What's happened, and what next
What happened
NO2 at Portsmouth's urban background monitor fell from 27 µg/m³ (2001) to 12 (2025); the roadside monitor on Anglesea Road fell from 31 (2018) to 19 (2025). The 200 µg/m³ hourly objective has never been breached at either site. On 2023 data, Alfred Road (43.7) and Hope Street (42.5) exceeded the 40.49 limit value; both fell on 2024 data, and Portsmouth passed the government's State 2 Assessment in January 2026 with no exceedances at any of twelve assessed locations, having failed State 1 in November 2023 with three. Five Air Quality Management Areas declared in 2005 remain in force, none revoked following compliance. Bus journeys reached 11.9 million in the year to March 2025, above the 11.0 million of 2019.
Why it matters
Nitrogen dioxide and fine particulates are linked to respiratory and cardiovascular harm. Portsmouth's density and island geography also make active travel unusually practical — 12.7% walk to work against 7.6% nationally, and 5.2% cycle against 2.1%.
What's being done
A Class B Clean Air Zone has operated since 29 November 2021, charging non-compliant taxis, HGVs, buses and coaches — but not private cars or vans. The council's draft Clean Air Strategy 2025–2040 sets interim NO2 targets of 30 µg/m³ by 2030 and 20 by 2035, against a WHO guideline of 10.
Has it worked?
Air quality has improved substantially — but the Clean Air Zone cannot be credited for it. The council's own study records that traffic on the exceedance roads has NOT fallen since the zone opened, that under 1% of entering vehicles are chargeable, and that there is "limited scope for further NO2 benefits". The largest single fall at the roadside monitor was 2019 to 2020, a year before the zone launched.
What happens next
States 3 and 4 of the government assessment are due at the end of 2026. If Portsmouth passes both, the council plans to decommission the Clean Air Zone in 2027 at the earliest. Three locations — including the former Alfred Road and Hope Street exceedance sites — remain classed as at risk.
Cost
Not published as a single figure for the zone.