Air Quality Standards: What They Measure and What the Numbers Mean

Air quality reports use abbreviations and threshold values that can be difficult to interpret without context. Here is what the core indicators actually represent.

PM2.5 and PM10

Particulate matter is measured by particle diameter in micrometres. PM10 includes coarse dust; PM2.5 refers to fine particles that bypass nasal filtering and enter the bloodstream. The WHO guideline for annual mean PM2.5 is 5 micrograms per cubic metre.

NO2 — Nitrogen Dioxide

Produced primarily by road traffic. The EU annual limit is 40 micrograms per cubic metre. High NO2 concentrations are consistently recorded near motorways and in city centres with dense traffic.

SO2 — Sulfur Dioxide

Generated by burning fossil fuels with high sulfur content. Regulations triggered significant reductions after the 1980s, when acid rain visibly damaged forests across central Europe.

O3 — Ground-Level Ozone

Unlike stratospheric ozone, ground-level ozone forms from reactions between vehicle exhaust and sunlight. Concentrations peak on hot, sunny days in summer, particularly in suburban and rural areas downwind of cities.

CO — Carbon Monoxide

An odourless gas produced by incomplete combustion. Indoor sources such as faulty boilers pose the most acute risk, but outdoor monitoring remains standard near heavy traffic corridors.

Reading an air quality index correctly depends on knowing which pollutant is being measured and over which time period the average is calculated — hourly, daily, or annual figures lead to very different interpretations.