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Heat Stress, how to measure it, and what it means for us
Heat Stress, how to measure it, and what it means for us
Wendell Porter
September 2026
Most of us are used to checking the outdoor temperature to see what kind of day we will experience. This typical measurement does not tell the whole story. A thermometer measures the outdoor air temperature and does not take into account the amount of water vapour in the air. This thermometer reading is called the dry bulb, or sensible temperature. A short explanation is needed that will show how important water vapour and other factors can also be when it comes to our physical comfort and well-being.
When we are at rest and in a room, our bodies exchange heat to the surroundings via different pathways compared to being active or being in direct sunlight. Much like you feel heat radiating from an open fireplace, your body also “feels” the temperature of the surrounding walls, ceiling, etc. This is why, in winter, in poorly insulated buildings we feel colder than the thermostat says we should. We are feeling the effects of the cold walls, ceiling and floor. This radiative pathway represents about 60% of our heat exchange with the room. Sweating, or evaporation, represents an additional 22% of heat exchange with the environment. Direct interactions between our skin and the air and what we are sitting or standing on (conductive heat transfer) make up the remaining (about 18%) of heat exchange with the environment.
However, if we are working, exercising, or just walking outside, this changes the heat pathways significantly. In these cases, over 80% of our heat exchange happens via evaporation! The wet bulb temperature is used to explain why this is a critical difference.
The wet-bulb temperature is the lowest temperature that air can reach through the cooling effect of evaporation. Simply put, humans exposed to wet bulb temperatures of 35C or greater can no longer cool themselves. This will lead to death, rather quickly if you are walking somewhere in direct sunlight, more slowly if you are resting in the shade.
Measuring all of the factors that cause heat stress has lead to the development of what is known as wet-bulb globe temperature (WBGT).
The table below translates WBGT categories into actionable conditions for outside activity (moderate work and hard work). Relative humidity is extremely important in determining these categories. Often high relative humidity is thought to be 70 to 80 or even 90%. However, a temperature of 38C with a relative humidity of 30% in direct sunlight results in a WBGT of 32.7C which is well into the “black” zone of the WBGT chart where any outside exercise should be forbidden.
Now the problem is that most of us do not have real-time access to WBGT’s! We can use the chart below which combines relative humidity, temperature under full sunlight conditions to calculate a WBGT which are color-coded to the categories in the chart above. It’s easy to see that at temperatures above 40C, there are very few conditions that enable us to work or exercise outside without threatening our health, or life.
In general, we need to be more aware of our surroundings. Where are we going today? What is the weather forecast? Make sure to take a water bottle and a hat, of course. But what if your car breaks down? Will you be in an area with limited cell coverage? These questions need to be in our everyday thinking so that we avoid getting into trouble with heat stress.
I know from personal experience that the short-term effects of heat stress can come on quite suddenly. Prompt attention can lead to a quick recovery, but there are also risks of long-term heat exposure. What about people that can’t avoid the effects of heat in their working lives? Most construction jobs are outside and certainly, most agricultural and mining jobs are outside, too. What happens to people that experience low, but persistent levels of heat stress every day? The long-term cumulative effects can include:
· Permanent loss of kidney function
· Increased risk of heart attacks and strokes
· Cognitive decline, trouble speaking, or learning
· Increased headaches, lack of muscle coordination
People more at-risk of heat-related health problems
Extreme heat can affect anybody. Those more at-risk of heat-related health problems include:
People over 65 years
Babies and young children
People with existing medical conditions, such as heart disease, kidney disease, diabetes, or mental illness
People on certain medications including diuretics (fluid tablets),) and drugs with anticholinergic properties
People who work or exercise outdoors
People who are socially isolated or living alone
People with limited ways to keep cool or living in buildings that heat up easily
People who are experiencing homelessness.
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