Further improvement in London's air quality demands more than the Ultra Low Emission Zone policy

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Conflicts of Interest

Identified Weaknesses

Rating Explanation

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Paper Summary

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London's Clean Air Zone: First Round Knockout, Second Round Flop!

The 2019 Ultra Low Emission Zone (ULEZ1) significantly reduced nitrogen dioxide (NO2) and nitrogen oxides (NOx) in central London and had positive spillover effects, though its impact on fine particulate matter (PM2.5) was limited. However, the city-wide expansion in 2023 (ULEZ3) showed no detectable impact on air pollution, and overall NO2 and PM2.5 levels remain well above World Health Organization guidelines, indicating ULEZ alone is insufficient to achieve truly clean air.

Possible Conflicts of Interest

None identified

Identified Weaknesses

Exclusion of ULEZ2 Expansion

The first ULEZ expansion in 2021 (ULEZ2) was excluded from the analysis due to its overlap with the COVID-19 lockdown, which limits a complete evaluation of the ULEZ policy's phased implementation.

Limited PM2.5 Impact

ULEZ1 showed very limited impact on fine particulate matter (PM2.5), especially at urban background sites, and ULEZ3 had no detectable impact. This is crucial as PM2.5 is a major health concern and road transport is a relatively minor source compared to industrial, commercial, and domestic emissions.

No Detectable Impact from ULEZ3 Expansion

The city-wide ULEZ expansion (ULEZ3) in 2023 showed no statistically significant changes in NO2, NOx, or PM2.5. This indicates a significant limitation in the policy's effectiveness for further air quality improvement.

Air Quality Still Below WHO Guidelines

Despite ULEZ implementation, NO2 and PM2.5 levels across London remained substantially above World Health Organization air quality guidelines, highlighting that the policy alone is insufficient for achieving health-protective air quality.

Confounding Factors for ULEZ3 Evaluation

The ULEZ3 implementation occurred post-pandemic, when commuting patterns had already shifted, and a substantial scrappage scheme might have encouraged compliant vehicle adoption before ULEZ3, potentially masking the direct impact of the expansion itself.

Rating Explanation

This paper employs a robust causal inference framework with weather normalization and a synthetic control method to evaluate a major urban policy. The findings are significant for policy-making, clearly demonstrating the initial success of ULEZ1 but the subsequent lack of impact for ULEZ3, and highlighting the complexity of urban air pollution. While the exclusion of ULEZ2 is a minor limitation, it is justified by confounding factors.

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