Interpreting Particle Matter (PM) sensor data can be challenging. A question we get often asked is: why is the PM2.5 value sometimes identical to the PM10 value? The quick answer is: because the are no particles above 2.5µm present. Another question we often get is: what is the difference between Number Concentration and Mass Concentration.

To start understanding the first question, i.e. why is the PM2.5 value sometimes identical to the PM10 value, it is important to look at the definition of PM2.5 for a PM sensor: PM2.5 covers the particles size 0.3µm to 2.5µm. And PM10 means all particles between sizes 0.3µm and 10µm. So if the PM2.5 and PM10 numbers are identical, it means that all the particles are smaller or equal to 2.5µm and that there are no particles between 2.5µm and 10µm.

The second question is about the difference between Number Concentration and Mass Concentration. What is important to understand here is that a particle of 0.3µm is much smaller and much lighter than a particle of 10µm. So even if there are a lot of particles of size 0.3µm, their mass will always seem negligible. The opposite is true for the 10µm particles, even a very small number of particles will have a huge impact on the mass concentration. Being able to look at both numbers is therefore important. For example, when looking at filtration efficiency, number of particles in the air is more indicative than mass concentration. However, mass concentration is the standard way to express PM values (in µg/m3); the higher the mass, the more dangerous it is to breathe.