This post originally appeared on the Green Building Advisor website.
I’m pretty much a nerd; I own four different indoor air quality monitors plus a radon monitor and portable CO2 monitor. I have the EPA AirNow app downloaded on my phone, and I’m considering purchasing my own outdoor IAQ monitor, just to finish off the collection. The past few days, they’ve all come in handy.
Canadian wildfires have moved across my home state of Minnesota, blanketing the entire region with a visibility reducing haze and the strong odor of something similar to a campfire, but just a little different. We have spent the last three days under an air quality alert with one more day of the choking stench to go.
Wildfire smoke and the risks to our health
Wildfire smoke is at a minimum an irritant and can cause serious health problems for those at higher risk. Some of the pollutants present are small particulate matter (PM2.5), carbon monoxide, volatile organic compounds, formaldehyde and other gases and chemicals. Prolonged or repeated exposure can cause chronic respiratory diseases and a whole host of other heath issues. The EPA has a lot more information on the topic which can be found on their website.
The biggest problem with wildfire smoke is PM2.5. These very small airborne particles can get stuck deep in our lungs, with some of the smaller particles bypassing our lung tissue and ending up directly in our bloodstream. The EPA recommends an annual average exposure limit of no more than 9 µg/m³ and a 24-hour average exposure limit of 35 µg/m³, which is not to exceeded more than once per year on average over a 3-year period. The World Health Organization suggests even lower levels. The good news is PM2.5 can be monitored and filtered inside the home.
How the outside can get inside
My home is leaky, around 9 ACH50, which means somewhere in the neighborhood of one-quarter to one-half of the volume of air inside my home is being exchanged with outside air every hour. As you might imagine, some of the PM2.5 particles are finding a way into my home.
A tighter home has an advantage, natural air exchange will be limited as long as all the windows are closed, and exterior doors are not opened frequently.
Other ways for the outside to get inside is by balanced and supply only ventilation strategies, they should be turned off or highly filtered. Balanced and supply ventilation use fans to force outdoor air into the home. I would also limit the use of exhaust ventilation, the use of bath fans or kitchen range hoods, oh, and vented dryer, all of which cause a home to become negatively pressurized. To equalize the pressure, outside air finds deficiencies in the building envelope, entering the home through those cracks and holes.
Monitoring IAQ
My IAQ monitors, far left is the AWAIR Element, next is the AirThings View Plus. The middle monitor is Qingping Air Monitor, and then the CPS IAQPRO, which I use during energy audits and building investigations. The last meter is the Radon Eye.
The only way to know the levels of PM2.5 (and other potentially harmful gases and chemicals) in the home is to monitor. Like I mentioned at the beginning of this article, I own 4 air quality monitors, of which 3 are operating in my home -the fourth is part of my energy audit/building investigation tool kit. Unless you have several monitors, I recommend monitoring the areas in which you spend the most time. My monitors are located in my bedroom, in the living room/kitchen area, and in my office. During this latest bout of wildfire smoke, these were my PM2.5 readings:
This first PM2.5 reading is from my basement office and taken with an AirThings View Plus. The 24-hour readings ranging from about 17 µg/m³ to nearly 30 µg/m³. Typically, my office’s PM2.5 reading is 2-3 µg/m³.
The next reading is from the living room/kitchen area, this meter is a Qingping. PM2.5 reading were as high as 31 µg/m³. Again, usually the reading in this area is in the low single digits. My significant other and I spend much of our evenings in this area, she is sensitive to poor air quality, mostly allergies, but she doesn’t feel well when wildfire smoke is present.
The last reading is from the AWAIR Element, which is located in the bedroom. This room had the lowest PM2.5 reading in the home, bouncing between 0 and 2 µg/m³. There’s a reason why this area has such a low reading, which I will get to in a bit.
Improving IAQ during wildfire periods
Once I knew I had elevated levels of of PM2.5 in my home, I was able to take some action to greatly reduce the problem. We were already trying to limit the number of particles from the wildfire by closing the house up. We closed and locked all the windows, tried to limit opening doors to the outside and did not turn on exhaust equipment. With my home being as leaky as it is, that only helped so much.
The primary strategy to improve IAQ during periods of poor outdoor air quality is to filter the indoor air. There are a few methods that could work, one is to install a really good furnace filter, something in the range of MERV 13 and operate the furnace fan continuously. There are a few drawbacks with this strategy. The first being that operating a furnace fan continuously during periods when air conditioning is also ongoing creates a reduction in the effectiveness of the AC to dehumidify the home. The furnace fan pushes much of the condensate that the AC has removed back into the indoor air.
Another issue is that many HVAC duct systems aren’t designed to handle the resistance of air moving through a MERV 13 filter. This type of filter is great at removing indoor pollutants but restricts the air moving across the ACs coil, which can result in the coil freezing up. I’ve had this happen in my home when I try to use filters with high filtration levels.
One final issue with using a furnace and filter to remove indoor pollution, some furnace fans use large amounts of energy. I’ve tested some that use in excess of 700 watts. In my market, this can add $75 per month to an electric bill.
Instead of operating my furnace fan to filter the air inside my home, I chose to build a couple DIY filtration systems using an inexpensive box fan and filters that can be purchased at any big box store.
The first air filter is called the Corsi/Rosenthal air cleaner. I’m not going to get into how to build one, but Allison Bailes did in this GBA article from 2020. I built mine last summer for about $100. My unit operates continuously in my bedroom and is the reason for the low PM2.5 (0 to 2 µg/m³) readings while the rest of my home has elevated levels.
I used a simpler DIY filter system to help lower the level in my living room/kitchen area. I simply taped a 20 in. x 20 in. x 4 in. pleated filter to a 20 in. x 20 in. box fan. The filter is a HDX FRP 9 (the equivalent of about a MERV 12) I purchased at Home Depot for about $35. I have heard that using a box fan with a filter taped to the intake side can put a strain on the fan motor, my hope is the larger 4 in. filter will help reduce the wear and tear.
One final option is to simply purchase a manufactured air filtration device. I have to admit, these do look much better than the DIY versions. They are also much more expensive and often rely on proprietary filter replacements, increasing the maintenance costs.
The DIY single filter and box fan worked
After starting the new box fan and filter combination, the PM2.5 levels in my living room/kitchen dropped from the high 20s µg/m³ to the high single digits over the course of about 4 hours. It’s been maintaining those levels even though the outdoor PM2.5 levels, according to the EPA AirNow app, have been around 60 µg/m³ (there is a formula for converting the AQI number the app provides to an estimated concentration of PM2.5 in µg/m³).
These affordable DIY filtration systems should provide a few years of air cleaning during wildfire events. Hopefully I’ll complete my renovation work, which includes tightening the envelope, the addition of an ERV and pre-filter system, and an update of my heating and cooling ductwork. Whether these steps are enough to maintain healthy levels for indoor air remains to be seen, but until then, I’ll operate my DIY air filter systems.