About one third of the world’s population experiences at least 20 days per year of extreme heat conditions that can be considered deadly.

Block quote: In heatwaves we need to change the way we think about our environment.

The most vulnerable groups are children, older adults and individuals with type 2 diabetes especially those with associated co-morbidities such as obesity and cardiovascular disease.

The increased vulnerability is probably due to disease- or age-induced inability to dissipate whole body heat. Humidity reduces heat loss because we sweat less. This is because the air is already saturated with water vapour and this prevents sweat evaporation, which is our primary heat loss mechanism.

Hot weather not only increases mortality, but also leads to less obvious health problems, such as a higher risk of kidney disease, reduced sleep quality, increased violence and higher suicide risk and reduced work capacity.

Excess heat can also inhibit preventive medicine, especially vaccination. Vaccines can only save lives if they remain cool. Most vaccines are developed for a temperature range of 2-8°C (35-46°F) but are designed to withstand short periods of higher temperatures. However, most live vaccines will spoil immediately if frozen. In heat wave conditions, vigilance around keeping vaccines cool increases and freezing becomes a problem in up to 75 percent of vaccine supply chains.

Health risks don’t just end when the temperature returns to the standard range. Increased morbidity can continue for three to four days after a maximum temperature peak.  One of the reasons is that, even when outdoor temperatures at night time decrease from peak daytime levels, indoor temperatures in our homes remain elevated. For healthy adults an indoor temperature range of between 18 and 24C (64.4 and 75.2F) is associated with minimal risk to health.

The extent to which our homes remain baking ovens depends on the level of insulation, surrounding green coverage, occupation density and use of heat generating appliances such as lights and cook tops. In the United Kingdom, it has been estimated that overheating affects more than 20% of households, underlying the danger that extreme heat events may have in vulnerable homes.

In heatwaves we need to change the way we think about our environment. For example, in most climates that experience four seasons, houses are designed to protect from extremes of cold not heat. In conditions of extreme heat, winter chilling drafts can be turned into summer cooling ones by removing heat retaining curtains and opening windows to encourage flow-through ventilation.

Changing health practices, for example around hydration, comorbidities and vaccination may not be that straightforward.