Why humid heat is harder on the body in Kisumu, Kenya, and why roofs matter

Heat is harder on the body in a humid place like Kisumu because the body cools mainly by sweating, and damp air slows the evaporation that makes sweat work. The Kenya News Agency (KNA) reported on 19 October 2026 that average relative humidity in Kisumu, in the Lake Victoria basin, is about 70 per cent. The report covered the county's proposed Heat and Cooling Action Plan for 2027–2032.

The plan has not been adopted; the county is working towards adopting it, and the plan was discussed at public forums held from 14 to 16 October in the Muhoroni, Nyakach, Nyando, Kadibo and Seme sub-counties. Proposals include 35 ward-level cooling stations and a three-tier warning system with levels called Normal, Heat Alert and Heat Emergency.

Monthly average temperatures in Kisumu have exceeded the late-1970s baseline by more than 2°C over the past two decades, KNA reported. Researchers have identified a local heatwave threshold: daily maximum temperatures above 37°C for more than three consecutive days. On 8 October Celeste Saulo, Secretary-General of the World Meteorological Organization (WMO), said El Niño was "firmly established and strengthening rapidly". The WMO said El Niño was forecast to peak in December and was likely to bring above-average temperatures to much of the globe.

Sa'ad Lahri of the Division of Emergency Medicine at Stellenbosch University in South Africa wrote in a paper that the body holds its core temperature steady through conduction, convection, radiation and evaporation. Sweating, the paper says, is the dominant cooling mechanism, "but high humidity greatly reduces evaporation". The paper appeared on 30 April 2026 in the African Journal of Primary Health Care & Family Medicine, and its stated aim is "a clear and concise overview of the health effects of extreme heat".

Once humidity exceeds roughly 75 per cent, Lahri writes, sweat cannot evaporate effectively and heat accumulates.

The World Health Organization (WHO) lists the factors in heat stress as high temperature, high humidity, low wind and high thermal radiation. Its heat and health fact sheet, dated 31 July 2026, says that when the body cannot shed heat generated by its own metabolism, the risk of heat exhaustion and heatstroke rises. It also cites studies putting heat-related deaths at approximately 489,000 each year worldwide between 2000 and 2019.

Lahri describes heat exhaustion as a moderate heat-related illness with fatigue, weakness, headache, dizziness, nausea and a racing pulse, and says core temperature may be normal or mildly elevated. Heat stroke, Lahri writes, occurs when core temperature rises above 40°C with central nervous system dysfunction such as confusion, agitation, seizures or coma. WHO calls heatstroke a medical emergency with a high case fatality rate.

The UK's National Health Service lists excessive sweating with pale, clammy skin among the signs of heat exhaustion, and hot skin without sweating, confusion and restlessness, and seizures among the signs of heatstroke.

KNA cites experts who warn that extreme heat poses particular risks to pregnant women, young children, older people, persons with disabilities and outdoor workers. WHO says outdoor and manual workers are exposed to excess heat because of their work, and that the urban and rural poor are often disproportionately exposed to overheating.

WHO lists inappropriate housing materials, for example metal roofs, among the things that amplify human exposure to excess heat, and says informal settlements are often hotter than other urban areas in some cities because of building materials. Felix Odhiambo, Senior Energy Planner and Chief Heat Officer at the county's Department of Infrastructure, Energy and Public Works, said roughly 93.5 per cent of structures in Manyatta, Obunga, Nyalenda and Bandani rely on corrugated iron-sheet roofing, which he said traps daytime heat and radiates it inward at night. The plan proposes community-guided improvements to existing homes, including cool-roofing technologies, insulation and neighbourhood greening, and enforcing building regulations that encourage suitable materials and designs that improve ventilation.

The plan also covers food and fish. It proposes solar-powered cold storage hubs serving markets including Jubilee and Osiri and selected fish landing sites, to reduce post-harvest losses. Odhiambo said the facilities could use phase-change materials for thermal storage, letting them retain cooling capacity from daytime solar energy. A permanent Kisumu County Heat and Cooling Action Committee would connect meteorological information with local health surveillance.

WHO's fact sheet gives household advice. It says to use electric fans only when temperatures are below 40°C, because above that fans heat the body, and to open windows after dark to cool the home with night air. It recommends cool showers or baths and drinking water regularly, one cup an hour and at least 2–3 litres a day, and says to check on vulnerable people, especially those over 65.