It can take as much as 24 hours for the body to absorb enough fluid to fully rehydrate. By using shirts and vests that incorporate active cooling on about 40 percent of the body surface, the danger of heat stress can be greatly reduced. (CoolShirt Systems photo)

How Cooling Boosts Productivity

Training and company policies must help employees decide that they can protect themselves from heat stress. Reducing heat stress will improve productivity and reduce accidents in the workplace.

In 2012, the National Fire Protection Association updated its terminology in relation to personal protective equipment. Moving forward, PPE designed to withstand electric arc flash hazards is no longer always referred to as Flame Resistant (FR), but instead is considered Arc-Rated (AR). This has led to some confusion because there are important differences between arc-rated vs. flame resistant clothing.

As the name implies, arc-rated clothing fabric is subjected to a series of arc flashes to determine how much energy it can block before it is likely to cause the wearer a second-degree burn 50 percent of the time. This amount is known as the Arc Thermal Protective Value (ATPV). However, if the fabric breaks open prior to reaching the ATPV, the arc rating is labeled as the Energy break open threshold (EBT).The EBT is the amount of energy a material can withstand before the fabric breaks open and exposes skin or lower layers of clothing to the hazard. The ATPV and EBT are measured in calories of heat energy per square centimeter (cal/cm²). The higher the arc rating value, the greater the protection.

According to all current standards for arc flash protection, clothing must be flame resistant to qualify for arc rating. This makes the difference between arc-rated vs. flame resistant clothing clear: Equipment rated FR may not provide sufficient protection for workers in environments where arc flash hazards exist. These employees must wear the appropriate level of AR PPE in order to lessen the chances of serious injury or even death.

How the Body Releases Heat

  • Sixty-five percent of the body's heat is released through radiation. This occurs when ambient air temperature is lower than the body's skin temperature. Radiation is the movement of heat energy from a warmer object to a cooler object as heat radiates from the sun to the earth.
  • Convection accounts for approximately 10 percent of heat loss. Convection is the transfer of heat energy from a warmer object or space to a cooler object or space through differences in density and the action of gravity.
  • Approximately 23 percent of heat loss is due to evaporation of perspiration from the skin. Evaporation is the cooling of a surface through the process of a liquid changing to a vapor and leaving that surface.
  • Conduction will add another 2 percent to the heat loss total. Conduction is the transfer of heat energy from a warmer object to a cooler object through direct contact.

When the ambient temperature of the surrounding air is 95° F or higher, radiation, convection, and conduction stop working. Evaporation is all that is left to cool the body. Protective clothing used by welders, firefighters, hazmat personnel, and nuclear and electrical workers will make the heat situation even worse.

Heat-Induced Errors
A performance study by NASA using telegraph key operators showed that in temperatures of 80° F, the operator will make five errors an hour and 19 mistakes after three hours. At 90° F, the operators made nine mistakes per hour, and 27 after three hours. At 95° F, the mistakes went to 60 in one hour and 138 in three hours. Although errors made by telegraph key operators may not be critical, this same hot environment will produce a proportional amount of errors regardless of the task. 

When a person is in a hot environment, up to 48 percent of the blood is pumped by the heart to the skin for cooling. The first effect is to release heat, but water is also released through perspiration. If an individual loses 2 percent of body weight due to perspiring, that person is considered to be in a heat exhausted state; performance is affected and the effects of dehydration on the decision-making process could be related to an increase in work-related accidents.

Studies have concluded that with 2 percent of body weight loss, visual motor tracking, short-term memory, attention, and arithmetic efficiency were all impaired.

When Heat Overtakes the Body
At the ambient temperature of 95° F, the body no longer keeps up with its internal heat generation levels, and inner core temperature begins to rise. The only mechanism to release body heat from the inner core is for up to 48 percent of the body's blood to be pumped to the skin to create perspiration. This creates two problems: blood loss to the organs, muscles, and brain; and dehydration.

When the brain, muscles, and major organs are receiving half of the blood they normally receive, the heart must work much harder to try to deliver the same volume of blood to those organs to keep them nourished by beating up to 150 times a minute. Factor in a thickening of the blood due to fluid loss (dehydration), and you can understand why heart attacks are a major byproduct of heat stress.

When an employee performs heavy physical work, fluid intake may not overcome the effects of sweat output. Employees who perform duties in fully encapsulated protective clothing may have increased sweat rates of 2.25 liters per hour. Other studies link job-related accidents to "orthostatic intolerance." Carter, et al.3 established that with a 3 percent dehydration state due to heat exposure, subjects experienced a significant reduction in cerebral blood flow velocity when changing from a seated to standing position that can cause workers to lose consciousness.

It can take as much as 24 hours for the body to absorb enough fluid to fully rehydrate. Godek, Bartolozzi, et al.4 have shown that fluid intake alone does not reduce core body temperature.

Cooling Workers
Work should be curtailed while fluid is replaced, or the dehydration rate must be slowed by using personal cooling methods such as misting fans, ice vests, or active cooling products, which pump cooled fluid through tubing or a bladder sewn to a garment that the employee wears under the protective clothing. If not, the inner core temperature will continue to rise for up to 30 minutes after work is stopped, unless other means are used to cool the blood that has been pumped to the skin for cooling.

Pre-cooling, continuous cooling, and post or rehab-cooling will increase productivity of workers. Rehab cooling has been proven to bring workers' vital signs to normal quickly and safely, getting them back to work sooner. 

The "OSHA Technical Manual" (Section III: Chapter 4) talks about how active cooling products using water are useful in preventing heat stress by using conduction to enhance the body's capacity to cool. In fact, it has been demonstrated that water is 28 times faster in cooling a subject than cooled air. These products slow the rate at which the core body temperature rises by using conduction to greatly increase the body's capacity to cool the blood that is pumped to the skin during times of elevated core temperatures. This slows the fluid loss caused by sweating. By using shirts and vests that incorporate active cooling on about 40 percent of the body surface, the danger of heat stress can be greatly reduced. 

Training and company policies must help employees decide that they can protect themselves from heat stress. Reducing heat stress will improve productivity and reduce accidents in the workplace.

1. Wasterlund DS, Chaseling J, Burstrom L: The effect of fluid consumption on the forest workers' performance strategy. Appl Ergon 35:29-36, 2004 
2. Gopinathan PM, Pichan G, Sharma VM: Role of dehydration in heat stress-induced variations in mental performance. Arch Environ Health 43:15-17, 1988 
3. Carter R 3rd, Cheuvront SN, Vernieuw CR, Sawka MN: Hypohydration and prior heat stress exacerbates decreases in cerebral blood flow velocity during standing. J Appl Physiol 101:1744-1750, 2006 
4. Godek S, Bartolozzi A, Burkholder R, Sugarman E, Dorshimer G: Core temperature and percentage of dehydration in professional football linemen and backs during preseason practice. J Athl Train 41(1):8-17, 2006

This article originally appeared in the August 2016 issue of Occupational Health & Safety.

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