Screening for heat stress in workers and athletes

Heat-related illnesses lead to almost 6000 hospital visits per year, or two emergency department visits per 100,000 visits. Heat-related illnesses are most high among men (72%) aged 15 to 19 years and are most often associated with athletic activities (1). Global climate change will continue to have an impact on heat stress in people, on animal survival, and on food availability. The World Health Organization has estimated that over the past 30 years, 150,000 lives have been lost annually due to global heat waves and climate change (2). Weather changes have a great impact on athletes and workers by slowing efficiency and decreasing productivity (3, 4). When heat waves occur, overall mortality is also affected among the old and the young, due to their increased risk of dying from cardiovascular, respiratory, cerebrovascular, and some cardiovascular diseases, such as ischemic heart disease, heart failure, and myocardial infarction (3).

Despite increasing temperatures and environmental levels of carbon dioxide, one third of metropolitan areas do not have heat stress response plans (5). Cases of heat stress are handled on a case-by-case basis rather than by being addressed collectively as a public health concern. The National Institute for Occupational Safety and Health initiated heat stress criteria in 1986 and amended them in 2008 regarding worker heat stress safety (6). Few people are aware of these criteria, of the risk factors associated with a predisposition to heat stress, or of the signs or symptoms to look for that represent risk factors for heat stress. The rules and standards proposed by the Occupational Safety and Health Administration for workplace heat stress are recommendations and still not enforceable (7).

Screening for heat stress susceptibility in workers and athletes is an issue in need of public health criteria with enforceable rules based on evidence-based research. Screening for heat stress risk factors should consider physical demand level; cardiovascular, pulmonary, and endocrine status; previous history of heat stress; age; diseases; and medications including illegal drug use. Such screening should be part of the recruitment of any person expected to work or play in extremely hot environmental conditions. Older people are at increased risk for cardiovascular and thermoregulatory instability. The aging American workforce, undiagnosed disease, poor hydration while working or playing, and global warming are all factors that contribute to the need to screen workers and athletes for the risk of heat stress (8).

This article first reviews some physiological aspects of temperature regulation and then provides a formula for heat stress testing, discusses guidelines for screening, and reviews accommodations for those susceptible to heat stress.

PHYSIOLOGY OF TEMPERATURE REGULATION

The human capacity to regulate temperature equilibrium depends on various factors that ultimately impact the body's ability to maintain a steady core temperature (6). Each person's ability to maintain a steady inner core temperature can be influenced by environmental factors, age, gender, cardiovascular health, and even the medications they take (9). The type of clothing barrier that individuals wear influences their ability to cool down core body temperature and adjust to extreme environmental heat as well (10). Excessive clothing barriers can be extremely important in cases of athletic heat exhaustion and heat stroke mortality.

Under normal physiological conditions, the human body can maintain a stable core temperature even when stressed by extreme environmental temperatures as long as proper hydration is continuously provided (11, 12). Skin temperature is several degrees cooler than core temperature. It is regulated by the constriction or dilation of subcutaneous blood vessels, the rate of sweating production, heart rate, and cardiac output. Cardiovascular fitness as well as environmental factors play an important role in coping with extreme environment. Tolerance to temperature homeostasis via dehydration is more robust when individuals are physically fit and young (12–14). However, this ability to adapt to heat declines with age, the presence of disease, and a poor level of fitness (15). Any condition that affects the ability to sweat will also affect a person's ability to maintain a constant body temperature.

Regarding medications, any medication that has the potential to affect the cardiovascular system, sweating ability, vasodilatation, volume depletion, electrolyte equilibrium, venous pooling and cardiac output, or core temperature via the central nervous system has the potential to increase the risk for heat stress and heat stroke. Most notable among these medications are those listed in Table 1 (16). The table also reflects the mechanism whereby these medications might impact thermoregulation. Any individual taking these medications is at greater risk for developing heat stress, irrespective of age, weight, or environmental factors. Older individuals are more sensitive to these effects and are more likely to be on some of these medications due to their increased risk for chronic disease.

Table 1.

Legal and illegal medications that impact heat stress susceptibility^∗

Substance	Mechanism of action
Legal
Alcohol	HR decrease, volume depletion
Amphetamine	HR increase, increase in sweating
Anticholinergic	HR lability, abnormal sweating
Antihistamine	HR lability, abnormal sweating
Antihypertensive	HR lability
Benzodiazepine	HR lability, abnormal sweating
Calcium channel blocker	HR lability, venous pooling, abnormal sweating
Diuretic	Volume depletion, decreased vasodilation
Laxative	HR lability, abnormal vasodilation
Neuroleptic	HR lability, abnormal sweating
Phenothiazine	Electrolyte imbalance
Thyroid agonist (improperly controlled)	HR increase, increased sweating
Topiramate	Electrolyte imbalance
Tricyclic antidepressant	Electrolyte imbalance
Illegal
Cocaine	HR increase, increased sweating
Phencyclidine (PCP)	Abnormal temperature regulation
Lysergic acid diethylamide (LSD)	HR lability, abnormal sweating

∗Modified from Glazer, 2005 (16).

HR indicates heart rate.

FORMULA TO MEASURE HEAT STRESS RISK

A worthwhile risk assessment model for heat stress that combines both environmental factors and physiological risk factors has been posed in a simplified formula. The formula incorporates both environmental factors (environmental stress index, or ESI) and personal factors (personal stress index, or PSI) as follows:

• ESI = 0.63T_a – 0.03RH + 0.002SR + 0.0054 (T_a × RH) – 0.073(0.1 + SR)⁻¹

• PSI = 5(T_ret – T_re0) × (39.5 – T_re0)⁻¹ + 5(HR_t – HR₀) × (180 – HR₀)^–1

Variables represent ambient temperature (T_a), relative humidity (RH), and solar radiation (SR), which require only a few seconds to reach equilibrium. The ESI is the first stress index using direct measurements of solar radiation, heart rate (HR), and rectal temperature (T_re). T_re0 and HR₀ represent the initial T_re and HR, and T_ret and HR_t are measurements taken at a specific time.

This method considers environmental variations in temperature, humidity, and radiation as well as the individual's heart rate and aerobic tolerance and is therefore a good objective measure of risk for heat stress in individuals (17). Tachycardia accompanies volume depletion, and that is reflected by the heart rate in this formula. The formula can detect febrile states because it measures rectal temperature.

The PSI and ESI scales are integrated mathematically to reflect cardiovascular strain relative to environmental stress on a scale between 0 and 5. The higher the value, the higher the strain. This formula and coordination of work-rest cycles in training have been used to prevent the risk of future heat stress injuries in military recruits.

One limitation of this risk assessment is that it doesn't account for certain disease states, such as cardiac or respiratory disease compromise. It does not measure stroke volume, cardiac output, central venous pressure, or pulmonary wedge pressures, which are the benchmark measurements of cardiovascular and pulmonary compromise in diseased individuals. For this reason, the sensitivity and specificity of this risk assessment formula to screen for heat stress–susceptible persons with severe cardiopulmonary disease is somewhat limited, and other measures suggested later in this article should complement this risk assessment instrument.

RECOMMENDATIONS FOR SCREENING WORKERS AND ATHLETES

Several types or workers are at risk for heat stress in the US since they work outdoors for long hours in the sun or are in direct contact with flames or hot equipment (6). These include firefighters, bakery workers, farmers, construction workers, miners, boiler room workers, factory workers, and workers in manufacturing. Athletes at risk for heat stress include football players, runners, soccer players, hockey players, triathlon competitors, weight lifters, swimmers, hurlers, and hunters due to the accelerated cardiovascular state and outdoor nature of these activities.

In addition, certain individuals have higher risks, including those with reduced mental capacity, Alzheimer's disease or dementia, trisomy disorders, dysrhythmias, and congenital heart abnormalities. Persons with reduced mental capacity may not drink fluids on a regular basis, so they must be put on frequent rest cycles and be reminded to drink fluids in hot environments. Aging individuals face brittle vasoregulatory mechanisms. All these factors can be screened for during physical evaluations.

A person's desired level of job activity or athletic activity (physical demand level) is very important to assess as part of the history. Great physical exertion in a hot environment results in more stress on the cardiopulmonary system. Therefore, heat stress screening should be more thorough in individuals who anticipate heavy exertion in hot environments. In such situations, additional objective tests may be needed to improve the sensitivity and specificity of the screening process.

The categories of physical demand of work or play are easy to understand if they are categorized into light (requiring 0 to 19 lb of exertion), medium light (requiring 20 to 30 lb of exertion), medium (requiring 31 to 40 lb of exertion), medium heavy (requiring 41 to 70 lb of exertion), and heavy (requiring 71 to 100 lb of exertion) physical demand categories. These categories correspond to sedentary administrative-type jobs, light cleaning jobs, heavy cleaning jobs, manufacturing jobs with lifting duties, and construction work, respectively. The athletic corollary would be casual walking, light jogging, sprinting, marathon running, and playing football/participating in a triathlon, respectively.

During screening, the medical history should focus on the health and resilience of the cardiovascular and pulmonary systems and the medication history. It is also important to assess whether the individual has had prior episodes of heat stress or heat stroke. Some specific aspects of the screening include obesity, fitness level, cardiac function, and diabetes, as discussed below. Table 2 summarizes the recommendations for heat stress screening based on physical demand level for individuals who are working or playing in a heat-stressed environment.

Table 2.

Recommendations for heat stress screening for workers and athletes

		Components of screening needed
Weight to be lifted or exerted (lb)	Physical demand level of work or activity	Medical questionnaire∗	BP and BMI	EKG >35 yrs	3-step test	Stress test
0–19	Light	Yes	Yes	No	No	No
20–30	Medium light	Yes	Yes	No	Optional	No
31–40	Medium	Yes	Yes	Yes	Yes	No
41–70	Medium heavy	Yes	Yes	Yes	Yes	Optional
71–100	Heavy	Yes	Yes	Yes	Yes	Yes

Copyright Ramphal 2011.

∗Focused questionnaire to include cardiopulmonary function, fitness level, prior episodes of heat stress, and medications.

BP indicates blood pressure; BMI, body mass index; EKG, electrocardiogram.

Obesity

Recent studies indicate that approximately one third of our children and adults fall into the overweight or obese category. A high body mass index (BMI) adds additional burden on the cardiopulmonary system (6, 7, 18). Thermoregulation of the core temperature can be more challenging if there is increased peripheral insulation. Obesity increases the core temperature and decreases a person's ability to effectively cool down rapidly with the usual cardiovascular thermoregulatory mechanisms. In addition, obesity is an independent risk factor for increased cardiovascular disease (14).

The association seems to be independent of hormonal factors such as leptin. Therefore, screening for BMI is easy and reasonable to do and should be included as part of the screening protocol for persons who are about to engage in activities that expose them to heat, which will further stress the cardiovascular system. As recommended by the Federal Motor Carrier Safety Administration for truck drivers, a BMI >30 kg/m² should alert the screening professional that the individual requires health counseling about dietary and exercise goals, assuming there are no cardiac issues that will prevent participation. A BMI >40 kg/m² should trigger a need for more frequent rest periods in a heat-stressed environment for various reasons. At this level, data correlate a high BMI with a risk for labile hypertension, higher mortality from sudden death, and sleep apnea (19). Such persons can work in the heat for very short periods of time with frequent break sessions for cooling until they decrease their BMI to a safer level.

Fitness level

Fitness level is an important part of the intake history because a sedentary individual is more likely to succumb to heat stress than an active one. If it is necessary to assess a person's cardiovascular resilience, a simple 3-minute step test and pulse recovery monitoring can easily be done. This test must be performed in a standardized manner using a 12-inch step. The participant steps up and down to a 96-beat-per-minute metronome beat for 3 continuous minutes. Pulse recovery from baseline after 3 minutes of rest is standardized according to the scoring in Table 3, which determines the individual's level of fitness. Fitness levels are not static and can be improved with gradual exercise. If individuals are not fit, their tolerance to heat will be inadequate, and they should not be exposed to long periods in the heat before they improve their fitness and cardiac conditioning.

Table 3.

Scoring on the 3-minute step test for men and women based on age^∗

	18–25	26–35	36–45	46–55	56–65	65+
Men
Excellent	50–76	51–76	49–76	56–82	60–77	59–81
Good	79–84	79–85	80–88	87–93	86–94	87–92
Above average	88–93	88–94	92–88	95–101	97–100	94–102
Average	95–100	96–102	100–105	103–111	103–109	104–110
Below average	102–107	104–110	108–113	113–119	111–117	114–118
Poor	111–119	114–121	116–124	121–126	119–128	121–126
Very poor	124–157	126–161	130–163	131–159	131–154	130–151
Women
Excellent	52–81	58–80	51–84	63–91	60–92	70–92
Good	85–93	85–92	89–96	95–101	97–103	96–101
Above average	96–102	95–101	100–104	104–110	106–111	104–111
Average	104–110	104–110	107–112	113–118	113–118	116–121
Below average	113–120	113–119	115–120	120–124	119–127	123–126
Poor	122–131	122–129	124–132	126–132	129–135	128–133
Very poor	135–169	134–171	137–169	137–171	141–174	135–155

∗Scores are the age-adjusted standards based on guidelines published by the YMCA (http://www.exrx.net/Testing/YMCATesting.html). In the test, the participant steps up and down a 12-inch step to a 96-beat-per-minute metronome beat for 3 continuous minutes, and pulse recovery from baseline after 3 minutes of rest is recorded.

Cardiac function

Cardiac screening tests, such as a stress echocardiogram or electrocardiogram, which are considered the gold standard for cost-effective screening (20), are recommended only in certain circumstances. If a person is going to be in a medium or heavier physical activity, it is advisable to screen that person for cardiac risk factors with objective tests before he or she is exposed to the stress of a hot environment, in order to mitigate the risk for adverse cardiac events. In addition, at age 35, the cardiopulmonary system of average adults begins a slow aging process that can be mitigated only with exercise and is unpredictably impacted by environmental and genetic factors. For this reason, those older than 35 years should be screened with basic cardiovascular screening tests if they have any cardiovascular risk factors or if they are going to be challenged by physical demands of medium-heavy levels or greater, as indicated in Table 2.

Diabetes as related to cardiac function

Diabetes is highly associated with ischemic heart disease and cerebro-vascular accidents, events that can be provoked by heat stress (21). For this reason, those exposed to heat should be screened for diabetes, even if they are in a sedentary role. A urinalysis can be used for most screening. For individuals engaging in an activity above a medium physical demand level, the more sensitive serum glucose test is appropriate. A urinalysis positive for glucose also indicates the need for serum glucose testing. A serum glucose level >200 mg/dL indicates the need to stabilize the glucose before the individual is allowed into a heat-stressed environment, as he or she has an increased risk for negative cardiovascular events (22).

ACCOMMODATION OF HEAT STRESS–SUSCEPTIBLE PERSONS

The Americans with Disabilities Act Amendments Act of 2008 (ADAAA) requires employers to accommodate employees who have gone through a disabling disease process that makes them incapable of accomplishing their usual work due to medical disability (23). The expanding focus of the ADAAA places the burden on employers to accommodate workers after clearance from their personal physician. Guidelines from the Federal Motor Carrier Safety Administration, which appear in Table 4, specifically address recovery from cardiovascular and pulmonary instability and are applicable to a wide range of situations, including heat stress screening.

Table 4.

Guidelines for truck driver certification that apply to heat stress screening^∗

Variable	Guideline
High blood pressure	Disqualify if blood pressure exceeds 160/100
Blood glucose	Disqualify if serum glucose ≥200 mg/dL and refer to PCP
Urinanalysis	Defer to serum glucose if dipstick is positive for glucose
Disqualifying medications	Refer to PCP for possible alternative change of medications†
Body mass index	Refer for counseling if >40 kg/m2
	Screen for sleep apnea
Cardiac arrhythmias	Disqualify if there is a malignant arrhythmia
	Clear with benign arrhythmia after medication control and normal baseline stress echo biannually
Myocardial infarction	Wait 2 months then clear with normal baseline stress echo and normal biannual echo; ejection fraction must be >40%
Cardiac stent placement	Wait 1 week then clear with normal baseline stress echo; repeat every 2 years
Cardiac bypass	Wait 3 months then clear with normal baseline stress echo; repeat every 2 years; ejection fraction must be >40%

∗Based on 2010 guidelines from the Federal Motor Carrier Safety Administration, available at http://www.fmcsa.dot.gov/rules-regulations/rules-regulations.htm.

†Disqualifying medications are specific for underlying cardiac or pulmonary diseases that may not be capable of substitution.

PCP indicates primary care physician; echo, echocardiogram.

Rest/work cycles based on physical demand and temperature, as shown in Table 5, can also be applied. The threshold limit values (TLVs) in the table are based on the assumption that nearly all acclimatized, fully clothed workers with adequate water and salt intake should be able to function effectively under the given working conditions without exceeding a deep body temperature of 38°C (100.4°F). They are also based on the assumption that the Wet Bulb Globe Temperature (WBGT) of the resting place is the same as or very close to that of the workplace. Where the WBGT of the work area is different from that of the rest area, a time-weighted average should be used (24).

Table 5.

Work/rest regimen based on work load and temperature^∗

	Work load
Work/rest regimen each hour	Light	Moderate	Heavy
Continuous work	30.0°C (86°F)	26.7°C (80°F)	25.0°C (77°F)
75% work, 25% rest	30.6°C (87°F)	28.0°C (82°F)	25.9°C (78°F)
50% work, 50% rest	31.4°C (89°F)	29.4°C (85°F)	27.9°C (82°F)
25% work, 75% rest	32.2°C (90°F)	31.1°C (88°F)	30.0°C (86°F)

∗From American Conference of Governmental Industrial Hygienists (24).

CONCLUSION

In our increasingly warmer global climates, specific screening is needed for heat stress to prevent morbidity and mortality in the workplace and during sports activities. Most people who are screened using these simple tests and guidelines will avoid undesirable outcomes. The recommendations for screening individuals superimpose existing tools and thus increase the sensitivity and specificity in identifying those at risk for heat stress. These methods also help accommodate individuals who might need to recover from an illness that temporarily affects cardiovascular fitness. Occasionally, there will be those with congenital problems such as Kawasaki's disease, which can cause unforeseen cardiac complications that remain undetected until the fatal cardiac arrhythmia occurs on a hot summer day on the athletic field. Yet, the goal of primary care providers is to prevent 99.9% of preventable illness. By using the screening tools recommended in this article, we can be better equipped to accomplish that goal.