How to Ventilate Your House

Every day we learn more about the importance of good air quality. Here are some tips to help you improve air quality inside your house.

How to Measure

First, you are going to want to be able to measure air quality in your house. There are a few different things you want to measure:

  • PM2.5 measures the amount of particles that are 2.5 micrometers or smaller. Compared with larger particles, PM 2.5 does the most damage because they are so small that they can get embedded far into your lungs, which is why they are measured and tracked. You want to keep this value as low as possible.

  • AQI is an aggregate index that measures PM 2.5, PM 10 (10 micrometer particles), carbon monoxide, sulfur dioxide, and nitrogen dioxide in the air. The formula for calculating it is complicated but basically takes the largest value out of those. You want to keep this value below 50.

  • Carbon dioxide builds up in a stuffy room with no ventilation and can affect cognitive performance, make you sleepy, and have other bad side effects. 400 is the ambient air; a value over 1000 is bad.

To measure PM2.5 and AQI, I like the Temtop M10, though it's very easy to buy a more expensive piece of gear that you can connect to Wifi, or smart devices.

To measure CO2, I like the Aranet4, which can run forever on a single battery.

If you don't want to buy them, try to borrow one from a friend, or see if your library will buy them and loan them out; you'll get a sense for the ranges of air quality in your house after a few days.

When it's nice outside

The easiest case is when the weather is nice outside, and there's no fires or smoky air, and you don't live near a freeway. Open your windows! Bringing in outside air is the only way to lower CO2 counts (you can't just run an air purifier).

The best way to ventilate quickly is to open windows on both sides of your house, called "cross ventilation," so a breeze can flow from one end to the other. A good sign that this is working is that doors between the two windows are slamming shut.

If it's not nice outside

Sometimes you can't or shouldn't keep your windows open - it's too hot or too cold, or the air quality outside is too bad. If you live within 1000 feet of a freeway, you are likely breathing in polluted air almost all the time you are outside or have your windows open, which has been linked to lower birth weight, worse cognitive performance, higher rates of respiratory illness.

You are going to want to pump in outside air anyway. If it's hot or cold, you can use your AC, or your heat, which will take care of this for you, by swapping in new air and swapping out old air.

If you have either of these and can't or don't want to change the temperature, they should have a "fan" mode, which does not apply heat but will just bring in new air. You should be running this mode almost all of the time you have your windows closed.

Your heat or your AC will have a filter between the outside air and the fan. Mine is 16x25x1, and looks like this:

Filter for heating

You need to buy a good filter. Filters are rated on the MERV system, which sorts by how much particles they are supposed to catch. Your average filter will be about MERV 5. You want to buy a MERV 13 level filter, which is only a little bit more expensive but is going to be vastly better at stopping particulates.

Plan to replace your filter every 2-3 months, more if you run the fan more often, less if you run the fan less often.

You can supplement outside air with an air purifier like the Coway AP1512-HH. Each air purifier comes with a rating for how big of a room it is designed to clear; this one is for 360 square feet. When it ships to you, the filters are already in the purifier, but covered in plastic. Be sure to remove the plastic or the filters won't work.

I keep my air purifiers (we have four) on the lowest level all of the time. Note that these will be ineffective at lowering carbon dioxide levels; you need outside air for that. Plan to change the odor filter every 6 months and the HEPA filter once a year, more often if you live in a particularly polluted area.

Sources of bad air inside your house

The primary source of bad air inside your house is your stovetop. When we cook, the AQI inside our house spikes up above 300, which is worse than in Beijing. If the fan above your stove vents to the outside, you almost always want to run this at the highest level, in addition to opening your windows and doors, and running your house fan, in order to bring the air quality back to a decent level as quickly as possible.

Other sources of bad air include cleaning products and using the toilet. Understand if bathroom fans vent to the outside (they should, if your house is up to code) and run them. Open windows when you clean.

Your stove, your oven, and your shower will also make your kitchen hot, which can be uncomfortable if you are trying to keep the windows closed. On hot days consider using the BBQ instead, if you have one.

That's it

Modern homes in particular are designed not to leak any air if you don't want to. While this is good news for efficiently heating or cooling your house, it is bad news for air quality inside. Measure air quality inside your house and understand which tools are best for bringing in better air.

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