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!

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According to my laser particle counter, the number of particles in t he 2.5 micron size range is typically in the 10-90 thousand per cubic meter (in a non-smoggy city) compared with 10 times that number for the 1 micron size range and 10 times that number again for the 0.3 micron size range. ( I could not find comparable data from the EPA because their measurements are in weight measures). All of these represent perhaps one percent of particulates, most of which are below 0.1 micron. Thus, it would appear that the molecular nature of the 2.5s is a… Read more »

Peter

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… Read more »

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).

Noam

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… Read more »

Noam

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?

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

Noam

So how come PM2.5 is sometimes higher than PM10? this should have been impossible if PM10 includes PM2.5…

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

Peter

We too have PM2.5 readings measured in mg/m3 which are higher than the PM10 reading taken at the same time, also measured in mg/m3. Is this a result produced by the margin of error within the equipment itself? How else is this possible?

That’s very strange. PM2.5 concentration (in mg/m3) are very rarely higher than PM10 concentrations (in mg/m3). It could be an issue with the equipment, or the units being displayed, but in general PM2.5 concentration is going to be lower than PM10, as per my comment above!