Why Do We Use Wet Bulb Globe Temperature?

The term “wet bulb temperature” has gained attention in recent years due to ongoing climate shifts. This is a reality already witnessed in various regions worldwide over the Summer and Fall of 2023. This escalation raises concern, as elevated wet bulb temperatures pose a severe risk to human well-being, amplifying health vulnerabilities and tragically leading to loss of life.

Let’s dive into the essence of wet bulb temperature and its significance for athletic trainers. 

What is the Significance of Wet Bulb Temperature?

Commonly, discussions on temperature focus on air temperature, also termed as dry bulb temperature. Yet, wet bulb temperature has been increasingly making its presence felt, notably in news stories. 

Wet bulb temperature denotes the amalgamation of heat (dry air temperature) and humidity, offering a gauge for evaluating heat stress conditions. The nomenclature stems from its measurement process—placing a moistened cloth over a thermometer’s bulb. The water on the cloth evaporates, causing the thermometer to cool. This resultant lower reading represents the wet bulb temperature.

When humidity is high, impeding evaporation, the wet bulb temperature edges closer to the dry bulb temperature (note: the wet bulb temperature can never surpass the dry bulb temperature). Variations in wet bulb temperature hinge on humidity levels, serving as a reflection of the human body’s capacity to regulate temperature via perspiration (sweat).

Why is Wet Bulb Temperature Important?

Wet bulb temperature is important due to the lethal combination of heat and humidity it signifies, potentially imperiling human lives.

When wet bulb temperature surpasses the human body’s temperature (roughly 36 degrees Celsius), perspiration loses its capacity to evaporate, rendering natural cooling ineffective. This initiates body overheating, and a mere 6 hours at this level can cause fatalities, even during rest.

Wet bulb temperatures at and above 35 degrees Celsius are considered fatal. For instance, 35 degrees Celsius equates to 35 degrees Celsius alongside 100% humidity, 39 degrees Celsius with 75% humidity, or 45 degrees Celsius at 50% humidity.

The Dangers of Humidity and Heat Synergy

Living organisms, humans included, endure within specific temperature thresholds. Elevated air temperature triggers internal temperature regulation mechanisms, involving evaporative cooling (sweating, panting, etc.). The efficacy of this cooling hinges on wet bulb temperature. Elevated wet bulb temperature obstructs sweat evaporation, preventing efficient body temperature reduction.

Normal human internal body temperature ranges from 36.5 to 37.5 degrees Celsius. Excessive internal temperatures lead to hyperthermia, or overheating. 

This underscores the wet bulb temperature’s significance. Beyond its 35-degree Celcius threshold, the body’s temperature continually climbs. Symptoms range from profuse sweating and rapid respiration to nausea, headaches, and dizziness. Left unchecked, plummeting blood pressure may trigger seizures, organ failure, and eventual death.

A Potential Lower Threshold for Dangerous Wet Bulb Temperature

Historically, a lethal wet bulb temperature threshold of around 35 degrees Celsius was accepted. However, contemporary studies indicate a possibly lower threshold. A 2022 Journal of Applied Physiology publication posits the threshold around 31.5 degrees Celsius, even for healthy individuals. Currently, the United States National Weather Service deems 31 degrees Celsius wet bulb temperatures as “extremely dangerous.”

Who’s at Greatest Risk from Elevated Wet Bulb Temperatures?

Elevated wet bulb temperatures disproportionately endanger the elderly, outdoor workers, and individuals with preexisting health conditions.

The elderly face diminished self-regulation of internal temperature, often compounded by illnesses or medications. Even healthy young individuals remain vulnerable –  high wet bulb temperatures can induce hyperthermia even in shaded, breezy settings with hydration available. Those with underlying health conditions are also susceptible due to increased stress on cardiovascular, respiratory, and renal systems, which may already be compromised.

Today’s extreme heat events are unfolding sooner than anticipated. While currently limited to brief durations, concerns mount over their prolonged persistence as the consequences of global warming escalate. The best way to keep your athletes safe is to have good heat policies in place and to follow their guidance. 

The National Athletic Trainers’ Association has several heat-related consensus and position statements: 

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