Physiology: Heat Production, Dissipation, and Dehydration Regulating Body Temperature        A body at work generates heat faster than at rest, often more than needed.  The body protects itself by dissipating excess heat to the environment through mechanisms that include vasodilatation and sweating.  When the body's core temperature exceeds 98.6 degrees Fahrenheit, vasodilatation begins as the heart increases blood flow to microscopic vessels (capillaries) in the upper layers of skin.  Excess heat is then transferred to the cooler exterior environment.  If, however, increased blood circulation to the skin cannot sufficiently cool the body, or if the surrounding air is warmer than the skin, then the brain signals sweat glands to release sweat onto the skin.  The sweat then evaporates, carrying additional heat from the body as it undergoes the phase transition from liquid to vapor.  High ambient humidity decreases the rate of sweat evaporation and consequently the body's capacity to dissipate heat through this mechanism.       Vasodilatation and sweating, however, can impair worker strength and comfort in three ways.  First, as blood drains from the internal organs and muscles to the skin, less oxygen is carried to the brain and muscles.  Workers then experience higher levels of muscle fatigue and lower mental alertness.  Second, water loss through sweating exacerbates fatigue by decreasing blood volume and increasing viscosity.  This further decreases the amount of oxygen carried to muscles.  In addition, as water volume diminishes, the body's ability to cool itself through vasodilatation and sweating is impaired and its core temperature rises.  Third, though most diets provide ample electrolytes, prolonged periods of intense sweating can result in electrolyte deficiencies that cause severe heat cramps.  To maintain comfort and health when working in a hot environment, it is critical for workers to replace both the water and electrolytes they lose through sweating.  If workers' fluids are not replenished at the same rate they are lost, or if they lack the electrolytes necessary to process water, their body cannot effectively lower their core temperature and the possibility of Heat stress rises.   Heat Stress as a Cause and Consequence of Dehydration           Heat stress is a condition in which the body has problems dissipating excess heat.  Results of inadequate heat dissipation range from general discomfort to symptoms of heat rash, heat syncope, heat cramps, heat exhaustion, and heat stroke.           Heat rash, the least severe of heat illnesses, occurs when  sweat clogs  pores.  Though heat rash usually causes only temporary discomfort, it can develop into an infection.  Beyond the more subtle impairments to productive capacity that results from fatigue, heat-related illnesses may also lead to complete disruptions of work.           Heat syncope, or fainting, may affect people who are not acclimatized to working in hot environments.  Syncope generally is the sudden loss of consciousness due to lack of sufficient blood and oxygen to the brain.  Heat stress can cause it by "diverting" blood to the lower body or extremities at the expense of the brain.            Heat cramps are often extremely uncomfortable and can be completely disabling.  They typically occur during or after hard work and are caused by electrolyte deficiencies that result from extended periods of intense sweating.  Symptoms include painful spasms of leg, arm or abdominal muscles, heavy sweating, and thirst.             Heat exhaustion also occurs during work but results from dehydration, lack of acclimatization, reduction of blood in circulation, strain on circulatory system, and reduced flow of blood to the brain (see EPA's, A Guide To Heat Stress in Agriculture).  Symptoms of heat exhaustion include fatigue, headache, dizziness, muscle weakness, nausea, chills, tingling of hands or feet, confusion, loss of coordination, fainting and collapse.              Heat stroke may occur suddenly if heat exhaustion is ignored, and it can be fatal.  At this extreme state of heat stress, the body typically has a core temperature exceeding 104 degrees F and can no longer cool itself.  A person suffering heat stroke needs immediate attention and should be taken to a medical facility as soon as possible. Additional Readings:   WARNING!! HEAT STROKE ADVISORY U. S. DEPARTMENT OF LABOR  A GUIDE TO AGRICULTURAL HEAT STRESS Steve Sutter HEAT STRESS IN AGRICULTURE EPA Popular Literature Outdoor Action Guide to Heat-Related  Illnesses & Fluid Balance by Rick Curtis, Princeton University  Fluid and Electrolyte Supplementation for Exercise-Heat Stress Michael N. Sawka, Ph.D., Scott . Montain, and William A. Latzka