HOW TO PROTECT STUDENTS FROM SEVERE HEAT-RELATED ILLNESSES: THE HEAT INDEX VERSUS WET-BULB GLOBE TEMPERATURE

1.????OVERVIEW

?Heat-related illnesses are historically the leading cause of weather-related death across the United States according to the NWS (National Weather Service 2021). High-risk individuals such as students, workers, athletes, and the elderly are particularly susceptible to severe heat-related illnesses.?Heat-related illnesses include heat stroke, heat exhaustion, heat cramps, sunburn, and heat rash. They often occur when a person is exercising, working, or engaging in an activity when temperatures and humidity are high (Kerr 2011). To help mitigate the risk of severe heat-related injuries, the Wet-Bulb Globe Temperature (WBGT) and the Heat Index (HI) are indices used to assess the local environmental conditions.

This paper will discuss the history of the two indices and how they compare to each other, the hurdles of implementing widespread usage of the Wet-Bulb Globe Temperature, and a brief overview of heat-related illnesses.

2.????HISTORY

The Wet-Bulb Globe Temperature (WBGT) was invented by the United States Military during the early 1950s in response to the number of heat-related illnesses and casualties occurring in the United States Armed Services during the 1940s and 1950s. After implementing the Wet-Bulb Globe Temperature, the military made modifications to training sessions when environmental conditions were extreme by adjusting water intake and rest periods for service members. The United States military began to see a dramatic drop in heat-related illnesses among service members (ACSM 1984, ACSM 1987).

Three measurable atmospheric variables are used to calculate the Wet-Bulb Globe Temperature.

WBGT = 0.7*Tw + 0.2*Tg + 0.1*Td ?????[1]??????

Tw is the natural wet-bulb temperature, which is used to estimate evaporative cooling

Tg is the black globe temperature, which is used to estimate radiant heat

Td is the dry-bulb air temperature

The Wet-Bulb Globe Temperature Globe temperature has been widely implemented across the globe for the measurement of heat stress during physical activities. Organizations such as the U.S. Occupational Safety and Hazard Administration (OSHA), the National Athletic Trainers Association (NATA), the National Collegiate Athletic Association (NCAA), the American College of Sports Medicine (ACSM), the Korey Stringer Institute of UCONN (KSI), the International Standards Organizations (ISO, the United States Military, the National Football League (NFL), the Major League Soccer (MLS), the International Olympic Committee (IOC), and local school districts have all adopted the Wet-Bulb Globe Temperature to mitigate illnesses caused by severe heat.

The Heat Index (HI) was invented in 1979 by Robert G. Steadman for the National Weather Service after extensive biometeorological studies. The Heat Index is a simplified energy balance model that combines several measurable atmospheric variables (Dry Bulb Temperature and Relative Humidity) with non-modifiable environmental atmospheric conditions and behavioral and physiological assumptions of the human body (human body mass and height, clothing, etc.). A sample of these assumptions that are made in the Heat Index equation includes a model human that is 5’7’’ tall, 147 pounds, wearing pants and a short-sleeve shirt that covers ~84% of the body, walking outdoors at 3.1 mph, with light winds of 5 knots.

The Heat Index is designed as a measurement of the “feeling” a certain combination of temperature and humidity has on the human body. Therefore, it is an objective measurement depending on the person in a specific environment (Steadman 1979, Rothfusz 1990).

Compared to the Wet-Bulb Globe Temperature equation, the equation for Heat Index is far more complex.

HI = C1 + C2*Td + C3*RH – C4*Td*RH – C5*Td2 – C6*RH2 + C7Td2*RH + C8*Td*RH2 – C9*Td2*RH2 ??[2]???

C1 = -42.379

C2 = 2.04901523

C3 = 10.14333127

C4 = 0.22475541

C5 = 6.83783 x 10-3

C6 = 5.481717 x 10-2

C7 = 1.22874 x 10-3

C8 = 8.5282 x 10-4

C9 = 1.99 x 10-6????????????

Td is the dry-bulb air temperature

RH is the relative humidity

The Heat Index measurement has been widely adopted by the United States and Canada (referred to as the Humidex). The Heat Index is the primary heat stress measurement that the public is exposed to due to its use in the media and news agencies.

3.????Heat-related Illnesses

Excessive heat is historically the deadliest meteorological phenomenon in the United States. In the chart below (Figure 1) the National Weather Service and The Center for Disease Control and Prevention (CDC) track the fatalities each year by their environmental category, with Heat being the leading cause of death over a 30-year average. These fatalities are caused by heat-related illnesses.

Figure 1 Weather-related fatalities for the year of 2020

The CDC quantifies heat-related injuries into six categories. These include, from least to most severe: Heat Rash, Sunburn, Heat Cramps, Heat Exhaustion, and Heat Stroke. Each heat-related injury has a definition and treatment plan as recommended by the CDC (Figure 2). These illnesses are common in students and athletes across the country, especially during the summer and early fall when students and athletes are beginning their conditioning for their activities. During this time, heat acclimatization in students and athletes is reduced, so there is a greater risk for injuries to occur due to severe heat.

Figure 2 CDC Heat-Related Illnesses and Treatments

4.???Exertional Heat Illness

Severe heat-related illnesses in students and athletes are especially dangerous due to the additional equipment and clothing worn during activities. These heat-related injuries are categorized as Exertional Heat Illnesses (EHI) and include, from least to most severe: Heat Rash, Heat Edema, Muscle Cramps, Heat Syncope, Heat Exhaustion, and Exertional Heat Stroke. In Table 1, a summary of each illness is shown along with common symptoms. The illnesses are listed with severity increasing as you descend the table, with the bottom three conditions being of greatest concern for students and athletes (Armstrong 2003, Armstrong et al 2007, Binkley 2002, Rynkiewicz 2018, Saenz et al. 2019). EHI is an ever-present danger during both practice and competitions for students and athletes and is the third leading cause of death behind Head and Cardiac Injuries (Maron et al. 2009, NATA 2015).

It is important to determine the heat stress risk on the body to mitigate these illnesses by utilizing either the Heat Index or Wet-Bulb Globe Temperature. While both measurements provide a metric for heat stress on the body, the Wet-Bulb Globe Temperature provides a better representation of the heat risk at a given time and specific location.

5.????Differences between the Wet-Bulb Globe Temperature and the Heat Index

Both the Heat Index and the Wet-Bulb Globe Temperature provide estimations of the effects of heat on the body. That is where the similarities between the two of them end. As mentioned earlier, the Heat Index is how hot it “feels” when relative humidity is factored into the ambient air temperature and makes assumptions on the environmental conditions, clothing, and activity levels. The Heat Index calculation assumes that the weather station taking the measurement is in an ideal environment with a shady, light-wind location with no direct sunshine. When taken in an outdoor environment these conditions are most likely not met and can lead to an underestimation of the Heat Index. When measured in direct sunlight, the Heat Index values can increase up to 15°F over their stated measurement, which is a severe disadvantage when most outdoor activities do not take place in such ideal conditions

The Wet-Bulb Globe Temperature is a location-based measurement that not only takes into consideration more atmospheric variables than the Heat Index but is designed for physical activities outdoors. The Wet-Bulb Globe Temperature is location-based and is highly variable depending on where and when the measurement is taken. This is because the Wet-Bulb Globe Temperature is affected by air temperature, humidity, wind speed, sun angle, and cloud cover (Figure 3). By identifying the onsite microclimate for outdoor activities, the Wet-Bulb Globe Temperature becomes a better measurement for heat stress mitigation.

Figure 3 Environmental variables in WBGT vs The Heat Index

Given the high variability of these atmospheric phenomena, a single Wet-Bulb Globe Temperature measurement cannot provide an accurate heat stress assessment over a large geographic region. However, compared to the Heat Index, the Wet-Bulb Globe Temperature provides a more accurate representation of the risk at a specific location. This is because the Heat Index values are calculated at static weather stations across the country and could be tens of miles away from an outdoor event and may not represent the current conditions at a location.

The key difference between the Wet-Bulb Globe Temperature and the Heat Index is that the Wet-Bulb Globe Temperature makes recommendations on how to mitigate the risk of heat-related illnesses when coupled with activity modification. The Wet-Bulb Globe Temperature has four categories of heat risk which correspond to a specific range of Wet-Bulb Globe Temperature values (Figure 4). As these values increase, the human body requires more rest and hydration to maintain proper functionality. An important thing to note is that the Wet-Bulb Globe Temperature, in conjunction with a guidance table, accounts for the individual’s heat acclimatization in its recommendations.

Figure 4 WGBT thresholds. Table from the Korey Stringer Institute, UCONN.

For example, a Texas football player at the beginning of spring training would not be acclimated to the heat and would require more water and breaks compared to the end of the season. This is due to the body’s gradual improved heat tolerance over time and throughout activity in a hot environment. In addition, the chart also utilizes data on heat safety thresholds to split the heat risk thresholds into geographic regions. These regions or categories (Figure 5) take into account the local extreme Wet-Bulb Globe Temperature measurements, climate, and geography.

Figure 5 Heat Safety Regions.

When comparing the heat thresholds and geographic categories, the Wet-Bulb Globe Temperature values do not correlate across all the color thresholds as shown in Figure 4. For example, a Wet-Bulb Globe Temperature of 82°F in Texas would not have any modifications needed for outdoor activities. However, if the same Wet-Bulb Globe Temperature occurred in Maine, activity modifications would be needed. This is due to the acclimatization of students and athletes to severe heat. In extreme Wet-Bulb Globe Temperature cases, activities are recommended to cease due to the high risk of heat-related injuries occurring (Grundstein 2015, GHSA 2014).

6.????Hurdles for the adoption of Wet-Bulb Globe Temperature over the Heat Index: Media Presence

The Wet-Bulb Globe Temperature calculation is older than the Heat Index calculation and yet the general public is widely unaware of what it is and how to use it. This lack of awareness is generally due to the fact that the Heat Index is widely used by the media and weather companies. This puts the Wet-Bulb Globe Temperature at a severe disadvantage since it is not distributed across multiple sources. The National Weather Service office in Tulsa, Oklahoma, and other offices have started issuing Wet-Bulb Globe Temperature area forecasts as part of an experimental project to calculate the Wet-Bulb Globe Temperature from standard meteorological sensors used by the National Weather Service and other government agencies. The difficulty they face is calculating the black globe temperature (Tg) without having the black globe sensor. The standard black globe sensor is a 6” diameter copper sphere, painted black, with a temperature probe inside it. Since the black globe temperature (Tg) is affected by several atmospheric variables, a derived estimation is needed to calculate it from standard atmospheric sensors. This project is a first step towards implementing the Wet-Bulb Globe Temperature across the United States (Dimiceli 2011). By determining how to calculate the Wet-Bulb Globe Temperature from the standard sensors that the media and weather companies utilize, the Wet-Bulb Globe Temperature can become a more common metric. However, the Wet-Bulb Globe Temperature still has some obstacles to overcome.

The Wet-Bulb Globe Temperature’s values are typically lower than the actual air temperature or the Heat Index values. This can lead to some confusion about how to use the Wet-Bulb Globe Temperature since it is not a “feels like” temperature like the Heat Index. The general public can grasp a “feels like” temperature easier than an abstract measurement like the Wet-Bulb Globe Temperature. As an example, a weather forecast states that the high for the day is 95°F and the Heat Index will be 107°F most people immediately understand the meaning of the heat risk for the day. Whereas the Wet-Bulb Globe Temperature measurements, on a hot day, are typically in the 80-90°F and do not sound as ominous as the Heat Index values. There is a significant misconception that the Wet-Bulb Globe Temperature is a measurement of heat and not heat stress risk. So low-temperature values compared to the actual air temperature do not appear as dangerous as they should. For example, a football game will occur in the late afternoon with a forecasted high temperature of 100°F, with light winds and no cloud cover. The Heat Index value during the peak temperature will be 103°F and a Wet-Bulb Globe Temperature of 90°F. When comparing the Heat Index side by side to the Wet-Bulb Globe Temperature it does not appear alarming since the temperature is less than the actual air temperature. The lack of education, understanding of, and appreciation of the Wet-Bulb Globe Temperature and the heat stress risks it represents is the biggest obstacle for the general public to overcome (Robinson 2021). One solution to this problem would be to implement the color-coded categories (Figure 4) in the media along with the Heat Index values. This way the general public will be exposed to both measurements at the same time and can learn the correlation between them.

7.????Hurdles for the adoption of Wet-Bulb Globe Temperature over The Heat Index: Legislation

The Wet-Bulb Globe Temperature has made some progress in being implemented in the United States due to several states mandating the use of Wet-Bulb Globe Temperature over the Heat Index for Heat Modification Policies in schools and colleges (Figure 6). Heat Modification Policies are school district policies required by law that schools must promote a method of monitoring the environment for heat-related concerns, and must comply with recommendations for activity modifications for the safety of students and athletes. So far there are thirty-one states plus the District of Columbia that require some form of environmental monitoring, either the Heat Index or the Wet-Bulb Globe Temperature, to make activity modifications based on severe heat risk. Eleven states require the Wet-Bulb Globe Temperature to be used over the Heat Index for activity decision-making. In the remaining twenty states and the District of Columbia, Heat Modification policies are typically left to the governing organization of the school’s athletic programs or the local school boards. While many of these school districts have heat modification policies in place, several of them are based on the Heat Index.

Figure 6: Heat Modification Policies required by Law. Data provided by the Korey Stringer Institute, UCONN

The policies that are based on the Wet-Bulb Globe Temperature usually refer to national athletic organizations that recommend the Wet-Bulb Globe Temperature over the Heat Index. These organizations include: the National Athletic Trainers Association (NATA), the National Collegiate Athletic Association (NCAA), the American College of Sports Medicine (ACSM), the National Football League (NFL), the Major League Soccer (MLS), the International Olympic Committee (IOC), the Korey Stringer Institute of UCONN (KSI), etc. It should be noted that in some states, such as Texas and Louisiana, the legislative branch requires widespread school mandates to be voted on, whereas other states, such as Georgia, chose to let the governing high school association make statewide decisions.

Another issue with not having the Wet-Bulb Globe Temperature use mandated by legislation is that the schools have no legal obligation to follow the recommendations made by their governing organizations. In the end, their governing bodies are not equipped to enforce their policies upon every school in their state. Even if the governing organization had heat modification policies in place, based on either the Heat Index or Wet-Bulb Globe Temperature, they are just recommendations. The enforcement and adoption of these policies lie in the hands of the state’s legislative branch and/or the state’s athletic governing organization. For example, in Georgia, the Georgia High School Association can levy fines on schools that do not follow the mandated guidelines (Kerr 2019).

8.????Hurdles for the adoption of Wet-Bulb Globe Temperature over the Heat Index: Fear of Cancelations of Sporting Events

One of the greatest attributes of the Wet-Bulb Globe Temperature is the recommendations it makes for activity modification when the Wet-Bulb Globe Temperature is dangerously high. By recommending longer rest times, water consumption, and equipment modifications, students and athletes are better protected from heat-related illnesses. When the Wet-Bulb Globe Temperature reaches a certain threshold, it recommends that all activity ceases due to the extreme risk of injury. This safety feature of the Wet-Bulb Globe Temperature has led to some steep criticisms that utilizing the Wet-Bulb Globe Temperature will cancel sporting events and practices for students and athletes. Climatologically, the Wet-Bulb Globe Temperature has a distinct diurnal pattern (average rise and fall over a 24-hour period) for its temperature range. When looking at the Wet-Bulb Globe Temperature equation [1] incoming solar radiation plays a key role in the black globe temperature and dry-bulb air temperature. Due to this diurnal pattern, the Wet-Bulb Globe Temperature is highest during the peak hours of incoming solar radiation. This roughly corresponds to the hours of noon to three o’clock. After this peak in Wet-Bulb Globe Temperature, the Wet-Bulb Globe Temperature will then begin to drop into the afternoon and evening hours as the incoming solar radiation decreases (Figure 7). Air temperatures will continue to increase throughout the day until peaking in the late afternoon and early evening hours.

Figure 7: Avg Air Temp. vs Avg Wet Bulb Globe Temp. vs Solar Radiation

Since most sporting event practices and games typically occur in the early morning or late afternoon/evening, the highest Wet-Bulb Globe Temperature values typically fall outside of these times. In addition, by knowing the temporal pattern of the Wet-Bulb Globe Temperature, decision-makers can adjust practices and sporting events to better protect students and athletes from heat-related injuries (Grundstein 2014).

9.????CONCLUSION

While both the Wet-Bulb Globe Temperature and the Heat Index assess the measure of heat stress of the environment on the body, only the Wet-Bulb Globe Temperature is designed to measure the risk in real-time and to make recommended modifications to activities when coupled with activity modification. By making modifications to activities, students and athletes are better protected from heat-related injuries and especially Exertional Heat Illnesses. These severe heat-related injuries pose a great risk to students and athletes and activity modification is one way to help mitigate the risk.

The Wet-Bulb Globe Temperature is widely used by countries outside of North America to access heat risk, whereas the Heat Index is the primary heat stress measurement used in North America. However, some states across the United States have adopted the Wet-Bulb Globe Temperature as their primary heat stress measurement for physical activities. These states have overcome the hurdles against the Wet-Bulb Globe Temperature and passed legislation to better protect students and athletes in their states.

When comparing the mathematical equations of the Heat Index versus the Wet-Bulb Globe Temperature, the Heat Index is a far more complex equation mathematically, utilizing non-modifiable assumptions of the environment. It does not take into consideration the effects of wind, cloud cover, and direct solar radiation on the human body. Due to the high variability of these environmental factors, the Wet-Bulb Globe Temperature provides a better assessment of heat risk during outdoor events. Changes in the microclimate greatly affect the Wet-Bulb Globe Temperature, so one measurement cannot cover a widespread area of service. Local measurements of the Wet-Bulb Globe Temperature are the best method for mitigating heat-related injuries.

Education and widespread utilization of the Wet-Bulb Globe Temperature represents the only way that this measurement will become utilized by the general public. Once the misconceptions and hurdles of the Wet-Bulb Globe Temperature are understood and dismissed the Wet-Bulb Globe Temperature will become the unit of measurement for heat safety.

10.??Acknowledgments

I am grateful to Dr. Andrew Grundstein, Department of Geography, University of Georgia, Athens Georgia, for editorial guidance and advice on this paper. Special thanks to Gerry Creager, Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma, for encouraging me to expand my poster abstract for the American Meteorological Society’s 2022 Annual Conference into this paper and for editorial guidance and drafting.?To Lori Weeden, Department of Environmental Earth and Atmospheric Sciences, University of Massachusetts Lowell, Lowell, Massachusetts, for editorial guidance and reviewing.?Also, to Dr. Rebecca Stearns and Dr. Christianne Eason, Korey Stringer Institute, University of Connecticut, Storrs, Connecticut, for information and guidance regarding heat-related illnesses and exertional heat illnesses. As well as information on Wet-Bulb Globe Temperature laws and policies across the United States.?

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