Study reveals underestimated heat risks in climate change, highlighting challenges for aging population

By Dr. Liji Thomas, MDNov 30 2023Reviewed by Benedette Cuffari, M.Sc.

Global warming and urbanization, combined with an aging population in many countries, have increased the risk of numerous catastrophic changes that could occur in the near future, including challenges to human health, reduced productivity, and poorer quality of life.

A new modeling study recently published in Nature Communications provides more accurate estimates of human survivability and introduces the concept of livability under both current and future conditions of climate change. Physiological parameters were incorporated into the model, thereby improving its accuracy of prediction.

Study: A physiological approach for assessing human survivability and liveability to heat in a changing climate. Image Credit: Quality Stock Arts / Shutterstock.com

Background

Several epidemiological studies have reported a higher risk of cardiovascular and respiratory deaths from heat exposure. These predictions are based on actual outcomes and living conditions used to predict future risks under different global temperature scenarios. However, these approaches often fail to prioritize humidity, which reduces evaporative cooling and is expected to rise with global temperature.

In contrast, physiological studies are based on the human energy balance, as they model relationships between environmental heat and health outcomes while also emphasizing the crucial role of humidity. Nevertheless, these studies do not directly observe outcomes like death, hospital admissions, or actual heat levels, as they rely on thermal chamber conditions.

Earlier studies utilized a threshold of 35 °C wet-bulb temperature (T_w) without incorporating individual variations in body temperature, metabolism, state of activity, age, and other physiological states. Sherwood and Huber framed the T_w survivability threshold, in which six hours of exposure would lead to death. The rationale for this threshold was the knowledge that, at this point, the possibility of heat exchange is voided.

The human body becomes an adiabatic system at this temperature that no longer heats or cools, even under the best conditions. This leads to heat strain, which taxes the cardiovascular, thermoregulatory, and renal systems.

The results may include heat exhaustion, heat stroke, and cardiovascular collapse, especially in older individuals and those with pre-existing conditions. In contrast, fitter and acclimatized people and behavioral adaptations may mitigate the effects of heat strain.

This led many other studies to predict the limits of human survival under extreme heat conditions, and the extent to which humans could adapt to future rises in global temperature. However, these studies fail to account for inter-individual differences such as clothing, body size, level of activity, and variations in sweating, thus introducing the possibility of gross errors.

The current study predicts the risk to human survival by integrating physiological and environmental variables and interventions designed to mitigate or reduce the impact of heat stress and evaluate the feasibility of human life and work under much warmer conditions. To this end, the investigators assumed a range of T_w thresholds beyond which core temperature will be incompatible with life.

What did the study show?

The T_w threshold was found to significantly underestimate the risk to human survival under hot, dry conditions by 0.9-13 °C, despite relatively accurate estimates in very humid shaded conditions. Estimated T_w compatible with survival was 4-9 °C lower for the young, whereas T_w is reduced by 7-13 °C for older individuals as ambient temperature rises beyond 40 °C and relative humidity falls below 25%.

In fact, older adults would not live beyond six hours, even in the shade, at a T_w of 22 °C once relative humidity drops to 10%. At this point, the air temperature would be 46.4 °C, which is significantly lower than the value of 60 °C with the T_w of 35 °C assumption. However, beyond survival, higher solar exposure and humidity result in the loss of livability.

Based on maximum safe metabolic rate that a person can generate without a sustained positive rate of heat storage even with a maximal thermoregulatory response.”

At this limit, no activity is considered safe, as it leads to uncompensable heat stress, which initiates an irreversible rise in body temperature.

Thus, at air temperatures of 25 °C in shaded humid conditions, young adults could reach maximum metabolic equivalents (METs) of at least five, which is the equivalent of dancing without heat stress. In drier conditions, young adults may safely achieve just over eight METs, such as by running, to a greater extent than older adults. However, in the sun, the maximum decreases to four to seven METs, equivalent to doing household chores or climbing stairs.

With warmer air temperatures and over 75% humidity at a T_w of 35.5 °C, no activity is safe for the young, as they can only sit, which is equivalent to 1.5 METs. This limit is reached at 34 °C for older adults.

The steepest reduction is with age, especially in dry and hot conditions, where maximum METs fall by 2.5 to three for older adults compared to young and healthy adults. In cooler and more humid conditions, this difference narrows.

Accordingly, there is a wider range of conditions where older people can only rest safely (i.e., survivable but not livable).”

Further modeling suggests that at current carbon emission levels, maximum METs will fall by over 5% under warm conditions in the most humid tropical regions. Large parts of the world already have hot conditions with low maximum METs.

Thus, areas like North India and Bangladesh may expect survivable but not livable conditions much more frequently by the end of this century at six months every decade. The decrease in safe activity in some of these regions will impact older adults much more than expected from climate change alone, thus highlighting the effect of aging on heat risk and survivability.

What are the implications?

The physiological modeling approach used in the current study indicates a marked overestimation of livable and survivable temperatures using the conventional 35 °C T_w assumption as compared to physiological limits. This is due to the omission of the effects of sweating in response to extreme heat.   

Reductions in safe activity for younger and older adults between the present and future indicate a stronger impact from aging than warming.”

Younger adults are most likely to survive future heat stress conditions over this century due to better sweating responses. However, with increasingly hot and dry conditions, survivability drops steeply for young and old.

The conventional 35 °C T_w thresholds fail to show this effect, whereby survivability limits are reached within six hours through environmental and/or sweating restrictions.

Livability is expected to decline by the end of this century. The most significant challenges will occur in low-income, resource-straitened, and populous countries, thereby threatening human welfare on a large scale. The results of these changes in safe activity levels could increase heat stroke death rates and promote competition for livable land areas by increased migration or displacement from unlivable regions, thereby exacerbating conflicts.

Findings underline the need for continued research efforts and investments in heat risk management, adaptive capacity, and technological innovation for personal heat protection in vulnerable global regions.”