Air filtration: One easy trick to increase your life expectancy 2 months

Recommendations for air purifiers/filters and air quality in general

Dec 29, 2021

Making your air cleaner with filtration and ventilation provides huge long-term health benefits by reducing particulate pollution, which causes increased risk of cardiovascular disease, diabetes, respiratory disease, and other chronic health problems, and globally kills an estimated 3-10 million people per year.

And it’s also easy to address, making it arguably the easiest way to improve your health and extend your lifespan. For an average person in the US, you can extend your healthy lifespan by about 2 months just by using air purifiers or better HVAC air filters in your home.

It also greatly reduces the transmission of airborne pathogens including Covid-19, flu, etc. It provides a ~4x Covid risk reduction - for comparison, that is more effective than a surgical mask, without the hassle or discomfort. This is a great risk reduction strategy for your home or office, and I view this as the most underused Covid mitigation - and it also protects against other airborne disease including the cold and flu.

Here's what I recommend (basically taken from dynomight):

Key things to do, roughly in order of priority:

Filter your air:
- For individual rooms: Use HEPA air purifiers and run on medium 24/7. If you want a recommendation, the Coway AP-1512HH Mighty is great.
- For your whole home: Change your HVAC air filters to MERV-13 filters.
Ventilate when cooking - use a range hood or at least open a window.
Wear N95 or better masks in dirty air (and in crowded indoors places for Covid). This includes when outdoor air quality is bad due to e.g. wildfires, but I've also started wearing masks when my cooking creates a lot of smoke, and when taking the subway (since underground subways have terrible air quality). See my masks guide for recommendations.
Check your local outdoors air quality online with AirNow. Consider getting an air quality monitor to check your indoors air quality. See Wirecutter recommendations.
Put a HEPA cabin air filter in your car.

Key things to avoid:

Don't use ultrasonic humidifiers - use evaporative or steam humidifiers instead
Don't smoke (obviously)
Don't use incense, and don't blow out candles - extinguish them with a lid or avoid them entirely
Reduce indoors usage of aerosols

https://dynomight.net/air/ is a great explanation of why air quality matters and what you can do about it personally, and quantifies the harms and the impacts of these interventions. Because that article does it so well, I won’t go into that, and my article will focus on specific recommendations on how to filter your air.

Air purifier recommendations
HVAC recommendations
Ventilation
Public policy thoughts

Air purifier recommendations

The 8 Best Air Purifiers 2021 | Reviews by Wirecutter

Standalone air purifiers are a cheap and easy way to improve air filtration in your home or office.

Top recommendation: I recommend the Coway AP-1512HH Mighty (also called Coway Airmega) as the best air purifier model overall - it has great filtration, enough airflow for medium sized rooms, is quiet, and is by far the cheapest and most cost-effective air purifier for medium and larger rooms (including initial purchase price, filter replacements, and electricity).

For small rooms (about 100 sqft or less) or tight budgets, the IKEA Förnuftig is a good option - it’s the cheapest air purifier for small rooms.

How many: Use one in each room that you spend a lot of time in (e.g. bedroom, living room, office). Each Coway Mighty is good for about 200-300 sqft of space, so I recommend more than one for large rooms, but one is a lot better than none.

How to run them: I recommend simply running air purifiers on the medium setting 24/7. Running them 24/7 reduces risk of Covid as well as numerous long-term health problems caused by ambient particulate pollution, and they are pretty quiet on medium.

Other models: See my Air purifier models comparison for a review and comparison of other top models.

https://www.nytimes.com/wirecutter/reviews/best-air-purifier/ and https://www.tomsguide.com/best-picks/best-air-purifiers have extensive reviews.

In general, look for a HEPA or HEPA-like air filter that provides enough airflow for your room. Avoid "air purifiers" that are based on generating ozone - ozone is a pollutant so these are harmful and worse than useless.

Cost: The Coway Mighty costs about $200 up-front (typically varies from $180 to $230). Since it can filter about 200-300 square feet of space, that’s less than $1 per square foot. Annual operating costs are about $50 to run on medium 24/7, for filter replacements and electricity. This is by far the cheapest compared to other air purifier models, both per air purifier and per square foot. The total cost over 5 years averages to about $75 per year.

Maintenance: You should clean off the pre-filter monthly - vacuum it or wipe it off. And you'll typically need to replace the filters every 6-12 months. The filters do still work a lot longer than this according to Wirecutter, so it’s not that big of a deal if you’re late on the replacement schedule.

Tips for the Coway Mighty specifically:

New filters sometimes have a strong smell that can last for about a day.
Replacement filters: Official filters or cheaper 3rd party filters from Cabiclean or Durabasics. In my personal experience so far I prefer Cabiclean. These 3rd party filters are tested and recommended by Wirecutter, they fit well and filter just as well as the official filters, but there are Amazon comments about poorly fitting filters, so I think the quality control is not as good. I use the third party filters and they’ve generally fit well so far, but if I noticed a poor fit (gaps on the side) then I’d return it.
The filter replacement schedule is the HEPA filter (the big white filter) every 12 months and the odor filter (the thin black filter) every 6 months. But it’s typically ok to use filters longer than that, they don’t lose effectiveness very fast.
Leave the ionizer feature off - it’s a waste of electricity (see below).
You can turn off its display lights by holding down the Ionizer button for three seconds.

Sizing: Make sure you pick an air purifier (or multiple) that's large enough for each room you want to filter. Each model has a Clean Air Delivery Rate (CADR) which for HEPA filters is basically the amount of clean air it outputs per minute in Cubic Feet per Minute (CFM). You want to aim for enough filtration to clean the volume of air in your room every ~12-15 minutes - in other words, at least 4-5 Air Changes per Hour (ACH). To calculate this:

See the HEPA Filter Calculator near the bottom of https://www.microcovid.org/blog/hepafilters to calculate what CADR you need based on your room's size.
Rule of thumb: for a normal room height (8ft), you want CADR (CFM) > 2/3 * square footage

Note that the CADR rating listed on the product description is for the air purifier’s high speed setting, but this is too loud for everyday use, so you will normally be using the medium setting, which is typically about 60% as much CADR.

This amount of filtration provides good protection from both Covid and for particulates (PM2.5). Half this amount of filtration (i.e. 2-3 ACH) would probably give about 60-80% of the Covid protection (this is just a guess) and 90% of the particulate protection (based on my calculator).

Large room recommendation: For large rooms, you can use a big air purifier or multiple medium-sized air purifiers. Compared to one large-room air purifier like the Blueair Blue Pure 211+, two Coway Mighty air purifiers are less noisy, cheaper to run, and provide higher total CADR, but take up more space and require more maintenance hassle.

DIY option: In a pinch (e.g. if you've found yourself in the middle of wildfire season and all the air purifiers are sold out), you can simply strap HEPA or MERV-13 or similar filters over a box fan - see Experiments on a $50 DIY air purifier you can make in 30s - dynomight.net. This can be improved a lot with this slightly more complicated DIY design called a Corsi-Rosenthal box.

Caution: the efficiency varies a lot depending on the fan and filters. A typical filter-over-box-fan setup provides a pitiful CADR, only enough for about 100 sqft; and while the initial price is low, the total cost to run it is higher than commercial air purifiers because the box fan consumes a lot more electricity (your mileage may vary depending on the type of fan and filters you use, of course). I think this is roughly because box fans are designed to move air 10x faster (in CFM) which naturally should use a lot more electricity, but their performance suffers with the huge air resistance created by the filters so they end up delivering much less CADR than even a small-room air purifier. It's also a lot noisier. It does filter the air that goes through the filter well, the problem is just that it doesn’t move that much air, i.e. very low CADR.

Therefore, I would only recommend this as a fallback option if you cannot buy an air purifier, or if you only plan to run an air purifier very occasionally - running it 24/7 is best but if that’s not possible it's still valuable to at least have an air purifier you can use in occasional high-smoke or high-Covid-risk conditions.

Some Corsi-Rosenthal boxes work very efficiently and provide high CADR, while others perform poorly - it varies a ton depending on how you build it and what fan and filters you use. So I would be cautious about relying on them unless you have some way to measure them or are exactly copying a well-tested design.

Ionization - ignore and turn it off: Many air purifiers, including the Coway Mighty, come with an ionization feature. In general I recommend ignoring this and turning it off. What actually works is the HEPA filter. Ionization is generally ineffective at cleaning the air, and a HEPA filter already captures virtually all particles anyway. And ionization uses more electricity which is a waste.

Avoid any product that relies only on ionization and doesn’t have a highly effective filter - the evidence indicates they are ineffective.

Ozone generators - avoid: Separately, there are also "air purifiers" that function by generating ozone - ozone is a pollutant and is actively harmful to health so definitely don't use those.

Volatile Organic Compound (VOC) filtration: Most air purifiers have a token amount of VOC filtration that is basically ineffective. If you have specific concerns about VOCs - e.g. if you live near a farm, chemical plant, power plant, or refinery - Wirecutter's recommendation is the Austin Air HealthMate HM400. You can measure VOCs with an air quality monitor to check whether this is a problem for you.

HVAC recommendations

The Furnace and Air Conditioner Filters We Would Buy

Standalone air purifiers are a cheap and easy way to improve air filtration in a room, but you can also easily upgrade the filtration of your HVAC system to provide better filtration for the entire home/building.

My main recommendation is to simply change the filters in your existing HVAC system with MERV-13 filters.

If you have a standard forced-air system that’s blowing air through your home with a fan, it is most likely already equipped with a lower-MERV filter. You're supposed to change your air filters regularly, around every 3 months, since they gradually get clogged up over time. So start changing your filters if you weren’t before, and just change them to MERV-13 filters.

My suggested brand is 3M Filtrete. There are many different sizes, so just search on Amazon: "3M filtrete MERV 13 (dimensions)x(you)x(need)". For example, for my furnace filter, I searched “3M filtrete MERV 13 12x24x1” to get this filter.

Replacing these filters is easy: you just need to open the panel in front of the filter and swap the old filter for the new one; and cheap: MERV-13 filters typically cost only slightly more than lower-rated ones.

See https://www.nytimes.com/wirecutter/reviews/furnace-and-air-conditioner-filters-we-would-buy/ for more explanation of HVAC filters and recommendations.

More about HVAC filters

The rest of this section is just explaining where the above recommendation comes from, and more advanced steps you can take.

For HVAC filters, filtration is usually rated with MERV on a scale of 1 to 20, where higher means better filtration. For a typical home HVAC system, I would suggest aiming for MERV-13. It’s the lowest level that provides decent protection against Covid and fine particulates like smoke. It’s also the highest level that a typical home HVAC system is likely to work well with. MERV-12 and MERV-14 are also good candidates.

If you find all these ratings confusing, see this appendix for a quick explainer and a table that compares the effectiveness at different ratings.

High-MERV filters often require a purpose-built HVAC system - MERV-14 to 16 filters are typically used in hospitals and some commercial and industrial environments, but generally not in homes. Adding a high-MERV filter to your standard home HVAC system might not work well - they create higher air resistance which can reduce airflow and stress the blower, or cause more air to bypass the filter. The amount of air resistance a filter creates is called pressure drop, and this value may be reported by the filter manufacturer. If you want to be very careful, you could hire an HVAC professional to evaluate your system and measure the pressure drop your system can handle, but this is likely to cost a few hundred dollars and is unnecessary for most cases. I’d suggest MERV-14 if you know your HVAC system can handle it; going to MERV-15 and higher adds relatively little benefit for a home setting. Higher MERV filters also generally cost more and may need to be replaced more often.

But if you’re in control of the HVAC system and want to take further steps, I’d look at ensuring that the airflow rates are sufficiently high. The main metric you should think about for overall effectiveness of your filtration system is the Clean Air Delivery Rate (CADR) for smoke. The CADR metric includes both the air flow rate through the filter, and the effectiveness of the filter at removing particles. You can roughly think of it as <filter effectiveness> * <airflow rate (CFM)>. (The true CADR is not quite that because it’s actually measuring the rate over and above the natural rate at which the particles are falling out of the air, but this is not important for our purposes.)

You want this to be high enough compared to the size of the building to provide 4-6 Air Changes per Hour - the calculation here is the same as for HEPA air purifiers (since the filter effectiveness is already part of the CADR number):

See the HEPA Filter Calculator near the bottom of https://www.microcovid.org/blog/hepafilters to calculate what CADR you need based on your room's size.
Rule of thumb: for a normal room height (8ft), you want CADR (CFM) > 2/3 * square footage

Once you've upgraded to a filter that is at least moderately effective at filtering COVID and smoke - MERV-13 or MERV-14 - the air filtration/ventilation rate is the main thing to focus on. MERV-14 is substantially better than MERV-13, but you get quickly diminishing returns past that. E.g. even though a HEPA filter looks much more effective than MERV-16 (99.97% vs 95%), at that point you're already catching most particles and it makes little difference, while increasing airflow rate through the filter makes a big difference - the CADR metric reflects this.

Ventilation

In this article I’m focusing on filtration, which is the primary way to deal with particulates, including respiratory viruses. Ventilation (air exchange with outdoor air) is another important factor in indoor air quality, especially for gaseous pollutants, but I’ll just touch on it briefly here.

If you don’t own the building, there’s not much you can do. If you do own the building, a good solution is to install an Energy Recovery Ventilator (ERV), with a MERV-13 filter for the incoming outdoor air; and seal up any gaps in the building envelope to make it as air sealed as possible. Installing an ERV probably requires professional installation from an HVAC contractor.

An ERV lets you exchange indoor air for outdoor air, which removes pollutants from your indoor air - in particular, gaseous pollutants like VOCs which are hard to remove with air filters. And the air sealing reduces the inflow of particulate pollutants, which lowers the equilibrium concentration of pollutants with your indoor air filters running, and improves your climate control and costs.

This is better than relying on passive air leakage through cracks and openings in your home, because:

It gives you control over the exchange rate - passive air leakage may be too much or too little depending on the wind and weather
It lets you filter the incoming outdoor air to reduce particulate pollutants.
It improves the temperature and humidity of the incoming air, which improves comfort, reduces heating/cooling needs, and reduces moisture problems.

Public policy thoughts

I think improving indoor air quality and filtration standards would create huge long-term value for both Covid and other respiratory illnesses. The Clean Air Act has done a ton for outdoors air (although there's still a lot more we could do, even in the US), but as far as I can tell little attention is paid to indoors air.

This seems like a tractable problem both locally and nationally:

You personally can improve your home air quality, and this is probably the easiest way to increase your life expectancy: Dynomight
Workplaces can improve employee health and cognition by improving office HVAC systems: Questions to ask your employer about Covid and Air Quality (NYT), Workplaces need fresh air (The Atlantic), Experts Urge Strict Workplace Air Quality Standards, in Wake of Pandemic (NYT)
It seems like NYC and other cities could just install a bunch of air purifiers in their subway stations, and it would be cheap with massive returns on investment in long-term health.
At a city/state/national/global level, public policy can Cut air pollution to allow humans to live two years longer - Vox: Reaching WHO guidelines for particulate pollution would save more than 2 years of life for the average person globally. And at the same time we can Stop Every Respiratory Virus at Once - The Atlantic: In the 1800s and 1900s, we virtually eliminated outbreaks of waterborne diseases by reengineering our drinking water and sewage systems, and foodborne diseases with food safety policy. We can and should do the same for airborne diseases - not just COVID but flu and the common cold too.

Want more links?

Changelog of major updates

Jan 2022: added recommended MERV-13 filter brand and guide on changing your HVAC system’s filter. (Thanks to Eric for his recommendations.)
Dec 2021 (updated since my original Google Doc version of this guide): added HVAC and public policy sections, and expanded analysis and comparison of air purifier cost and CADR

This article is an evergreen, living document that I will continue updating over time.

Like with all my posts, feedback/questions/comments are very welcome - feel free to send me an email or message, or comment on this post.

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