Why should we Care about the Wet Bulb Temperature?

Wet bulb temperature is a term that you might have heard crop up in the news over the last few years - and that’s because it’s something that’s becoming a growing threat. As our climate continues to warm, the dangerous wet bulb temperature threshold is increasingly likely to be broken - and already has been broken in some regions across the world. This is concerning because high wet bulb temperatures are a threat to human health, making health issues more likely and even resulting in human deaths. 

👉 In this article we’ll discuss what the term wet bulb temperature actually means, why it’s a growing concern, and what we can do to mitigate its effects.

Usually when we talk about the temperature we’re talking about air temperature - also known as dry bulb temperature. But you might also have heard about wet bulb temperature - it’s increasingly cropping up in the news these days, and for good reason (but more on that later), let’s first explore what wet bulb temperature actually means. 

Wet bulb temperature is a measure of heat (ie. dry air temperature) and humidity. It’s a method of assessing heat stress conditions. The name stems from how the measurement is actually taken - you basically place a wet cloth over the bulb of a thermometer, and as the water from the wet cloth evaporates the thermometer will cool down. The resulting lower temperature is what's known as the wet bulb temperature. 

If the humidity level is high (ie. the air is already saturated with a lot of moisture) less evaporation will take place which means that the wet bulb temperature will be higher and closer to the dry bulb temperature (note: the wet bulb temperature can never be higher than the dry bulb temperature).

Wet bulb temperature is an important measurement because the combination of heat and humidity can prove fatal for humans. 

When the wet bulb temperature rises to above the temperature of the human body (around 36 degrees celsius), sweat is no longer able to evaporate which means that the human body is unable to cool itself down. 

Once this wet bulb temperature is breached the human body will begin to overheat. It only takes around 6 hours at this temperature for death to occur, even if you simply sit there. 

Living organisms such as humans can only survive within a certain temperature range. When the air temperature is too high our bodies work to regulate our internal temperature through evaporative cooling (ie. sweating in humans and horses, and saliva and panting in other mammals). However, the effectiveness of this evaporative cooling mechanism depends on the wet bulb temperature. When the wet bulb temperature is too high this essentially means that the sweat on our skin can no longer evaporate, meaning that we are no longer able to cool down our body temperature.

In simple terms hyperthermia is overheating. It’s where the body absorbs more heat than it is able to expend. The most common cause of hyperthermia is heat stroke, which results from excessive exposure to heat, or a combination of heat and humidity. 

This is why wet bulb temperature is so important, because above the wet bulb threshold (generally understood to be around 35 degrees celsius) the temperature of the human body will continue to rise. The effects on the body are extreme: initial symptoms include heavy sweating, rapid breathing, and a fast pulse. This can then progress to nausea, vomiting, headaches, low blood pressure and dizziness. If the condition continues the dropping blood pressure may cause seizures, unconsciousness, organ failure and eventually death.

Traditionally, it’s been accepted that the fatal wet bulb temperature threshold is around 35 degrees celsius, however recent studies suggest that the threshold may actually be lower.  A 2022 study, published in the Journal of Applied Physiology, showed that the wet bulb temperature is actually considerably lower - at around 31.5 degrees celsius, even for healthy subjects. 

👉 The United States National Weather Service considers wet bulb temperatures of 31 degrees celsius to be an “extreme danger”.

Those who are most vulnerable to high wet bulb temperatures are the elderly, those who work outdoors, and those with underlying health conditions. 

This is because the elderly have a reduced ability to self regulate their internal body temperature and are more likely to suffer from disease or to take medication which can complicate the situation. 

Those working outdoors are at an increased risk due to their direct exposure to heat and their inability to escape the conditions. Additionally, even though hyperthermia can occur without any exertion required, physical exertion can speed up the process and makes the condition more likely. 

Finally, as is the case with the elderly, those with underlying health conditions are more prone to hyperthermia due to the excess stress that high temperatures and humidity put on the cardiovascular, respiratory and renal systems - systems that may be damaged by health conditions and unable to cope with increased stress.  

Certain regions of the world experience higher wet bulb temperatures more than others and studies have found that parts of South Asia (most notably India and Pakistan), southwestern North America, and areas around the Persian Gulf have already experienced conditions that breach the threshold for deadly wet bulb temperatures. 

👉 Research found that there have been around 1,000 incidences of a wet bulb temperature of over 31 degrees celsius, and around a dozen cases where it reached over 35 degrees celsius (if only for a few hours) in Pakistan, India, Saudi Arabia, Mexico and Australia. 

Scientists originally predicted that we’d reach these wet bulb temperature thresholds several decades from now and even towards the end of the century, but what we’re actually seeing is that these extreme heat events are happening now. 

So far incidences where the wet bulb temperature has breached 35 degrees celsius have only lasted for a few hours at a time, but the concern is that the duration for which they last will increase as the effects of global warming continue to worsen.

There is a direct link between rising wet bulb temperatures and climate change. Heatwaves are one of the main impacts of our warming planet and scientists estimate that heatwaves are now at least 100 times more likely to occur. Not only this, but climate change is also making heatwaves more intense and increasing temperatures by as much as 3.5 degrees celsius (vs a world without man made carbon emissions).

Therefore, if we want to prevent increasing wet bulb temperatures - and human deaths - it is essential that we prevent further global heating. If we fail to do so it will mean that some places on Earth become uninhabitable - something that will upend livelihoods, put strain on national health services, and even force migration.

Individual action

At an individual level, actions that should be taken to prevent reaching a state of hyperthermia include staying in the shade, hydrating, limiting any physical exertion, and wearing light clothing. However, where the wet bulb temperature threshold is breached for an extended period of time it will be essential to lower the body temperature through some other means - for example by finding access to air conditioning.

Detection

The areas that are most vulnerable to dangerous wet bulb temperatures tend to lie within subtropical regions, and are more likely to be developing countries. Often such areas don’t have reliable weather stations which makes it difficult to predict and to understand such extreme weather events. 

NASA has developed satellites such as the Atmospheric Infrared Sounder (AIRS) and the ECOsystem Spaceborne Thermal Radiometer Experiment (ECOSTRESS) to provide useful data on heat stress. This will allow scientists to better understand the process and to develop climate models that will allow us to develop more effective mitigation strategies.

Eliminate carbon emissions

Ultimately the only way to prevent the increasing frequency and severity of wet bulb temperatures is to prevent further global warming. This means cutting out all greenhouse gas emissions and working to prevent climate change. 

Until we do this, the Earth’s climate will continue to warm, and the impacts will become increasingly severe. This means more heatwaves, hotter heatwaves, and heatwaves that last longer - making the risk of deadly wet bulb temperatures more likely.

As climate change heats up our world, wet bulb temperatures are a growing issue. With heatwaves now 100 times more likely, we can expect to see increasing incidences where the dangerous wet bulb temperature threshold is breached - and in fact, we’re already seeing evidence of this acceleration. 

Heat stress is already a leading cause of weather related deaths across the globe, and  breaching these dangerous wet bulb temperature thresholds means that these figures are set to rise. The only way to prevent these harmful levels of heat and humidity is to prevent further global warming - this is why it's so important that we work towards eliminating carbon emissions and reducing our environmental footprint.

At Greenly we can help you to assess your company’s carbon footprint, and then give you the tools you need to cut down on emissions. Why not request a free demo with one of our experts - no obligation or commitment required. 

If reading this article has inspired you to consider your company’s own carbon footprint, Greenly can help. Learn more about Greenly’s carbon management platform here.