Particulate matter, or PM, isn’t just one contaminant or pollutant. It’s a range of particles of dust, dirt, and liquids that become suspended in the air. Some of these are large enough to see, like smoke, smog, or soot, but the most harmful are smaller, invisible particles1. These can get into your lungs and even your bloodstream. The healthier the air, the fewer PMs.

PM comes from both man-made and natural sources outside. The man-made variety can be generated by industry, construction work, landfills, agriculture, motor vehicles with either petrol or diesel engines, and friction from brakes and tires.

Whereas the natural sources include wildfires, pollen-producing plants, spray whipped up from water, soil, and even volcanoes and other seismic activities. 'Secondary particles’ are created when gases react in the air to form PM. Nitrogen oxides emitted by traffic and some industrial gases can become solids or liquids in this way.

As particulate matter is so vast, scientists found a way to measure it. PM1 for example, refers to the size of the particle.

Because particulate matter is categorised by size, PM2.5 includes all PM particles size 2.5 and under. So if your sensor reports high PM2.5 levels, it includes PM2.5 and under. Therefore, PM2.5 levels will always be higher than PM1 levels.

Airthings sensors use a laser scattering-based optical particle counter. As the name says, it uses a laser to detect and count both small and large particles floating in the air. This type of sensor is extremely accurate. More information about accuracy can be found in the sensor specification sheets here.