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Background In hot weather, electric fans can potentially provide effective cooling for people, with lower greenhouse gas emissions and cost than air conditioning. However, international public health organisations regularly discourage fan use in temperatures higher than 35°C, despite little evidence. We aimed to determine humidity-dependent temperature thresholds at which electric fans would become detrimental in different age groups.Methods We used biophysical modelling to determine the upper humidity-dependent temperature thresholds at which fan use would become detrimental (ie, worsen heat stress) for healthy young adults (aged 18-40 years), healthy older adults (aged ≥65 years), and older adults taking anticholinergic medication. We also obtained hourly environmental data for the period Jan 1, 2007, to Dec 31, 2019, for 108 populous cities to determine the number of days fan use would be effective for cooling, standardised to a 31-day hot weather month. We established simplified temperature thresholds for future fan use recommendations on the basis of temperatures below which fan use would never have been detrimental between Jan 1, 2007, and Dec 31, 2019, across all prevailing levels of ambient humidity. FindingsAccording to our model, fan use would have been beneficial on 30•0 (96•6%) of 31 hot weather days for healthy young adults and 29•4 (94•9%) of 31 hot weather days for both older adults and older adults taking anticholinergic medication between Jan 1, 2007, and Dec 31, 2019. Adherence to the current WHO recommendation of fan use below temperatures of 35°C only, fan use would have been recommended on 27•2 days (87•7%) of 31 hot weather days. According to our simplified thresholds for fan use (at temperatures <39•0°C for healthy young adults, <38•0°C for healthy older adults, and <37•0°C for older adults taking anticholinergic medication), fan use would have been recommended on 29•6 (95•5%) of 31 hot weather days in healthy young adults, 29•4 (94•8%) days in healthy older adults, and 28•8 (93•0%) days in older adults taking anticholinergic medication between Jan 1, 2007, and Dec 31, 2019. Interpretation Electric fan use, particularly for healthy young adults, would not have worsened heat stress on the majority of study days between 2007 and 2019. Our newly proposed thresholds for fan use provide simple guidelines that improve future heatwave fan use recommendations.

Discussion | Relative to healthy counterparts, physically active middle-aged and older men with well-controlled type 2 diabetes had attenuated heat-loss capacity during exercise in the heat, due primarily to impaired sweat evaporation, which exacerbated thermal (body temperature) and cardiovascular (heart rate) strain. These preliminary findings indicate that exercise heat stress may pose a health concern in patients with type 2 diabetes, especially because physical activity is recommended for diabetes management. 5 However, participants with type 2 diabetes demonstrated a greater improvement in heatloss capacity than did healthy controls after heat acclimation. A randomized clinical trial of repeated brief, supervised exercise is warranted to determine whether heat acclimation during heat exposure offsets diabetes-related thermoregulatory impairments and health complications. Study limitations include the small sample size, the inclusion of only men and patients with well-controlled diabetes, and the specific exercise and environmental conditions.

ObjectivePregnant women are advised to avoid heat stress (eg, excessive exercise and/or heat exposure) due to the risk of teratogenicity associated with maternal hyperthermia; defined as a core temperature (Tcore) ≥39.0°C. However, guidelines are ambiguous in terms of critical combinations of climate and activity to avoid and may therefore unnecessarily discourage physical activity during pregnancy. Thus, the primary aim was to assess Tcore elevations with different characteristics defining exercise and passive heat stress (intensity, mode, ambient conditions, duration) during pregnancy relative to the critical maternal Tcore of ≥39.0°C.DesignSystematic review with best evidence synthesis.Data sourcesEMBASE, MEDLINE, SCOPUS, CINAHL and Web of Science were searched from inception to 12 July 2017.Study eligibility criteriaStudies reporting the Tcore response of pregnant women, at any period of gestation, to exercise or passive heat stress, were included.Results12 studies satisfied our inclusion criteria (n=347). No woman exceeded a Tcore of 39.0°C. The highest Tcore was 38.9°C, reported during land-based exercise. The highest mean end-trial Tcore was 38.3°C (95% CI 37.7°C to 38.9°C) for land-based exercise, 37.5°C (95% CI 37.3°C to 37.7°C) for water immersion exercise, 36.9°C (95% CI 36.8°C to 37.0°C) for hot water bathing and 37.6°C (95% CI 37.5°C to 37.7°C) for sauna exposure.ConclusionThe highest individual core temperature reported was 38.9°C. Immediately after exercise (either land based or water immersion), the highest mean core temperature was 38.3°C; 0.7°C below the proposed teratogenic threshold. Pregnant women can safely engage in: (1) exercise for up to 35 min at 80%–90% of their maximum heart rate in 25°C and 45% relative humidity (RH); (2) water immersion (≤33.4°C) exercise for up to 45 min; and (3) sitting in hot baths (40°C) or hot/dry saunas (70°C; 15% RH) for up to 20 min, irrespective of pregnancy stage, without reaching a core temperature exceeding the teratogenic threshold.

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