Recently, in mid-July 2026, Canadian wildfire smoke entered Michigan, resulting in the worst air quality I can recall in my lifetime. For two days, we sat in the hazardous range listed within the EPA AQI rating. Everything was foggy, like a normal heavy-fog day, but the air outside tasted like metal and smelled like a distant wood fire, but not the pleasant kind. In addition, we had just come off a heatwave that week, and some of the days were fairly hot during the smoke event.
I had no interest in doing this or writing this, as I do not take pleasure in it. Still, I decided to take advantage of the bad situation and to run indoor air quality experiments to see how outdoor air was affecting my indoor air quality and solar power output.
My first action Wednesday night when the smoke started to descend into Michigan was to shut down my Energy Recovery Ventilator (ERV), think lungs for your house, a system that brings in fresh air and exhausts stale air. The reason I did this was the hazardous level of smoke coming in; I was worried that even my robust MERV 13 filter on the ERV and MERV 16 filter would not filter out the additional pollution a wildfire brings beyond particulate matter like Pm2.5 and PM10.
By shutting down my ERV, I was making a conscious trade-off: I knew I may increaseAirthings monitor in hallway near thermostat showing CO2 spikeshousehold CO2, certainly, and maybe to a minor extent indoor VOCs and humidity, but the trade-off was not letting the nasty wildfire pollution into my home. In addition, I set my heat pump filter (same as your furnace/AC filter) to high recirculation mode because it is a MERV 16 filter designed to filter out wildfire smoke that gets pulled into the house, even with the ERV off.
Now, while I do not have an outdoor air quality monitor to know exactly the air quality directly at my house, I do have several indoor air quality monitors, so I have tracked my air quality before, during, and after this event.
One of the first things I noticed on the first major smoke day was that we used our local ventilation systems, such as a range hood to vent while we were cooking or a bath fan to vent while we were using the bathroom or taking showers.
The first spike on the left was from turning on the range hood to cook breakfast; the 2nd spike was from lunch made in the oven with the range hood on again; and the 3rd and 4th spikes were from running the bath fan during showers.
Spikes in PM2.5 from local ventilation on hazardous level (500 AQI) smoke day show on Awair air quality monitor inside
So basically, what’s concerning here is that one turns on their ventilation systems to remove pollution, but during extreme wildfire events, you’re actually bringing more pollution into the house. My home is about 978 CFM and/or four (4) air changes per hour, which is not the tightest home, but that’s what our current code is here in the State of Michigan. Other states and other green building programs are usually around that level as well. But that’s enough leakage to worry about if you bring on an exhaust-only fan like a constantly running bath fan, which the vast majority of our homes, even newly built ones, and multifamily buildings are using as we speak. It’ll draw in that polluted air through the cracks and crevices in the home, and keep it in there. You will see some more examples of this below.
Now, one thing I didn’t get to experiment with was whether turning off my high-
efficiency filtration system in my heat pump would have caused my particulate pollution numbers to go up, since my home isn’t super airtight. This was because it was very hot out, and so I had to keep my heat pump cooling my house, which meant the filter was running as it should be, so the only way to experiment with that would’ve been to turn off my heat pump and suffer through heat just to see how my house handles smoke without any mechanical systems running. Certainly, if we have a hazardous wildfire event in one of the swing seasons, when I don’t need to be heating or cooling, I could test that out and update.
Speaking of a wish list of things to test, as the hazardous smoke wound down, someone on LinkedIn told me they decided to switch their ERV system to positive pressure to help keep the smoke from entering the home by pushing the air out the natural leaks in all homes and then using their MERV 13 to filter the incoming air. I wish I had tested this more during the hazardous smoke, but I did switch over to this by moving the exhaust dial to 70 CFM and maintaining 100 CFM on the intake during the next few days when we spent most of the time in the EPA IQA unhealthy and very unhealthy range, and it seemed at least effective there.
Now, that being said, our Green realtor friend Wayne, who works in the Chicagoland area, shared an update of his house using his air quality monitor on LinkedIn during the event. He told me he wasn’t home at the time, and his energy recovery ventilation
system was running and drawing in the outdoor air through a MERV 7 filter and only being filtered through the standard low MERV heat pump conditioning system, so you can see from his post that his house quickly became polluted until he shut the system down the numbers went down from the red to yellow and so they were still elevated in higher threshold above what people should be consistently exposed to. He also shared that his house is pretty tight, around 1 air change per hour, showing even in tighter homes some pollution was coming in and it was not being cleaned by a MERV 7 filter system.
Now how does a more advanced Passive House fare? Dale Hulst, who recently built MINet0-1, got to experiment with this as he currently has his home listed on the market but has it monitored.
“The big smoke event was a great test of the house. It performed well, but not as well as I’d hoped given my MERV16 final filter on the Energy Recovery Ventilator. I decided to upgrade the recirculation plenum filter (the system with three duct fans that I added late in the game). I upgraded from MERV 11 to MERV 13, toward the end of the smoke event, on July 20 in the afternoon, and thought I saw improvement, but it was hard to know for sure as the outdoor air was significantly cleaner by that time. We’re likely to see some bad 300+ AQI days again, which will be
a better test.” – Dale Hulst.
Dale also shared with me that this particular Energy Recovery Ventilator has a MERV 13 intake filter ahead of the MERV16 final filter which also includes carbon for removing VOCs from the incoming fresh air.
“My goal is to show the new buyers / owners they can safely run the ERV in a wildfire event. With the fresh filters and upgraded MERV13 in the recirc system, I’d like to think I could run it. Some of the PM2.5 in the house may have been from the little bit of infiltration that gets into the house through the doors, etc. (0.03 CFM/FT2 at 50 pascals per final blower door test). If the ERV is taking out everything intentionally being brought in, my hope is the recirc filter will take out whatever is unintentionally leaking in.”- Dale Hulst.
So just going on beyond a couple anecdotal examples here with myself, Wayne and Dale, where I grow really concerned is when I look back over the last 15 years I’ve been providing consultations to help people at where I have just helped people achieve our building code in Michigan, which is four air changes per hour (4 ACH) and typically includes running a negative pressure bath fan to meet the ventilation standards, which are typically referred to ASHRAE 62.2.
Beyond that even the Green Building Certification programs we help consultant on such as LEED or Green Communities are fairly similar at the lower levels where typically people are just at a bare minimum meeting somewhere between two and four air changes per hour in their home or multifamily units, then having a bath fan run to meet the basic ventilation standards such as ASHRAE 62.2 2010 or maybe 2016 and then putting in a MERV8 filter in the HVAC system.
In all these cases and all these homes that are otherwise designated to meet a new building code or even a green standard, one can infer that people went into their houses thinking they were protecting themselves from the wildfire but heavy smoke at these levels are shown here above with their bath fan that is running and would’ve been otherwise used to keep their air healthy in their house was instead pulling in the wildfire smoke, and the MERV 8 in their HVAC system filter wasn’t enough to filter it but only spread it around. Maybe some of these people were savvy enough to notice and shut down their bath fans, install HEPA filters, or build a DIY Corsi–Rosenthal Box fan at some point, but that likely is not the case for most people who are not thinking about these things.
My main point is that, unfortunately, our current codes and Green Building standards are not protecting us against this new threat, which was probably unforeseen here in the Midwest and Northeast.
I would argue this is an opportunity to reflect on and review our building codes. Look at our Green building programs and significantly increase their requirements while providing the resources available for home renovations in new construction to achieve these outcomes.
The good news is that, even if, let’s say, wildfire smoke rarely came again or ever came again, the benefits of making our homes wildfire smoke-resistant would extend beyond just keeping smoke out to include improved home health, comfort, and energy cost savings.
So what do we need to do?
Airseal, airseal and then airseal again, ideally to Passive House levels
Boost HVAC filtration to MERV 13-16 (but be warned if you do!)
Get Smart; systems need to know air quality and respond to smoke events
Change policy/programs
First and most important, we need to air-seal our homes and reduce leaks, ideally aiming for Passive House standards to the best of our abilities. This can be done easily on new construction, but once the home is built, it gets harder and harder depending on its condition, unless there’s a gut renovation. Beyond keeping smoke out during extreme events, we also have humidity control, which reduces winter dryness and summer/swing-season mold/allergy issues, reduces energy loss, and could lead to improved comfort, especially during extreme heat or cold. Also, in many areas with air leaks, pests can enter the home, and water can get in those areas as well, so targeting air leakage has multiple benefits beyond just smoke.
Next, we need to boost the filtration capacity of our ventilation, heating, and cooling systems. The problem with doing this, though, is that the heavier the filtration is, the more drag it puts on the system’s efficiency due to something called static pressure. With new construction, this can be easily addressed through Manual D, where you properly design the ductwork and include a 4- to 5-inch filter with MERV 16 as part of the total system design and proper install. Then the HVAC is properly commissioned to verify that the installation works and the system is performing. Some HVAC systems even self-commission and can report issues during operations.
For existing homes, this becomes a lot more complicated because in many cases you are stuck with the ductwork you have, as it can be very expensive to get to tearing out drywall, tearing out walls, and tearing out and replacing ductwork; rarely do any of the energy efficiency programs or weatherization pay for this work but getting a good HVAC contractor assess the existing system for performance is still important and improvements can be paid or funded through some programs. You would have someone review the ductwork during a GreenHome Inspection, maybe do duct blaster testing or pressure pan testing to see how leaky it is, and check to see if ducts are panned in the joists. Then, check the length and size of the ductwork, and have the HVAC contractor or assessor complete a static pressure test on the existing system to see what its current pressure is. From there, a plan can be put together that is specific to each property to determine if it’s possible to update to at least a MERV 13 filter, which is what is needed to start filtering out wildfire smoke, or even a MERV 16 without dragging down the static pressure and reducing the life and efficiency of the system. Each home is going to be on a case-by-case basis, depending on the situation, budget, and the homeowner’s or building owner’s desires. If a home is using ductless air source heatpumps (aka mini-splits) to heat and cool, or radiant, you might have to try to do the filtration through the ventilation system instead.
Now for ventilation systems, these are usually a detriment to health and extreme wildfire smoke, but the first thing we need to do is make sure they have thick filters in them and are also designed appropriately with good static pressure without major bends or kinks in the ductwork and lots of 90° or sharper turns. I haven’t seen too many ventilation systems with MERV 16 on them other than Dale’s noted above, but I know many like what I have; the Panasonic Intellibalance one has at least a 13 on it, which helps. But more than this, we need smart ventilation and demand-controlled systems that can automatically: sense bad air, shut down, and recirculate, as most people aren’t thinking about these kinds of things on a daily basis and want to rely on the system to ensure their safety before it is too late. This would mean that the ventilation system is communicating with an outdoor air quality monitor on the house, in the area, or, if necessary, with the general EPA outdoor air quality data. Then the system would also monitor indoor air quality and either shut down the intake based on how bad it gets outside and what’s being sent inside, or maybe even throttle it back. Hence, it was a balance between some ventilation and reducing higher levels of particulates in the house. But as I said, if you’re relying on your ventilation system to filter the air because your HVAC is older and can’t have the updated filter, or you’re using mini-splits or radiant heating and cooling, then you’ll want to rely on your ventilation system to shut down, but still provide filtration for the home, and I’ve never seen any system do that short of a Build Equinox CERV unit which has the ability to detect the outdoor water wildfire smoke and do just that shut down from being a ventilation system to just a filter system which then we’ll filter the air in the home only for any of the wildfire smoke that migrate in through the cracks and crevices.
To be fair, this can all be expensive, so in the short term, what we highly recommend is a low-cost air quality monitor that can review particulates, and then a DIY filter fan that you can build a couple and set up in a room during emergencies.
But moving forward, I think it’s imperative that our codes and green building programs change, as well as our incentive structures, to help fund these kinds of advanced upgrades during Home retrofits, or especially during new construction, where it’s a lot easier to take care of.
So I promised to talk about solar power. Certainly less of a worry because the majority of people don’t have solar power, but we all breathe; if you do, or you’re considering it, wildfire smoke is going to be a detriment to solar power output just the same.
Solar before smoke on a typical July day
Now, thankfully, I had one really nice full sunny day right before the wildfire smoke event, and then during one of the most hazardous days, the weather told me, at least according to the weather, I couldn’t see because of all the smoke that it was truly a full sunny day without any clouds. But otherwise, most of the smoky days we had also had clouds, so that skewed some of the data, but if you look at the sunny day right before the event and the sunny day during the event here, you’ll see all in all we had a 17 kW decrease in solar
percent again from extremely rare hazardous smoke output or otherwise a 27%
change.
This similair % drop is confirmed by Dale Hulst of MINet0, who also confirmed his solar data with me; you can see on average his home peaks at 70 kW and on the smokiest yet sunniest day (same as mine) he dropped to 50 kW, so 28%, pretty close and his system is a bit bigger and all south facing so of course the bigger they are, the harder they fall or state larger systems will see larger % drops.
So overall, unfortunately, as more wildfire smoke is forecasted to occur. we will also lose energy output with solar power, ironically, as solar is meant to reduce carbon emissions and help prevent climate change, which worsens wildfire smoke in forest fires. The good news is, despite the smoke output, I still was able to stay off grid most of the day and most of the peak energy usage time, and the other good news is the smoke will depress the heat from the sun, which is another thing that diminishes solar panels: extra heat.
someone on LinkedIn told me they decided to switch their ERV system to positive pressure to help keep the smoke from entering the home by pushing the air out the natural leaks in all homes and then using their MERV 13 to filter the incoming air. I wish I had tested this more during the hazardous smoke, but I did switch over to this by moving the exhaust dial to 70 CFM and maintaining 100 CFM on the intake during the next few days when we spent most of the time in the EPA IQA unhealthy and very unhealthy range, and it seemed at least effective there. 
