I just learned some important things about indoor air quality after watching Why Air Quality Matters, a presentation by David Heinemeier Hanson, the creator of Ruby on Rails. It seems like something that is both important and under the radar, so I'll brain dump + summarize my takeaways here, but I encourage you to watch the whole thing.

• He said he spent three weeks researching and experimenting with it full time. I place a pretty good amount of trust in his credibility here, based on a) my prior experiences with his work and b) him seeming like he did pretty thorough research.
• It's easy for CO2 levels to build up. We breathe it out and if you're not getting circulation from fresh air, it'll accumulate.
• This has pretty big impacts on your cognitive function. It seems similar to not getting enough sleep. Not getting enough sleep also has a pretty big impact on your cognitive function. And perhaps more importantly, it's something that we are prone to underestimating. It feels like we're only a little bit off, when in reality we're a lot off.
• There are things called volatile organic compounds, aka VOCs. Those are really bad for your health. They come from a variety of sources. Cleaning products and sprays are one. Another is that new car smell, which you don't only get from new cars, you also get it from stuff like new couches.
• In general, when there's new construction, VOCs will be emitted. That's what lead to DHH learning about this. He bought a new house. His wife got sick. It turned out the glue from the wood panels was emitting VOCs and making her sick.
• People in the world of commercial construction know all about this. When a hotel is constructed, they'll wait eg. a whole month, passing up revenue, to let the VOC's fizzle out. But in the world of residential construction, for whatever reason it isn't something people know about.
• If you want to measure stuff like CO2 and VOCs, professional products are expensive, consumer products are usually inaccurate, but Awair is consumer product for $150 that is good.
• If you want to improve indoor air quality, air purifiers are where it's at. They do a good job of it. You could use filters on eg. your air conditioner and stuff, but in practice that doesn't really work. High quality filters make your AC much less effective. Low quality filters are, well, low quality.
• Alen is the brand of air purifier that DHH recommended after testing four brands. I spend about 10-15 minutes researching it. Alen seems to have a great reputation. The Wirecutter doesn't recommend Alen seemingly because you could get similar quality for about half the price.
• I decided to purchase the Alen BreatheSmart 75i today for $769. a) I find it very plausible that you could get similar quality for less money, but since this is about health and it is a long term purchase, I am happy to pay a premium. b) They claim they offer the industry's only lifetime warranty. For a long term purchase, I think that's important, if only due to what it signals.
• I considered purchasing more than one. From their website it seemed like that's what they recommend. But after talking things through with the saleswoman, it didn't seem necessary. The product weighs about 20 pounds and is portable, so we could bring it to the bedroom to purify the bedroom before we go to sleep.
• I currently live in a ~1000sqft apartment and was initially planning on purchasing the 45i instead of the 75i. The 45i is made for 800sqft and 75i 1300sqft. The saleswoman said it's moreso a matter of time than ability. The 45i will eventually purify larger spaces, it'll just take longer. That'd probably be fine for my purposes, but since this is a long term purchase and I don't know what the future holds, I'd rather play it safe.
• The Alen BreatheSmart does have an air quality sensor, but I decided to purchase an Awair as well. a) The Alen doesn't detect CO2 levels. At first I was thinking that I don't really need a CO2 sensor, I could just open the window a few times a day. But ultimately I think that there is value in having the sensor in my office. It sends you a push notification if CO2 levels pass whatever threshold, and I think that'd be a solid step up from me relying on my judgement and presence of mind to open windows. b) My girlfriend has been getting a sore throat at night. I think it's because we've been using the heat more and the heat dries out the air. We used an air purifier last night, but I think it'd be useful to use the Awair to make sure we get the humidity level right. (We do have a Nest thermostat which detects humidity, but it's not in our bedroom.)
• In general, I'm a believer that health and productivity are so important that on the order of hundreds of dollars it isn't worth trying to cut costs.
• Air quality is something you have to pay attention to outside of your house as well. The presentation mentioned a study of coffee shops having poor air quality.
• Older houses have a lot more draft so air quality wasn't as big a problem. But newer homes have less draft. This is good for cutting your electric bill, but bad for air quality.

Added:

• Cooking gives off a significant amount of bad particles, especially if you have a gas stove.
• You are supposed to turn your vent on about five minutes before you start cooking. Most people don't turn it on at all unless it smells.
• Apartment kitchens often have vents that recycle air instead of bringing in fresh air, which isn't what you want.
• If you're using a humidifier, use distilled/filtered water. If you use water from the sink it will add bad particles to the air.
• I've found that random appliances like the dish washer and laundry machines increase VO2 and/or PM2.5 levels.

Update:

• I decided to return my Alen air purifier. a) It doesn't really do anything to reduce CO2 or VO2. b) It does a solid job of reducing PM2.5, but I have found that if I'm having issues with it I resort to opening my window anyway. That may change when it gets hot outside. But I'm planning on buying a house soon, and when I do I'm hoping to install stuff into the HVAC system instead of having a freestanding purifier. And if I do need a freestanding air purifier, it seems to me now that a ~$300 one would make more sense than the ~$800 Alen.
• The Awair I could see not being worth it for some people, but I'm still happy with it. You'd think that you could purchase it, figure out what things trigger your air quality to get screwed up, return the Awair, and moving forward just be careful about opening a window around those triggers. But I've found that random things screw with the air quality that I'm not able to predict. Plus it provides me with a peace of mind that makes me happy.