MERV Ratings
Minimum Efficiency Reporting Value (MERV) is a rating system used to report the effectiveness of air filters. You can find out a lot more at MERV or at the EPA What is a MERV rating site. The chart below unlike some other charts that show lower ratings as red is showing everything as a color gradient of green. The reason I didn’t show the low MERV ratings as red or yellow is because in some cases filters with a low MERV rating are still capable of filtering air well enough to be effective. I tested a cheap Honeywell MERV 7 filter and it performed really well.
PSE (Particle Size Efficiency)
Modern furnace filters should now be labeled with their PSE rating. 3M Filtrete have started adding their PSE rating to all their filters. Unfortunately other brands like Nordic Pure and Honeywell have not added the PSE rating to their filters. I made the mistake of buying 6 Nordic Pure filters at MERV 7 and they didn’t work to filter the air nearly as well as a similar MERV rated 3M or Honeywell brand. I also bought a Nordic Pure MERV 13 filter that does work but it was expensive, 4 inches deep and 3M Filtrete with a lower rating worked better. At this time I won’t be using any more Nordic Pure filters for air filtration purposes. Based on my tests I just don’t think they’re as good as 3M or Honeywell for what I’m trying to do.
Merv Rating scale, numbers obtained from, EPA What is a MERV rating site
You can see in the chart above that the MERV standard doesn’t set a minimum for anything MERV 4 and under. This was a missed opportunity and has led to all sorts of marketing nonsense. In effect I could take a T-Shirt from my wardrobe and sell it as MERV 4 because there’s no way to verify it. Essentially anything MERV 8 and under doesn’t technically need to filter any PM2.5 matter from the air. This is what a lot of the air purification companies are using to scare people. I think The folks over at 3M created their MPR rating because of this lack of clarity.
The following image is from a MERV 5 3M Filtrete.
3M MERV 5 Larger Image
In the image above you can see the PSE rating at the bottom of the filter. If we take the 0.30-1.0 micron reading it has a PSE rating of 12. This indicates that it will filter out particles in this size range at a rate of 12%, i.e. If 100 particles get pushed at the filter, 12 get trapped and 88 get passed. What this means is that as the air gets passed through the filter each pass removes 12% of particles at 0.3-1.0 microns in size.
Unfortunately Honeywell have not started adding the PSE ratings to their filters yet.
Can a low MERV rated filter clean my air
If it’s a good quality filter like 3M Filtrete, or Honeywell brands then yes a lower MERV rated filter can clean your air.
The following table is for 3M Filtrete filters only. Assuming we’re using a standard 20 inch box fan with a CFM of 500 after attaching the 20x20 inch filter the table shows how any hours it takes to have the air cleaned in a 1500 sq ft house. Of course this assumes perfect air circulation around the house etc but you get the idea, if you’re in the house most of the day and you have a fan in your room with a MERV 5 filter it will filter the air, slowly, but it will clean it.
The difference in price between a MERV 5 and a MERV 13 filter is $10 - $15. Personally I like to use the higher MERV rated filters in addition to a low MERV filter where the low MERV rated filter is in front of the high MERV filter, this means the low MERV cheap filter will need replacing more often than the expensive high MERV filter which means I can double the life of the more expensive filter.
| MERV | MPR | PSE: 0.30-1.0 µm | Hours |
|---|---|---|---|
| 5 | 300 | 12% | 14 |
| 11 | 1000 | 41% | 4 |
| 13 | 1900 | 62% | 2 |
One thing I would like to do some research on is if Brownian motion helps us by moving the dust particles around the house.
MERV vs HEPA Confusion
There is a substantial amount of confusion, half truths and general ignorance around MERV and HEPA rated filters. I cannot tell if the confusion stems from marketing departments inventing terms like True HEPA or from the fact that consumers think their one inch thick filter they bought in Target or Home Depot is a HEPA filter. Personally I think the body that invented MERV ratings, (ASHRAE) could take a lot of the blame because of the way they defined the MERV rating scale. The EPA could also take some of the blame because on their HEPA page explaining HEPA, they digress into explaining MERV ratings, in fact the bulk of the page is spent explaining the MERV ratings scale. This conflates two independent standards and makes it easy for marketing departments to take advantage of the confusion and sometimes use scare tactics to market what I believe in lots of cases are overpriced gimmicks.
Are the Filters at Home Depot HEPA filters?
Almost all the furnace filters you find at the big box stores or target, Costco etc are not HEPA because they do not satisfy the HEPA standard.
HEPA
MERV and HEPA are two unrelated standards for air filtration. Generally HEPA filters are for clean room environments and these have strict ISO standards to be followed.
A MERV rated filter cannot and should not be called a HEPA filter unless it filters out 99.97% (USA) 99.95% (UK) of all particulate matter at 0.3 microns. If you want to know why 0.3 microns was chosen as the one size to rule all sizes I cover it in the article.
So what MERV rating do I need to filter my air?
This is where the fun starts. The marketing departments of a lot of the air filtering companies have taken full advantage of the gaps in the MERV rating scale and added a ton more confusion with terms like True HEPA etc.
The answer as always with anything complex is, it depends, it depends on what you’re trying to do. If you just want the particulates removed from the air in your home then the MERV rating can vary depending on how fast you want the air cleaned and how much you want to spend but you do not need an expensive filter to clean your air. For our use case all I want is to get the air in my home clean in every room at minimal cost. The following experiments show that a mid range filter can be very effective at cleaning the air in a room.
Experiments
I decided to run some experiments and to prove to myself that this works. The following are hardly hard science but they proved convincing enough to me that a low MERV filter and a simple box fan can be a very effective way to quickly clean the air in a room.
Experiment 1
What I found is that a cheap MERV 7 filter will clean the air in a 144 sq ft room in less than two hours. The image here was the first experiment I ran with two simple MERV 7 filters that I got from Home Depot. The reason it does not fit the fan was because all the 20x20 filters were sold out at the time because the wildfires etc had increased demand for filters.
| Time | PM2.5 | Event |
|---|---|---|
| 8:40 | 8 | Started two incense sticks |
| 9:20 | 403 | Incense sticks out |
| 9:25 | 447 | no fan |
| 9:26 | 447 | fan1 on |
| 9:44 | 226 | fan1 & fan2 on |
| 10:09 | 42.7 | both fans off |
| 10:52 | 33 | both fans on |
| 11:32 | 2.2 | both fans off |
| 11:49 | 1.9 | both fans off |
| 12:05 | 2.5 | both fans off |
| 12.06 | 2.7 | both fans on |
I deliberately turned the fans off to see if the curve would flatten and it did, you can see this at 10:09. So filtering air with really basic filters actually works and it does not have to cost $100 to do it.
First filter tested
So I ran some more experiments
The graph below is an experiment I did by polluting a room using incense sticks. I lit multiple incense sticks in a room roughly 144 sq ft in size and then waited until they went out. I did this for two filter types with different MERV rating and charted how fast they clean the air.
Lasko 20x20 with 3M Filtrete as media
Note for the Filtrete MPR 2200 and Filtrete MPR 300 + MPR 2200 I waited for about 15 minutes before turning on the fan.
So at this stage I know for approximately $60 (much too expensive) I can achieve the following sort of performance using a 20 inch box fan and a 3M Merv 13 MPR 2200 filter.
Lasko 20x20 with 3M Merv 13 (MPR 2200) filter
Conclusion
A cheap 20 inch box filter and a MERV 13 filter is an extremely effective way to clean your air. More interestingly you can also clean the air using a much cheaper MERV 5 filter you can clean your air over a longer period of time, this is significantly cheaper.
Caveat Emptor
I have only tested 3M Filtrete, Honeywell and Nordic Pure filters. Only 3M and Honeywell cleaned the air in the tests I was conducting using the low MERV filters.
Do your own tests and convince yourself.
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