The Insider’s Guide to Cleaner Indoor Air
Tags: Bay Area, San Francisco, Air Quality, Technology, Lifestyle

The Insider’s Guide to Cleaner Indoor Air

Photo: oatawa/iStock/Getty Images Plus

Residents of the West Coast have been experiencing a continuous onslaught of air pollution unlike any we’ve previously seen. For many of us, we are trapped indoors with generic recommendations to “stay at home,” “keep doors and windows closed,” and “reduce outside activity.” But is that truly enough?

I called Beijing home for many years, which is one of the most polluted cities in the world, and have lived through multiple weeklong hazardous air events both there and here in the states. Through the years, I learned a lot about how air quality works and have evolved my thinking on how to best improve in-home air quality.

Just a normal hazy day on the Great Wall of China. Photos from this point: Fred Chang

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Staying indoors is the least you can do

If you own a cheap, small particulate pollution (PM2.5) meter, you probably know that most indoor environments only reduce the concentration of particulate pollution by between 10%–25%. While that’s definitely better than nothing, it’s not much solace if concentrations are at the unhealthy >100 micrograms/cubic meter levels that the Bay Area has been experiencing for weeks.

Assess your indoor space

Is it big? Does it span multiple floors? Are individual “air spaces” sealed off from other areas or is it one big open-plan kitchen and living area? Where do you and your family spend most of your time?

The answers to each of these questions will determine what strategies you should use for improving the air quality in that space.

Assess your risk

Do you live with people who are at high risk of respiratory distress or have a history of asthma? Does the air pollution level cause you to have a sore throat or headache? It might be worth spending more money or energy on this if you fall into a more sensitive category.

The quick and easy way is to buy the biggest, most expensive air purifiers and place one in each room in your home. That’s for people who can afford it. For the rest of us, I present to you some thoughts gleaned from many years of experimentation.

Just a 340 microgram day in Beijing…

Step 1: Buy/borrow a cheap air quality monitor so that you know what is happening as you make improvements

There’s no way to tell whether your improvements are working unless you have one of these. If you can’t buy one (say, because they’re out of stock), borrow one from a friend when you are making adjustments — you only need it for a few days to figure out what’s working and what’s not.

I should probably add some more insulation here…

Step 2: Reduce the dirty air coming in

This one is pretty basic — if you have very old or very leaky doors and windows, spend a couple dollars on window/door “insulation foam tape.” It’s about $10 for a few yards, and it’s easy to apply. You can find it at your local home improvement store or on Amazon.

To apply it, look for places where you can see outside light streaming in or feel outside air leaking into your home. Apply insulation tape to make the door/window close more seamlessly.

Some people choose to tape up their windows to reduce the infiltration of polluted air. You can do that, too — just keep in mind that you will get carbon dioxide buildup if you do that, so you will need to periodically air out your space.

Step 3: Use existing central heating/air conditioning or buy air purifiers

This is where the real money comes in.

If you have central heating/air conditioning that can be run on “fan-only” mode, the first thing you should do is ensure that you have it equipped with a filter with the highest MERV rating that your furnace can suitably handle.

Note that higher MERV ratings mean more particles trapped, but also less airflow — and potentially more stress on the fan/blower of your central air system.

Keep in mind that MERV 14+ is probably not feasible for most home systems. The ratings specify “the % of particles of a certain size” that are removed in one pass. The important thing here is that a single-pass removal of all particulates is NOT what we are aiming for. We are trying to remove the highest percentage of particulates over time. In some cases, having a lower MERV rating might give a better outcome because more somewhat-filtered air is reaching all areas of your home.

Indoor PM2.5 concentration vs. outdoor PM2.5 concentration on a Xiaomi Air Quality Monitor.

How do you measure success?

To know whether your setup is working or not, you should measure the air quality outdoors, and then measure the air quality at various points in your home very quickly thereafter.

Do this both before and after swapping the air filter in your furnace/air conditioning and compare the results. If your new filter is doing its job, you should see the indoor air quality levels improve, as indicated by lower readings on your air quality monitor.

Keep in mind that it often takes 10–20 minutes for filtered air to reach various points in your house.

What if I don’t have central air?

Not to fret — I don’t have central air either. In this case, your air system is relatively simplified: You just need to consider how much space you have and how much you are willing to spend on air purifiers to filter it.

If you have a small space (say, 300 square feet) and a shoestring budget, check out my previous article, which has some DIY instructions for strapping a furnace filter on a fan.

This air purifier is basically a bathroom exhaust fan with a filter mounted on it — roughly $30 in China. By the way — this is when you know to change your air purifier’s filter — when the output is dirtier than the incoming air…

If you want to spend some money, check out Wirecutter’s recommendations (the best brands to look at are Coway, Blueair, Levoit).

Notes to keep in mind when purchasing an air purifier:

There is a component of “tolerable noise level” for all of these filters. The larger the fan, usually the quieter it will be to move the same amount of air. Of course, the trade-off for silence is space used and replacement filter size (and cost). Each filter has a certain area it will cover (at a certain fan speed) — the largest typically cover around 500 square feet. In Beijing, I used to use five or more small air purifiers positioned at various spots throughout the house instead of one large one. A HEPA filter is not really that important. Similar to the point above around MERV ratings on furnace filters, you don’t want the most nonporous filter after one pass — you want the most cleanly filtered air over time.

When should I run my air purifiers?

The Wirecutter editors recommend running air purifiers 24/7, but personally, I don’t think that’s the right approach. My general experience is that you should turn on air purifiers for the spaces that you are in, and turn off air purifiers in spaces that you aren’t using. That way you can save on energy costs.

If you’re willing to put some elbow grease into better air quality…

Then you’ll want to do something similar to what Tesla’s Model X does in “Bioweapon Defense Mode” — create a positive pressure environment.

Warning: This is not for the faint of heart. But the results are amazing.

Conceptually, the way to do this is by taking dirty air from outside your home, piping it into a filter, and then forcing that air into your indoor home airspace to seal up all the small cracks where bad air is coming in. In China, many companies offer off-the-shelf installations of self-contained “fresh air” units — yes, including drilling a hole in your wall!

In the U.S., you may have to get a little more scrappy by building something similar to what I’ve pictured below:

Portable AC duct (taking in air from a window adapter) + Xiaomi air purifier (with a hole cut out for the AC duct)

Home air quality remediation, expert mode.

With this setup running on medium fan speed in a 1,000-square-foot space or larger, I get a sustained 90% reduction in particulates with that single air purifier.

Hope this information helps some of you breathe easier. Leave me a comment if you have any questions.