Heat plus high ozone: mortality risks stack up

Heat and ozone tend to arrive together, because the sunlight and stagnant air that drive a hot spell also drive ozone formation. Most health research studies them separately, which risks missing what happens when they coincide — a scenario climate change is expected to make more common. Compound events may act differently from either exposure alone, and public warning systems are generally built around one hazard at a time. Establishing whether co-exposure carries a distinct risk is the first step towards deciding whether they need to be managed jointly.

Five databases — Embase, PubMed, Scopus, Web of Science and ProQuest — were searched from inception to May 2025, with the protocol registered in PROSPERO in advance. Of 20348 articles identified, 78 met the inclusion criteria for the systematic review, and 28 provided data suitable for pooling. A random-effects meta-analysis estimated the relative risk of health outcomes on compound heat-ozone days compared with non-co-exposure days. The authors also examined the spatial distribution of the evidence and assessed risk of bias across the included studies.

Co-exposure to heat and high-level ozone was associated with an increased overall mortality risk, with a pooled relative risk of 1.17 and a 95% confidence interval of 1.14 to 1.20. Cause-specific estimates showed a relative risk of 1.20 for all-cause mortality, 1.14 for cardiovascular mortality and 1.20 for respiratory mortality. Raised risks appeared across different age and gender groups and in urban areas. The authors identified a risk of bias arising from inadequate adjustment for confounders and unclear methods for imputing missing data.

This is the first pooled synthesis of the compound heat-ozone literature, and it supports the idea that the two exposures together track with higher mortality than days without both. It does not disentangle how much of that risk belongs to heat, how much to ozone, and whether the combination is more than the sum of its parts. Inconsistency in how studies defined a compound event, which health outcomes they measured and where they were conducted limits how firmly the pooled figures can be interpreted. The authors call for more comprehensive work rather than treating the question as settled.

This is a reason to treat hot, high-ozone days as a combined risk period rather than thinking about temperature alone, particularly in urban areas where the raised risks were observed. The estimates come from observational studies of population mortality, so they describe deaths across a region during such conditions, not an individual's odds on a given day. Respiratory and cardiovascular deaths were the categories showing elevated pooled risks. Nothing here specifies a threshold or an action, and the authors themselves flag the quality limits of the underlying studies.