Heat stress is a very dangerous condition that affects many employeess, particularly those performing physical activities in hot and humid working conditions, indoors or outside, particularly during the summer months. Heat stress not only is dangerous to workers' health, but it also decreases workers' productivity and the quality of their work.
Heat stress can manifest itself in many ways, including heat rashes, heat cramps, heat exhaustion and ultimately, heat stroke. Although heat rashes and heat cramps can be both uncomfortable and affect workers' ability to do their jobs, heat exhaustion and heat stroke are much more serious and require immediate attention.
Numerous warning signs for heat-related illnesses exist. Recognizing these warning signs allows a worker to take the appropriate steps to get the situation under control. Some of the warning signs are headache, lightheadedness, dizziness, unusual fatigue, irritability, confusion, nausea/vomiting, cramping and diarrhea. These signs manifest due to the body redirecting blood from internal organs and muscles to the skin in an attempt to shed the heat. Plus, the sweating action means a loss of fluids and possible dehydration.
The illustration on pg. 54 shows the temperature and humidity levels where heat stress is likely to occur and where it becomes dangerous. At temperatures between 85 and 95 degrees F with humidity levels from 30 percent to 60 percent, heat stress is possible. When temperatures rise above 95 degrees F and humidity above 60 percent, workers are in even greater danger. Some heat-related illnesses include:
Heat rash — Skin irritation caused when sweat cannot freely evaporate from the skin and sweat glands become blocked. Wearing clothes that allow sweat to evaporate and taking breaks to allow skin to dry will help.
Heat cramps — Sweat results in loss of body fluid and salts. Depletion of salt in muscles can cause muscle spasms and cramping. Staying hydrated and replacing salts will help.
Heat exhaustion — The loss of large amounts of fluids (and sometimes salt) causes symptoms like fatigue, headache, dizziness and nausea. Staying well hydrated, replacing salts and taking regular rest breaks will help. Heat exhaustion can be the precursor to heat stroke.
Heat stroke — The body's temperature rises so rapidly and high that the normal cooling mechanisms cease to function and the body is unable to cool down. The victim's skin is hot and usually dry. Immediate medical treatment and first aid is needed. Move the victim to a cool area, remove or wet clothing and vigorously fan the person to increase the cooling effect while waiting for medical personnel to arrive.
HOW THE BODY COOLS ITSELF
The body continually generates heat that must be released in order to maintain the proper core temperature of 98.6 degrees F. The body responds to the increased temperature by circulating more blood to the skin, which allows the body to radiate more heat into the air. If heat loss from increased blood circulation is not adequate, the brain signals the sweat glands in the skin to release sweat onto the skin surface. As the sweat evaporates, it draws the energy to vaporize from the rest of the sweat on the skin, leaving cooler moisture on the skin, which helps reduce the overall core temperature. This is called evaporative cooling.
There are four basic mechanisms to cool the body:
Radiation — Almost 65 percent of all body heat is lost through radiation. This occurs when the heat from the skin is absorbed by the surrounding cooler air.
Convection — Approximately 10 percent of body cooling comes from heat transfer due to moving cool air (from a breeze or fan) across the skin to encourage evaporation and heat loss.
Conduction — Approximately 2 percent of body heat is pulled away by direct contact with a cooler object. The heat on the skin is transferred to the cooler object in contact with the skin.
Evaporation cooling — Moisture on the skin's surface (sweat) evaporates, taking heat away and leaving cooler moisture and skin.
Temperatures in many parts of the country can reach well into the 90s and relative humidity can surpass 60 percent. This particularly is common in the summer months. Should the ambient temperature reach or surpass 95 degrees F and the relative humidity get above 60 percent, the effectiveness of all these cooling mechanisms significantly is hampered. Radiation ceases to function as the air and skin temperatures equalize. Convection stops as the moving air now is hot rather than cool. If there is nothing cool to contact, conduction is not available. And if the air is saturated with moisture, the evaporation of sweat will not happen.
In these extreme heat and humidity conditions, the body no longer can keep up with internal heat levels and the body's core temperature rises. The body's reaction is to pump more blood to the skin, which takes blood and fluids from the brain, muscles and major organs. The heart pumps harder to keep up and the blood thickens due to fluid loss. Now workers not only are vulnerable to heat exhaustion and heat stroke, but also to heart attack.
High temperatures also can impact workers' ability to perform their jobs.
Other factors in addition to the environment can affect the level and the speed at which heat stress conditions occur, including:
Activity level — This likely is the most important factor that determines how much internal body heat is generated. The more active the person, the more heat and sweat is generated. It is important to know that when workers stop to rest, the rate of heat production drops dramatically but the removal of the heat already produced may take quite a while.
Age — Older workers have a decreased maximum possible heart rate, which limits their ability to transfer heat to the skin. They also have a reduced ability to produce sweat: They start sweating later and sweat at a lower rate.
BMI — A heavier layer of fat tissues insulates the core, which increases sweat production and can lead to faster dehydration. Also, heavier workers tend to have a lower skin area-to-weight ratio, which puts them at greater risk.
Hydration — Not being properly hydrated at the start of working and not consuming enough water and salt while working will accelerate dehydration and lead to heat stress symptoms.
TIPS TO REDUCE THE IMPACT OF HEAT AND HUMIDITY
A number of actions can be taken to help reduce the effect of heat stress. Learn the signs and symptoms of heat stress because being trained to know when heat stress is possible and recognizing the symptoms will provide the opportunity to take action. Also, it is a good idea to work in teams to help monitor each other.
Workers do have some ability to adjust to hot work conditions. Gradual exposure to heat gives the body time to become accustomed to the higher environmental conditions. Both new workers and workers returning from time off need time to adjust to the hot work environment.
Taking regular rest breaks helps allow the body to cool down, and drinking plenty of cool water regularly during work activities can minimize the effects of heat and humidity. Also, taking advantage of a shady spot whenever possible can help reduce the likelihood of heat stress.
Finally, clothing also can impact susceptibility to heat-related illness. Wear lightweight and light-colored garments and fabrics that don't trap heat. Fabrics that promote evaporative cooling by transferring sweat from the skin to the outer surface of the garments for easier and faster evaporation can be beneficial.
Finally, avoid caffeine and alcohol. These beverages promote water loss — thereby increasing the risks of heat-related illness — and should not be consumed before or during work activities.
Each of these actions can help reduce the effects of heat stress, but the two most effective ways of reducing heat stress are taking regular rest breaks and drinking plenty of water, particularly when the thermometer rises above 95 degrees F and at high humidity levels.
WORKERS IN SPECIAL CIRCUMSTANCES
For some workers, particularly those in the oil and gas industries and others, such as electricians or welders, wearing flame-resistant (FR) clothing — regardless of weather conditions — is a “must.” Like most other types of work clothing, FR clothing can inhibit the transfer of body heat from the skin to the surrounding environment. Lighter weight and looser weave fabric can help with this activity. Fabrics also can help by moving sweat off the skin and onto the surface of the fabric where it can evaporate and promote evaporative cooling.
FR clothing's ability to transfer sweat from the skin to the fabric surface and spread the moisture across its surface can vary greatly from fabric to fabric. Although the attire can play some role in reducing heat stress, research and other testing indicate that most FR garments, in high heat and high humidity conditions, play a relatively minor or inconsequential role in causing or reducing heat stress.
As long as workers stay hydrated and take rest breaks — just like all other workers in high temperature and high humidity situations — they should be safe from heat-related illness.
Heat stress is 100 percent avoidable and preventable as long as you take proper precautions and recognize the signs. By understanding how your body controls its temperature, you're better able to recognize the symptoms of heat stress and take action if you or coworkers develop the symptoms.
To minimize the potential for heat-related problems — particularly in hot, humid working conditions — get acclimated, take regular rest breaks, drink plenty of cool water, wear the right clothes for the situation and know the warning signs so you can take the appropriate actions.
Mark Saner, a technical manager for Workrite Uniform Co., has over 30 years of experience in the fire and safety industry. He is a member of ASTM, serving on committees F 18, “Electrical Protective Equipment,” and F 23, “Protective Clothing” He is a member of NFPA and serves as a member of NFPA 2112, “Standard on Flame-Resistant Garments for Protection of Industrial Personnel Against Flash Fire,” and participates with NFPA 70E and NFPA 1975.