Air quality reporting compresses a chemistry lab’s worth of measurements into one number, and it’s worth knowing what got compressed. The pollutants that matter most fall into two families: particles you inhale, and gases that inflame.
The particles: PM2.5 and PM10
“PM” is particulate matter, and the number is a size in microns. PM10 — dust, pollen, sea salt, brake wear — is coarse enough that your nose and throat trap most of it. PM2.5 is the one to watch: at under 2.5 microns (a thirtieth of a human hair), these particles ride past every defence, deep into the lungs and across into the bloodstream. They come chiefly from combustion — traffic, wood burning, industry, and wildfire smoke, which is why a fire hundreds of kilometres upwind can turn a clear-skied city hazardous. PM2.5 is the pollutant most strongly tied to long-term cardiovascular and respiratory harm; the WHO’s guideline annual average is a strict 5 µg/m³, which most of the world’s cities exceed.
The gases: ozone and NO₂
Ground-level ozone (O₃) isn’t emitted by anything — it’s cooked in the air when sunlight works on traffic and industrial fumes, which is why it peaks on hot, still, sunny afternoons and why summer heatwaves and ozone episodes arrive together. It’s a lung irritant that makes exercise outdoors measurably harder. Nitrogen dioxide (NO₂) is the traffic gas, hugging busy roads in sharp gradients — concentrations can halve one street away from the main road.
How the index works
The European Air Quality Index EarthPulse displays takes the measured level of each pollutant, scores each against health-based bands, and reports the worst one — the logic being that lungs don’t average. The bands run Excellent (0–20) through Good, Moderate, Poor, Very Poor to Hazardous (100+). A “Moderate” day typically means sensitive groups — asthma, heart conditions, the very young and old — should ease off strenuous outdoor exercise, while “Poor” and above starts applying to everyone.
The practical reading habit: glance at the index for the verdict, then look at which pollutant is driving it. PM2.5 spiking with wildfire smoke calls for different behaviour (close windows, filter air) than an ozone afternoon (exercise in the morning instead). EarthPulse shows the component numbers next to the index for exactly that reason.