Occupational Safety - Working in Hot Environments
Heat stress, the result of exposure to hot environments, is a potential safety and health hazard in many industries and occupations. This report discusses heat stress, its affect on the human body, and possible controls to reduce the exposure.
Introduction
Between five and ten million workers are exposed for at least part of the work year to hot working conditions. Heat stress, the result of exposure to hot environments, is a potential safety and health hazard in many industries and occupations. Outdoor workers who may suffer from heat stress include those who work in agriculture, construction, merchant marine, and surface mining. Indoor heat exposure is a concern in work places, such as bakeries, brick-firing, and ceramics factories, chemical manufacturing facilities, electrical utilities, food canneries, glass products factories, iron and steel foundries, laundries, nonferrous foundries, mines, rubber products factories, smelters, and steam tunnels. Heat exposure is also a concern in the fire fighting service.
A World Health Organization's panel of experts recommended that a body core temperature of 38°C (100.4°F) be considered the limit of permissible exposure for work in hot enviroments. This is consistent with the fact that body core temperature in excess of this limit increases the likelihood of heat disorder or illness.
This report discusses heat stress, its affect on the human body, and possible controls to reduce the exposure.
Heat Disorders
Physiological and psychological effects of heat stress include lack of efficiency in performing heavy tasks, and deterioration in skilled manipulations and psychological and mental tasks. Furthermore, heat accelerates the onset of fatigue. The combination of increases of carbon dioxide and heat stress has been shown to have more harmful effects than each of these factors taken alone. Noise and heat also have been shown to have interactive (synergistic) effects.
Heat Stroke
Factors that predispose an individual to heat stroke include: sustained exertion in heat by un-acclimatized workers; obesity and lack of physical fitness; recent alcohol intake; dehydration; individual susceptibility; and chronic cardiovascular disease.
Heat stroke is a life-threatening medical condition. Symptoms of classic heat stroke include:
- Altered mental status and rectal temperatures >104°F (>40°C) and often greater than 107.6°F (42°C);
- Major disruption of the central nervous system (unconsciousness or convulsions);
- Lack of sweating. Sweating may be present in exertional heat stroke;
- The skin is usually flushed and hot;
- The pulse is elevated;
- Hyperventilation is also common, and
- Nausea, vomiting, and diarrhea may be present. Heat stroke is linked to failure of the thermoregulatory center (hypothalamus).
Treatment requires immediate immersion in chilled water with massage, or a wrapping in wet sheets with vigorous fanning by cool dry air. Overcooling must be avoided and shock may be present. Preventative measures include medical screening of personnel and ensuring that the placement of workers in hot environments is based on their health and physical fitness. A period of acclimatization of five to seven days by graded heat and work exposure should be required and workers should be monitored during sustained conditions of possible heat stress.
Heat Syncope
This is a heat disorder with the most common symptom of fainting while standing erect and immobile, making postural changes, or exercising strenuously in hot environments. Nausea, sweating, and dimming of vision may precede fainting. It is caused by the pooling of blood in dilated vessels of the skin and lower parts of the body. Lack of acclimatization predisposes a person to heat syncope. It is treated by removing the patient to a cooler area. Recovery is usually prompt and complete. Proper acclimatization is the best preventative measure. Intermittent activity will assist venous return to the heart.
Heat Exhaustion
Heat exhaustion is a complex of symptoms, including fatigue, nausea, headache, and giddiness associated with moist clammy skin, rapid heart rate, postural hypotension (low to normal blood pressures that fall on standing) and usually normal to slightly elevated rectal temperature of 99.5 to 101.3°F (37.5 to 38.5°C). Symptoms of heat exhaustion are similar to those of heat stroke; so all heat exhaustion victims must be evaluated to rule out the diagnosis of heat stroke.
Heat exhaustion occurs more commonly among workers who are un-acclimatized to heat and who are without adequate water and salt (electrolyte) replacement. Water-depletion heat exhaustion occurs when water replacement is not adequate to replace water lost in sweat. Dehydration is present and thirst may be an important symptom. The worker may have abnormally high levels of blood sodium and urine volume may be small. Salt-depletion heat exhaustion occurs when sweat fluid losses are replaced with water, but salt or electrolyte intake is inadequate to replace sodium, chloride, and other electrolyte losses. Dehydration is usually not present, and urine volume is normal.
Allowing the worker to rest in a cool area and providing oral fluid and salt replacement may treat mild heat exhaustion. Severe cases of heat exhaustion where severe water depletion requires intravenous fluids need to be handled in an emergency room for careful replacement of body water.
Preventative measures include:
- Acclimatizing workers during a 5- to 7-day breaking-in schedule.
- Supplementing dietary salt during acclimatization.
- Drinking water frequently during work shifts.
Heat Cramps
This heat disorder is characterized by painful spasms of muscles used during work (arms, legs, or abdomen). Onset occurs during or after work hours. The disorder is caused by loss of body salt in sweat. Water intake dilutes electrolytes. Water enters muscles causing spasms. Predisposing factors include heavy sweating during hot work and drinking large volumes of water without replacing electrolytes.
Treatment includes consumption of salted liquids by mouth or more prompt relief by intravenous infusion of fluids containing electrolytes. Preventative measures include adequate salt intake with meals and, for un-acclimatized workers, provision of salted drinking water or an equivalent electrolyte replacement beverage.
Heat Rashes
Heat rash or miliaria is caused by sweat duct obstruction and resultant sweat retention within the sweat gland. Obstruction of the sweat duct leads to duct rupture within the skin and an inflammatory reaction surrounding the duct. Since sweat gland rupture may occur within different layers of the skin, there are three types of heat rash: miliaria crystallina, miliaria rubra, and miliaria profunda.
Miliaria crystallina is a mild asymptomatic condition consisting of small clear vesicles resulting from sweat gland rupture within the surface layers of the skin.
Miliaria rubra (prickly heat) commonly affects the skin of the trunk and areas that rub against one another. Extensive cases of miliaria rubra involving large numbers of sweat glands can impede body heat dissipation and lead to more serious heat disorders.
Miliaria profunda results from sweat gland rupture deep within the skin. The lesions, which appear only after prolonged periods of miliaria rubra, are small, white- to flesh-colored papules and occur most commonly on the trunk.
Treatment usually includes drying lotions or powders, and keeping the skin clean to prevent infection.
Methods for Controlling Heat Stress
Total heat stress can be reduced only by modifying one or more of the following factors: metabolic heat production, heat exchange by convection, heat exchange by radiation, or heat exchange by evaporation. The environmental heat load can be reduced by engineering controls (e.g., ventilation, air conditioning, screening, insulation, process, or operational modification), as well as by using protective clothing and equipment.
Metabolic heat production can be modified by changes in work practices and use of labor-saving devices. Heat stress control for an individual worker depends on a number of factors involving appropriate behavior and environmental control:
- Bodily heat production.
- Number and duration of exposures.
- Heat exchange components as affected by environmental factors.
- Thermal conditions of the rest area.
- Clothing and protective equipment.
Control Methods for Various Sources of Heat
Methods for controlling sources of heat include:
Metabolic - Body Heat Production
Decreasing the physical workload, increasing the number and length of rest periods, and using power tools and equipment may control metabolic heat production.
Radiation Heat Exposure
Using one or more of the following engineering controls may reduce the heat load on workers from radiation heat sources:
- Interpose line-of-sight barriers between workers and the radiation heat source, insulate the heat source, use heat reflective clothing, and cover exposed body parts.
- Increase the distance from the source or decrease the time of exposure. Isolate the heat source.
- Reduce emissivity of hot surfaces.
- Use radiation shielding. Reflective shields include aluminum and stainless steel sheeting or other bright-surface metallic materials. Absorptive shielding absorbs infrared radiation effectively. It is constructed of two or three sheets, preferably flat black, separated by air spaces. Heat can be removed by water running between two sheets. Transparent shielding, using either special glass or metallic mesh, is also used. Special glass is either heat-absorptive or infrared-reflecting. Metallic mesh uses chains and wire mesh to provide partial reflectance and reduce heat reaching the operator. There is also flexible shielding that uses fabrics treated with aluminum to reduce radiant heat. When worn as aprons or other garments, they reflect radiant heat by up to 90 percent
Convective Heat Exposure
If air temperature exceeds 95°F (35°C) air temperature should be reduced, if possible; otherwise, air movement across skin should be reduced, and clothing should be worn. If air temperature is below 95°F (35°C), air movement over skin should be increased by providing ventilation and reducing the amount of clothing. Types of heat reduction for convective heat exposures include:
- Local exhaust ventilation is effective at reducing the heat load from convection - it does not reduce radiation heat stress. Canopy hoods with natural draft or mechanical exhaust ventilation are used commonly over furnaces and similar equipment.
- Localized cooling at workstations or 'spot cooling' provides cool air in sufficient quantity to surround the worker with an independent atmospheric environment.
- General ventilation is a common method of heat removal in hot industries. It makes use of wall openings and roof openings for the entrance of cool outside air and roof openings, commonly of the gravity type, for the discharge of heated air. It is an essential part of the overall heat control strategy. However, it cannot offset direct radiant heat exposures. It sometimes fails to function as needed because it fails to provide adequate make-up air.
Evaporative Cooling
To increase evaporative cooling, humidity should be decreased, airflow over the skin increased (increase ventilation), and the amount of clothing reduced.
Managing Worker Heat Stress
Methods to manage worker stress include:
Employee and Supervisor Training
In hot work environments, employees and supervisors who may be exposed to heat should be provided with information on the following:
- Causes of heat disorders and recognition of symptoms.
- Heat acclimatization.
- Electrolyte and water replacement.
- Synergistic effects of heat and alcoholic beverages, drugs, toxic agents, and physical agents.
- Use of protective clothing and equipment.
Medical Examinations and Emergency Medical Care
Physical examinations are recommended for employees to determine fitness to work in hot environments. A history of adaptation to hot environments in previous jobs can be used to help predict the future adaptation to heat stress. Supervisors and certain other personnel should be trained to recognize the signs and symptoms of heat disorders, so they can administer first aid.
Acclimatization
Acclimatization refers to the adaptive process by which an individual worker can adjust the thermoregulation processes of his or her body so body core temperature does not rise excessively during hot work. The Natonal Institute for Occupational Safety and Health (NIOSH) recommends that workers who have had previous experience working under heat stress have an acclimatization regimen with exposure for 50 percent on day one, 60 percent on day two, 80 percent on day three, and 100 percent on day four. New workers should begin with 20 percent exposure on day one and a 20 percent increase on each successive day. The term 'exposure' here refers to the percentage of time spent in hot work during the workday.
Work-Rest Routine
The NIOSH criteria recommends that a work and rest routine be implemented to reduce peaks of physiological strain and to improve recovery during rest periods. This regimen should include the following:
- When possible, schedule hot jobs for the cooler part of the day (early morning, late afternoon, or evening).
- Schedule routine maintenance and repair work in hot areas for the cooler seasons of the year.
- Alter the work-rest routine to include more rest time.
- Provide cool areas for rest and recovery.
- Use extra employees to reduce the exposure time of each member of the work team.
- Allow workers to interrupt work when they feel extreme heat discomfort.
Water and Electrolyte Replacement
Fluid intake during a work period should approximate the amount of sweat produced. Work in hot environments should result in sweat production of 1 to 3 gal (3.78 to 11.36 L) per shift. If this loss is not replaced, severe dehydration occurs. Thirst is not an adequate drive to achieve sufficient replenishment of fluids. An ample supply of fluid should be on hand and workers should be encouraged to drink every 15 to 20 minutes.
Rehydration will occur more rapidly if beverages containing sodium - the major electrolyte lost in sweating - are provided. The rehydration beverage should also contain sucrose or glucose because these carbohydrates provide energy for working muscles, stimulate fluid absorption in the stomach, and improve beverage taste.
Supplementary Body Cooling and Protection
Modifying clothing or equipment is one effective means of alleviating heat stress, without the need for large ingestion of drinking water.
Water-Cooled Garments - Water-cooled garments include: a water-cooled hood that provides cooling to the head; a water-cooled vest that provides cooling to the head and torso; a short, water-cooled undergarment that cools the torso, arms, and legs; and a long, water-cooled undergarment that cools the head, torso, arms, and legs. None of these apparel cool the hands and feet.
Air-Cooled Garments - Air-cooled suits and/or hoods that distribute cool air next to the skin are also available.
Ice Packet Vests - The available ice packet vests may contain as many as 72 ice packets; each packet has a surface area of 9.9 in2 (64 cm2) and contains about 0.1 lb (46 g) of ice. These ice packets are generally secured to the vest by tape. They are most often used for short duration tasks or emergency repairs.
Wetted Overgarments - An auxiliary cooling garment may be made by using a wetted cotton terry cloth coverall or a two-piece cotton cover that extends from just above the boots and from the wrists to a V-neck, when used with impermeable protective clothing, can be an effective. The effectiveness of this garment depends on the relative humidity and the wind speed of the hot environment.
Forecasts of Hot Weather and Heat Alert Programs
In many hot industries, heat stress is a seasonal phenomenon. Sudden heat waves early in the summer can create dangerous situations. For other facilities, the main problem comes during the hottest episodes of the summer.
Depending on the severity of the measured or forecast episodes of heat stress, changes in work practices may be required, such as distribution of the total work load, scheduling of the hottest jobs for the coolest part of the work shift, or other administrative measures. In plants where heat illnesses and disorders occur mainly during hot spells during the summer, a Heat Alert Program should be established.
During heat alerts, supervisors should be instructed to open windows, doors, skylights, and vents according to instructions for greatest ventilating efficiency at places where high air movement is needed. Drinking fountains, fans, and air conditioners should be checked regularly and in good repair, and workers should know how to use them. Jobsite and medical personnel should be prepared to administer heat disorder first aid.
The following operating procedures should be followed during a heat alert:
- Postpone tasks that are not urgent.
- Increase the number of workers in each team to reduce each worker's heat exposure.
- Increase rest periods for workers to recover, in air-conditioned rest areas where available.
- Turn off heat sources that are not absolutely necessary.
- Remind workers to drink proper fluids frequently.
- Check oral temperature of workers during their most severe heat exposure period, primarily if there is an indication of a problem.
- Limit overtime.
- Send any worker who shows signs of heat disorder, however minor, for medical observation.
- Monitor jobsite and rest area temperature, air velocity, and humidity.
The National Weather Service provides daily environmental measurements that are useful additions to measurements made at the worksite. The National Weather Service data includes measurements at three-hour intervals of air temperature, wet-bulb temperature, dew-point temperature, relative humidity, wind velocity, sky cover, ceiling, and visibility. A summary of daily environmental measurements includes maximum, minimum, and average temperatures, as well as wind velocity (direction and speed), extent of sunshine, and sky cover. These data can be used for approximate assessment of the worksite environmental heat load for outdoor jobs and for some indoor jobs where air conditioning is not in use.
Conclusion
Exposure to extreme heat in the occupational environment may have serious health consequences. Prevention of heat-related injury is best achieved by the following:
- Analysis of risks posed by constant or changing environmental conditions and appropriate adjustment of work rates and exposure times.
- Education in and monitoring of workers' fluid intake, drug and alcohol use, and rest periods.
- Provision of protective facilities and equipment, such as cooling areas and a readily available water supply.
Workers at risk of heat-related injuries because of acute or chronic medical illness, drug use, body build (and predisposition to heat stress), or lack of adequate acclimatization should be identified. The special needs of workers whose jobs require wearing protective clothing in hot environments must be recognized. The early recognition and treatment of heat stroke significantly reduces mortality and morbidity and requires the availability of treatment facilities at the worksite.
COPYRIGHT ©2008, ISO Services, Inc.
