Impact of Heat-Related Illness on Behavior and Cognitive Recognition

Exposure to hot indoor or outdoor work environments even for short periods of time can lead to cognitive and behavioral impairment.

• By Bernard Fontaine
• Jul 12, 2022

Heat stress and heat-related illness is one of the most common occupational health problems for workers both indoors and outdoors. Approximately 13.3 million US workers performed work in extreme heat every day in July 2017. Globally, the World Health Organization (WHO) estimates 125 to 175 million people are exposed to excessive heat waves due to climate change. Workers who are affected by hyperthermia include smelter and foundry workers; farmers and ranchers; agricultural workers; loggers; laborers in construction, oil and gas, warehousing; manufacture and textile workers; hazardous material emergency responders and firefighters.

Studies published in the literature support the concept that workers can experience cognitive and behavioral effects while exposed to hot and humid work environments. The significance depends on numerous confounding factors. Human performance is influenced by a range of different environmental factors related to air temperature, relative humidity, wind speed or air circulation along with the duration and frequency of exposure. Other physiological factors include but not limited to, heat acclimation, hydration, rest-work periods, protective equipment use, current health status and physical condition.

Core body temperature should be maintained around 37°C to maintain comfort. When the outdoor or indoor air temperature and humidity begins to influence the core body temperature, workers begin to sweat and dehydrate. Failure to remove or reduce the heat load using engineering or administrative controls impacts the body to compensation for the external climate. Uncontrolled work environments or heavy work without proper acclimatization can lead workers to experience heat induced physiological strain that may lead to health impairments such as heat stroke, rhabdomyolysis (rhabdo), heat exhaustion, heat cramps, heat collapse, heat rashes, and heat fatigue. Additionally, there are two types of human responses linked to hyperthermia including: behavioral and cognitive responses.¹ Simple work tasks are less likely to be affected by heat stress as compared to complex tasks such as tracking, monitoring, and multiple tasking.²

Cognitive and Behavioral Evidence

Despite the overwhelming evidence on the physical effects of heat stress, less is known about the effect on behavior and cognition. Research indicates that prolonged periods of demanding cognitive activity or work-induced fatigue can alter cognitive functioning. The addition of hot and humid environmental conditions over time can exacerbate cognitive function and negatively affect performance outcomes.³

Cognition refers to mental abilities and processes, and includes memory, knowledge, attention, reasoning, problem solving, and comprehension. Hyperthermia, even if mild and only occurring for a short period, may cause cognitive impairment. Hyperthermia can adversely affect attention, memory, and processing of information,. Some cognitive processes may be affected more than others. For example, short-term memory processing may be more adversely affected.⁴

Functional neuroimaging supports observations of numerous connections in cognitive pathways, that can be affected acutely in hyperthermia. In general, connections appear to be increased around the limbic system, consistent with observed changes in memory and learning ability. The dorsolateral prefrontal cortex of the brain (involved in memory, cognition, and reasoning), and the intraparietal sulcus (involved in processing and memory) shows increased activity in acute cases of hyperthermia. Connection in other parts of the brain, including the temporal, frontal and occipital lobes, appears to be reduced by acute hyperthermia.⁵

Experimental studies using computerized tasks (i.e. cognitive tests) help to better understand the relationship between endogenous thermal load and cognitive performance. The existence of an inverted U-shaped relationship between hyperthermia development and cognitive performance is suggested, and highlights core temperatures of ~38.5°C [101.3°F] as the potential 'threshold' for hyperthermia-induced negative cognitive performance.

Hancock et al. conducted a meta-analysis of heat stress and reported that thermal stressors affect human performance negatively.⁶ Stubblefield et al. measured the effects of hyperthermia during a heat stress test (HST) on four specific cognitive functions (including working memory, attention, response speed, and processing speed). Results showed that hyperthermia reduced working memory performance over time. However, response speed, processing speed and attention were less influenced by high body temperature.⁷ The detrimental effects of thermal stress on working memory, information retention, and information processing were confirmed by other researchers. Gaoua et al. indicated impairment in working memory can occur during excessive heat exposure.⁸

Epidemiological and Clinical Findings

Out of 157 retrieved articles, 39 articles were considered for review. Only four articles evaluated workers. Overall, the results identified heat stress as a detrimental factor for decreased cognitive functions for reading comprehension, memory, focus, mathematical processing, tracking test, reaction time, perception and decoding text and numeric message, visual alertness, mental computing, text reading, hidden figures test and verbal fluency.⁹

In a cross-sectional study conducted in Malibel Saipa Company (2013), workers were assigned into two groups: one group was exposed to heat stress (n=35), working in casting unit and the other group worked in the machining unit (n=35) with a normal air conditioning. The Wet Bulb Globe Temperature (WBGT) was measured at three heights of ankle, abdomen, and head. The effects of heat stress on attention and reaction time using Stroop tests were conducted before starting the work and during the work.¹⁰

A significant positive correlation was observed between WBGT and test duration [p=0.01] and reaction time of Stroop test 3 [p=0.047], and between number of errors in Stroop tests 1-3, during the work [p= 0.001]. Stroop test 3 showed a significant higher score for both test duration and reaction time of workers than in the control group.

Another study investigated the effect of heat stress on cognitive performance by measuring blood concentration of stress hormones among foundry workers. Seventy workers within the exposed group (n=35) and unexposed control group (n=35) were studied. The WBGT index was measured for heat stress assessment. Cognitive performance tests were conducted using the Stroop's Color Word Test (SCWT) before and during work hours.

Serum level of cortisol and plasma level of adrenaline and noradrenaline, blood samples were taken during working hours from both groups. Results showed a significant relationship between heat stress and test duration, error rate and reaction time. Laboratory test results revealed significantly higher concentrations of cortisol, adrenaline and noradrenaline in the exposed foundry workers than in the unexposed control group. A positive correlation was made between cortisol, adrenaline, noradrenaline, WBGT index, test duration, and reaction time, and number of errors during work. The study concluded that heat stress can increase in blood level of stress hormones, resulting in cognitive performance impairment.¹¹

Other evidence shows a relationship between the effects of heat stress, heat-related illness and deep body temperature. A number of occupational exposure limits (OELs) have been established including the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) and the National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL).Other occupational exposure limits include a special emphasis on the most recent date derived by Hancock and Vasmatzidis. This limit, which employs an attentional resource approach, defines exposure duration thresholds. Although this approach appears to be promising, much remains to be understood before any OEL becomes accepted.

From a psychological viewpoint, there is additional evidence of the impact of extreme heat on both behavior and psychiatric conditions. Let’s look at violence. There is scientific evidence linking extreme heat and aggression. One standard deviation of temperature increase can lead to a 4% increase in interpersonal violence and 14 percent increase in group violence.^12,13 Burke and colleagues suggest that the inhabited world is expected to warm 2 to 4 °C by 2050, amplified rates of human conflict could represent a large and critical impact of anthropogenic climate change.” Increases of 2 to 10 °C due to “urban heat islands” caused by asphalt and concrete structures and limited green space compared to adjacent suburban and rural communities may contribute to increased summer violence in inner cities.

Suicide-a form of violence turned on oneself increases during extreme heat. Research shows an increase in suicide rates of 0.7 percent in the US and 2.1 percent in Mexico during periods of 1°C increase over average monthly temperatures. Projections based on global warming, assuming no reduction in green-house gas emissions, it is estimated that by 2050 there may be between 9000 to 40,000 additional suicides in the US and Mexico. These rates are comparable to the effects on suicide incidence due to economic recessions and unemployment.^14,15

The same study on suicides also analyzed over 600 million social media communications and found an increase in depressive language and suicidal ideation correlated with increased temperatures-indicating a decrease in mental well-being. Meyer postulated a possible mechanism for these behaviors and suggested that serotonin may play a physiological role.

Finally, sleep is an essential function for overall well-being and health with adverse impacts of sleep deprivation on mood, depression, and cognition. Normal sleep onset and maintenance is triggered by a drop in core body temperature. Increased heat contributes to insomnia and worsens with increased humidity.¹⁶ This has implications for workers who live in areas where heat is trapped indoors and for people who don’t have access to air-conditioning in residential places.

Conclusions

Exposure to hot indoor or outdoor work environments even for short periods of time can lead to cognitive and behavioral impairment. Much depends on the environmental conditions, work tasks, hierarchy of controls, acclimation, human physiology, frequency and duration of exposure. From this perspective, interventions to slow or reduce heat loads and protect both against hyperthermia changes (e.g. cooling strategies) should be used to benefit and preserve behavior and cognitive performance. Guidance from the ACGIH, federal OSHA and NIOSH can reduce the risk of cognitive and behavioral effects that may lead or contribute to other health and safety concerns. Training on heat stress can provide more clarity into constructing the hierarchy of controls to reduce exposure while increasing productivity and human performance.

If hyperthermic workers are kept well hydrated and acclimated, cognitive and physical impairment may be minimal, suggesting some of the dysfunction is due to dehydration and conditioning. The evidence also shows that advancing age is associated with a reduced cognitive ability. Younger workers tend to recover quicker from acute cognitive dysfunction. Information on long-term health effects from heat exposure has not been researched. Some heat stressed workers may be left with persistent changes in attention, memory, or personality. Most cases of heat-related illness are mild but in more severe cases dementia may result from classic heat stroke.