Neon pink fan with five blades.
September 25, 2020

We recently discussed the three things you need to need for a healthy indoor environment. Indoor air quality rests on three principles: first eliminate sources where you can, then ventilate to dilute pollutant sources, and finally, filter the air to capture pollutants. 

The need for filtration loomed large at the beginning of September with the wildfires in the Western US, just as it did earlier in 2020 with the fires in Australia, just as it did in California and BC in 2017, 2018, 2019, and 2020. 

Ventilation - natural or mechanical - does not do a lot for us when it comes to dealing with smoke or other pollutants like dust. What we need in that case is a tightly sealed envelope with steady filtration of the particulate matter.

COVID-19 precautions include filtration to capture any virus-laden droplets and aerosols, reducing the overall load in a closed space. 

To learn more, check out our short course: Healthy Indoor Environments (bonus: worth CEUs!)

This is the drill: Ventilate, filter the system, add portable air cleaners

Ventilation systems can include filtration, and forced-air heating systems typically have a furnace filter. But these may not be adequate for smoke clearance, or as a COVID reduction measure, first of all, because they are usually ‘mid-efficiency’ or lower on the MERV (Minimum Efficiency Reporting Value) scale. In addition, it is a challenge to quantify how much ‘cleaned’ air is making it to each room. This is especially true of large buildings like schools. 

The goal is to reduce the amount of virus people are exposed to, and the amount of time they are exposed to it. 

Air Filters Get Sorted by MERV Rating 

So, back to filtration: MERV ratings indicate the effectiveness of the filter on a scale of 1 to 20. The higher the rating, the better the filter, meaning the material can trap smaller particulate matter (smoke and virus-laden aerosols). Home air filters are rated anywhere between MERV 5 and 13, a typical furnace filter is MERV 8 or 11. MERV 16+ filters are used in hospitals, cleanrooms, and nuclear power plants. 

Get ready to nerd out.

A MERV rating is determined by a filter's effectiveness at filtering 12 laboratory-created particles. The particles are divided into three different size ranges:

  • E1: 0.3 to 1.0 micrometers (µm)

  • E2: 1.0 to 3.0 µm

  • E3: 3.0 to 10.0 µm

To get a MERV rating, a filter is put through a series of 72 ‘spray’ tests. The final rating is based on how many particles were successfully filtered out of the air for each test. The worst test result in each category (E1, E2, E3) determines a filter's M(inimum)ERV rating. 

Why Do We Care About MERV Ratings?

It’s all about particulate matter (PM), the tiny pieces of solids or liquids that are in the air, all the time, but sometimes in heavy concentrations. We’re talking:

  • Dust

  • Dirt

  • Soot

  • Smoke

  • Drops of liquid

PM is measured in micrometers (µm) - the units that filters are tested against (surprise). Coarse material that you can perceive, like dust is classed as PM10, particulate matter of 10µm or smaller. This can irritate your eyes, nose, and throat. Folks who live with asthma know particulate pollution all too well, even low levels of PM10 can trigger an attack. 

PM2.5 are tiny particles in the air that are 2.5µm or smaller. In heavy concentration, they can create a haze, or smog. 

Breathing in particle pollution can impact your health immediately or over the long term. PM2.5 is more dangerous because it can get deep into your lungs and can even get into your blood. How tiny are we talking? We’re talking tiny.


MERV Ratings, Rated 

MERV 8: pollen, dust, dust mites, mold, and bacteria. It is not designed to capture PM2.5

MERV 11: pollen, dust, dust mites, mold, and bacteria and pet dander. MERV 11 captures some PM2.5

MERV 13: Pollen, dust, dust mites, mold, bacteria, and pet dander. Cooking oil smoke, smoke, smog, AND virus carriers. MERV 13 captures up ≥85% of PM2.5

You’ve probably also heard the term HEPA filter. High-efficiency particulate air (HEPA) is an efficiency standard. A HEPA air filter must remove more than 99% of PM2.5 particles from the air that passes through. Room-sized, stand-alone HEPA filters are hard to come by these days. The run on those was maybe not so bad as toilet paper or hand sanitizer, but stocks have been depleted. 

To reduce or eliminate smoke, particulate matter, and droplet/aerosol virus load, you need filters with at least MERV-13. This will capture  ≥85% PM2.5.

3 Approaches to Box Fan Air Filters

HEPA filters are expensive, and can be hard to find. It’s hard to confirm that a whole-building system is properly ventilated and/or filtered. So people have come up with some great and inexpensive solutions using box fans or other types of room fans. How well do they work? John Semmelhack, airflow whiz, uses one, and here’s a tweet about results in his house:


Three configurations seem to be working well:

MERV-13 filter taped to the face of the fan - source - Yale School of Public Health


MERV-13 filter ‘tented’ and sealed to the face of the fan - source: Tom Builds Stuff and Marshall Hansen Design

MERV-13 filter ‘box’ sealed to the face of the fan - source: Yale School of Public Health


MERV-13 filter ‘box’  sealed to the face of the fan, with vertical outflow Comparetto Comfort Solutions

Fans not made for filters

The challenge with filters is they create a barrier between the fan and the room, slowing down the amount of air that gets delivered. Filters that are more tightly woven, catching more of the smaller PM2.5 also make it harder for air to pass through. This creates a decrease in air flow and a higher pressure drop. 

So there are concerns about overheating the fan motor when you add a high-MERV filter. The pressure drop makes the fan work harder to push air through the fan. The tent and box versions reduce pressure drop by increasing the amount of filter surface area, decreasing the effect of air resistance. Bonus points: more filter surface area means you don’t have to replace the filters as often.

According to John Semmelhack (Think Little), a 2” thick MERV filter will be better for air flow than a 1” thick filter. Another tip found somewhere on Twitter: If your fan and filter sizes don’t match, use a piece of cardboard as an adapter plate. 

Some Indoor Air Quality and Air Filter Resources

If you’re keen on graphic analysis, here’s a cool little app called AMID-RA from researchers out of Duke University, based on work by Dr. Shelly Miller (@ShellyMBoulder) and Dr. Jose-Luis Jimenez (@jljcolorado). The app can help estimate the risk of aerosol transmission of SARS-COV-2 in an indoor setting. The thing that you, the occupant, cannot know is the air exchange rate per hour, but you can see where it should be to lower the risk. 

Harvard and CU Boulder have developed a portable air cleaner calculator for schools that compares portable air cleaners. 

Find out more about some DIY air purifier tests and observations here

Oh, and look, here’s another scholarly article showing the efficacy of masks in controlling aerosol particle emission. So filter your breathing to control outward emission, and filter the air to control what gets breathed in.

In-depth article comparing low-cost box filter devices to high-end dust collection systems for workshop systems - geek right out on all the details provided by Shop Hacks!

Credit Due:

There are some folks who have been on top of aerosol transmission from the get-go. I read/listen to as much of their science-based work as possible to inform the content of Blue House Energy courses and blog posts that relates to indoor air quality, healthy buildings, and ventilation. Shout out all of the excellent research and outreach being done by people who have dedicated their lives to healthy buildings and people. 

In no particular order, here are some of them by Twitter handle:














Check out our Healthy Environments Course to learn more!


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