Nitrogen oxides (NO2 and NOx)

General

• NO2 sources in the UK: 33% construction and industry, 30% road transport, 16% non-road transport, 4% agriculture, 4% wood burning, 0.3% waste, 13% other (Healthy Air Coalition, 2025)
• Nitrogen Dioxide (NO2) from burning for fuel in power plants, vehicles, industrial facilities, traffic, agriculture. 42% world pop in levels above annual guideline (State of Global Air, 2024)
• WHO's 2021 Global Air Quality Guidelines recommend an annual mean NO2 limit of 10 µg/m³, four times stricter than the 2005 guideline of 40 µg/m³ (WHO, 2021)
• The US EPA's Integrated Science Assessment concluded there is a causal relationship between short-term NO2 exposure and respiratory effects, including worsened asthma (US EPA, Oxides of Nitrogen ISA, 2016)
• Road traffic is the dominant source of urban nitrogen dioxide, with exposure shaped by traffic intensity, fleet composition and street layout (Mucha & Mainka, 2026)
• The fraction of NOx emitted directly as NO2 (primary NO2) rose with the growth of diesel vehicles, slowing improvements in roadside NO2 (AQEG, 2007)
• Real-world vehicle NOx emissions have often exceeded laboratory test values — a key reason roadside NO2 limits proved hard to meet (AQEG, 2021)