What is wet bulb temperature and how is it related to heat stress?
We're all quite familiar with air temperature. We know that we can measure it by setting out a thermometer, and that we can use the reading from the thermometer to decide how we should dress for the day and whether we should avoid going outdoors.
The numbers we get from those thermometers are known as the dry bulb temperature, since they're based on the way mercury inside a glass bulb reacts to the heat.
It's a clever invention and one that has been incredibly useful. But there's just one problem: the human body isn't a glass tube filled with mercury.
Because of this, the temperature measured by a thermometer doesn't reflect the way we actually experience ambient heat, which is highly influenced by humidity and wind speeds.
Wet bulb temperature takes those factors into account, which is what makes it so useful. Because the dry bulb temperature may tell you how hot it is, but it's the wet bulb temperature that measures how hot it feels.
Measuring the Wet Bulb Temperature
Wet bulb temperature is a method used to capture both the ambient air temperature and the way it interacts with humidity and air movement.
It starts with a temperature sensing device (a thermometer or thermocouple) covered in a wet cloth, and air is passed over the surface actively or passively. A sling psychrometer (or similar device) is then used to measure the resulting temperature.
The intention is to capture the effect of evaporation, which also happens to be the body's natural cooling mechanism. Sweat evaporates from the surface of the skin, helping to manage the body's temperature. However, if there is too much atmospheric humidity, water or sweat won't evaporate as easily. At 100% humidity, no evaporation takes place at all. At that point, the wet bulb temperature will be equal to the dry bulb temperature.
(Learn more in Are Heat Edemas a Safety Issue?)
Wet Bulb Temperature and Heat Stress
Wet bulb temperature is an important factor when making decisions about occupational heat exposure and heat stress.
Air temperature is still part of the picture, but wet bulb temperatures more accurately capture how the atmosphere will affect the human body - and which control measures should be in place to help workers cool down.
(Learn about Implementing Control Measures for Heat Hazards)
The role of humidity is especially significant. The greater the humidity levels, the more difficult it is for the body to regulate its temperature. Very humid heat, then, should be treated as an additional safety hazard that needs to be addressed.
Making decisions based on the dry bulb temperature is one of the reasons some workers or their supervisors underestimate the risk of succumbing to heat stress. That's why it's important to consider all the factors that can contribute to a hot atmosphere, not just the ambient temperature.
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