Why is particulate matter smaller than 2.5µm not given much importance when discussing air quality? That being said, what are considered safe concentrations of PM0.3, PM0.5, and PM1.0?

We generally see the following sized particles discussed the most by air quality initiatives, and companies which make air quality monitors: 10µm, and 2.5µm.

Why is this the case, when the scientific material all talks about particulate matter SMALLER than 2.5µm being harmful for you?

My outdoor air quality sensor shows the following: https://imgur.com/a/jcwNXvr

As you can see, the the 0.3µm, 0.5µm, and 1.0µm readings are quite high, even though the main indicator is showing the air quality as being “clean,” on the basis of the PM2.5 readings.

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 PM10 is particles 10 microns and below. PM2.5 is 2.5 microns and below. (That means PM10 includes PM2.5.)

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I agree with your assessment of harmfulness … the smaller, the more capable of entering the bloodstream, and causing havoc.

My experience - if you smell firewood burning, it’s more than sufficient to register on the meter. In my case, if I smell it, my meter is never saying “clean” (defined as < 50 ).