Bath Air-Quality Change: Reading the 2024 Clean Air Zone Measurements
Matched monitoring sites recorded lower nitrogen dioxide in 2024 than in 2019. Here’s how to read the comparison alongside vehicle compliance and business indicators.
Published
Bath’s 2024 Clean Air Zone measurements show a substantial fall in nitrogen dioxide, the pollutant targeted by the scheme. Bath and North East Somerset Council reports that the average annual concentration across 65 matched monitoring sites inside the zone was 40% lower than in 2019. Understanding that result means keeping the monitoring locations, vehicle measures and city-centre business indicators in their separate geographical and statistical contexts.[1]
What Changed at the Matched Monitoring Sites?
The council’s annual report compares the same monitoring locations over time. Its Table 3, labelled provisional, gives an average annual nitrogen dioxide concentration of 32.4 micrograms per cubic metre inside the zone in 2019 and 19.4 in 2024. The urban monitoring group outside the zone also recorded a fall. These are averages across selected monitoring sites, rather than measurements of every street or estimates of each resident’s exposure.[1]
| Monitoring Group | Matched Sites | 2019 Annual Mean NO2 | 2024 Annual Mean NO2 | Council-Reported Reduction |
|---|---|---|---|---|
| Inside the Clean Air Zone | 65 | 32.4 µg/m³ | 19.4 µg/m³ | 40% |
| Bath urban area outside the zone, including Batheaston and Bathampton | 56 | 25.7 µg/m³ | 15.3 µg/m³ | 41% |
Comparison method: this table brings together the council’s matched-site averages and its reported percentage reductions for calendar 2024 against 2019. The report excludes sites with less than 25% data capture, annualises results with 25–75% capture and averages results with more than 75% capture. Annualisation estimates an annual concentration from incomplete-year measurements. Co-located triplicate tubes count as one monitoring location.[1]
The denominator matters. The report lists 72 diffusion-tube locations inside the zone providing annualised data in 2024, but the historical comparison uses 65 matched locations. Substituting the larger network total would change the comparison’s basis. The council chose 2019 as its air-quality baseline because it precedes both the zone and the pandemic’s disruption to travel.[1]
Annual Objectives and Roadside Exposure
Across the CAZ and the surrounding Bath urban monitoring group, the report records no site with a 2024 annual mean above 36 µg/m³. The council’s nitrogen dioxide monitoring page identifies a national annual objective of 40 µg/m³ and a local council objective of 36 µg/m³. An average below an objective across a group would still leave open the possibility of individual hotspots, which makes the site-level finding useful.[1][3]
Location affects what a reading represents. The council explains that monitors sit on roadside signposts, lampposts or footways, and nitrogen dioxide concentrations decrease rapidly with distance from the road. A nearby business therefore shouldn’t treat a roadside annual reading as a measurement of its indoor air or of conditions throughout the working day.[3]
Diffusion tubes also need adjustment. The council applies a bias adjustment factor established by placing tubes alongside automatic reference monitors. This corrects their tendency to read differently from the reference instruments. Separately, the annual report says its results have not been adjusted to remove the influence of weather. Bias adjustment and weather normalisation answer different questions.[1][3]
Vehicle Compliance Has Its Own Denominator
For businesses using vans, the fleet figures offer another comparison. Van and light goods vehicle compliance rose from 63% during the zone’s launch week in March 2021 to around 91% by the end of 2024. That percentage concerns the vehicle category’s compliance with the zone’s emissions requirements. It measures something different from the percentage reduction in roadside nitrogen dioxide.[1]
The report also says chargeable non-compliant vehicles fell from 6% of all traffic entering the zone in launch week to 0.9% by the end of 2024. That all-traffic denominator includes private cars, which were outside the charging categories. Use the van-specific rate when assessing changes in the van fleet, and the all-traffic share when considering how common chargeable non-compliant vehicles were overall.[1]
What the Business Indicators Can Tell Us
The report’s footfall series uses selected city-centre counter locations, including Milsom Street and Southgate Street. It reports annual footfall in 2024 as 1% lower than in 2023. People can be counted at more than one location, so the series is useful for tracking pedestrian activity but cannot establish a total of unique visitors or tell a shopkeeper how much those pedestrians spent.[1]
Property vacancy evidence has another boundary: the council collects the figures for its own assets. Those properties form a particular portfolio. Its vacancy measure cannot establish the vacancy rate for all privately and publicly owned commercial premises across Bath.[1]
District statistics require similar care. The Office for National Statistics’ area page for Bath and North East Somerset, code E06000022, covers a unitary authority and lists places including Keynsham and Radstock. A business indicator for that authority has a broader geography than Bath city centre, the charging zone or either matched monitoring group.[1][2]
The Remaining Question Is Attribution
The measurements establish improvement between the selected years. Assigning a precise share of that improvement to the CAZ remains harder. The report describes a longer downward trend, natural replacement of older vehicles and pandemic-related changes in travel. Weather also influences concentrations, and its effects remain in these annual results.[1]
When comparing a later report with 2024, check whether it retains the same monitoring locations and adjustments. For a particular Bath premises or street, start with the council’s monitoring-location information and the relevant annual site result. That preserves the local detail which a zone-wide average necessarily smooths out.[1][3]
Sources and checks
These sources were used for this article. Availability, prices and service details can change after the stated check date.
- https://www.bathnes.gov.uk/sites/default/files/Clean%20Air%20Zone%20full%20report%202024.pdf
Checked 20 October 2026.
Table 3 reports matched-site annual mean NO2 concentrations of 32.4 µg/m³ in 2019 and 19.4 µg/m³ in 2024 inside the CAZ, using 65 sites. The urban group outside the CAZ uses 56 matched sites and records 25.7 µg/m³ in 2019 and 15.3 µg/m³ in 2024. The executive summary reports reductions of 40% inside the CAZ and 41% in the urban group outside it. The comparison excludes sites below 25% data capture, annualises sites with 25–75% capture and averages sites above 75% capture. Triplicate tubes count as one site, and the full 2024 CAZ diffusion-tube network contains 72 locations. No site in the CAZ or CAZ_Boundary group recorded a 2024 annual mean above 36 µg/m³. The air-quality results have not been adjusted to remove weather effects. Van and LGV compliance rose from 63% at launch to around 91% by the end of 2024. Chargeable non-compliant vehicles declined from 6% of all traffic at launch to 0.9% by the end of 2024. The footfall series covers selected city-centre locations, reports a 1% annual decline from 2023 to 2024 and can count individuals at multiple locations. The property vacancy figures concern council-owned assets. Longer-term fleet replacement, weather and pandemic-related travel changes limit causal interpretation of the before-and-after comparison.
- Bath and North East Somerset (E06000022) - ONS
Checked 20 October 2026.
The ONS page concerns the Bath and North East Somerset unitary authority, identified by E06000022. The authority page lists Keynsham and Radstock among its constituent places, supporting the distinction between district and Bath city-centre geography.
- Nitrogen dioxide monitoring data | Bath and North East Somerset Council
Checked 20 October 2026.
Roadside monitor readings depend on location, and nitrogen dioxide concentrations decrease rapidly away from the roadside. Diffusion-tube bias adjustment uses co-location studies with automatic reference monitors. The council identifies national and local nitrogen dioxide objectives of 40 µg/m³ and 36 µg/m³ respectively. The council’s monitoring information provides site locations and annual average nitrogen dioxide results.