International Journal of Biometeorology

, Volume 56, Issue 1, pp 11–20

A retrospective analysis of American football hyperthermia deaths in the United States

Authors

    • Department of GeographyUniversity of Georgia
  • Craig Ramseyer
    • Department of GeographyUniversity of Georgia
  • Fang Zhao
    • Department of GeographyUniversity of Georgia
  • Jordan L. Pesses
    • Department of GeographyUniversity of Georgia
  • Pete Akers
    • Department of GeographyUniversity of Georgia
  • Aneela Qureshi
    • Department of GeographyUniversity of Georgia
  • Laura Becker
    • Department of GeographyUniversity of Georgia
  • John A. Knox
    • Department of GeographyUniversity of Georgia
  • Myron Petro
    • Department of GeographyUniversity of Georgia
Original Paper

DOI: 10.1007/s00484-010-0391-4

Cite this article as:
Grundstein, A.J., Ramseyer, C., Zhao, F. et al. Int J Biometeorol (2012) 56: 11. doi:10.1007/s00484-010-0391-4

Abstract

Over the period 1980–2009, there were 58 documented hyperthermia deaths of American-style football players in the United States. This study examines the geography, timing, and meteorological conditions present during the onset of hyperthermia, using the most complete dataset available. Deaths are concentrated in the eastern quadrant of the United States and are most common during August. Over half the deaths occurred during morning practices when high humidity levels were common. The athletes were typically large (79% with a body mass index >30) and mostly (86%) played linemen positions. Meteorological conditions were atypically hot and humid by local standards on most days with fatalities. Further, all deaths occurred under conditions defined as high or extreme by the American College of Sports Medicine using the wet bulb globe temperature (WBGT), but under lower threat levels using the heat index (HI). Football-specific thresholds based on clothing (full football uniform, practice uniform, or shorts) were also examined. The thresholds matched well with data from athletes wearing practice uniforms but poorly for those in shorts only. Too few cases of athletes in full pads were available to draw any broad conclusions. We recommend that coaches carefully monitor players, particularly large linemen, early in the pre-season on days with wet bulb globe temperatures that are categorized as high or extreme. Also, as most of the deaths were among young athletes, longer acclimatization periods may be needed.

Keywords

American footballHyperthermiaClimateUnited States

Introduction

Over the last 50 years (1960–2009), there have been 123 documented cases of American-style football players in the United States dying from heat-related illnesses (Mueller and Colgate 2010), including 58 well-documented cases from 1980–2009. Football players are particularly susceptible to heat-related illnesses in part because of the timing of pre-season practice, often in the late summer when conditions are hot and humid. A climate study of optimal practice times in Alabama found that there were no suitable times for outdoor practices in full uniform in August (Francis et al. 1991). Additionally, the level of clothing football players wear may increase metabolic heat production and inhibit cooling (e.g., Fox et al. 1966; Mathews et al. 1969; Kulka and Kenney 2002; McCullough and Kenney 2003; Brothers et al. 2004; Armstrong et al. 2010). A full football uniform, for example, may impede evaporative heat loss by 60–70% (Mathews et al. 1969). Finally, football players tend to have physical characteristics that may increase heat storage, such as greater muscle mass, lower body surface area to body mass ratios, and higher body fat percentages (Godek et al. 2004).

A variety of safety guidelines for practicing under different environmental conditions have been established by organizations such as Sports Medicine Australia, The American College of Sports Medicine, and the American Academy of Pediatrics (Armstrong et al. 1996; Binkley et al. 2002; American Academy of Pediatrics, Committee on Sports Medicine and Fitness 1991, 2000; Sports Medicine Australia 2010). These guidelines rely on a derived variable called the wet bulb globe temperature (WBGT; Yaglou and Minard 1957) that integrates the influence of air temperature, humidity, and solar radiation. Depending on the threshold, recommendations are made about the level of risk and whether changes in how the practice is conducted are needed. Cooper et al. (2006), however, noted that these standards are generic and do not account for the particular sport, the equipment worn, or fitness of the athlete.

Football-specific thresholds based on air temperature and humidity were established for different levels of dress including full pads, light pads, and shorts only by Kulka and Kenney (2002). Using subjects that walked on a treadmill wearing different types of clothing and under different meteorological conditions, they identified levels of uncompensable heat stress in which the body becomes unable to maintain a thermal balance. A limitation of this method is that it makes assumptions about the level of activity and does not account for other meteorological factors such as solar radiation or wind that may affect exertional heat stress (Kulka and Kenney 2002). Coyle (2003) expressed thresholds for uncompensable heat stress for the different levels of dress in terms of WBGT using empirical relationships between the black globe temperature and dry bulb temperature.

Several studies have examined environmental conditions and health outcomes in football players. Cooper et al. (2006) investigated the use of the WBGT and safety thresholds established by the American College of Sports Medicine (ACSM) at five universities in the southeastern United States. They noted that the highest risk of heat-related illness was in the first few weeks of practice in August. Most illnesses (97%) occurred under high-risk or extreme-risk categories. The clothing-specific standards of Kulka and Kenney (2002) were studied during football training sessions. Godek et al. (2004) noted that in many cases players in partial or full football uniforms were able to train at levels that exceeded thresholds without any signs of heat-related illnesses because periods of inactivity allowed the athletes to cool and maintain a thermal balance.

In particular, the meteorological conditions during football hyperthermia deaths have rarely been examined. Only two studies have performed retrospective analyses relating weather conditions to hyperthermia deaths in football players. Fox et al. (1966) studied nine deaths from 1959 to 1962. They noted that all were interior linemen in full football uniforms and most died during the first 2 days of practice. Deaths occurred under a range of conditions from higher temperatures and lower relative humidities to lower temperatures and higher relative humidities. Kulka and Kenney (2002) added data on three deaths in 2001 to Fox et al.’s study and placed these values in the context of risk thresholds that accounted for level of clothing. They found that all cases fell at or above the level of uncompensable heat stress for exercise in a full football uniform.

Our study builds upon Fox et al. (1966) and utilizes the largest available dataset to investigate the geographic, temporal, and meteorological characteristics of football hyperthermia deaths in the United States. In particular, our study answers the following questions:
  • Where do football hyperthermia deaths occur?

  • When do deaths occur (annual, intra-seasonally, time of day, time of practice season)?

  • Which athletes (age, position, weight, level of dress) are most vulnerable?

  • Under what meteorological conditions and safety thresholds do the deaths occur?

Unlike other injuries, hyperthermia deaths are completely avoidable with proper oversight and precautions. It is hoped that this study provides empirical evidence that will better guide safety efforts. We conclude this paper with some recommendations based on the results of our study.

Materials and methods

Data on heat-related deaths of football players were obtained from the National Center for Catastrophic Injury Research (NCCIR; Mueller and Colgate 2010). The dataset includes information on the death date, age of deceased, height, weight, school, city, state, and the sport being played that caused the death. The first death recorded in the database occurred in 1955 and the last in 2009. We used 1980–2009 as a study period because deaths before 1980 lacked information in the majority of the data fields.

In order to compile a more complete record of the data, extensive research was conducted to fill in missing data in the NCCIR dataset and to add additional information. It is important to note, however, that no additional deaths were added to the original NCCIR database. Media reports were the principal source of information but lawsuit publications and obituaries were also used. Most of the news articles were obtained using the Lexis–Nexis academic database. Local newspapers were contacted for several cases where no information was found in the Lexis–Nexis database.

Additional data fields were added to the existing database that would allow better analysis of the conditions under which the death occurred. These data fields included the exposure date, time of day the athlete was practicing or participating in football-related outdoor exercise, the point in the football season the incident occurred (first day of practice, etc.), position, pre-existing medical conditions, type of activity (practice, conditioning workout, etc.), clothing worn, and latitude/longitude coordinates of exposure location. The exposure date is a critical field for assessing the meteorological conditions that were present during the onset of hyperthermia symptoms. In many cases, the exposure date and death date were different. Clothing ensembles were categorized according to Kulka and Kenney (2002) as full football uniform (helmet, undershirt, shoulder pads, jersey, and game pants with thigh, knee, and hip pads), practice uniform (helmet, undershirt, shoulder pads, jersey, and shorts), and shorts only.

Meteorological data were used to assess the environmental conditions under which the fatalities occurred. All hourly observations of temperature, dew-point temperature, relative humidity, wind speed, and cloud cover were from the Integrated Surface Hourly (ISH) dataset (Lott et al. 2001). However, we retrieved data for 1991–2005 from the National Solar Radiation Database (NSRDB) because of ease of use of the dataset (NREL 2007). Meteorological observations were obtained from the nearest station to the location where the exposure occurred. Fatalities were only examined, however, if an observing station was within 50 km and recorded data during the approximate exposure time. The average distance between the station and the heat-related death was 19.2 km, with a standard deviation of 13.8 km.

We reconstructed meteorological conditions during the approximate exposure period. While the exact exposure time was not available for the majority of the cases, whether it occurred at a morning or afternoon practice was often indicated. Thus, two time periods were defined: a morning session was assumed to occur somewhere between 8:00 am and 12:00 pm local daylight time (LDT), and an afternoon session was assumed to occur somewhere between 2:00 and 6:00 pm LDT. For these time periods, the maximum temperature, relative humidity and dew-point temperature at the time of maximum temperature, the average temperature, average dew-point temperature, average relative humidity, and average cloud cover were calculated. Two derived meteorological variables, the WBGT and the Heat Index (HI), were computed using the average morning or afternoon data. As noted earlier, WBGT is a widely used measure of environmental conditions assessed for safety standards in athletics and industry (Cooper et al. 2006). We used the software package Heat Stress Advisor (Zunis Foundation 2010) which uses algorithms developed by Coyle (2000) to convert standard meteorological variables (e.g., air temperature, relative humidity, cloud cover) to the wet bulb globe temperature (WBGT). The Heat Index is based on the apparent temperature by Steadman (1979) and modified for operational purposes by the U.S. National Weather Service (NWS). The NWS issues heat-health warnings based on the HI and it is therefore familiar to people in the United States. An empirical equation by Schoen (2005), requiring air and dew-point temperatures, is used to compute the HI.

Results

Spatial and temporal patterns

Heat-related football deaths were first examined geographically (Fig. 1). The fatalities were widespread across the eastern U.S. in a belt that extends along the central East Coast southward and westward across the Southeast into Texas. Only five deaths occurred in the drier western U.S. (southern California and Arizona), whereas no deaths were reported in New England.
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Fig. 1

Football hyperthermia deaths, 1980–2009. Filled circles are deaths that occurred in locations with meteorological observing stations within 50 km

Over the study period, there were 58 reported deaths of football players from heat-related causes. On average, there were almost two deaths per year with a standard deviation of 1.66 deaths. The greatest number of recorded fatalities was six in 2008. There is an apparent increase in the number of deaths over time, particularly since the mid-1990s (Fig. 2a). The 1980–1994 period had an average of 1.07 deaths per year compared with the 1995–2009 period with an average of 2.8 deaths per year.
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Fig. 2

Time series of a total football hyperthermia deaths and b deaths per 100,000 players. Deaths were normalized using data on the number of high school football players from the period 1980–2006

A number of factors individually or in combination may help to explain the broad increase in fatalities over time. First, there has been an increase in athletes participating in football which would increase the pool of players who may be susceptible to harm. Available data from National Federation of State High School Associations (NFSH) indicates that the number of high school football players has increased by almost 18% from 1980–2006 to a total of 1,105,583 players (NFSH 2010). Using the NFSH data as a metric for overall participation, we note that the pattern of deaths per 100,000 football players is similar to that of overall deaths, with greater values after the mid-1990s (Fig. 2b). Thus, the increase in players participating in football does not seem to be a major factor in the increase in deaths. A second explanation is that there has been a change in the characteristic of the athletes that makes them more susceptible to heat-related illnesses. Football players at all levels of competition and particularly linemen have increased in size over time (Wang et al. 1993; Noel et al. 2003; Secora et al. 2004; Kraemer et al. 2005). Wang et al. (1993), for instance, showed a distinct increase in the size of high school linemen from a body mass index (BMI) of 27.7 in 1963 to 32.0 in 1989. Additionally, Noel et al. (2003) notes that Division I college football players have become bigger and fatter than they were in the 1980s and early 1990s. It is possible, then, that the greater size would increase heat storage and therefore the possibility of heat-related illnesses. Finally, several studies have identified climate conditions becoming hotter and more humid across much of the central and eastern U.S. over different study periods such as 1961–1995 (Gaffen and Ross 1999), 1949–1995 (Gaffen and Ross 1998), and 1976–2004 (Dai 2006). Climate change over our study period was investigated using an approach similar to Gaffen and Ross (1998) in which we computed trends in the number of August days that exceed a local threshold (85th percentile) for maximum and minimum daily apparent temperatures (NCDC 2010; Fig. 3). There are few statistically significant trends for maximum apparent temperatures but widespread positive trends are evident in minimum apparent temperatures, particularly across parts of the southern and eastern U.S. The implication is that there are a greater number of days where morning practices have stressful meteorological conditions and thus an increased risk for heat-related illnesses.
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Fig. 3

Trends in frequency of August days that exceeded the local 85th percentile in a maximum apparent temperature and b minimum apparent temperature from 1980–2009. Filled (open) circles are positive (negative) trends at p ≤ 0.05 and crosses are stations with no trend

Intra-seasonally, the largest number of deaths occur in July–September, with August comprising by far the greatest number of deaths with approximately 66% (Table 1). A closer examination of August shows that 71% of August deaths occur in the first 2 weeks of the month (Table 1). As early August is often the starting time for pre-season training, it is possible that acclimatisation to hot and humid conditions may be an important component in these heat-related deaths. In 13 cases, news reports specifically noted that the death occurred within the first 3 days of practice. Finally, the time of day of the exposure occurred was reported in 36 cases. Over half the cases (~58%) occurred in the morning, when temperatures are generally lower but relative humidities are higher.
Table 1

Temporal patterns of football hyperthermia fatalities

Time of day

n

Time of season

n

Month

n

Sub-month (Jul–Sep)

n

Morning (8:00 am–12:00 pm LDT)

21

1st practice

9

Jan–May

1

Jul 1–15

7

Afternoon (2:00–6:00 pm LDT)

15

2nd practice

3

Jun

0

Jul 16–31

9

Unknown

22

3rd practice

1

Jul

10

Aug 1–15

27

  

1st practice with pads

1

Aug

37

Aug 16–31

11

  

Unknown

44

Sep

9

Sep 1–15

4

    

Oct

1

Sep 16–30

2

    

Nov–Dec

0

  

LDT Local daylight time

Athlete characteristics

Athlete characteristics including age, weight, position, and clothing worn during the exposure period were examined (Table 2). The average age was 16.76 years with a standard deviation of 2.83 years. Most of the athletes were age 18 or younger, comprising almost 86% of the total deaths. The athletes were heavy, with an average weight (BMI) of 113.96 kg (33.60) and a standard deviation of 21.76 kg (5.13). All but three athletes exceeded 90 kg and 95% of them would be classified as overweight or obese based on the BMI. The BMI does not distinguish between adipose and lean tissue, and can be misleading in muscular athletes, but Laurson and Eisenmann (2007) note that the BMI correlates well with fat mass for boys aged 11–17.
Table 2

Characteristics of football hyperthermia fatalities. Percent of High School (HS) linemen is rounded to the nearest percent and based on data in Laurson and Eisenmann (2007)

Age

n

Weight (kg)

n

BMI

All linemen n

Linemen ≤18 years n (%/%HS linemen)

Position

n

Clothing

n

11

1

<70

2

<20

1

1 (4/1)

Lineman

32

Full uniform

4a

12

0

70–<80

1

20–24

1

0 (0/28)

Backs

4

Practice uniform

8

13

4

80–<90

1

Overweight (25–29)

7

2 (9/41)

Other

1

Shorts only

8

14

4

90–<100

8

Obese Class I (30–34)

18

8 (35/21)

Unknown

21

Unknown

38

15

7

100–<110

5

Obese Class II (35–39)

9

8 (35/7)

    

16

10

110–<120

7

Obese Class III (≥40)

6

4 (17/2)

    

17

18

120–<130

10

Unknown

 

3

    

18

4

130–<140

5

       

19

1

140–<150

0

       

20

1

150–<160

2

       

21

2

160–<170

1

       

22

1

Unknown

16

       

aOnly two deaths with the athlete in full uniform had corresponding data on time of death and were within 50 km of a meteorological observing station

For further analysis, the positions were aggregated as linemen (offensive and defensive), backs (two fullbacks, one running back, one defensive back), and other (one wide receiver). Among American-style football players, offensive and defensive linemen tend to be the largest players on the field with higher levels of body fat percentage and greater absolute strength (Pincivero and Bompa 1997). Offensive and defensive linemen tend to mirror each other by position, with offensive linemen attempting to protect the passer and clear the way for runners to advance the ball while defensive linemen try to disrupt the passer and tackle any running backs before they can gain yardage. Offensive and defensive backs along with wide receivers tend to have lower body fat levels and lower absolute strength, but are faster and have greater aerobic capacity (Pincivero and Bompa 1997).

In our dataset, linemen had an average weight (BMI) of 120.07 kg (34.75) compared with 105.91 kg (31.85) for backs. For reference, we compared the BMI of 23 linemen ≤18 years of age with an extensive dataset of 3,683 high school linemen compiled by Laurson and Eisenmann (2007) (Table 2). The linemen who died from heat-related illnesses were large, with approximately 87% classified as obese (BMI >30) compared with 30% in the Laurson and Eisenmann (2007) dataset. Also, linemen represent a large and disproportionate percentage of deaths (86%) relative to their numbers on the field (45%). Finally, we considered the level of dress of the athletes. Of the 20 cases with data, 8 were in in shorts only, 8 were in practice gear, and 4 were in full football uniforms.

Meteorological conditions

Average meteorological conditions on exposure days were examined for 33 cases that included the approximate time of day when the athlete was practicing and were within 50 km of a meteorological station (Table 3). Days were separated into morning (8:00 am–12:00 pm LDT) or afternoon (2:00–6:00 pm LDT) for analysis. Temperatures on exposure days ranged from a moderate 23°C up to a very hot 39°C. Conditions were generally humid, with dew-point temperatures averaging 19°C in the afternoon and 21°C in the morning. Afternoon practices were hotter with average temperatures of approximately 33°C, almost 5°C greater than the average morning temperature of 28°C. Morning practices, however, had considerably greater relative humidity values that were nearly 20% greater (67 vs 48%) than those in the afternoon. The WBGT and HI were calculated for each death to integrate the influences of several meteorological variables. Average WBGT values ranged from approximately 24 to 34°C. Afternoon WBGT values of 30°C were about 2°C greater than morning values of 28°C. The small difference in WBGT between morning and afternoon occurs because the WBGT is very sensitive to humidity levels, which contribute about 70% of the overall value. The HI ranged from 23 to 40°C, with average afternoon values of 35°C about 5°C greater than morning values.
Table 3

Average meteorological conditions on days with hyperthermia deaths

 

Air temperature (°C)

Relative humidity (%)

Dew-point temperature (°C)

WBGT (°C)

Heat index (°C)

Afternoon (14 cases)

Mean

32.9

48.3

18.9

30.2

35.0

Median

32.4

51.7

19.9

30.0

35.6

Maximum

39.2

77.9

24.8

33.5

40.3

Minimum

26.1

16.8

9.7

25.2

26.4

Standard deviation

3.9

18.4

4.9

2.4

3.7

Morning (19 cases)

Mean

27.8

66.6

20.5

27.7

30.2

Median

27.1

71.2

22.0

27.7

29.9

Maximum

35.4

86.0

25.7

31.8

36.5

Minimum

23.1

33.3

11.0

23.6

23.5

Standard deviation

3.0

14.2

3.8

2.4

3.8

Meteorological conditions were next considered in the context of established risk thresholds for athletic activity. Sports Medicine Australia, The American College of Sports Medicine, and the American Academy of Pediatrics define categories based on the WBGT (Table 4). All 33 deaths occurred under conditions considered high or extreme by both the American College of Sports Medicine and Sports Medicine Australia, which use similar thresholds. Indeed, over 60% of the deaths occurred on days where the WBGT suggested that the practice should be canceled. Under American Academy of Pediatrics guidelines, 42% of deaths occurred on days where practices should have been canceled and another 39% on days when activity should have been stopped for athletes not acclimatized to high temperatures and/or high humidity. Overall, the vast majority of deaths occurred in categories where the threat for heat-related illnesses were high. Very few deaths occurred under low or moderate risk conditions. The NWS has established risk thresholds based on the HI (Table 4). Deaths occurred under conditions with no safety advisory (15%), caution (30%), and extreme caution (55%). Of interest is that 45% of deaths occurred on days with no safety warning or one at the lowest level of risk for a heat-related illness (i.e. caution), and no deaths occurred on days categorized as the most dangerous (i.e., danger and extreme danger). The lower level of threat suggested by the HI is likely related to assumptions used in its calculation, including that an individual is in the shade, of average size (1.7 m and 67 kg), is dressed lightly (i.e. wearing long trousers and a short sleeve shirt), and engaged in light activity (i.e. walking at about 1.4 m s−1) (Steadman 1979; Rothfusz 1990). Clearly, these assumptions are not representative of football players who tend to be larger, exposed to sun, may be dressed in protective gear, and involved in greater levels of activity. Thus, the HI may underestimate the environmental risk for athletes such as football players. Finally, it is important to note that average values were used to represent the integrated conditions over the practice period and that the athletes may have been exposed to higher instantaneous WBGT or HI values.
Table 4

Statistics on football hyperthermia fatalities by risk category

American College of Sports Medicine and Sports Medicine Australia

Count (%)

American Academy of Pediatrics

Count (%)

NWS Heat Index

Count (%)

Uncompensable heat stress

Above/below

Low (<18°C)

0 (0)

No limits (<24°C)

1 (3)

No warning (<26.7°C)

5 (15)

Full uniform (24.7–28.4°C)

1/1

Moderate (18–23°C)

0 (0)

Longer rest periods (24–25.9°C)

5 (15)

Caution (26.7–32.2°C)

10 (30)

Practice uniform (28–29.6°C)

5/3

High (23–28°C)

13 (39)

Stop activity if not acclimatized (26–29°C)

13 (35)

Extreme caution (32.3–40.6°C)

18 (55)

Shorts only (31.6–33.1°C)

1/6

Extreme/Cancel event (>28°C)

20 (61)

Cancel event (>29°C)

14 (42)

Danger (40.7–54.4°C)

0

  
    

Extreme danger (>54.4°C)

0

  
Third, meteorological conditions were investigated with regard to the type of clothing the athletes were wearing during exposure. Kulka and Kenney (2002) established thresholds for uncompensable heat stress for full pads, practice gear, and shorts using air temperature and relative humidity. These thresholds are plotted as curves in Fig. 4. We analyzed 18 cases (8 with shorts only, 8 with practice gear, and 2 with full uniforms) that included the level of clothing, the approximate time of heat exposure, and were located within 50 km of a meteorological station. For comparison, 16 cases without data on clothing but with time of exposure were included. Maximum morning or afternoon temperatures along with the relative humidity at the maximum temperature, representing the worst possible conditions the athlete would have experienced, are plotted for each case in Fig. 4. All but one of the athletes dressed in shorts only fell below the established threshold for uncompensable heat stress, indicating that conditions should have been safe. It should be noted that two of the athletes had pre-existing medical conditions (i.e., a rare hemoglobin disorder and sickle cell anemia) that may have made them more vulnerable to a heat-related illness. Five of the eight deaths for athletes who were wearing practice uniforms fell above the threshold. Of the remaining three incidents, one was near the established threshold. Finally, athletes were wearing full football uniforms in two cases. Conditions fell above the threshold in one case and below on the other. When considering incidents with only data on time of exposure, approximately 88% occurred at levels that are unsafe for full uniform and 75% at levels unsafe when wearing practice uniforms. All but one of the deaths, however, occurred at levels below the uncompensable heat stress for shorts only.
https://static-content.springer.com/image/art%3A10.1007%2Fs00484-010-0391-4/MediaObjects/484_2010_391_Fig4_HTML.gif
Fig. 4

Football hyperthermia deaths during 1980–2009 in the context of uncompensable heat stress limits for different levels of clothing. The curves are derived from data in Kulka and Kenney (2002). Solid lines are fit to observed data and dashed lines are extrapolations. Thresholds are ordered, respectively, from bottom to top lines for full football uniform, practice uniform, and shorts only. Filled symbols indicate that the athlete had a pre-existing medical condition

Integrated exposure conditions over the morning or afternoon for clothing specific standards were investigated using WBGT thresholds adapted by Coyle (2003) from the temperature and relative humidity thresholds of Kulka and Kenney (2002). The WBGT categories involve a range of values because of the interaction between temperature and relative humidity. Nevertheless, the results using the lower and upper WBGT thresholds for each clothing category were identical (Table 4). Here, one of the two athletes in a full uniform practiced in conditions that exceeded the level of uncompensable heat stress, five of the eight athletes in practice gear were exposed to conditions that exceeded the threshold, and only two athletes in shorts exceeded the threshold.

An important question is whether the environmental conditions on days with fatalities were atypically harsh. For each of the 33 fatalities with an approximate time of exposure, we constructed a climatology of meteorological conditions using data from 1991–2005 for the particular day of the year on which the death occurred. Only 31 stations, however, had sufficient data for a climatological analysis. The air temperature and dew-point temperature on the day a death occurred were then standardized relative to the local climatology. In general, conditions on days with hyperthermia deaths were both hotter and more humid than normal (Fig. 5). Over 80% of days had temperatures greater than the mean and almost 42% of those days were ≥1 standard deviation above average. Similarly, about 68% of dew-point temperatures were above the mean and approximately 32% were ≥1 standard deviation above average. Finally, drier than normal conditions on some days did not necessarily alleviate the stressful meteorological conditions because most of those days (about 78%) were also hotter.
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Fig. 5

Average morning (8:00–12:00 am) or afternoon (2:00–4:00 pm) air and dew-point temperatures on days with fatalities standardized using climatological conditions for the particular day of the year that the death occurred upon. Stations with 15 years of data available for constructing the climatology are shaded black while those with 10–14 years of data are shaded gray

Discussion and conclusions

From 1980 to 2009, 58 football players died from heat-related illnesses. Our results create a temporal, demographic, and meteorological profile of hazardous conditions for football hyperthermia deaths. Consistent with findings by Cooper et al. (2006) and Fox et al. (1966), heat-related injuries are most prominent in the first half of August, when unconditioned and unacclimatized players begin pre-season practice under hot and humid conditions. Over half the deaths occur during morning practices which, although cooler, have higher humidity levels that increase heat stress. The level of dress has been a concern in studies of football players and heat stress as helmets and pads can inhibit cooling mechanisms. Many of the deaths, however, were among players wearing no pads and minimal clothing. Finally, consistent with Fox et al. (1966), large linemen, especially those with BMIs exceeding 30, are disproportionately represented among the deaths.

Meteorologically, the fatalities occurred on days that tended to be unusually hot and humid by local standards. More importantly, the conditions were categorized as extremely dangerous (i.e., high or extreme risk) for heat-related illnesses by the American College of Sports Medicine using the WBGT. The HI, however, tended to underestimate the degree of hazard as most deaths occurred under conditions that were assigned lower risk levels. Football-specific thresholds for uncompensable heat stress were a useful metric for athletes in practice uniforms but less so for those dressed in shorts only. Indeed, most of the deaths for athletes in shorts occurred below the level of uncompensable heat stress. A possible explanation for this discrepancy is that the deaths for athletes wearing only shorts all occurred early in the pre-season (all on or before 8 August), suggesting that a lack of acclimatization may have made the players more vulnerable to heat-related illnesses. Too few data on athletes wearing full pads were available to draw any definitive conclusions.

Coaches should carefully monitor players, particularly big linemen, early in the practice season. The complete suite of meteorological conditions, not merely temperature, should be accounted for when deciding whether to practice and the intensity of the practice. In particular, coaches should be aware that simply holding practice in the morning does not alleviate hyperthermia threats due to higher humidity levels. Derived measures of heat stress such as the WBGT, along with established safety thresholds from organizations like the American College of Sports Medicine, are useful guides for assessing levels of hazard and in dictating activity levels. Finally, the great number of deaths early in the pre-season practice schedule suggests that proper acclimatization is important. Most of the deaths were among youth athletes who may have less physical preparation or opportunity for acclimatization than college-level or professional athletes (Bergeron et al. 2005). Thus, longer acclimatization periods may be an important strategy for reducing the number of hyperthermia-related deaths. Bergeron et al. (2005) offer suggested acclimatization plans for both high school and youth football players. At high school level, for instance, they suggest a 14-day acclimatization plan where coaches carefully adjust the level of dress, and length and intensity of practice sessions as the players gain fitness and adapt to the ambient meteorological conditions.

Acknowledgment

We would like to thank Dr. Fred Mueller of the National Center for Catastrophic Injury Research for kindly providing data on heat-related deaths of football players.

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