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# Heat alone and heat with ozone linked to excess deaths in five Moroccan regions
- URL: https://www.coios.me/heat-alone-and-heat-with-ozone-linked-to-excess-deaths-in/
- Published: 2026-11-09T12:50:52.000Z
- Updated: 2026-11-09T23:34:52.000Z
- Description: Heat-only and compound heat-ozone days were linked to excess deaths in Morocco in 2021, but ozone-only days were not.
- Author: Daniel Ryan
- Tags: Tomorrow's world, A changing climate, Morocco, #preprint, #refines, #item, #source-2026-11, #score-50

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.*

Source

[Research square, 8 November 2026](https://doi.org/10.21203/rs.3.rs-10671369/v1?ref=coios.me)

Paper

Mortality associated with compound exposure to heat and ozone across five Moroccan regions

Authors

Adarbaz M, Khomsi K, Ilbadisy I, Houdou A, Mendili JE, Marnissi AE et al.

DOI

10.21203/rs.3.rs-10671369/v1

Full text

[Open copy](https://pmc.ncbi.nlm.nih.gov/articles/PMC13742801/?ref=coios.me) (open-access copy, PubMed Central)

Type

Preprint

Design

Time-stratified case-crossover with conditional quasi-Poisson models, daily all-cause mortality, five Moroccan regions, 2021

Verdict

Refines prior evidence

Driver

[A changing climate](https://www.coios.me/d-climate/)

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