How Heat Stress Impacts Worker Performance

 When the ambient temperature reaches 95 degrees Fahrenheit or higher, conventional methods of cooling the body—radiation, convection, and conduction—become ineffective. At this point, evaporation is the sole mechanism left to dissipate heat from the body.

Understanding Heat Stress

Education plays a crucial role in minimizing heat stress-related accidents in the workplace. According to NIOSH's 1986 publication "Occupational Exposure to Hot Environments," while workers can adapt to varying levels of heat, each individual has a threshold beyond which heat stress becomes dangerous. Research indicates that even mild dehydration, as little as 2 percent loss of body weight, can significantly impair focus and reaction times.

 Industries such as foundries, heavy machinery manufacturing, and shipbuilding often expose workers to high temperatures. At temperatures as low as 80 degrees Fahrenheit, the body manages heat by directing blood flow to the skin for cooling. Given that many people start their day already slightly dehydrated (with about 80 percent of the U.S. population experiencing this), heat stress is a significant factor in preventable accidents and injuries. 

Many businesses may unknowingly expose their workers to heat stress. The common justification, "We’ve always done it this way," often prevents the implementation of preventive measures.

 

Heat Dissipation Mechanisms

 Under normal conditions, the body loses heat through the following processes:

1. Conduction: Core palm cooling affords virtually 32 percent of heat loss through core palm cooling
2. Convection: Accounts for around 10 percent of heat loss. It involves the transfer of heat through the movement of air or liquid from warmer to cooler areas.
3. Evaporation: Contributes approximately 23 percent to heat loss. This process involves the conversion of sweat into vapor, cooling the skin.
4. Radiation: Responsible for about 65 percent of heat loss. This occurs when the body radiates heat to cooler surrounding objects or air.

 When temperatures exceed 95 degrees Fahrenheit, radiation, convection, and conduction become ineffective. Evaporation is the only method left to cool the body, and protective clothing, used by welders, firefighters, and others, can exacerbate the heat problem. 

A NASA study involving telegraph key operators highlighted the effects of heat on performance: at 80 degrees Fahrenheit, operators made five errors per hour and 19 errors over three hours. At 90 degrees Fahrenheit, the error rate increased to nine per hour and 27 over three hours. At 95 degrees Fahrenheit, errors soared to 60 per hour and 138 over three hours. These findings suggest that high temperatures can drastically reduce accuracy and efficiency, regardless of the task.

 

Impact on Physical and Mental Performance

In extreme heat, the body directs up to 48 percent of its blood to the skin for cooling, which reduces blood flow to vital organs and muscles. This can lead to dehydration and increase the risk of heat-related conditions. A study by Wasterlund and Chaseling found that dehydrated workers (losing 1 percent of body weight) experienced a 12 percent decrease in productivity.

Gopinthan et al. studied the impact of dehydration on mental performance and decision-making. They found that a 2 percent body weight loss impaired visual motor tracking, short-term memory, attention, and arithmetic efficiency. A 4 percent loss could reduce reaction times by 23 percent.

 

Warning Signs and Preventive Measures

 At 95 degrees Fahrenheit, the body's heat regulation system can become overwhelmed, leading to a rise in core temperature. This results in blood being redirected to the skin for cooling, which can lead to reduced blood flow to the brain, muscles, and organs. The heart must work harder, and dehydration can thicken the blood, increasing the risk of heart attacks.

 Workers engaged in strenuous activities or wearing heavy protective gear may sweat excessively, potentially exceeding fluid intake and leading to severe dehydration. Studies have shown that with a 3 percent dehydration level, workers may experience a significant reduction in cerebral blood flow, causing dizziness or loss of consciousness. Proper hydration alone may not suffice, as it can take up to 24 hours for full rehydration. 

Employers should consider implementing cooling methods such as misting fans, ice vests, or active cooling garments to manage heat stress effectively.

 

Training and Policy

Training is essential to mitigate heat stress-related incidents. Workers and supervisors must be aware of how heat affects performance and implement preventive measures. Policies that promote the use of cooling products and provide education on heat stress can significantly reduce related illnesses and accidents, improving overall workplace safety and productivity.