What do air quality measures like PM2.5 and NO2 mean?
They are the main pollutants monitored in air quality reporting, and they cause harm through different mechanisms and come from different sources.
PM2.5 — fine particulate matter. Particles smaller than 2.5 micrometres in diameter, roughly a thirtieth the width of a human hair.
Size is what makes them dangerous. Larger particles are filtered by the nose and upper airway; PM2.5 penetrates deep into the lungs, and the smallest fraction crosses into the bloodstream. Health research links it to respiratory and cardiovascular disease, stroke, lung cancer, adverse birth outcomes, and increasingly to dementia. It is generally regarded as the air pollutant causing the greatest burden of harm globally.
Sources: combustion of all kinds — vehicle engines, brake and tyre wear (which continues with electric vehicles), wood and coal burning, industry, agriculture via ammonia forming secondary particles, and cooking indoors.
PM10 is the coarser fraction, including dust and pollen.
NO2 — nitrogen dioxide. A gas produced by high-temperature combustion, overwhelmingly from diesel vehicles in urban areas, plus gas boilers and cooking. It irritates airways, worsens asthma, and contributes to the formation of both ozone and secondary particulates. Concentrations are highly localised, falling sharply within metres of a road — which is why roadside monitoring differs so much from background readings.
Ozone (O3) is different again: it is not emitted directly but formed when nitrogen oxides and volatile organic compounds react in sunlight. This makes it a summer and afternoon problem, and — counterintuitively — it is often higher in rural areas downwind of cities, because fresh NO emissions actually destroy ozone locally.
Sulphur dioxide has declined sharply in many countries following coal reduction.
Note that WHO guidelines are substantially stricter than most legal limits, so "within legal limits" does not mean safe. There is no established threshold below which PM2.5 is harmless.