Looking at peak values across the sensor network for a week we can assemble a leaderboard of the worst recorded air quality across Ireland in towns where there is a sensor (there may well be places that lack sensors that have poor air quality) Here is the leaderboard for data captured last week.

This measure is often averaged over 24 hours, or we can look at peak values. The sensors take a measurement every hour or so.

We often look at particulates less than 2.5 microns in diameter in microgrammes per meter cubed, this is known as PM2.5 and it is in units of μg/m^3

This is measured by classifying the particulates by size, and working out how much by mass of that size in microgrames there would be in a meter cubed of air.

These floating bits are called particulates, they can be made of all kinds of things including sea salt (which is probably totally harmless) to fossil fuel combustion products (mostly bad). For most purposes we don't need to care what they are, just how concentrated the particulates are.

Air quality is measured by looking at what is in it that isn't air. At the microscopic scale there are little bits of solid stuff floating about. We are talking about stuff that can form a cloud like smoke, but at concentrations usually below visible smoke.