| 15 Oct 2025
Global hotspots and mechanisms of extreme humid heat and air pollution co-occurrence
Abstract. Exposure to extreme humid heat and air pollution each represent significant, well-characterized environmental hazards to human health. But the questions of where, when, and why they may co-occur, and whether humid heat may exacerbate pollution relative to high temperatures alone, remain largely unexplored. Here, we identify regions worldwide where ozone (O3) or particulate matter (PM2.5) pollution tend to be higher during humid versus non-humid extreme heat – i.e., where increased moist heat stress tends to co-occur with increased pollution, revealing a compound hazard tendency – and characterize the meteorological and chemical drivers of this co-occurrence. We analyze 19 years of near-surface concentrations of ozone, PM2.5, and related species (NO2 and HCHO) in the Copernicus Atmosphere Monitoring Service global chemical reanalysis (CAMSRA), along with meteorological conditions from the European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5). We find that the global hotspots of worsened pollution during humid heat overlap with several global hotspots of extreme humid heat itself, and include multiple densely-populated areas. Altogether, more of the global population experiences worsened air quality during humid heat (versus dry heat) than experiences cleaner air quality. Overall, we find that humid heat and pollution co-occurrence hotspots typically occur where (1) the near-surface background chemical makeup is more urban (higher NO2, lower HCHO), and (2) humid heat is associated with stagnation and suppressed boundary layer heights (as is common in areas that experience severe humid heat), such that the local meteorological drivers of extreme humid heat are also conducive to pollutant accumulation.
This manuscript investigates an important and increasingly relevant issue, the co-occurrence of extreme humid heat and air pollution. The analysis relies heavily on spatial and temporal processing steps that lack sufficient validation, and key interpretations are not supported by robust statistical evidence or a nuanced understanding of the underlying mechanisms.
(1) The handling of different spatial resolutions between CAMSRA (0.75°) and ERA5 (0.25°) is problematic. Merely stating that "ERA5 grid cell at the center" of each CAMSRA cell is used is insufficient. This spatial mismatch introduces potential biases that need to be quantified and addressed more thoroughly. The method should be explained with consideration of how this choice affects results and whether it may bias the analysis toward certain regions.
(2) The use of 95th percentile for defining extreme heat days is reasonable but requires more justification. Alternative thresholds should be tested or discussed to demonstrate robustness of findings.
(3) The manuscript reports significant results based on data processing steps (e.g., compositing, anomaly calculations) that have not been adequately validated. The interpretation of statements like "increased humidity corresponds with worsened pollution" requires robust support. The authors should demonstrate the reliability of their methodology through independent validation or sensitivity testing.
(4) The use of a bootstrapping procedure for regional composites is positive, but its lack of application to the global-scale analysis is a significant oversight. Statistical significance should be consistently reported for all major findings to ensure the reliability and reproducibility of the results.
(5) The claim that “humid heat is associated with stagnation” is presented as a factual statement rather than a correlation. The authors must acknowledge the complex and often bidirectional relationship between humid heat and stagnation, drawing on existing literature on humid heat mechanisms. Stagnation creates humid heat, but humid heat also creates stagnation through various feedback processes.
(6) The suggestion of a causal link between background chemistry regimes and co-occurrence patterns lacks sufficient experimental design to support it. While the correlation between NO2/HCHO ratios and co-occurrence is interesting, it requires cautious interpretation. Correlation does not equal causation, and alternative explanations should be considered.
(7) Data and Method section requires substantial revision to address the issues outlined above, particularly regarding spatial resolution, threshold selection, and validation of processing steps. The results section should be revised to reflect the limitations of the methodology and avoid overstating the strength of the findings.