Thermal Physiology 103: Chronic Heat Exposure

According to the leading medical journal The Lancet, more than 1 billion workers worldwide are exposed to high heat episodes, and a third experience negative health effects from this exposure. In a related recent study, younger people, presumably healthy and better capable of tolerating hot conditions, were found to account for a disproportion share of heat-related deaths. Study authors attribute this disproportionate share of young people to “behavioral, social, and economic factors”, i.e., younger people form most of the labor force, working outside in hot conditions when older folks stay home.

In many places, those who work outdoors routinely deal with hot temperatures they cannot control (it’s called summer). Compared to acute heat injuries like heat stroke, chronic heat exposure wears down the body in different ways. Cardiovascular, neurological, and respiratory health issues, along with organ damage, are known effects of chronic heat exposure and can persist for years following injury. Long term heat exposure affects health in less direct ways too. As many who have worked a shift in extreme heat can attest, hot weather makes it harder to think and work, increasing the rate and severity of workplace accidents.

Cooked Kidneys – A Typical Chronic Heat Injury

Kidney injury isn’t commonly associated with workplace injuries. Kidney problems are most common in the elderly, diabetic, and people with high blood pressure or other comorbidities. Yet kidney injury is one of the more common health outcomes associated with chronic heat exposure; one research project found up to 15% of people who frequently work in high heat conditions developed some form of kidney injury. This link between heat stress and acute kidney injury in younger, health adults was initially identified in recruit populations undergoing physically demanding military training in hot environments.

The kidneys are two organs responsible for filtering waste products from the blood stream. They also play a role in reabsorbing and returning water, nutrients, and minerals back into the body. Acute kidney injury is a relatively sudden reduction in the kidney’s ability to filter blood and results from conditions that stress the kidneys, such as dehydration and exposure to high temperatures.

Acute kidney injury can lead to the more dangerous chronic kidney disease, which is a permanent reduction in kidney function resulting from damage over long periods of time. A person recovered from the mildest form of acute kidney injury is still at elevated risk of developing chronic kidney disease within a year by 43%. Severe cases of chronic kidney disease require lifelong dialysis or kidney transplant.

Pathology of Heat-induced Kidney Injury

Heat-induced kidney injury is strongly linked to dehydration.[1] Exposure to high temperatures and dehydration reduce blood flow to the kidneys, which appears to trigger damage. High temperature exposure causes body responses that shift blood flow away from internal organs and toward the skin surface in an attempt to shed heat, potentially damaging organs like the kidneys. Dehydration, and the associated increase in sodium concentration in the blood, make it more difficult for the kidney to reabsorb water and balance salt concentration in the blood (by volume, sweat is mostly just water. As the body loses water, sodium concentrations become elevated).

In other words, kidney damage is associated with the magnitude of heat exposure, the amount of dehydration, and length of time spent dehydrated. Medical studies find reducing body core temperature and limiting dehydration both reduce the risk of kidney injury. Worn by outdoor workers and operators, products like the Qore Performance ICEVEST® may be particularly beneficial in protecting against chronic heat injuries, since ICEVEST® addresses both factors associated with kidney injury by helping reduce core temperature and providing a ready source of water for rehydration.

The Qatar Case Study - Background

Among the most compelling recent examples of chronic heat injury in young, healthy workers comes from Qatar. To prepare for the 2022 World Cup, Qatar embarked on an ambitious building program including rapid construction during extreme summer heat. A rich Middle Eastern country situated along the hot and humid Persian Gulf, Qatar imports low-cost laborers from Nepal, India, and Bangladesh as the labor force for “3D” jobs- dirty, dangerous, and difficult jobs- like construction. Although these laborers are poor, they are healthy- physical checks are required for each workers prior to arriving in Qatar.

Addressing heat safety concerns, Qatari heat stress regulations prohibit labor between 10:00am and 3:30pm from June to mid-September and require work to stop when the wet bulb globe temperature (an outdoor heat index designed by the U.S. military) reaches around 90^oF (about the same temperature as the most extreme heat threshold used by the Army and Marine Corps- the “black flag” threshold). Despite these regulations, to meet the November deadline for hosting the games, up to 6,500 labors may have died as a result of the working in the heat, the rapid pace of construction, and lax safety enforcement.

The Qatari government strongly disputes that number, along with the negative narrative associated with construction during the buildup to the World Cup. The actual number of fatalities is contested, and the fraction of deaths attributable to heat more so. What is indisputable is that “heart attack” was listed on death certificates as the cause of 58% of worker deaths in Qatar during hot months, a rate far higher than the 15% experienced globally in young adults.

Through interviews and reporting, it is also indisputable that construction workers in Qatar suffered severely from the heat. In an expose, Time Magazine chronicles heat exhaustion, heat stroke, and heart attacks experienced by these workers; other media outlets, such as the Wall Street Journal and even ESPN, carried similar stories.

Qatar Case study - Lingering Effects

The World Cup construction program provides a lesson in the longer-term health effects of chronic heat exposure. The country of Nepal sourced approximately 13,000 workers for construction sites in Qatar. Many of these workers now experience a range of health issues, especially kidney failure. One telling statistic comes from the Koirala Institute of Health Sciences in Nepal. According to the institute, 31% of patientsundergoing dialysis at the center had left the country to work as laborers, as well as more than 30% of kidney transplant recipients.

In 2023, the executive director of Nepal’s National Kidney Center stated that 10% of patients at his center worked in the Persian Gulf. Despite being fit and healthy when they leave, they “go to the Gulf to earn money, but return with kidney disease,” due in large part to dehydration and long hours working under heat exposure.

The large percentage of kidney failure in returning Nepali workers generated concern at a research institution a world away and, in 2023, it was announced John Hopkins received a grant to study the relationship between heat and chronic kidney disease among this population. A resulting study found, despite being an overall young group, returning workers accounted for more than 25% of dialysis patients, implicating “heat and environmental exposures as potential risk factors for chronic kidney disease”.

While dramatic, the Qatar experience is just one of many examples emerging in medical literature of kidney failure resulting from heat exposure. Review papers have turned up similar findings in other hot locations like Brazil and India. One study from Thailand found men exposed to prolonged heat stress were more than two times as likely than their peers to develop kidney disease.

Lessons for Closer to Home?

It’s easy to dismiss the experience of migrant laborers in Qatar as unrelated to working conditions in the U.S., but the effects of chronic heat exposure on worker health have also been documented here at home.

One British study of nearly 29,000 people who work in high heat conditions found they were 11% more likely to develop chronic kidney disease over a seven-year period than peers who did not work in hot conditions. In the U.S., 1 in 40,000 workers in certain industries were found to be hospitalized each year for acute kidney injury, a statistic that may capture less than half the true number. Unsurprisingly, two hot locations- the San Joaquin Valley in southern California and the Rio Grande Valley in Texas- have been identified as kidney disease hot spots.[2] 

Despite the studies mentioned above, there has been little other systemic research on the effects of chronic heat exposure in the U.S. Some researchers speculate that better adherence to safety standards and the more temperature climate in most of the U.S. might prevent larger portions of the population from experiencing chronic heat related health issues.

Although the frequency of health issues linked to chronic heat exposure is small in the U.S., every year steadily rising temperatures put more outdoor workers in the sustained high temperatures linked to chronic injury. Adaptations that reduce body core temperature and maintain hydration are vital to ensuring long-term health for Americans working outdoors in the heat. Anyone working prolonged periods in high temperatures should take steps to stay cool and stay hydrated!