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How can I determine source of PM 2.5?

awinn17 | Posted in General Questions on

Hello! Since purchasing an Airthings AQM I’ve discovered we have fairly high PM25 levels. As of this writing the 30 day average is 15 microgram/m3 and the 7-day average is 18.

Before I go nuts cleaning duct work, buying air purifiers, etc. is there anything I can do to find the “source”? For instance, measure what’s in the air, determine what it is, to attempt to treat the cause before the symptoms?

edit: as an aside, how much might a home ventilator help? I have one that I’m about to install (to handle straggler radon post-mitigation and be ready as I get the house more sealed up). It should serve an additional role as particulate matter dilution. Any idea what I could expect there? Maybe nothing much measurable?


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  1. Expert Member


    The usual suggestions for dealing with contaminants is first source control, then removal by filtration and dilution. However sometimes dilution alone works well. What ventilation do you have? Without a good rate of air changes, even very small amounts of PM25 can cause the levels to build up.

    1. awinn17 | | #2

      Hi Malcom,

      Currently, only natural ventilation. I don’t have a blower door number to provide, and the ERV isn’t set up yet. It’s a 1.5 story cape style, upper and lower air handlers. Most of the load is on the lower system so that’s where this ERV will go. It’s a Braun AI (self balancing) for up to 160 CFM. The downstairs total volume including main room sharing upstairs volume is approximately 17,250 cu ft (give me a 10% margin of error here). I am expecting to run the system at about 2/3 capacity most times with availability for “boosting” it during events if needed. Sized it intentionally a bit large for that reason, and to (hopefully) make it run a bit quieter.

  2. Ryan_SLC | | #3

    That's odd.

    I live in SLC, UT which has some of the worst PM2.5 air in the winter.

    What is air quality around where you live? Are you matching exterior? is your exterior greater than outside? the same?

    Remember, PM2.5 is everything 2.5 and smaller. Filters that aren't high (MERV 14 like Filtrete Elite 2800) are reducing at the largest level only. So while a 3M filter at MERV 13 is going to clean your air, it's not capable of ever collecting 2.5 and below. You have to step up to true HEPA levels. Ultra fine particles of PM2.5 are technically PM.25 by definition, but they aren't 2.5

    I live in a 1980s house that isn't custom or built great. With a single furnace Filtrete 2800 filter I get less than 3 on my reader inside with the outside air being at times the worst in the world. My reader stops at 3.

    1. awinn17 | | #4

      Interesting. I am running MERV 11’s at the recommendation and reading I’ve done for equipment stress. It’s clearly not helping at the PM2.5 level. I have MERV 13’s on the way to trial.

      But- you sort of hit on a question I can’t answer- am I getting it from outside, because my house is leakier than I think? Or am I making it inside and trapping it in because my house is more sealed than I think?

      On the very cold days we had in Jan
      Uses PM2.5 hung pretty low, single digits. That tells me I’m making it and trapping it inside. In the more mild weather cracking a window open a few inches to the stops would bring it down usually, too.

      1. Ryan_SLC | | #7

        In my mind you can exchange the air for unknown air quality or you can know it's filtered well. You can stress your furnace by using during extra heating or you can have a restrictive filter at all times.

        Seems like a filter is the way to go. Fan is set to on for me and a 2800 Filtrete filter during the winter for us. Ozone in the summer can't be furnace filter, so tend to drop down to a 2200 Filtrete in the summer.

  3. Expert Member
    Michael Maines | | #5

    Do you cook at home? If so, do you have a range hood ducted to outdoors and use it? Cooking is one of the largest sources of PM2.5 particles.

    1. awinn17 | | #6

      We do cook. Not every day. We do not have a hood and external duct (and the recirc duct is laughably terrible). It’s high on my list of improvements to get an externally vented hood with sufficient sq foot coverage to catch it all

      1. benwolk | | #8

        Maybe check your PM2.5 levels on the days you cook and the days you don't. That could be the source as Michael suggests. I have a reasonably well sealed house at 3 ACH50 and when I cook without turning the vent hood on, I can see the PM2.5 and more specifically VOCs shoot up on my Awair Element IAQ monitor, even when it's located in the first floor main bedroom, not even in the kitchen.

        Without proper ventilation, it's possible those cooking particulates aren't getting diluted or filtered out and are hanging around in your space.

        15 doesn't seem terribly high to me though. Building a comparetto cube and running it on a timer if you don't want to listen to the box fan all day, would probably help bring those PM2.5 levels down.

        1. Expert Member
          DCcontrarian | | #10

          So you made me google "Comparetto cube." Also known as a Corsi-Rosenthal Box, which has a wiki page:

          I have one in my workshop, never knew it had a name.

          1. benwolk | | #19

            Ah, yea. I for some reason couldn't think of the Corsi-Rosenthal Box name yesterday and the alternative of comparetto cube was stuck in my head. Good reminder that I need to buy a set of filters to build one before the wildfires come this summer.

          2. Expert Member
            DCcontrarian | | #20

            So according to the Wikipedia article the Corsi-Rosenthal Box was "invented" in August 2020.

            According to my Amazon history I bought the filters for mine in May 2020. And I didn't invent it, I got the idea from the Internet, I believe from a woodworking site.

          3. Expert Member
            Michael Maines | | #24

            We talked with John Semmelhack, Neil Comparetto's business partner, about it near the beginning of the show here:

    2. awinn17 | | #11

      So I ran a test yesterday. I opened the windows and turned the HVAC off. I got it down to single digits. Closing the windows and resuming HVAC (changed to AC mode actually first time this year but I think that’s irrelevant) brought it back to a steady 30-33 overnight.

      I am running another test today. I have the HVAC shut off while we’re all away (except our dog) and it has risen to 53 as of this writing and looks to project rough stabilization around 55.

      This seems ridiculously high for zero activity.

      The only thing I left on is the pilot light in our direct vent fireplace and this sensor is right above it. It could cause a draft up to it- it makes a perceptible warm spot ~100 def F on the glass. Shouldn’t be combustion gasses since it’s DV (and CO2 reports as appropriately low).

      I’m completely stumped and kind of freaked by how high it wants to make itself.

      The crawl space is sealed (I did that DIY and I’d give it an 7-8 /10 job). Has a dehu running. My FIL does mold testing and I’m going to have him sample the air for molds. But that’s all I can do in my power for positive identification.

      1. benwolk | | #18

        Try running a test with turning the pilot light off at least a day before when you do the outdoor air flush and take the dog out of the house when you leave the HVAC shut off. With nobody home and windows closed and any combustion appliances turned off, you should hopefully have reduced as many variables as possible to identify what the source is.

        You might be surprised on the particulates that could be coming from your fireplace pilot light despite the direct vent and CO2 level readings. Move the sensor closer to the middle of the space as well and away from the fireplace.

        Do you also have a gas range and wall oven? You might be surprised that the valves might be leaky enough to cause the increase in PM2.5 readings. I'd suggest turning the valves at the appliances and the main gas valve off if possible if you do and want to reduce your variables as much as possible.

        Another source could be any air fresheners in your house. The EPA has a good list of sources here and you can run down the list to see what items you might have in your house that you haven't accounted for:

  4. DennisWood | | #9

    You need a range hood venting outside. Our range hood is automated to the induction cooktop so pm2.5 levels are stable during cook top use. That said, the wall oven is not vented and its use does spike pm2.5 in our home. You should resolve this first before doing anything else.

    Having your ERV running will help.

    1. awinn17 | | #12

      For sure. I avoid cooking things like oils and butters and especially high temp searing meats kind of things just because it trashes the air so badly. It’s high on my list of improvements, believe me. Unfortunately it means move the microwave, which means for us get rid of the double wall oven and replace with a microwave/oven combo, the. I can put in a proper under cab range hood and duct it outside. I 100% will not replace with a recirculating vent. Just has this knock down cost effect for us and I can avoid some of it for a while by modifying how we cook. I can rule it out as a primary cause of day to day poor quality (I commented above some tests I’ve run). But I can trace a direct correlation to cooking as well which highlights it as a major problem, for sure.

      1. Expert Member
        Deleted | | #25


  5. Ryan_SLC | | #13

    I'm not an expert on anything building. But my wife is a pulmonologist. So I like to play her expert proxy.

    If you go to 3M, the merv 11 filter doesn't even reach pm2.5 levels. Let's not worry about stress on your furnace.

    An 11 catches 80 or so percent of 3-10 microns. Those sizes are order of magnitude, exponential sizes within 3-10. PM2.5 starts at 2.5 and then goes into the very bottom ultra small. Merv 11 is getting very little, but some of the 3. Trying to hit 2.5 is noble, but capturing at the top of 2.5 is almost irrelevant.

    An 11 merv filter can't capture PM2.5. The next level at 12 can't capture PM2.5.

    You have to go to merv 13 to even start capturing some PM2.5 which signals it is the higher level sized junk.

    A cheap "True HEPA" filter like you see in the stand alone air filters clean to 99.97%? Well, it's the 0.7% that is not lint you can see. It's the PM2.5 and lower crap.

    At this point, if you did a mer 7 filter to free up the concern of stress on your furnace, I bet your air readings don't drastically decrease.

    If you're looking for clean air, your looking at filtering. Even if the range takes out the biggest contributor, you're clearly "building up" the bad air in your house.

    Peeps here know how to building fix it. I won't even think to pretend understand it.

    I think if you are serious about getting it low readings (you should of course), you're committed to both a higher whole house filter and some stand alone filtering. Stand alone 20" box fans with a 20X20 Filtrete 2800 or expensive room filters aren't free...but seem attractive. They aren't whole house air movers like the furnace filter though.

    You're paying for it in multiple ways. You're buying low burn avocado oil. You're buying increased energy cost on the furnace and stand alone fans. You're changing multiple filters more regularly. But just exhausting outside and bringing in "fresh" won't do it alone.

    You've definitely got an weird situation though. I'd chuck in a 2200 or just bite the worry and go 2800, set fan to always on...I'd bet you're poor air reading would take an immediate hike.

    There is convincing research out of our Asian friends that indoor air quality alone isn't enough to help if you live in outdoor high PM2.5 area. It's exposure, not total exposure alone. However, it sounds like you're the opposite. I'd bite the bullet and start the filter route.

    1. awinn17 | | #15

      That gives me lots of interesting things to consider. I've read up on the range of capture on filters as well and knew before writing that my MERV 11's weren't expected to help with <3 um particles.
      I've been running some tests scenarios to try and see what does and doesn't have an effect on the levels, to rule things out or consider them more strongly. As you mentioned I actually got some MERV 13 filters yesterday and the next scenario I plan to run is swapping filters from 11 to 13. Arguably it should drop to something nominally lower than the MERV 11's. A couple days should say a lot for that.
      I really want to address it at the source to avoid costly and time consuming projects that may amount to nothing. The ERV is overkill for my house, I know that. It's not installed yet, and if I can find the source and address it effectively, I may simply return it. I did get it to help with radon spikes, mostly in winter time, so the discussion I brough up regarding particle levels was just some thinking I was doing about potential fringe benefits. While I do think it would help to change that air entirely every 2-6 hours, I don't expect it to fix the problem.
      I am currently angling for some laboratory air quality testing, hopefully they can give me an idea of the item(s) so I know where to look.

  6. Ryan_SLC | | #14

    Final thought too, duct cleaning might not bad, but it's also kinda questionable.

    Anything that stuck in the duct at this point is staying there or it's not and getting pulled into the filter.

    EPA was/is against cleaning because you are freeing up that stuck crap. Maybe it all gets vacuumed out. Maybe it doesn't. Then you've kinda started the process all over again of the stuck dust breaking off too much or going back to "stuck."

    I'd worry about that item last, but know you might have bad air around for a while after too because of it.

    1. awinn17 | | #16


      I've wondered as much, actually. My duct work is mostly downstairs and the thought I actually came up after discussing with a pro disaster remediation company (to sterilize it properly, too) was just have them do the steel duct trunks and simply replace the flex. It would cost me about the same and the flex part of my system is trashy currently, since it had collected condensation and filled the jackets with water, and they're still sliced open. Losing me that heat and generally being kind of Yuck in the space.

      1. acrobaticnurse_Eli | | #22

        I generally go with the EPA's caution that most duct cleaners are scams with such cleaning being of questionable benefit and even potential harm, but if the flex duct is sliced open after having filled with condensation and is still connected to the HVAC system with air flowing through it that could be a case where ductwork does introduce pollutants. Replacing that could make a difference. I would worry less about the rigid ductwork unless it has the dirty flex duct flowing into it.

  7. DennisWood | | #17

    If you have room for a few inches of expansion in your HVAC setup, and want to reduce PM2.5 with low restriction, take a look at the X6672 Lennox 5" filter (16x25 Merv 16). It has a higher filtration rate while having relatively low static numbers. I'm using them in our HRV pre-filter system. It's more expensive, but you're replacing it every 12 months, not 3.

    Air Flow Rate (CFM) 659 initial resistance (IWC) .10
    Air Flow Rate (CFM) 989 initial resistance (IWC) .17
    Air Flow Rate (CFM) 1319 initial resistance (IWC) .24
    Air Flow Rate (CFM) 1649 initial resistance (IWC) .32
    Air Flow Rate (CFM) 1600 initial resistance (IWC) .31

    1. hockipuck | | #23

      Dennis, I don't have a price comparison but the DustFree MAC13 has decent static numbers also. The DustFree 16 has really low static numbers too. It's quite a bit bigger than the Lennox units though. That's what it takes to get to Merv 16 equivalency.

  8. acrobaticnurse_Eli | | #21

    I started using airgradient's indoor and outdoor air quality monitors last month. It's interesting being able to see the pm2.5 numbers for both inside and out and compare them over time. My indoor levels are consistently lower, likely due to having a hepa filter in the living room, main bedroom, guest bedroom, and now an ERV with a MERV 13 filter. Given the trend on the graph I can see that when my indoor levels do increase it's usually due to outdoor levels going up, maybe from neighbors using their wood stove, but indoor levels rarely get much above 5 and are mostly 0 to 1. We have zero indoor combustion appliances and run the vented range hood whenever using our induction range.

    On the image, the gray line above the bar graph is the outdoor pm2.5 level while the lower part is the indoor level.

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