What is the difference between PM2.5 and PM10 with respect to the atmospheric pollutants?

The difference between PM10 and PM2.5 is size. “PM” refers to particulate matter—particles in the air. Those particles are things like organic dust, airborne bacteria, construction dust, and coal particles from power plants (for example, check out this study from researchers in Shanghai who analyzed what those particles are really made of).

 

Now on to size. The “10” and the “2.5” refer to microns (AKA micrometers). Microns are tiny. Here’s an idea of how small microns are compared to human hair:

 

 

Next there’s a hidden (unlabelled) detail in the terms “PM10” and “PM2.5.” That is the “smaller than” piece. Each pollutant type is defined as that size and below. So PM 10 is particles 10 microns and below. PM2.5 is 2.5 microns and below. (That means PM 10 includes PM 2.5.)

 

What PM is not

Finally, it’s helpful to think of what PM is not. Particulate pollution does not include gas pollutants like ozone and NO2.

For fellow nerds curious to read more about PM 2.5:

  1. I describe how governments measure PM 2.5 here (including the trick of how they get rid of any particles in the air larger than 2.5 microns)
  2. I describe what PM 2.5 does to our bodies in this answer.
  3. In this answer, I detect PM 2.5 in my home in Beijing and test whether a simple DIY air purifier I made can clean it up.

 

Hope that helps!

Thomas is a new Assistant Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

7 thoughts on “What is the difference between PM2.5 and PM10 with respect to the atmospheric pollutants?

  1. If PM10 particles are defined as particles with a diameter below 10 microns, does a PM10 reading also include the PM2.5 fraction (because PM2.5 is lower than 10 microns…)? the fact that PM2.5 can sometimes be higher than PM10 suggest that PM10 are not just particles below 10 microns, rather probably particles below 10 microns and *above* 2.5 microns… Is this right?

    1. Referring back to the article, PM10 particles *includes* PM2.5 particles. PM10 refers to any particle that has a “mean aerodynamic diameter” of 10µm or below. 2.5µm is below 10µm so these particle sizes are included in PM10

        1. My guess is you’re saying the PM2.5 AQI is higher than the PM10 AQI. This is where AQI gets confusing. The scale used for PM2.5 and PM10 is not the same. A PM2.5 AQI of 50 is the same as a concentration of 12µg/m3. A PM10 AQI of 50 is the same as a concentration of 54. So you can’t use AQI to compare the two values, only concentration. See the AirNow AQI calculator here to see how to convert between AQI and concentration for PM2.5 and PM10

  2. Ok, I found this paper which gives the following definition:

    (Taken from the 2016 paper: “Beyond PM2.5: The role of ultrafine particles on adverse health effects of air pollution”)

    “Generally, airborne particles can be defined as ambient airborne particulate matter (PM) which is
    grouped as coarse, fine, and ultrafine particles (UFPs) with aerodynamic diameters within 2.5 to 10 μm (PM10), below-2.5 μm (PM2.5), and below-0.1 μm (PM0.1), respectively.”

    Btw, from this paper, I was disturbed to learn the following:

    “The association of adverse effects on human health with PM10, PM2.5 or PM0.5 exposure had been well described [105–110]. However, increasing evidences indicate that UFPs (PM0.1) especially those originated from human activities may have much more impact on human health than other particle fractions do in PM2.5.”

    This is disturbing for two reasons:
    1. Everyone are checking air pollution by using PM2.5 at most (including governments), which are not measuring UFPs. What if we are looking at the wrong particulate fraction in terms of bad health outcomes?

    2. What about Hepa filters and UFPs? Hepas are rated 99.X percent efficient at filtering particles above 0.3 microns, but what if the bigger health problem is indeed with particles below 0.3 microns? could it be we are filtering out the non-significant particulates (in terms of health) in our homes? on this regard, I don’t think I’ve ever seen an experiment demonstrating that (non-biological) particles between 2.5-0.5 microns are problematic to human health… all the studies that checked the outcome of PM2.5 exposure on human/animal health, are also including the PM0.1 (UFP) fraction, and maybe this is the most (or even only) important fraction regarding health.

    Ragarding #2, at least according to the following site (http://www.sentryair.com/blog/hepa-filter/hepa-filters-are-not-sieves/) 0.3 micron is the most difficult size for HEPAs to filter out. So, if a filter is rated 99% for 0.3 microns, then its filtering ability for above-0.3, or below-0.3 microns (i.e UFPs), is *higher* than 99% efficiency. This is comforting, and hopefully true… but if this was true, how on earth air purifier companies which use HEPA filters, are not happily mentioning this, that HEPAs are more than 99% efficient at absorbing smaller than 0.3 micron particles, which are considered the most dangerous of all? how is it possible that marketing teams are not using this info to their advantage? which makes me somewhat question the validity of this claim…

  3. The following statement is made in the article:
    “…That is the “smaller than” piece. Each pollutant type is defined as that size and below. So PM 10 is particles 10 microns and below. PM2.5 is 2.5 microns and below. (That means PM 10 includes PM 2.5.)…”

    It seems more intuitive that a filter for PM10 filters particles down to 10 microns, and a PM2.5 filters particles down to 2.5. If this is correct filter for PM2.5 will also filter out PM10, but a PM10 filter will not filter out particles PM2.5, which are smaller than PM10? Another way of reading the converse, state in the article is that a PM10 filter would not filter particles that are larger than PM10? Doesn’t seem corret

    1. Good question Peter! Because of the way filters work (they use 3 different mechanisms for filtering out particles), they are actually very good at capturing large particles, and very good at capturing the really small particles. It’s the particle size in between (around the 0.3µm range) that’s the most difficult to capture.

      Most filters are not classed based on the size of the particle they can capture, but on the percentage of the ‘most penetrating particle’ (MPP, which is typically a particle of 0.3µm in diameter).

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