Preventing heat stress

Heat stress in the workplace can cause reduced productivity and, at its worst, serious illness if you don't take proper precautions through education and prevention.

Introduction

Heat stress is an ongoing health problem for both indoor and outdoor workers. Its seriousness can range from severe thirst to a feeling of weakness and exhaustion or even uncontrolled heat stroke and death. The human body has an incredible capacity to adjust to hot weather conditions in just a few days, which is why acclimatization is a very important step in avoiding heat exposure.  To acclimatize workers, the National Institute for Occupational Safety and Health (NIOSH) recommends that the beginning worker or one returning from vacation or illness should work at least two hours a day in the heat for one week and gradually increase workloads from half to full capacity during those days.  As the worker becomes accustomed to the heat, body temperature will drop, and the pulse rate will decrease.  As the body gets used to the heat, the physical work will become easier.

Heat-induced occupational illnesses and injuries and occur when the total heat load, environment (outside) and metabolic heat (heat generated in the body) exceeds the capacity of the body to maintain normal body functions without strain.

Several conditions influence the heat equilibrium, including skin temperature, amount of evaporated sweat and the type, amount and characteristics of the clothing worn.

In analyzing any work situation, the following concepts should be considered.

• Conduction of heat to and from solid objects is usually not an issue in the workplace since workers generally are not in direct contact with surfaces hotter than normal body temperatures for any sustained length of time. Heat loss by conduction to air occurs when the air in contact with the skin is below body temperature.  Conversely, heat gain by conduction from air occurs when air temperature exceeds body temperature, as it would in a hot chamber or a warehouse during the summer months.
• Convective heat exchange occurs by radiation.  A body loses heat when the surrounding objects have surface temperatures lower than the temperature of the body surface and conversely gain heat by radiation when the temperature of the surrounding surfaces is above body surface temperature (i.e., steam pipes and blast furnaces.  Radiant heat exchange is independent of air motion.)
• Evaporation is generally the mechanism most used by the body for the dissipation of large amounts of heat generated by working muscles.  The Recommended Standard for Occupational Exposure to Hot Environments (NIOSH) explains in detail the interaction between gaining heat and controlling heat.

The heat stress exposure guidelines established by the American Conference of Governmental Industrial Hygienists are intended to represent working conditions under which most employees will not suffer adverse effects.  The Wet Bulb Globe Temperature Index (WBGT) is used to quantify environmental factors, including wet bulb, globe temperature, and dry bulb factors according to the following relationships:

WGBT = 0.7 WB + 0.2 GT + 0.1 DB (Outdoors with Solar Load)
WGBT = 0.7 WB + 0.3 GT (Indoors or Outdoors with no Solar Load)

Where,
WB = Wet bulb temperature
GT = Globe temperature
DB = Dry bulb temperature

Health standards

The exposure guidelines specify a scheduled work-rest regimen based on the derived WBGT and the workload associated with a task.  These guidelines do not apply when heavy protective clothing is worn. 

Adherence to the criteria should allow most acclimatized, fully clothed workers with adequate water and electrolyte intake to function under the given work-rest schedule without exceeding a deep core body temperature of 38ºC.  It is believed that maintenance of body core temperature at or below this level will not result in adverse health effects for most employees.  Some employees will still have problems.  The most susceptible are new workers starting a job that involves physical work in a hot environment, older people, and people who aren’t used to working vigorously in hot climates.

Workers’ physical characteristics and individual health conditions can affect the way they react to heat.  Studies show that those who are more physically fit are able to perform their workload in a hot situation.  An unfit worker’s body temperature is higher.  Studies, such as one by Shapiro in 1980, show that men tolerated heat stress better than women in hot, dry climates, but not as well as women in humid conditions.

Body size is predictive of risk.  Small people of either sex have a lower heat tolerance. This is due to less surface area of skin to dissipate heat.  Older people, 40- to 65-years-old, are less able to acclimate fully to work in hot environments.  The aging process results in a more sluggish response of the sweat glands and a decrease in the total body water content.  Obese people who exceed standard weight by 15 percent have a lower tolerance to heat than leaner individuals do.

Drug and alcohol abuse physiologically alters thermal regulatory functions and is a risk factor for heat stresses.  Certain chronic diseases can also affect heat tolerance.  Workers with blood pressure of more than 160/95 responded with less cardiac output and decreased skin blood flow in hot situations.  People at risk include those with organic diseases of the heart or vascular system, coronary artery, or cerebral vascular and peripheral artery disease.  Workers with active lung disease, skin disease, chronic liver, renal, and endocrine and digestive diseases can have heat stress problems when working in the heat.

Heat stress disorders

Heat fatigue:  People with this condition exhibit reduced performance capacity, a lowered standard of social behavior, and an inability to concentrate.  Heat fatigue is usually caused by moving from a tepid environment to a very hot environment.  Usually, when those affected get back to a cooler environment, the heat fatigue subsides.  An orientation or gradual introduction to the hot environment can help workers avoid this condition.
Skin Eruptions:  Employees who wear impermeable protective clothing are prone to skin eruptions caused by exposure to high humidity and a hot environment.  Under these conditions, the skin is continuously wet with sweat, but the moisture cannot evaporate. The sweat ducts become plugged, sweat is retained, and an inflammatory reaction occurs.  Heat rash can be helped by resting in cooler areas, frequent bathing, and allowing the skin to dry.  Wearing loose fitting clothing can also help.  Another type of skin condition is anhydrotic heat exhaustion which may occur when large areas of skin do not perspire when exposed to heat.  The skin takes on a goose- flesh appearance.  This condition may occur when someone with a previous history of extensive heat rash and sunburn has constant exposure to heat.  Usually, recovery of sweating occurs gradually on return to cooler climates.

Heat syncope:  This condition, fainting while standing erect and immobile in the heat, is generally caused by a pooling of blood in the lower extremities and dilated vessels of the skin.  It leads to hypotension (low blood pressure) and results in sudden unconsciousness.  To prevent heat syncope, employees should not be required to stand in the heat for a long period.  Treatment requires removal of the affected person to a cool, shaded area and keeping them in a recumbent position.

Heat cramps:  Cramps are extremely painful spasms of the larger muscles used during work (i.e., muscles of arms, legs, abdomen, or back).  They generally last less than two or three minutes.  The predisposing factor to heat cramps is heavy prolonged sweating and an electrolyte imbalance.  If there is an electrolyte deficit and prolonged sweat loss, once a fluid loss of 5 percent of body weight is reached, an electrolyte liquid replacement should be used.  Athletic drinks containing electrolytes leave the stomach slowly, produce a sensation of fullness, and may discourage people from ingesting the water needed to replace critical fluid losses.  Thirst is not a sensitive or reliable indicator of the need to drink fluids.  Cool water should be provided, and employees should be encouraged to drink at regular intervals, (approximately every 20 minutes) even though they may not be thirsty.  People on diuretics, salt-restricted diets, or other medications that alter hydration should be educated about the causes of heat cramps and review the need for salt.  Some employers used to provide salt tablets in the workplace in order to help people acclimate to heat, but so much salt is contained in the average diet that most people have sufficient salt intake.  Salt tablets can be difficult for some people to digest, causing stomach irritation and other illnesses, so they are no longer recommended.

Heat exhaustion:  Heat exhaustion is one of the more serious types of heat-related disorders.  Common symptoms are fatigue, headache, giddiness, diarrhea, nausea, and clammy and moist skin.  The worker may be disoriented, and treatment is usually symptomatic.  Rest in a cool area, fluid replacement under observation, and possibly the application of cold towels to the groin, armpits, and forehead may be indicated.  Medical care should be sought to discover the cause of the heat exhaustion and prevent its recurrence.

Heat Stroke:  Heat stroke can be life threatening.  Heat stroke occurs when the body's system of temperature regulation fails and body temperature rises to critical levels Symptoms include:  the absence of sweating; hot, dry skin; confusion; loss of consciousness and/or convulsions.  Heat stroke is a serious medical emergency and should be treated as such.  Hospitalization is recommended for anyone suffering heat stroke.  At least 24-hour observation is common.  Prevention is focused on worker health screening and employee education.

Summary

Heat-induced occupational illnesses and injuries occur in situations in which the total heat load exceeds the capacities of the body to maintain normal body functions without strain.

A preventative workplace program should be initiated in order to prevent heat stress disorders.   The program should include alerts to employees when the weather is hot enough to induce heat stress.  An education program or a heat stress disorder prevention program should discuss:

• Heat transfer
• The body’s reaction to heat
• Practical matters, such as clothing and pacing or work
• Fluid replacement
• Diet and salt intake
• Acclimatization
• Life style issues, i.e. alcohol consumption, drug abuse, extra jobs, adequate sleep, health status.
• Signs and symptoms of heat stress disorders
• An emergency action plan that complies with CRF 29 1910.38

You should review the various types of heat illness and first aid with your employees and hold a discussion about the countermeasures that can be used on site to prevent heat disorders.  The policies of the company on work in hot environments should also be explained.

Work practice modifications can be beneficial in avoiding heat stress.  Examples include having more frequent short exposures rather than few longer exposures, starting earlier in the day when it is cooler, increased rest times and restricted overtime.

References
Heat Stress and Strain.  Threshold Limit Values and Biological Exposure Indices, American Conference of Industrial Hygienists, 2001.