7 Reasons Your Filter Isn't Improving Your Indoor Air Quality

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Filter bypass because of incorrectly installed filter

Indoor air quality (IAQ) researchers have established that little bitty invisible pieces of stuff that float around in the air are bad for human health.  Of course, being scientists, they don't call it little bitty invisible pieces of stuff.  They call it particulate matter (PM) or particulates

And it turns out that the really small pieces — the stuff that's 2.5 micrometers (0.0000025 m, or 2.5 µm) or smaller, abbreviated PM2.5 — is worse than the bigger stuff because it can penetrate deeper into the lungs and more easily find its way into your blood than the bigger particulates.  The chart I included in my last article shows that PM2.5 is probably the worst indoor air pollutant overall.

And how do you deal with it?  One of the primary methods is to filter it out of the indoor air.  In most homes, the only filtering of the air that happens is in the heating and air conditioning system.  But if you think you're covered just because you have a forced air HVAC system and it has a filter, let me give you a few reasons why that filter that you so dutifully change may not be helping your IAQ.  (You do change your filter, don't you?)

1.  No filter

It's true.  If you don't have a filter, you're not gonna get much filtration.  And yeah, it really happens.  Sometimes someone removes the filter because it's in a difficult spot to reach, like a crawl space.  Sometimes they take it out and forget to put it back in.  Sometimes...well, who knows!  All kinds of things happen.  But not only are you not improving your indoor air quality if you don't have a filter, you're also getting your duct work, blower, air conditioner coil, furnace heat exchanger, and everything else in there dirty.

Missing filter in a damp, dirty crawl space

I took the photo above in a nasty, damp crawl space in Atlanta.  Notice that filter slot near the middle of the photo.  Not only is the cover missing but so was the filter.  I took the picture on a hot, muggy day in August.  Imagine all the nasty stuff getting sucked in there and sent into the house.

2. Filter bypass

Take a look at that photo at the top of this article.  That's some serious bypass.  They put a nice, deep media filter in that's capable of filtering out a lot of nasty stuff in the air.  But they installed it incorrectly so quite a bit of air going through the system was passing right by — instead of through— the filter.  Bypass means you're not cleaning the air nearly as well as you should.

Filter bypass

It doesn't take a fancy filter to get bypass, though.  Here's one with a standard one inch fiberglass filter that was jammed into the slot incorrectly.

3.  Not enough runtime

This one may blow your mind.  Filters can clean the air only when there's air going through them.  When the system is off, no filtering happens.  If you've been reading this blog for a while, you may be thinking, "Aha!  Yet another reason not to install an oversized heating and air conditioning system."  The problem there is that when you put in a smaller HVAC system, you also cut down the air flow and the filtration volume is the product of those two quantities:

Filtration volume = Air flow  x  Runtime

You can double the runtime by cutting the system size in half, but at the same time you're cutting the air flow in half so the filtration volume stays the same.

One way that some IAQ advocates tell people to get more runtime is to put the fan in the "On" position instead of in "Auto."  Weeeeeell, that's a nice idea and it will work in some places...at the cost of higher energy bills.  But in a humid climate during cooling season, you could be making your indoor air quality worse by raising the humidity.  I know.  I measured it in my home a few years ago.

The best thing to do here is size your system properly, don't put the fan in the "On" position, and get as much filtering as you can with your system.  If you have an efficient blower motor and are in a dry climate or running your system with a dry coil, using the fan "On" setting can help.  But the best thing to do is focus on minimizing the stuff in your indoor air that needs to be filtered to begin with by doing source control, air sealing, and mechanical ventilation, both local and whole-house.  If that still doesn't do it for you, it may be time to add a standalone fan with a filter that will run more and get you a higher filtration volume.

4.  Not enough flow

Now you know flow and runtime go together.  This other way to get low filtration volume is a common problem for a couple of reasons.  First, many return ducts and filters are sized too small.  That increases the pressure in the system, which reduces air flow.  Second — and I know this doesn't apply to you — some people don't change their filters often enough.  Again, the result is high pressure and low air flow.

Dirty filter, high static pressure, low air flow

The system in the photo above had both problems.  The pressure drop across the dirty filter and coil in this system was a super high 0.9 inches of water column (i.w.c.).  With a clean filter, it was still a too-high 0.6 i.w.c.  (Read more about this home.)  That pressure drop is about ten times too high.

5.  Low efficiency (MERV) filter

You can install a standard one inch fiberglass filter.  It's not going to do much for your IAQ because it's basically designed to keep dog hair, dead spiders, and lost socks from getting into the air handler.  The standard rating system for filters is Minimum Efficiency Reporting Value, or MERV.  That one inch filter is about a MERV 2.  The higher the number, the more stuff you filter out.

Here's a chart I saw in a presentation at the North American Passive House Conference in Boston last month.  It was put together by my friend John Semmelhack of Think Little in Virginia from data he found in various places.

Filter effectiveness by MERV rating and particle size

As you can see, you've got to step up your filter game to at least MERV-10 to get even half of the PM2.5.  But you really need to be at MERV-13 because it's better to remove more than 85% of those particular little bitty invisible pieces of stuff.  And that was Dr. Brett Singer's recommendation at his 2018 Building Science Summer Camp presentation.

But there's a caveat here:  Many (most?) systems with high-MERV filters decrease the air flow because of poor design.  My next three articles will explore this issue in depth and show you how to do it right.

6.  Not filtering outdoor air for mechanical ventilation

 Some ventilation systems are designed to use the heating and cooling ducts to distribute outdoor air, too.  With the central-fan-integrated-supply type of mechanical ventilation, a duct from the outdoors is connected to the return side of the duct system.  Occasionally, a designer or installer doesn't pay attention and connects that outdoor air duct to the return side downstream of the filter.

Oops!  When that happens, as it did below in this house with a filter grille, you're putting unfiltered outdoor air straight into your ducts, where it can make the ducts, blower, and heating and AC components dirty as well as sending more particulate matter into your indoor air.

Unfiltered mechanical ventilation air

 By the way, I haven't said this yet, but the biggest source of PM2.5 in most homes is outdoor air.  (If you smoke indoors or burn candles or incense regularly, those would be bigger.)  So make sure you filter that outdoor air before introducing into your home.

7.  Filter in the wrong place

 I have to think this one is anomalous but with the strange stuff I've seen in the wild, who knows.  The photo below was sent to me by my friend Jamie Clark in Kentucky.  The air moves through that system vertically.  There's nothing coming in from the right side...over there where the filter is attached.  So that filter does nothing because air doesn't move through it.

A filter with no air going through it won't clean the air


Filtration can be an important part of good indoor air quality.  Unfortunately, there are some significant obstacles to overcome if you want it clean up your air.  Professor Jeffrey Siegel has been studying the effectiveness of filtration in real homes and has found that the average home in their study has a filtration effectiveness of about 20%.  

How's your filter doing?


Related Articles

This Thermostat Setting Can Cost You Money and Make You Sick

A Few of My Favorite Filter Photos

Asthma and Poor Indoor Air Quality — The Trouble with Homes

Which Indoor Air Pollutants Matter Most?


Chart of filter particle trapping used with permission by John Semmelhack.  Photo filter installed outside the airstream used with permission by Jamie Clark.


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Please allow me to suggest another: Your house runs at a negative pressure relative to outdoors, and you get a serious amount of infiltration of outside air with no filtering at all.


Good point, Mark.  I should have mentioned that one.  I'd put that in the same category with number 6, not filtering outdoor air for mechanical ventilation.

And how do you know if you have a problem (or what the problem may be - like ozone where I live, or if the problem is solved)? You need to continually measure for all the issues you suspect. No measurement, just guesswork. Buy high-MERV HVAC filters every 3 months for guesswork? Install mechanical ventilation systems with their own ductwork and MERV 13 filters for guesswork? Not likely. We need fairly accurate, fairly affordable (think Nest thermostat) IAQ measuring devices that every home should have to show homeowners that there is a problem and then show problem solved when countermeasures are implemented. Otherwise - guesswork.

Here is a system I have been researching that may offer some of the IAQ monitoring desired. At least they seem to be approaching the IAQ issue as a central design goal... https://www.buildequinox.com/ I'm just a homeowner working on designing a future custom build for my retirement... not a rocket rocket scientist like Allison. If I am off the mark with what CERV2 is, I hope someone here will correct me... :-)


David, I respectfully disagree.  Do you need to measure everything to know that a house with an air-sealed floor or encapsulated crawl space is going to have few IAQ problems originating in the crawl space than a house with a leaky floor over a damp, musty crawl space?  Do we need to measure everything to know that we'll have better IAQ if we use a well-designed, MERV-13 filter in our HVAC system?  Do we need to measure everything to know we'll have better IAQ if we have a range hood with a good capture efficiency and use it regularly?

I absolutely agree that measuring what's in the air is important but home occupants already have enough to worry about and probably aren't going to pay attention to it anyway.  It's incumbent upon those of us in the design, construction, improvement, and assessment industry to see that occupants have the best possible chance for good IAQ in their homes.

That doesn't mean I'm against all measurement.  I regularly tell people to get a low-level carbon monoxide monitor and to measure humidity, too.  But to say that we're offering blind guesswork in giving advice about filtration or ventilation is a bit extreme.


Brad, the CERV is much more than the type of measuring device David was talking about.  It's a device that ventilates, filters, recirculates, dehumidifies, heats, and cools.  They focus on the IAQ side of things because the CERV can solve the problem of low runtime and thus low filtration volume as well as control humidity and bring in outdoor air for ventilation.  David was talking about a monitor that's fairly simple to use and understand and relatively inexpensive so everyone can measure what's in their air.

Allison - I respectfully disagree with you. Every house has a thermostat to measure air temperature; many houses have relative humidity gauges; many houses have CO detectors (although there is quite a bit of opinion as to whether or not these detectors are worth anything). The environment within a house is constantly changing. IAQ depends on a lot of factors that are in flux (amount of cooking, amount of time doors/windows are open, amount and duration of occupants, type/amount of outdoor pollutants or humidity, etc.) to say that you don't have to measure (the most important aspects, like PM2.5, ozone, CO, CO2, RH - not "everything") in order to get a good handle on IAQ is akin to saying all you need to do to maintain comfort is to turn the HVAC unit on with a wall switch whenever you are uncomfortable. How do you know you have "fewer IAQ problems"? You may have different problems that occur as an unintended consequence or is driven by outside forces your countermeasures were not engineered to combat. You may have the same problems, but just slightly reduced so you need to add more countermeasures. How do you know that all you really need is just a recycling air purifier instead of a full-blown mechanical ventilation system (so you don't introduce outside contaminants but still achieve clean air with interior air temp destratification)? Or whole-house dehumidification instead of just relying on the HVAC unit? Is the HVAC (or mechanical ventilation unit) air filter(s) doing a good job? How do you know when to replace them? Every situation is unique enough that it deserves unique monitoring. You don't manage what you don't measure.

I tend to agree with Allison on this one- not because I don't think measuring is worthwhile, but because I measure for a living and I can tell you that the cost of equipment to adequately measure these parameters exceeds the cost of the entire HVAC system. There are a few attempts at consumer grade devices (Foobot, for example) but the accuracy is so poor that I don't consider them measuring devices. They are indicators, like having a battery light on the dash to tell when your car's electrical system isn't right. But everyone has access to the EPA Airdata website (www.epa.gov/airdata/ ) or your state or county website to look at what the air quality is like in your region, and those monitors likely cost more than your house. So my recommendation is look at ozone and PM2.5 levels from EPA and get a CO detector and call it good.


Measuring temperature is easy.  We can do it with thermal expansion of liquids, the bending of bimetallic strips, thermistors, thermocouples, optical pyrometers, and more.  Measuring humidity and carbon monoxide are relatively easy, too.  We have so many ways to measure these things that ordinary people can feel confident in the results.

But when we move to indoor air pollutants, which ones of the hundreds or thousands are you going to measure?  And how reliable will the results be?  I think you're a bit too confident in our ability to measure the concentrations of hundreds of different chemicals reliably and inexpensively.  And I think you're too dubious about what we can do without such measurements.

This is why I love following your Blog. Great discussion and input on a very complicated topic. As economist Thomas Sowell was famous for saying... "there are no solutions, only trade-offs". I think that applies very well to building science. Heightened awareness of the next problem to "solve" is key.
IAQ looks to be a major one as homes become tighter. Looking forward to your next several Blogs on this topic. :-)

There is a point in technology where the measurement accuracy becomes "good enough" to recommend to a homeowner. Lots of different thermometers/thermostats on the market/recommended by building scientists but none truly measures human comfort (see Mean Radiant Temperature and the related MRT thermometer). Lots of RH meters recommended too, as well as CO detectors (even though their accuracy does not meet the safety levels of OSHA or BPI). The only research I've seen on IAQ monitoring devices for residences was by the brave work 2-years ago by Nata Adams and the recent work by Lawrence Berkeley Labs (https://eta.lbl.gov/publications/response-consumer-research-grade) where the test results were far less than stellar and not recommendable. No outcry from any authoritative public or private entities (DOE, HUD, BSC, PHIUS, Taunton Press) on the dearth of such devices as a precursor for recommending/implementing mitigation technology; no sponsorship to develop better/more affordable devices by the building science community. Just more emphasis on "ventilating right" (whatever that means since we're not measuring anything). All the discussion on ventilation schemas, filtration media, central vs distributed, EPA outside air quality measures, etc. become academic exercises - not solutions that you can recommend to a client because pre/post testing showed there was a an issue in their specific case but the implemented technical solution mitigated it. This is a paramount cause the whole building science community needs to get behind as it will be a proof/driver to the non-scientific clients of the world that changes need to be made (purchased) - like more extreme air tightness, better air filtration, better bath/kitchen fan technology, different heating/DHW/cooking technology - NOW. Otherwise there is no proof/incentive.

Unfortunately equipment air filters really have one duty, to protect the equipment. In the past 20 years the push was on to design and add IAQ devices to the equipment but little done to enable the equipment to handle the additional restricion to airflow of the filtering media or other air "cleaners".


Robert, yes, a whole lot of installations have problem with excessive pressure drop from restrictive filters.  The solution isn't in the equipment, though.  It's in the duct design.  Stay tuned.  I'll cover that in my next three articles.

You are quite correct, Bob. For this reason, I only recommend higher efficiency filtration if the filter surface area can be increased significantly or if a standard filter is kept in place to protect the equipment while a self contained bypass HEPA filter system is added to the return ducting.

Allison -- you are getting there step by step. I am afraid I am going to jump ahead. The VAST majority of filters we see in our studies are: Too restrictive. They are too restrictive because they are too small. I would never again specify a system with less than a 4 inch deep filter, even then the amount of filter face area would have to be more than twice the size that is regularly installed. The trade off between airflow and filtration is a difficult one.
On another note -- I might be wrong, but I think that the effectiveness of filtration increases with lower airflow. If I am correct then we need to change the equation above to
air velocity X area X run time.

Perhaps I should elaborate: Air Velocity * Filter Surface Area * Run Time * Capture Effectiveness (CE) where CE is a function of the filter material (eg. MERV) and Air Velocity


John, you have indeed anticipated where I'm going with this series.  My next article will cover the continuity equation and face velocity. 

Thanks for the link to your article.  I don't recall having seen that before.  For those of you who didn't notice it since he hotlinked his name with it, here it is:


There is a mathematical (physics) mistake within that article. Kudos to anyone who can find it

After decades of doing maintenance on systems my company installed, I found that the cleanest coils after years of use were coils that had hogs hair type of frameless filters installed. Being frameless, if sized a 1/16" over the size of the filter rack, there was no air bypass around the perimeter of the filter. Because these washable/vacuumable filters are plastic fibered, they have an electrostatic quality that attracts airborne particulate to them. Because of their continuous filtering through the depth of the filter, they do not get coated with particulate, affecting the air volume.
The major downside to these filters is that they tend to be flimsy when dirty. I always threaded steel insulation rod used to hold up insulation batts through the filters at 8" spacing and bent over the rods at the ends of the filter. This kept them nice and stiff. The other main factor is to make sure the size of the filter allows for a low velocity of air per square inch of filter to prevent particulate from being pushed through the filter.


Robin, the other downside to filters that rely on electrostatic charge is that they filter less and less as they get loaded.  Non-electrostatic filters, on the other hand, filter more and more as they load.  Of course, if you really want to filter everything, a piece of plywood works really well.  ;~)

Allison, I agree with the electrostatic factor. One of the reasons I like the hogs hair type is that they have enough space between the fibers that they tend to collect the most particulate on the surface before fibers below the surface actively collect particulate. After the surface fibers are coated, each layer of fibers below the surface pick up the slack. I have pulled very dirty hogs hair filters that were dirty to 75% of their thickness that had no signs of dust on the remaining 25% of fibers. The coils on systems with hogs hair filters were consistently cleaner than coils with well maintained electronic air cleaners. I do believe that the air having no way to bypass a frameless filter sized properly is a major factor for those cleaner coils.

Interesting article and I read some of your previous articles on dehumidicication. Over here in the UK we have mainly mini split systems over central air systems. You call them ductless systems. On these units unless they are ducted you cannot set the fan not to run when the compressor is off, if you put the fan to auto the fan modulates based on the temp difference between return and set temp. When it reaches the set temp only in heating does the fan shut off, otherwise it runs slow.

On the back of this, I have just returned from Florida where its been about 105F some days, humid as well! Never experienced anything like it! We went to Disney and Universal studios and I am suprised that they air conditioned the outdoor spaces such as where you board the rides which have no roof on but air conditioning ducts blowing conditioned air down to where the boarding area is for the rides.

Also all the stores there have the doors open and because they use Central air, the building ends up pressurised, their units must be so oversized because all the doors to the shops/rides are open yet the interior is very cold and you can feel a cold area outside at the front of the doors! Would be great to see an article specifically on this type of thing as this differs to the UK where we have different systems in use and use door curtains or keep doors closed when A/C is on.

I lived next to (150 ft away) from one of the largest urban freeways in the country for 7 years. Second year in, I developed strange symptoms in the wintertime. I had become sensitized to the constant barrage of pm2.5 and even smaller stuff and drink coffee without feeling completely on edge. Airports did the same thing a couple of times in those two pre-filtration years. Since I had no duct system, I built my own dedicated filterbox with an inline blower and moved approx 250CFM in a 1600 sq ft house all the time and kept the windows closed. It cost about 15 dollars per month, but the DYLOS counter LOVED the results--an approximately 75 percent reduction in particulates, and a 99 percent reduction in the positive pressurized room I set up. I also FELT MUCH BETTER. Research shows that the UFP (smaller than pm2.5) near a highway and downwind is very high--much higher than background. Even though much of the crap was below the threshold of visibility to a Dylos Pro, I am confident that the Microguard LR did a great job because HEPA and near-HEPA do even better at the smallest particles than the .5 micron stuff. Now that we moved away, I am back to a standard 1 inch filter that can't do anywhere near the same air-scrubbing, but seems sufficient in this cleaner environment.

I risk being shunned for advocating energy-consuming devices, but portable (or installed) indoor air fan/filter air purifiers are (in my experience) the single highest intervention that produces results for people with IAQ complaints. Sometimes adding a MERV-13 filter to a system works, but adding an independent fan/filter unit always works. The downsides are white noise and energy consumption. Whether the energy consumption penalty is offset by the difficulty or cost of designing (or redesigning) an installed HVAC system to handle improved filtration, I don't know. I look forward to learning more about that in the upcoming articles!

Bobby -- I for one agree with you. You get continuous filtration at what can be a relatively low cost if the unit has an efficient fan. When you try to filter with your HVAC system you are moving maybe 1000 or more CFM through a usually junk duct system with an often inefficient fan motor.

Allison, MERV is defined in ASHRAE Standard 52.2 and the particle size efficiency values in your table are somewhat different from the values in the standard. As usual, I am just nitpicking you.

I am taking exception to some of the statements above. Clean Air Delivery Rate (CADR) is the important metric for air cleaning and it is equal to the airflow rate times the filter efficiency. Thus, a HEPA filter is overkill since its efficiency is 99.98% when compared to high MERV filters that have efficiencies greater than 90%. The difference in efficiencies is negligible, but the airflow rate makes a huge difference in CADR when you compare a portable air cleaner or bypass HEPA to a high MERV filter on the return air for the whole house. Of course, you have to run the high MERV filter and blower to get this benefit, and there are other issues in humid climates as identified by others.

This is why I only recommend HEPA filtration when it is used in a self contained bypass system attached to the return ducting.

Roy - I agree with you about CADR being the important metric for filtration. However, there is another important factor and that is where the filtered air is drawn from and delivered to. A flimsy fiberglass filter in my portable air purifier will deliver more clean air to my room than a HEPA filter in a unit that is not running, or in a different HVAC zone. The main advantage of portable air purifiers is that they deliver the air where the occupant needs it. The second advantage, as discussed above, is that some systems can't handle a MERV-13 filter, much less a higher MERV filter.

As to sufficiency, MERV 13 still only filters

Roy -- Don't forget what happens with a duct system in the attic, R very little duct insulation, Duct surface area very high and continuous fan == Massive conduction loss (or gain)

That's a good point, John, about thermal conduction losses/gains in ducts in unconditioned spaces causing additional heating/cooling loads when running fan only for filtration. Throw duct air leakage in the mix and you can even have additional contaminant loading. This is why I now live in a house with a sealed (conditioned?) attic and these things are no longer issues. I just can't understand why anyone would put ducts and air handlers in spaces that are vented to the outdoors.

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