The Guardian • Issue #2090


Why wet bulb matters

Have your ever wondered why different days at the same temperature can feel more or less exhausting? It comes down to heat and humidity Our bodies cool down by sweating. When humidity combined with temperature reaches a certain level our bodies can no longer sweat. This results in heat stress, and can lead to organ failure and eventually death. This threshold is known as the wet-bulb temperature (WBT). The simple reading on a thermometer is referred to as a measure of dry heat.

A simple way of measuring WBT is to wrap a wet cloth around a thermometer. As the water evaporates off the cloth the temperature falls. The temperature at which it can no longer evaporate represents the wet-bulb temperature. In humans this is the point where evaporation can no longer occur through the body’s skin to reduce the core body temperature. It is a measure of heat stress on humans.

Scientists have carried out studies to identify WBT thresholds. They found that the limit for human tolerance was a WBT of 35°C. This would occur at various combinations of heat and humidity. For example, they cited 46°C of dry heat at 50 per cent humidity or 35°C at 100 per cent humidity.

A real-life study of young healthy people found the WBT to be considerably lower at a WBT of 30.6°C, not 35°C. According to Daniel Vecellio, one of the researchers, it would take 5-7 hours exposure for the situation to become dangerous.

The threshold WBT varies from person to person. Small children who are less able to regulate their body temperature are much more vulnerable to lower WBTs than say fit, healthy adults. Likewise, the elderly are more vulnerable. The risk of heatstroke, cardiovascular collapse, and other ill-effects also depends on such things as the length of exposure, level of physical activity, fitness, state of health, and hydration.

It is an important issue for trade union members who work outdoors. For example, in the building and construction industry unions have won limits on working in the sun on unionised sites. These vary from state to state and range from the around 32°C upwards. On union sites in extreme heat workers might work 40 minutes and then rest 20 minutes. These provisions are based on dry thermometer temperatures.

In the recent heatwave in the north west of Western Australia, temperatures ranged from 45°C upwards to as high as 49.9°C in Geraldton. Workers on non-union sites were vomiting on site due to the heat and humidity. They would go home sick and fatigued. If they walked off the job they would not be paid. It gets even worse for those handling steel which might be 70°C. Bosses told them to wear gloves! In such conditions it is important that workers walk off the job. Health comes first. Let the union sort it out with their boss.

WBT has serious implications for all outdoor workers and stands to get a lot worse with climate change.

Heat exhaustion and heatstroke also have serious implications for outdoor summer sports, in particular such sports as running and tennis. Without appropriate cooling, indoor sports can also have devastating consequences. The health and safety authorities have not kept up with the new climate change-related challenges. Apart from taking such measures as keeping cool and hydrated, the message is: join your trade union.

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