An increase in air temperature as well as humidity may lead to heat stress, and other factors such as physical exercise and clothing can also have an impact.
Heat stress occurs when the body’s means of controlling its internal temperature starts to fail. Some cultures have adapted to avoid heat stress – the Mediterranean siesta signals a work break during the hottest part of the day, or in the tropics where work begins earlier, ends later and is carried out at a slower pace. Heat stress is not something we experience very often in the UK, especially after recent summers, but if we do experience a heatwave – a prolonged spell when daytime and overnight temperatures are elevated and humidity is high - we need to be prepared. The Public Health England published the Heatwave Plan in May to raise awareness of the potential dangers to health from severe hot weather. The reason for this is that severe heat stress can be fatal particularly during a heatwave. The heatwave across Europe in 2003 killed more than 35,000 people during a two week period. Most of the deaths caused by such heatwaves are among the elderly, but it is noteworthy that more than 1,000 of the deaths during the 2003 heatwave were among people aged 35–64.
Our core body temperature of 37°C is maintained by our metabolism. The body defends itself from heat through three mechanisms: breathing, sweating, and changing the blood flow. The first reaction is to circulate blood to the skin, which increases skin temperature and allows the body to radiate some heat. Sweating also helps the body to cool off, but only when the humidity levels are low enough to allow the sweat to evaporate. This rate of evaporation depends on complex interactions between the air temperature, humidity and the wind. High humidity is especially troublesome since it inhibits evaporation of sweat.
Several indices have been developed for measuring the net effect of the ambient environment on human comfort or thermal stress. These indices are used to determine when to cancel sporting events and to establish international standards for rest periods in hot workplaces. Many of these indices require temperature and humidity measurements, and we will take a look at some instruments used to measure the latter on the next page.
Heat stress impact
Warm air can hold more moisture than cool air, so as global temperatures increase so does the humidity in the atmosphere. A recent study published in Nature Climate Change from the National Oceanic and Atmospheric Administration (NOAA) looked at military and industrial guidelines already in place for heat stress, and set those guidelines against climate projections for how hot and humid it's likely to get over the next century. Their findings were stark: "We project that heat stress-related labour capacity losses will double globally by 2050 with a warming climate," said lead
author John Dunne of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton. Work capability has already decreased by 10% in the past six decades during the hottest, more humid periods. Using a moderate projection of future temperature and humidity, they estimate that could drop to 80% by 2050.