A worker may be exposed to a """"net [overall] heat load"""" due to a combination of metabolic heat, environmental conditions (such as air temperature, humidity, air movement, and radiant heat), and clothing needs. Although it may be uncomfortable and have a negative impact on performance and safety, minor or moderate heat stress is not hazardous to health. The risk of heat-related diseases grows when the level of heat stress approaches the upper limit of human tolerance. Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices, 2022 TLVs and BEIs. American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 2022. p .229] The Glossary of Concepts at the conclusion of this document defines other terms associated with heat. In many businesses, heat may be a problem. For instance: In furnaces, foundries, steel mills, smelters, glass manufacturers, and other locations where the primary heat source is extremely hot or molten material. The primary heat source in outdoor jobs including construction, road maintenance, open-pit mining, and agriculture is summer sun. in canneries, bakeries, restaurants, and laundries. High humidity increases the strain of heat. A working environment that may be too hot for the body to handle is the primary cause of heat stress in every case. Information about the negative impact of hot surroundings on health can be found in this OSH Answers document. For details on how to prevent and manage heat exposure, please visit Hot Environments - Control Measures.
What responses does the human body have to heat?
The average body temperature of a healthy person is about 37 °C. Variations can be caused by the time of day, intensity of physical activity, or emotional condition, and are typically less than 1°C. Only when sick or when the environment is too hot for the body to handle does a temperature shift of more than one degree Celsius occur. The American Conference of Governmental Industrial Hygienists defined exposure limits for heat stress based on keeping the core body temperature within one degree of normal (37 degrees). The body tends to warm up along with the environment. The internal ""thermostat"" of the body raises perspiration production and increases blood flow to the skin to maintain a steady internal body temperature. In order to balance the heat burden, the body accelerates the rate of heat loss. When the rate of ""heat intake"" exceeds the rate of ""heat loss"" in an extremely hot environment, the body temperature starts to increase. Heat diseases, which can be very serious, are brought on by an increase in body temperature.
How does the body regulate its rate of heat gain and loss?
The body's internal heat is the principal source of heat under normal circumstances. It is produced by the physiological processes that keep us alive as well as the energy we expend while exercising and is known as metabolic heat. The main ways that the body transfers heat to its environment are through radiation, convection, and sweat evaporation. Through the mechanism of radiation, the body absorbs heat from hot items such as heated metal, furnaces, or steam pipes while radiating heat away to cold objects such as chilled metallic surfaces. A typical illustration of a radiant heat source is the sun. When the temperature of the surroundings is the same as the skin temperature (about 35°C), there is no radiant heat gain or loss. The mechanism through which the body transfers heat to the surrounding air is known as convection. When cold air comes into touch with the skin, the body loses heat and receives heat from hot air. Air velocity and the temperature difference between the air and the skin both boost convective heat exchange. The body cools as sweat evaporates from the skin. When the relative humidity is low and the wind speed is high, evaporation happens more quickly and the cooling effect is more pronounced. Because the air cannot hold any more moisture, the cooling effect of perspiration evaporation in hot and muggy environments is constrained. The amount of perspiration that the body produces in hot, dry conditions limits the cooling that results from sweat evaporation. Additionally, the body transfers a tiny quantity of heat through conduction and breathing. When the body comes into close contact with hot or cold items, it conducts heat either way. Because the respiratory system warms the air that is inhaled, breathing involves a heat exchange. This warmed air is exhaled, carrying part of the body heat with it. However, the heat transmitted through conduction and respiration is typically insignificant enough to be disregarded when calculating the body's heat burden.
What affects does heat have on the body?
Problems may occur when air temperature or humidity levels exceed what is deemed comfortable. The earliest impacts are felt as feelings. More heat exposure may have a negative impact on health as well as performance. People may experience the following as the temperature or heat load rises: a worsening of irritation. the inability to focus or perform mental tasks. loss of capacity for difficult jobs or skilled tasks. In moderately hot environments, the body """"goes to work"""" to get rid of excess heat so it can maintain its normal body temperature. The heart rate increases to pump more blood through outer body parts and skin so that excess heat is lost to the environment, and sweating occurs. These changes place additional demands on the body. Changes in blood flow and excessive sweating reduce a person's ability to do physical and mental work. Manual work creates additional metabolic heat and adds to the body heat burden.
Does everyone react to heat the same way?
No. The risk of heat-related illness varies from person to person. A person's general health influences how well the person adapts to heat (and cold). Those with extra weight often have trouble in hot situations as the body has difficulty maintaining a good heat balance. Age (particularly for people about 45 years and older), poor general health, and a low level of fitness will also make people more susceptible to feeling the extremes of heat. Medical conditions can also increase how susceptible the body is. People with heart disease, high blood pressure, respiratory disease and uncontrolled diabetes may need to take special precautions. In addition, people with skin diseases and rashes may be more susceptible to heat. Other factors include circulatory system capacity, sweat production and the ability to regulate electrolyte balance. Substances -- both prescription or otherwise -- can also have an impact on how people react to heat. The National Institute for Occupational Safety and Health (NIOSH) reports that several studies comparing the heat tolerances of men and women have concluded that women are less heat tolerant than men. While this difference seems to diminish when such comparisons take into account cardiovascular fitness, body size, and acclimatization, women tend to have a lower sweat rate than men of equal fitness, size and acclimatization. This lower sweat rate means that there can be an increase in body temperature.
Should workers be trained to recognize the symptoms of heat illness?
Yes. Workers should be trained to recognize the signs and symptoms of heat related illnesses. If possible, start a “buddy system” because sometimes it is hard for workers to notice their own symptoms.
What are the illnesses caused by heat exposure?
Heat exposure causes the following illnesses: Heat edema is swelling which generally occurs among people who are not acclimatized to working in hot conditions. Swelling is often most noticeable in the ankles. Heat rashes are tiny red spots on the skin with severe itching when in a hot, humid environment. The spots are the result of inflammation caused when the ducts of sweat glands become plugged. In most cases heat rash will disappear when the individual returns to a cooler environment. Heat cramps are sharp pains in the muscles that may occur alone or be combined with one of the other heat stress disorders. Cramps are caused by a salt imbalance from heavy sweating. Salt can build up in the body if water lost through sweating is not replaced. Inadequate fluid intake often contributes to this problem. The worker should move to a cooler area and should hydrate. Heat exhaustion is caused by loss of body water and salt through excessive sweating. Signs and symptoms of heat exhaustion include: heavy sweating, weakness, dizziness, visual disturbances, intense thirst, nausea, headache, muscle cramps, breathlessness, palpitations, and skin which is pale, cool, and moist. Do not leave the worker alone. Get medical attention, move to a cool area, and have the person drink cool water if they are able. Can lead to heat stroke if left untreated. Heat syncope is heat-induced dizziness and fainting induced by temporarily insufficient flow of blood to the brain while a person is standing. It can also be caused by vigorous physical activity for 2 or more hours before the fainting happens. It occurs mostly among unacclimatized people. It is caused by the loss of body fluids through sweating, and by lowered blood pressure due to pooling of blood in the legs. Recovery is rapid after rest in a cool area. Heat stroke is the most serious type of heat illness. It is a medical emergency. Signs of heat stroke include having a high body temperature (often greater than 40°C) and complete or partial loss of consciousness. There can also be confusion, irrational behavior, convulsions, and hot, dry skin. Sweating is not a good sign of heat stress as there are two types of heat stroke – non-exertional or """"classical"""" where there is little or no sweating (usually occurs in children, persons who are chronically ill, and the elderly), and """"exertional"""" where body temperature rises because of strenuous exercise or work, and sweating is usually present. Heat stroke requires immediate first aid and medical attention. Remove excess clothing. Drink and spray water. Delayed treatment may result in death.
What are symptoms and first aid steps for heat exhaustion?
Symptoms of heat exhaustion may start suddenly, and include: Nausea or irritability. Dizziness. Muscle cramps or weakness. Feeling faint. Headache. Fatigue. Thirst. Heavy sweating. High body temperature. First aid for heat exhaustion includes: Get medical aid. Stay with the person until help arrives. Move to a cooler, shaded location. Remove as many clothes as possible (including socks and shoes). Apply cool, wet cloths or ice to head, face or neck. Spray with cool water. Encourage the person to drink water, clear juice, or a sports drink.
What are the symptoms and first aid steps for heat stroke?
Heat exhaustion may quickly develop into heat stroke. Symptoms of heat stroke include: Hot, dry skin or profuse sweating. Confusion. Loss of consciousness. Seizures. Very high body temperature. First aid for heat stroke includes: Call 911 immediately. Heat stroke is a medical emergency. Stay with the person until help arrives. Move to a cooler, shaded location. Remove as many clothes as possible (including socks and shoes). Wet the person's skin and clothing with cool water. Apply cold, wet cloths or ice to head, face, neck, armpits, and groin. Do not try to force the person to drink liquids.
What are the illnesses caused by long-term (chronic) heat exposure?
NIOSH reports that certain heart, kidney, and liver damage are thought by some researchers to be linked to long-term heat exposure. However, the evidence supporting these associations is not conclusive. Chronic heat exhaustion, sleep disturbances and susceptibility to minor injuries and sicknesses have all been attributed to the possible effects of prolonged exposure to heat. Heat exposure has been associated with temporary infertility in both women and men, with the effects being more pronounced in men. Sperm density, motility, and the percentage of normally shaped sperm can decrease significantly when the temperature of the groin is increased above a normal temperature. Workers exposed to high heat loads should inform their family doctors of their exposure. Laboratory study of animals has shown that exposure of the pregnant females to high temperatures may result in a high incidence of embryo deaths and malformations of the head and the central nervous system. There is no conclusive evidence of teratogenic effects of high temperatures in humans. NIOSH recommends that a pregnant worker's body temperature should not exceed 39-39.5°C during the first trimester of pregnancy (Reference: Criteria for a Recommended Standard: Occupational exposure to heat and hot environments. Revised Criteria 2016. Cincinnati, Ohio: National Institute for Occupational Safety and Health, 2016)
What are some of the terms used in this document (Glossary of Terms)?
Acclimatization - Physiological changes which occur in response to several days of heat exposure and make the body accustomed to a hot environment.
Convection - Process of heat exchange between the body and the surrounding air or fluid as a result of bulk flow of that air or fluid.
Dehydration - Loss or deficiency of water in body tissues caused by sweating, vomiting or diarrhea. Symptoms include excessive thirst, nausea, and exhaustion. Heat cramps - Painful and often incapacitating cramps in muscles. Heat cramps are caused by depletion of salt in the body as a result of heavy sweating. Heat exhaustion - Weakness, lassitude, dizziness, visual disturbance, feeling of intense thirst and heat, nausea, palpitations, tingling and numbness of extremities after exposure to a hot environment. Heat rash (prickly heat or milliaria) - An itchy rash of small raised red spots on the face, neck, back, chest and thighs caused by a hot and moist environment. Heat strain - Physiological and behavioural responses of the body as a result of heat exposure. Heat stroke - Acute illness caused by overexposure to heat. Symptoms are dry, hot skin, high body temperature (usually over 40°C) and mental dysfunction. Heat syncope - Temporary loss of consciousness induced by insufficient flow of blood to the brain. Recovery is normally prompt and without any long-term ill effects. Metabolic rate - Rate of energy (heat) production of the body which varies with the level of activity. Nausea - The feeling that one is about to vomit as experienced in seasickness. Prickly heat - See Heat rash. Radiation (heat) - Transfer of heat between hot and cold bodies without contact between them. Relative humidity - The ratio of the water vapour content of air to the maximum possible water vapour content of air at the same temperature and air pressure."""