When I first started measuring my indoor air quality I was shocked to see the CO2 would get sky-high: over 4,000 ppm overnight in the bedorom.

I quickly noticed that times when I felt groggy or couldn't think quickly were tightly coorelated with high CO2 levels.

That makes sense: each cell of a body needs oxygen (O2) and produces carbon dioxide (CO2) as part of generating energy. If your cells don't have enough O2 or if there's too much waste CO2 around then your cells' functions grind to a hault. This is why you'll die if you stop breathing. You'll also die if you breathe the same air over and over because all of the O2 in that air will get replaced with CO2. The larger volume of air you're breathing the longer it will take to suffocate. In the room you're in right now you might be suffocating so slowly that you don't notice it because the higher the CO2 in your environment gets, the worse you function.

Here's what I tried and what finally worked to consistently keep CO2 below ~800ppm in my whole house.

More Plants? #

My first thought to lower CO2 was just adding more plants. Plants consume CO2 and produce Oxygen, right? Like, that's where all the CO2 goes and where all the O2 comes from.

Turns out I'd need about 700 plants to offset the CO2 from each person and that's pretty impractical.

Air filters? #

Adding air filters crossed my mind momentarily. Air filters clean the air and CO2 kinda feels like a dirty thing in the air so maybe an air filter would help?

But nah—CO2 is a really tiny gas that just goes right through air filters. Air filters are good for keeping pm2.5 (the measurement of dust particle concentration) down but they don't do anything for CO2.

Run the air conditioner to bring in more fresh air? #

My next thought was to run the air conditioner more. At the time I was in an apartment with a window AC unit and I thought when it ran it was bringing in air from outside, so if I just run it more it would bring even more air in from outside, right?

Turns out air conditioners don't bring in air from outside: they just move temperature. So they bring in air from outside, heat it up (or cool it down), and dump it back outside and then separately they bring in indoor air, cool it down (or heat it up), and dump it back inside.

Later when I'd moved into a condo with my wife I found that turning on the whole-house fan in our AC system did seem to lower CO2 overnight but I think that was just from mixing the air around—fresh air was probably just slowly leaking in through door and window cracks and added up over the entire house that was enough to keep a reasonable air exchange for three people.

Open a window? #

When the weather is nice, opening a window is the best way I found to lower indoor CO2. To lower it even faster I got a window fan to blow air in from outside and push air out from inside. I used to have really bad allergies in Dallas so I taped a HEPA filter to the window fan and that seemed to keep pollen out and the air fresh. If I did an air flush about once an hour I could keep CO2 below 800ppm pretty consistently. Success!

But what about in the Dallas summer when it's over 100ºF outside? Winters don't last very long but opening a window is also impractical when it's too cold outside.

How do they keep CO2 low on submarines? #

I also thought I could use the same system submarines use to keep CO2 low and O2 high in enclosed spaces. From a YouTube video called How Do Nuclear Submarines Make Oxygen?- Smarter Every Day 251 I got to learn how these systems work. Submarines use electrolysis on purified water to make O2 and H2. If O2 gets too low or if power fails the submarine crew can burn a candle made of Iron (Fe) and Sodium Chlorate (NaClO3) which liberates Oxygen. To scrub CO2 from the air submarines blow air through monoethanolamine (MEA) which absorbs CO2. The CO2-rich MEA can then dump it's CO2 into the ocean water.

The submarine solution to keeping CO2 low seemed unnecessarily involved and expensive for what I was trying to do. I have the advantage of being surrounded by high-O2 and low-CO2 air right outside! What I needed to figure out was how to get fresh air in and then immediately heat or cool it.

Energy Recovery Ventilators (ERV) #

Turns out there's a magical machine called an Energy Recovery Ventilator (ERV). An ERV takes in outdoor air through one side and indoor air through another rubs the two airstreams together in a box with hundreds of thin paper membranes. The thin paper membranes let the two airstreams exchange temperature and humidity.

In the summer in Texas the indoor air is typically around 75ºF and the outdoor air can be as hot as 110ºF and very humid. Outdoor air coming in through an ERV would recover about 70% of its temperature and humidity so incoming outdoor air 35ºF hotter than indoor air would come in through an ERV only 35*0.7= 10.5ºF hotter (which would be 75+10.5= 85.5ºF in a typical Texas summer). An ERV works just as well in the winter when you want your indoor air to stay hot and moist. An ERV is like an open window that brings fresh air in very quickly but temperature and humity very sowly.

I've only ever seen ERVs designed to be installed as part of an HVAC system but I figured out a way to install them in any room with a window. You'll need a piece of foam insulation, a few feet of insulated air ducts, and some duct tape. First cut the foam insulation to fit in a gap in your open window. Then cut holes in the foam for the ducts and tape the ducts in the holes. Next tape the foam with the ducts in your window.

Finally connect the outdoor air input and output streams from the ERV to the ducts. I've used both the Panasonic WhisperComfort and the Panasonic Intelli-Balance 100. These machines are overpriced for what they are (a box, two fans, and an ERV core) but I haven't yet found a better solution.

If you're building a house you can ask your HVAC guy to install an ERV as part of your HVAC system. If you have a house with attic or crawlspace access you can DIY it pretty easily: you just need to exterior penetrations (one for incoming outdoor air and another at least 10 feet away for outgoing indoor air), a power outlet, some ducts, and two (or more) interior vents for incoming and outgoing air.

If nobody else makes a standalone window ERV unit I'll probably have to start a KickStarter for one or something. If you'd be interested in that let me know at 𝕏.com/cgenco.

Now I can keep CO2 below 800ppm in my whole house year-round for minimal energy (an ERV is just two fans) and maximum comfort (temperature and humidity stay balanced).

Other dimensions of indoor air quality #

On this journey I discovered there's more to indoor air quality than just keeping CO2 low: temperature, humidity, pm2.5, and VOCs are all important to measure and keep at healthy levels.

I wrote about each dimension of indoor air quality at gen.co/air and I was on a podcast in 2024 talking about indoor air quality with all the same info.

Also if you want me to make a KickStarter for a window ERV unit let me know at 𝕏.com/cgenco.