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Heat Illness

What is heat illness?

Your body normally cools itself by sweating. During hot weather, especially when it is very humid, sweating just isn't enough to cool you off. Your body temperature can rise to dangerous levels and you can develop a heat illness.

Most heat illnesses happen when you stay out in the heat too long. Exercising and working outside in high heat can also lead to heat illness. Older adults (65+), young children (0-4), and those who are sick or overweight are most at risk. Taking certain medicines or drinking alcohol can also raise your risk.

Heat-related illnesses include

  • Heat stroke - a life-threatening illness in which body temperature may rise above 106° F (41.1° C) [see 1 below] in minutes (10-15 minutes). Symptoms include dry skin, a rapid, strong pulse, dizziness, nausea, and confusion. If you see any of these signs, get medical help right away.
  • Heat exhaustion - an illness that can happen after several days of exposure to high temperatures and not enough fluids. Symptoms include heavy sweating, rapid breathing, and a fast, weak pulse. If it is not treated, it can turn into heat stroke.
  • Heat cramps - muscle pains or spasms that happen during heavy exercise. You usually get them in your abdomen, arms, or legs.
  • Heat rash - skin irritation from excessive sweating. It is more common in young children.

You can lower your risk of heat illness by drinking fluids to prevent dehydration, replacing lost salt and minerals, and limiting your time in the heat.

U.S. National Library of Medicine - Heat Illness, Frequently Asked Questions (FAQ) About Extreme Heat

In recent years, excessive heat has caused more deaths than all other weather events, including floods. A heat wave is a prolonged period of excessive heat, often combined with excessive humidity.

The heat index is the temperature the body feels when the effects of heat and humidity are combined. Exposure to direct sunlight can increase the heat index by as much as 15° F.

Heat Wave Safety Checklist

Exercising in hot weather puts extra stress on your body. If you don't take care when exercising in the heat, you risk serious illness. Both the exercise itself and the air temperature and humidity can increase your core body temperature.

To help cool itself, your body sends more blood to circulate through your skin. This leaves less blood for your muscles, which in turn increases your heart rate. If the humidity also is high, your body faces added stress because sweat doesn't readily evaporate from your skin. That pushes your body temperature even higher.

Heat-related illnesses are largely preventable. By taking some basic precautions, your exercise routine doesn't have to be sidelined when the heat is on.

Stay safe during hot-weather exercise by drinking enough fluids, wearing proper clothing and timing your workout to avoid extreme heat.

WBGT

A WBGT device is a measurement tool that uses ambient temperature, relative humidity, wind, and solar radiation from the sun to get a measure that can be used to monitor environmental conditions during exercise. Establishing WBGT guidelines that dictate modifications in activity (work:rest ratios, hydration breaks, equipment worn, length of practice) at given WBGT temperatures play a huge factor in helping to prevent EHS.

As environmental temperature and humidity increase, there is an increase in the heat stress that is placed on the exercising individual. Exercise in the heat causes athletes to rely on evaporation of sweat from the skin as the primary method of dissipating heat that is produced by the working muscles. As humidity increases, the ability to dissipate heat through evaporation is further hindered, thus causing the body to have an increased body temperature, which increases the risk of EHS.

WET BULB GLOBE TEMPERATURE MONITORING

3rd Annual Collaborative Solutions for Safety in Sport National Meeting Wet Bulb Globe Temperature FAQs

Activity guidlines

National Athletic Trainers Association Parents’ and Coaches’ Guide to Dehydration and Other Heat Illnesses in Children


Parents’ and Coaches’ Guide to Dehydration and Other Heat Illnesses in Children, Inter-Association Task Force on Exertional Heat Illnesses Consensus Statement

American College of Sports Medicine position stand: Exertional heat illness during training and competition

American colleage of sports medicine: Exertional Heat Illness during Training and Competition, TABLE 2. WBGT levels for modification or cancellation of workouts or athletic competition for healthy adults.
American College of Sports Medicine position stand: Exertional heat illness during training and competition

American College of Sports Medicine position stand: Exertional heat illness during training and competition, Table 3. Modifying practice sessions for exercising children.
American College of Sports Medicine position stand: Exertional heat illness during training and competition

Zunis Foundation

ACSM Risk Category °F °C
Wet Bulb Globe Temp [see 2] NWS Heat Index Temp Wet Bulb Globe Temp NWS Heat Index Temp
Dangerous Zone >90 >115 >32.2 >46.1
Very High Risk Zone 82 - 90 98 - 115 27.8 - 32.2 36.7 - 46.1
High Risk Zone 73 - 82 80 - 98 22.8 - 27.8 26.7 - 36.7
Moderate Risk Zone 65 - 73 65 - 80 18.3 - 22.8 18.3 - 26.7
Low Risk Zone < 65 < 65 <18.3 <18.3

Even so, these guidelines, which are approximations, may eventually require minor adjustment as more data becomes available.

These risk zones ("low risk zone," "moderate risk zone," "high risk zone," "very high risk zone" = "event delay threshold zone", and "dangerous zone") are to be considered advisory. Untrained and unacclimatized athletes are much more likely to experience heat injury at any risk level than are those who are better prepared. Even so, extremely fit and acclimatized athletes can occasionally experience serious exercise-related heat injury, even in the "low risk zone." The chief benefit of these zone markings is to give the athlete, coach or event organizer an overview of relative heat stress likely to be encountered during an event held at a specific time and place.

Reading our charts

NSW Heat Index


NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, National Weather Service Heat Index Chart

Tennessee Secondary School Athletic Association heat index policy


Tennessee Secondary School Athletic Association heat index policy

Kentucky High School Athletic Association heat index policy


Kentucky High School Athletic Association heat index policy

Michigan high school athletic association - Heat/Hydration


Michigan high school athletic association - Heat/Hydration

Indiana High School Athletic Association HEAT INDEX INFORMATION & CHART


Indiana High School Athletic Association HEAT INDEX INFORMATION & CHART

Otsuka Pharmaceutical


How to use the WBGT heat index

Footnote

[1] Some experts use 104° F (40° C) as threshold level. (Inter-Association Task Force on Exertional Heat Illnesses Consensus Statement, American College of Sports Medicine position stand: Exertional heat illness during training and competition)
[2] ISO 7243:2017 Ergonomics of the thermal environment -- Assessment of heat stress using the WBGT (wet bulb globe temperature) index

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