Researchers analysed daily all-cause deaths in five Moroccan regions in 2021 using a time-stratified case-crossover design. Extreme days had maximum temperature or maximum daily 8-hour ozone at or above the regional 90th percentile. In pooled analysis, heat-only days had a relative risk of 1.18 and compound days 1.16 (95% CI 1.10–1.22). Ozone-only days showed no significant excess, and interaction metrics showed no sign of heat-ozone synergy. Region-specific results were heterogeneous, and only Oriental showed significant associations for all three exposure types. The data cover a single year, which the authors note limits compound-exposure risk estimates.
Why it is interesting: Evidence on compound heat and ozone exposure from North Africa is scarce. In this Moroccan series, heat-only and compound days were linked to excess deaths, with no sign of heat-ozone synergy.
Abstract — the authors' own words, CC BY
Abstract
Background
: Compound heat-ozone events are projected to become more frequent and intense under climate change, with North Africa among the most exposed regions, yet evidence from this region remains scarce.
Methods
: To quantify pooled and region-specific excess mortality associated with heat, ozone, and their co-occurrence across five Moroccan regions in 2021. We used a time-stratified case-crossover analysis of daily all-cause mortality using conditional quasi-Poisson models. Extreme heat and ozone were defined as days with maximum temperature (Tmax) and maximum daily 8-h average ozone (MDA8) ≥90
th
of region-specific percentile, and compound heat-ozone events as same-day co-exceedance or higher ozone within the subsequent.
Results:
In the pooled analysis, heat-only and compound events were associated with statistically significant excess mortality (RR=1.18, 95% CI:1.11-1.1.25 and RR=1.16, 95% CI:1.10-1.22 respectively), ozone-only days showed no statistically significant excess mortality. The region-specific analysis showed a substantial heterogeneity, with Oriental was the only region with significant associations across all exposures. A significant heat-only association was also found in Tanger-Tétouan-Al Hoceima (RR=1.22, 95% CI: 1.11-1.35). The analysis of interaction metrics showed no sign of synergistic interaction between heat and ozone. Results were stable across different lag specifications and sensitivity analysis excluding extreme COVID-19 periods and individual regions.
Conclusion:
extreme heat alone or compounded with high ozone was associated with excess mortality in Morocco. This risk was spatial heterogeneous and not amplified by ozone. These results support region-specific early warning system and stress the need for longer time series to refine compound exposures risk estimates
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