Exercise-Related Heat Exhaustion

Preparedness Spotlight: Beat the Summer Heat

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine
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Outdoor concerts. Beach trips. Pool parties. The sizzling summer days are back, bringing all the enjoyable activities that come with this time of year.

But the steamy temperatures can be dangerous: Extreme heat kills more than 600 people in the United States every year, according to the Centers for Disease Control and Prevention (CDC), even though heat-related deaths are preventable.

Heat-related illness, or hyperthermia, can happen on hot days when a person’s body temperature rises faster than it can cool. This can cause damage to the brain and other critical organs.

Heat-related illnesses can affect anyone, but those most at risk include infants and children up to 4 years old; people 65 years and older; and those with medical conditions such as diabetes and heart disease.

Behaviors, such as drinking alcohol and taking medications that weaken the ability to regulate body temperature, can further endanger a person during extreme heat.

Types of Heat-Related Illness

The following are some common heat-related illnesses and their symptoms.

  • Heat cramps are muscle pain or spasms that occur during or after exercise or physical work in the heat.

What to do: Stop physical activity and go to a cool place. Seek medical attention if the cramps last longer than one hour, or if you have heart problems or are on a low-sodium diet.

  • Heat exhaustion occurs when your body loses too much water and salt through sweating, particularly through physical activity or exercise, and you do not replace the fluids. Symptoms include muscle cramping, fatigue, headache, nausea or vomiting, and dizziness or fainting.

What to do: Drink water and go to a cool place. Seek medical help if you are throwing up or if your symptoms worsen or last longer than one hour.

  • Heat stroke is the most serious heat-related illness. It occurs when you spend a prolonged amount of time in extreme heat, and your body is unable to cool off. Milder heat-related illnesses, such as heat cramps or heat exhaustion, could lead to heat stroke, but heat stroke can occur without prior symptoms. Signs of heat stroke include a body temperature greater than 103 degrees F, inability to sweat, headache, dizziness, nausea, confusion or unconsciousness.

What to do: If a person is suffering from a heat stroke, call 911 immediately. Move the person to a cooler place. Apply cool, damp cloths or, if you are able, put the person in a cool bath to help lower the body temperature. Do not give the person anything to drink.

Prevention

The best way to prevent heat-related illness is to remain in an air-conditioned location during hot weather. This could be your home or another area with air conditioning. Here are other ways to prevent illness in extreme temperatures:

  • Drink water or other nonalcoholic and caffeine-free beverages frequently.
  • Wear light-colored, lightweight, loose-fitting clothing.
  • Avoid spending time in the sun, especially during hotter times of the day. Refrain from exercising or participating in strenuous activities, if possible, or do them during cooler parts of the day.
  • Do not leave children or pets alone in cars. Cars heat up quickly, which can be deadly. Also, be sure that children cannot lock themselves in an enclosed space, such as a car trunk.
  • Dehydration is common in pets, so be sure to provide them with fresh, clean water, and place the water in a shady area if they are outside. Also, be careful not to overexercise a pet, and give the animal access to a shady area.

During heat waves, remember to check on people at risk for heat-related illnesses, such as elderly or homebound people. If you are attending an outdoor festival, concert or other large gathering, locate the first-aid area before or when you arrive.

For more information on extreme heat and heat-related illness, visit the CDC’s website.

Read the latest news and information from CEPAR's Hopkins on Alert.

Source: https://www.hopkins-cepar.org/on-alert/beat-the-summer-heat

Heat Stroke and Safety Tips for Matagorda County

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine

Heat stroke and sky-high temperatures seem to be on everyone’s mind these days, and there’s no denying that the Matagorda County summer heat is a force to be reckoned with.

Here is some great information we compiled from the Centers for Disease Control and Prevention (source) and Johns Hopkins Medicine (source) that may help you prevent, recognize, and respond to complications from over-exposure to the unforgiving Texas sun.

Exposure to abnormal or prolonged amounts of heat and humidity without relief or adequate fluid intake can cause various types of heat-related illness. Children and teens adjust more slowly than adults do to changes in environmental heat.

They also produce more heat with activity than adults, and sweat less. Sweating is one of the body’s normal cooling mechanisms.

Children and teens often do not think to rest when having fun and may not drink enough fluids when playing, exercising, or participating in sports.

Children and teens with chronic health problems, or those who take certain medicines, may be more susceptible to heat-related illnesses. Children and teens who are overweight or wear heavy clothing during exertion, such as marching band or football uniforms, are also more susceptible.

Heat cramps

Heat cramps are the mildest form of heat illness and consist of painful muscle cramps and spasms that occur during or after intense exercise and sweating in high heat.

What to look for:

  • Heavy sweating during intense exercise
  • Muscle pain or spasms

What to do:

  • Stop physical activity and move to a cool place
  • Drink water or a sports drink
  • Wait for cramps to go away before you do any more physical activity

Get medical help right away if:

  • Cramps last longer than 1 hour
  • You’re on a low-sodium diet
  • You have heart problems

Heat exhaustion

Heat exhaustion is more severe than heat cramps and results from a loss of water and salt in the body. It occurs in conditions of extreme heat and excessive sweating without adequate fluid and salt replacement. Heat exhaustion occurs when the body is unable to cool itself properly and, if left untreated, can progress to heat stroke.

What to look for:

  • Heavy sweating
  • Cold, pale, and clammy skin
  • Fast, weak pulse
  • Nausea or vomiting
  • Muscle cramps
  • Tiredness or weakness
  • Dizziness
  • Headache
  • Fainting (passing out)

What to do:

  • Move to a cool place
  • Loosen your clothes
  • Put cool, wet cloths on your body or take a cool bath
  • Sip water

Get medical help right away if:

  • You are throwing up
  • Your symptoms get worse
  • Your symptoms last longer than 1 hour

Heat stroke

Heat stroke, the most severe form of heat illness, occurs when the body’s heat-regulating system is overwhelmed by excessive heat. It is a life-threatening emergency and requires immediate medical attention.

What to look for:

  • High body temperature (103°F or higher)
  • Hot, red, dry, or damp skin
  • Fast, strong pulse
  • Headache
  • Dizziness
  • Nausea
  • Confusion
  • Losing consciousness (passing out)

What to do:

  • Call 911 right away because heat stroke is a medical emergency
  • Move the person to a cooler place
  • Help lower the person’s temperature with cool cloths or a cool bath
  • Do not give the person anything to drink

Sunburn

Sunburn is an often painful sign of skin damage from spending too much time outdoors without wearing a protective sunscreen. Years of overexposure to the sun lead to premature wrinkling, aging of the skin, age spots, and an increased risk of skin cancer. In addition to the skin, eyes can get burned from sun exposure. Sunburned eyes become red, dry, and painful, and feel gritty. (more)

What to look for:

  • Painful, red, and warm skin
  • Blisters on the skin

What to do:

  • Stay the sun until your sunburn heals
  • Put cool cloths on sunburned areas or take a cool bath
  • Put moisturizing lotion on sunburned areas
  • Do not break blisters

Heat Rash

Red clusters of small blisters that look pimples on the skin (usually on the neck, chest, groin, or in elbow creases).

What to look for:

  • Red clusters of small blisters that look pimples on the skin (usually on the neck, chest, groin, or in elbow creases)

What to do:

  • Stay in a cool, dry place
  • Keep the rash dry
  • Use powder ( baby powder) to soothe the rash

Stay safe!

It’s important to stay safe, be aware, hydrate, and take care of each other. When in doubt, always seek medical attention right away. Your 24-Hour MRMC Emergency Department is standing by to assist with emergency situations, and the Weekend Urgent Care Center is open on Saturdays and Sundays from 9:00am to 6:00pm.

This update was generated by MRMC’s Public & Media Relations Team information shared by our employees, community members, and/or partners. If you have any questions or comments, or if you believe that the information displayed here is incorrect in whole or in part, contact the Public & Media Relations Team directly by clicking here.

Source: https://www.matagordaregional.org/heat-stroke

Heat stroke vs. heat exhaustion: Differences and treatment

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine

Heatstroke and heat exhaustion are illnesses caused by exposure to extreme heat. If left untreated, heat exhaustion can progress into heatstroke, which may be life-threatening.

According to the Centers for Disease Control and Prevention (CDC), above-average temperatures or unusually humid weather kills more than 600 people in the United States each year.

As temperatures rise, it is important to know how to avoid heat-related illnesses. Learn about the symptoms and treatments of heatstroke and heat exhaustion below.

These conditions both result from overexposure to extremely hot weather. However, only heatstroke can cause damage to the body’s systems.

Heatstroke

Heatstroke, also called sunstroke, is the most serious heat-related illness. It occurs when the body’s temperature is 104ºF or higher, and it is a life-threatening medical emergency.

If not treated immediately, heatstroke can damage multiple organs and systems, including the:

  • brain and nervous system
  • circulatory system
  • lungs
  • liver
  • kidneys
  • digestive tract
  • muscles

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine

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Exposure to abnormal or prolonged amounts of heat and humidity without relief or adequate fluid intake can cause various types of heat-related illness. Children and teens adjust more slowly than adults do to changes in environmental heat.

They also produce more heat with activity than adults, and sweat less. Sweating is one of the body's normal cooling mechanisms.

Children and teens often do not think to rest when having fun and may not drink enough fluids when playing, exercising, or participating in sports.

Children and teens with chronic health problems, or those who take certain medicines, may be more susceptible to heat-related illnesses. Children and teens who are overweight or wear heavy clothing during exertion, such as marching band or football uniforms, are also more susceptible.

There are 3 types of heat-related illnesses:

  • Heat cramps
  • Heat exhaustion
  • Heat stroke

What are heat cramps?

Heat cramps are the mildest form of heat illness and consist of painful muscle cramps and spasms that occur during or after intense exercise and sweating in high heat.

What is heat exhaustion?

Heat exhaustion is more severe than heat cramps and results from a loss of water and salt in the body. It occurs in conditions of extreme heat and excessive sweating without adequate fluid and salt replacement. Heat exhaustion occurs when the body is unable to cool itself properly and, if left untreated, can progress to heat stroke.

What is heat stroke?

Heat stroke, the most severe form of heat illness, occurs when the body's heat-regulating system is overwhelmed by excessive heat. It is a life-threatening emergency and requires immediate medical attention.

Symptoms and first-aid measures for heat injuries

The following chart contains the most common symptoms of heat-related illness. Specific treatment will be determined by your child's doctor and may include some, or more, of the following:

ConditionSymptomsFirst-aid and treatment
Heat cramps
  • Painful cramps, especially in the legs
  • Flushed, moist skin
  • Move to a cool place and rest. Do not continue to participate in the activity.
  • Remove excess clothing and place cool cloths on skin; fan skin.
  • Give cool sports drinks containing salt and sugar.
  • Stretch cramped muscles slowly and gently.
Heat exhaustion
  • Muscle cramps
  • Pale, moist skin
  • Usually has a fever over 100.4° F (or 34° C)
  • Nausea
  • Vomiting
  • Diarrhea
  • Headache
  • Fatigue
  • Weakness
  • Anxiety, and faint feeling
  • Move to a cool place and rest.
  • Remove excess clothing and place cool cloths on skin; fan skin.
  • Give cool sports drinks containing salt and sugar.
  • If no improvement or unable to take fluids, take your child to an emergency department immediately. IV (intravenous) fluids may be needed.
Heat stroke
  • Warm, dry skin
  • high fever, usually over 104° F (or 40° C)
  • Rapid heart rate
  • Loss of appetite
  • Nausea
  • Vomiting
  • Headache
  • Fatigue
  • Confusion
  • Agitation
  • Lethargy
  • Stupor
  • Seizures, coma, and death are possible
  • Move to a cool place and rest.
  • Call 911 or your local emergency medical service. Heat stroke is a life-threatening medical emergency and needs to be treated by a doctor.
  • Remove excess clothing and drench skin with cool water; fan skin.
  • Place ice bags on the armpits and groin areas.
  • Offer cool fluids if alert and able to drink.

How can heat stroke be prevented?

Some general guidelines to help protect your child from heat-related illnesses include the following:

  • Drink plenty of fluids during vigorous or outdoor activities (including sunbathing), especially on hot days. Drinks of choice include water and sports drinks; avoid alcohol and fluids with caffeine, such as tea, coffee, and cola, as these can lead to dehydration.
  • Make sure your child dresses in light colored, lightweight, tightly-woven, loose-fitting clothing on hot days.
  • Schedule vigorous activity and sports for cooler times of the day. Take rest periods in shady or cool areas.
  • Makes sure your child is protected from the sun and wears a hat and sunglasses, and uses an umbrella. Use a sunscreen that is at least SPF (sun protection factor) 15.
  • Increase time spent outdoors gradually to get your child's body used to the heat.
  • Teach children to take frequent drink breaks and “wet down” or mist themselves with a spray bottle to avoid becoming overheated.
  • Try to spend as much time indoors as possible on very hot and humid days.
  • Teach your child to warm-up and cool-down before and after exercising.
  • If your child has a medical condition or is taking medicine, consult his or her doctor for further advice for preventing heat-related illnesses.

Source: https://www.hopkinsmedicine.org/health/conditions-and-diseases/heatrelated-illnesses-heat-cramps-heat-exhaustion-heat-stroke

10 Essential Facts About Heat and Your Health

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine

In all weather conditions, the body works hard to maintain a normal temperature of 98.6 degrees. When temperatures are drastic, whether it’s a scorching summer day or a frigid winter morning, it can be even more difficult for your body to maintain this ideal temperature.

“Even at rest, the human body produces a lot of heat energy,” says Eric Buete, MD, medical director of AFC/Doctors Express in Sarasota, Florida. When it’s cool, your body will expel this heat through radiation. “The heat simply radiates from the body to the surrounding air,” explains Dr. Buete.

When it’s hot, your body sweats to keep you cool. Perspiration comes to the surface of your skin. As it evaporates, you begin to feel cooler.

When it’s humid outside, it’s harder for the perspiration on the surface of your skin to evaporate because the air is already saturated with moisture.

That’s why people often say it’s not the heat, but the humidity that makes it unbearable to be outside on a hot day — but both play a role in your body's overheating.

What You Need to Know

Here are 10 important facts about heat and your health:

  1. Extreme heat is dangerous. In a typical year, as many as 175 Americans die from extreme heat, according to the National Weather Service.
  2. Men sweat more than women. While women have more sweat glands, according to the U.S. National Library of Medicine, men's sweat glands are more active, leading them to sweat more. The more you sweat, the more easily you can become dehydrated, which can lead to other health issues, Buete says.
  3. People have about 4 million sweat glands throughout their bodies, according to the International Hyperhidrosis Society. There are two types of sweat glands: eccrine and apocrine. Both produce fluids. The area of the brain known as the hypothalamus controls your body temperature by regulating sweat output and blood flow to the skin. The foul odor associated with sweat comes from the apocrine glands found in the armpits and genital region; the sweat from these glands produces a smell when it comes in contact with bacteria on the skin.
  4. Heat exhaustion occurs when your body overheats. Warning signs of heat exhaustion include: dizziness, nausea, vomiting, fatigue, heavy sweating, and headache. To treat heat exhaustion, it’s important to move to a cool location, drink lots of water, and soak in a cool bath or use cool compresses.
  5. Being overheated can lead to heat stroke, which can be serious and life-threatening. Heat stroke can occur when your body temperature reaches 104 degrees; at this point, your body cannot regulate temperature on its own, according to the American Academy of Family Physicians. Signs you may be having heat stroke include muscle cramping, fast heart beat, vomiting, flushed skin, headache, and mental confusion. Call 911 if you see someone experience these symptoms. As with heat exhaustion, someone experiencing heat stroke should be moved to a cooler place and cooled down with a bath of cool water or compresses.
  6. You can protect yourself from heat stroke by staying hydrated. “Drink before you are thirsty,” Buete says. In extreme heat, it’s best to avoid caffeine and alcohol, the American Academy of Family Physicians advises. Wear loose clothing that allows the air to circulate around you when exercising, and avoid exercising outdoors during the hottest part of the day, which is from 11 a.m. to 64 p.m. Instead, schedule your workout as close to sunrise or sunset as possible.
  7. Infants and small children are at higher risk for heat-related illnesses for several reasons, Buete says. They can’t control their environment (if they’re left in a room that is hot, for example); they have a high metabolic rate, which means their bodies are producing heat constantly; and they aren’t able to perspire as sufficiently as adults, Buete says. Never leave a child in a parked car, even with the windows open.
  8. Others at higher risk for heat-related illness include those who are morbidly obese, the elderly, and people who are immobile, Buete says. People with diabetes can be heat sensitive, too, says Matthew Corcoran, MD, of APG Endocrinology in Egg Harbor, New Jersey.

    “If you have diabetes and you become dehydrated from the heat, it can affect your blood-sugar levels,” says Dr. Corcoran. Be sure to keep insulin and other diabetes medications the heat, as hot temperatures can ruin them, he adds. People with multiple sclerosis may also find that their symptoms worsen when they’re hot.

    When heat raises a person’s body temperature, it becomes harder for the central nervous system to work properly.

  9. Some medications can put you at an increased risk for heat stroke as well, according to the American Academy of Family Physicians. These include allergy medicines or antihistamines, blood pressure and heart medications, diuretics, laxatives, antidepressants, and seizure medications. Talk to your doctor about what precautions you should take if you’re taking any of these.
  10. About 3 percent of the population has hyperhidrosis, a condition characterized by overactive sweat glands, which cause profuse sweating, according to The Center for Sweat Disorders at Johns Hopkins Medicine. The often-embarrassing condition can be inherited or caused by other health conditions or medications, and can occur without a trigger from heat. Treatments for hyperhidrosis include oral and topical medications, Botox injections, as well as a procedure that uses electricity to turn off the sweat glands. If these treatments are not effective, your doctor may consider surgery.

Source: https://www.everydayhealth.com/news/essential-facts-about-heat-your-health/

Sickle Cell Trait and Heat Injury Among US Army Soldiers

Exercise-Related Heat Exhaustion | Johns Hopkins Medicine

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There is concern that sickle cell trait (SCT) increases risk of exertional collapse, a primary cause of which is heat injury.

However, to our knowledge, no population-based studies among active individuals have addressed this, representing a critical evidence gap. We conducted a retrospective cohort study of SCT-tested African-American soldiers who were on active duty in the US Army anytime between January 2011 and December 2014.

Using Cox proportional hazards models and adjusting for demographic and medical factors, we observed no significant associations between SCT and either mild heat injury (hazard ratio (HR) = 1.15, 95% confidence interval (CI): 0.84, 1.56; n = 45,999) or heat stroke (HR = 1.11, 95% CI: 0.44, 2.79; n = 46,183).

Risk of mild heat injury was substantially higher among soldiers with recent prescriptions for antipsychotic agents (HR = 3.25, 95% CI: 1.33, 7.90). Risk of heat stroke was elevated among those with a prior mild heat injury (HR = 17.7, 95% CI: 8.50, 36.7) and among overweight and obese individuals (HR = 2.91 (95% CI: 1.38, 6.17) and HR = 4.04 (95% CI: 1.72, 9.

45), respectively). In a setting where universal precautions are utilized to mitigate risk of exertion-related illnesses, SCT is not associated with either mild heat injury or heat stroke.

There has been vigorous controversy over whether college athletes and military service members should be universally screened for sickle cell trait (SCT) in an effort to protect them against exertional collapse (1–5).

Persons with SCT are heterozygous for the sickle cell mutation in the hemoglobin subunit beta gene (HBB), resulting in the presence of wild-type hemoglobin A, as well as hemoglobin S. SCT is most prevalent among persons with African ancestry. An estimated 7.3% of African Americans and 1.

6% of Americans overall carry the trait (6). While SCT is largely benign and of no consequence to athletic performance, it has been associated in the literature with an increased risk of exertional sudden death events in both athletes and warfighters (7, 8).

While there is speculation on potential risk factors contributing to exertional collapse in this population, the mechanism for this increased risk has yet to be elucidated (9).

Proponents of screening suggest that it would alleviate the increased risk of exertional collapse and death reported to be associated with SCT. Potential actions might include increased education of and accommodations for athletes and warriors with SCT (5), although there is no published evidence on the impact of such efforts.

Others, however, have expressed concerns about mass SCT screening on ethical grounds and because, until very recently, the literature on the association between SCT and exertional collapse and death had been dominated by case reports and review-type articles (1–4, 10–17).

There has been a dearth of current, population-based epidemiologic studies on fully SCT-tested populations to provide adequate evidence to support policy-making (9).

In a recent study, we leveraged data on an SCT-tested subset of the US Army population to address this evidence gap (18). In that study, no difference in overall mortality was observed when comparing soldiers with and without SCT, but there was a modest elevation in the risk of exertional rhabdomyolysis associated with SCT.

Another critical health event that can precipitate exertional collapse and requires study is heat injury. Heat-related injuries are an important source of morbidity and mortality among athletes and warfighters (17, 19–23).

Concern that SCT increases the risk of heat injury has been expressed (1, 16, 17), but no population-based studies of SCT-tested active individuals have been conducted.

We therefore utilized a large database of administrative and health data on US Army soldiers to test whether SCT is associated with risk of heat injuries.

METHODS

We conducted a retrospective cohort study using data from the Stanford Military Data Repository, which includes largely deidentified administrative and health-care records for all persons who were active-duty US Army soldiers between January 2011 and December 2014 (see the Web Appendix, available at https://academic.oup.com/aje). During this time frame, soldiers were subject to doctrinal regulations emphasizing universal precautions, including exercise acclimatization in conjunction with promotion of adequate hydration and appropriate work/rest cycles for a given heat strain, as measured by the wet bulb globe temperature (24).

This study was approved by the Stanford University Institutional Review Board and the Defense Health Agency’s Human Research Protection Office.

Study population

Because SCT is most prevalent among African Americans (6), we limited the eligible study population to SCT-tested soldiers who reported African-American race and were on duty anytime between January 2011 and December 2014 (n = 48,384). Self-reported race was identified using official personnel records.

To ensure the capture of incident heat injury events, we utilized 2 sets of eligibility rules supporting dedicated analyses of heat stroke and milder heat injury events.

For both of these analyses, all African-American SCT-tested soldiers who were newly enlisted during this time were included in the analyses (n = 12,406).

This criterion was the policy that substantial prior heat injury is generally a disqualifying condition for service (25).

For “experienced” soldiers (i.e., those who entered military service prior to the start of our data set in January 2011), we first utilized a washout process to ensure that any follow-up care we might observe during 2011 for prior heat injury events would not be counted as pertaining to incident heat injuries.

Specifically, we excluded experienced soldiers with fewer than 13 months of observed service—that is, those who were discharged from military service for any reason before February 2012 (n = 2,166). This criterion permitted a sufficient screening and exclusion period.

The 13th month for experienced soldiers, January 2012, constituted the initial month of time at risk for outcomes.

For the heat stroke analysis, only those experienced soldiers with a minimum of 13 months of observed service and no evidence of heat stroke during 2011 were eligible.

Those with evidence of mild heat injuries during 2011 were accepted for the heat stroke analysis, as mild heat injury served as a predictor in this analysis.

We thus employed a study population for the heat stroke analysis comprised of new and eligible experienced soldiers with a sample size of 46,183.

Because heat stroke is ostensibly the most serious type of heat injury, we further limited the study population for the mild heat injury analysis to a subset of the population that was eligible for the heat stroke analysis, as follows.

First, experienced soldiers with evidence of either heat stroke or mild heat injury in 2011 were excluded. Second, any soldiers diagnosed with heat stroke prior to or in the same month as an incident mild heat injury were excluded from the mild heat injury analysis.

Finally, any soldier experiencing only heat stroke as a heat injury was excluded from the mild heat injury analysis. The last criterion was implemented to be consistent with the Cox modeling requirement that censoring is uninformative.

These additional exclusions resulted in a total sample size of 45,999 new and experienced soldiers for the mild heat injury analysis.

Measures

The outcomes of interest were 1) heat stroke per International Classification of Diseases, Ninth Revision, Clinical Modification, code 992.0 and 2) milder heat injuries collectively identified with International Classification of Diseases, Ninth Revision, Clinical Modification, codes 992.

1–992.9. The outcomes were identified using data from the Military Health System Data Repository (Web Appendix), which includes records from in- and outpatient encounters taking place in both military and civilian facilities. SCT was defined by laboratory tests confirming the hemoglobin AS phenotype.

Demographic predictors included sex, age, and military pay grade. We used 4 categories for age: ≤22 years, 23–27 years, 28–35 years, and ≥36 years. We employed 7 categories for pay grade: private (E1–E3); specialist or corporal (E4); junior sergeant (E5–E6); senior sergeant (E7–E9); warrant officer (W1–W5); junior officer (O1–O3); and senior officer (O4–O10) (26).

Participants’ body mass index (weight (kg)/height (m)2) measures were categorized per standard classifications (27). As only 120 soldiers (0.25% of the total study population) were underweight, we included underweight soldiers, along with those missing body mass index data, with the normal-weight soldiers in the reference group.

The most recent body mass index measure for each soldier was employed. To control for physical conditioning, we included a binary variable denoting high scores versus normal or low scores on the most recent Army Physical Fitness Test (28).

Information on tobacco use reported at an outpatient encounter during the preceding 6 months was updated monthly. Three binary variables indicated the presence or absence of at least 1 dispensed prescription for statins, antipsychotic agents, or stimulants in each observed month and/or the prior month.

Season was modeled with 4 categories and updated monthly: winter (December–February); spring (March–May); summer (June–August); and fall (September–November).

Statistical analysis

We fitted Cox proportional hazards models, and ties for the total time observed before the outcome were handled using the Breslow method.

The model employed was as follows, with the hazard of heat injury modeled as a function of the demographic, health-related, and season-related variables as described above:  h(t)=h0(t)exp(β1X1+β2X2+…+βpXp).

Time at risk began at the first observed service month for new soldiers and in January 2012 for experienced soldiers due to the aforementioned screening process. For the heat stroke analysis, censoring occurred after the first observed heat stroke event; mild heat injuries did not interrupt observation.

For the mild heat injury analysis, censoring occurred after the first observed mild heat injury. Otherwise, censoring occurred at the end of military service, if observed, or in December 2014. Mean observed time was 31.0 person-months per subject, and we observed 119,213 person-years in total.

All models included as predictors the main exposure of interest—SCT—plus all of the demographic, military, and health-related variables described above.

All statistical analyses were conducted using Stata 14 software (StataCorp LP, College Station, Texas), and all reported P values are 2-sided.

RESULTS

There were 543 incident cases of mild heat injury and 61 incident cases of heat stroke. Table 1 contrasts the characteristics of soldiers who did and did not experience each incident event in the respective analyses. Table 1.

Demographic and Health-Related Characteristics of Active-Duty African-American US Army Soldiers Tested for Sickle Cell Trait, According to the Presence or Absence of Incident Mild Heat Injuriesa and Heat Strokeb, United States, 2011–2014

Characteristicc . Mild Heat Injury Analysis (n = 45,999) . Heat Stroke Analysis (n = 46,183) . Subjects With Mild Heat Injury (n = 543) . Subjects Without Mild Heat Injury (n = 45,456) . P Valued . Subjects With Heat Stroke (n = 61) . Subjects Without Heat Stroke (n = 46,122) . P Valued . No. . % . No. . % . No. . % . No. . % . 
Female sex  212  39.0  12,681  27.9 

Source: https://academic.oup.com/aje/article/187/3/523/4084319