risk. Look into weather strips and other types of insulation to augment the natural seal of a door or window. In a pinch, hang wet towels or extra sheets over exterior doors and windows for additional insulation. While it might be tempting to open up a window to allow for airflow to reduce the smell of smoke, you risk letting much more smoke into your home.

Preventing Infiltration is much more difficult, and hard to do when in an active wildfire scenario. If you live in a home, consider hiring a contractor to perform an energy audit. This will involve checking the perimeter of your home for any potential air leaks. While energy audits are designed to make your home more energy efficient, they will also detect leaks and cracks that allow for smoke infiltration. If you find yourself with a leaking home in a wildfire situation, do not attempt to perform an outdoor energy audit yourself. If you can tell where smoke is entering your home, hang a sheet or wet towel to act as an additional filter. Run an indoor fan to circulate the air in your home, which will mitigate some of the harmful effects of smoke.

The easiest way to prevent Mechanical Ventilation is to stop using devices which allow for smoke to enter your home. Avoid using kitchen fans or bathroom vents during a wildfire. If you happen to have a chimney, make sure the flue is completely shut, and if possible, replace the flue cap with a solid cover.

One major cause of Mechanical Ventilation can be your HVAC system. Considering that wildfires are common during the summer, it’s likely that you’ll be running your AC during wildfire season. But whether or not this puts your home at risk for smoke will depend on the type of air conditioning system you have. In order to answer the question “do air conditioners filter wildfire smoke?” we first have to know which type of air conditioner you have.

Which Types of AC Bring Smoke In?

Central Air Conditioning: Also known as split-system air conditioning, central air conditioners use an outdoor compressor to route cool air through indoor vents. The compressor works entirely with indoor air, which means that Central AC doesn’t bring outside air into your home. However, because Central AC requires vents throughout your home, it’s possible for a Central AC system to circulate smoke or other pollutants via the ductwork.
Window Air Conditioning: Our forte here at Windmill, Window Air Conditioning works by intaking indoor air, cooling it, and then shooting heat exhaust out the window. Window Air Conditioners don’t intake outside air, but with improper insulation can allow outdoor air into your home. Poor insulation normally reduces the efficiency of your AC, but in a wildfire situation can bring smoke into your home. Window AC units do filter smoke, but only if it is already inside your home.
Portable Air Conditioning: Some portable air conditioners—single hose portable ACs—work like a combination of a window unit and central air. The unit sits inside your home, and uses a fan to draw in air like a window AC. Then, using its hose, the unit vents the exhaust outdoors like a central air system’s ducts.
However, other portable air conditioners—notably dual hose portable air conditioners—actually intake outdoor air to cool. A dual hose portable AC should not be operated in wildfire conditions, because it will bring smoky air into your home. Even though dual hose portable AC units filter and cool the outdoor air they use, it is still dangerous to bring smoky air into your home.

There are other types of air conditioning—like through-the-wall units—but those operate similarly to one of the three types above. Very few types of air conditioning actively bring in outside air, so most are safe to operate in wildfire conditions. However, you need to be careful. When not in use, air conditioning provides a path into your home for smoke.

If you have central air conditioning, consider closing the vents when the unit is not in use. This will prevent any smoke that enters your home from circulating. For window air conditioning units, try running them on the fan mode. This will not bring in outside air, but circulate the air inside your home. For portable air conditioners, consider uninstalling them when not in use. Some portable ACs also provide the ability to close the hose, limiting the outdoor air that can seep into your home. And remember, if you have a dual hose portable AC, do not use it in wildfire conditions.

How Do Air Conditioner Units Filter Air (and Smoke)?

Regardless of the type of air conditioner you use, the filtration process will be roughly the same. AC units use fans to intake air. Before that air reaches the cooling mechanism of the AC, it’s run through a filter. Different ACs have different kinds of filters. Central air filters are often disposable, while some window air conditioner units, like the Windmill, have reusable filters. The filters comb dust motes and other particles out of the air, removing them from circulation in their home. Notably, you must replace or clean a filter in order to fully remove the filtered material from circulation; otherwise it just builds up in the AC unit until the filter clogs.

A filter will only catch particles that are larger than the holes in the filter. Think of a filter like a birdcage—if the bars are too big, the bird can fly right through. A standard AC filter is designed to catch dustmotes, pet dander, and pollen. While those aren’t large particles, they are significantly larger than most of the fine particles in smoke.

Some air conditioners utilize a second filter system to catch more pollutants. The Windmill uses a second filter made of activated carbon, which has adsorptive properties. The activated carbon filter is more effective at filtering out smoke’s dangerous fine particles.

Many consumers are curious about installing a HEPA filter in their AC units. HEPA, which stands for High Efficiency Particulate Air, is the standard in medical settings. HEPA filters entered the public consciousness during the COVID-19 pandemic, when people became concerned with filtering infected air out of their home.

While HEPA filters are good in medical environments, they don’t work with most air conditioning systems. The reason has to do with their MERV rating. MERV (minimum efficiency reporting values) ratings are a standardized system for identifying how well a filter can capture particles between .3 and 10 microns in size. For reference, a particle of wildfire smoke can average between .4 and .7 microns in size. A HEPA filter has a MERV rating 17 or higher, which means it can capture 99.7% of particles between .3 and 10 microns in size.

So why is a HEPA filter bad for your home AC? The highest standard MERV rating for a central AC box filter is 13. A more restrictive filter—especially a highly restrictive HEPA filter—will reduce the airflow and impede the functionality of your HVAC system. If you’re concerned about quality in your home, consult with an HVAC expert to see about installing HEPA-grade filters in a separate air purifier.

Do Air Conditioners Filter Wildfire Smoke?

So now we return to our initial question: do air conditioners filter wildfire smoke? The answer is a heavily qualified yes. Most air conditioners cool and recirculate indoor air. In doing so, the air is run through the air conditioner’s filter, which will remove larger particles. However, as we’ve explained, an air conditioner filter is insufficient to fully filter smoke particles out of the air. Furthermore, since your air conditioning likely only intakes indoor air, the amount of smoke filtration it provides is minimal.

Instead of focusing on whether or not an AC unit filters wildfire smoke, your primary concern should be insulating your home against smoke entering in the first place. As we discussed, there are three main ways smoke enters your home: Natural Ventilation, Mechanical Ventilation, and Infiltration.

If you want to obtain additional smoke protection, consider purchasing an air purifier or filtration device for your home. Many air purifiers or portable air cleaners work with HEPA filters, making them an excellent measure to reduce smoke in your home and improve indoor air quality. If you live in an area that is downwind of fires, invest in N95 respirator masks and take other precautions to limit your smoke inhalation.

When facing a smoke emergency, limit activity which will produce additional fine particles. Smoking, frying or broiling food, and using a gas or wood-burning stove or furnace can all contribute to indoor air pollution. In addition, limit your use of aerosols, and avoid vacuuming unless your vacuum has a HEPA filter attached to it. Even if you can smell smoke, avoid burning candles or incense to cover the odor; these flammables release a large amount of fine particles into the air which limits your ability to filter out smoke.

Remember, wildfires are a danger to those beyond their immediate vicinity. When facing a wildfire or an intense smoke event, follow the directions of local authorities and government environmental agencies. Wildfire smoke is a serious risk, but with preparation and caution, you can mitigate the danger to you and your loved ones.

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