Climate Change Extreme Heat and Health: Understanding the Scope of the Problem

Sameed Khatana, MD, MPH, is an assistant professor in the Division of Cardiovascular Medicine at the Perelman School of Medicine, University of Pennsylvania. This thought piece was written for the 2023 Perry World House Global Shifts Colloquium, “Living with Extreme Heat: Our Shared Future.” The colloquium was made possible in part by a grant from Carnegie Corporation of New York.

Extreme heat is likely the largest contributor of deaths resulting from extreme weather events related to climate change in the United States. The human body tolerates only minor temperature elevations above normal without developing significant dysfunction. Temperature regulation in the human body is a complex interconnected system that is driven primarily by the brain in response to receptors throughout the body. To preserve vital organs, in response to signals from the brain, the heart will beat faster and harder to transport heat absorbed in blood to the periphery of the body, where it can be released through sweat or breathed out from the lungs.

With prolonged exposure to heat, the human body experiences dysfunction through various mechanisms that can ultimately lead to organ failure and death if body temperature cannot be lowered. One of the most severe consequences of heat exposure is heat stroke, which is defined as an elevated core body temperature accompanied with central nervous system dysfunction. In the absence of treatment, up to 80 percent of people with heat stroke will die. However, even in the absence of heat stroke, various adverse health effects still occur. Cardiovascular events, such as heart attacks and strokes, are some of the most important. As the heart beats harder and faster in response to heat exposure, individuals with a history of heart disease who have less cardiac reserve are more prone to experiencing health consequences. Other individuals, such as those with lung or kidney disease, diabetes, or those taking certain medications, are also at risk of adverse health events when experiencing prolonged heat exposure.

However, quantifying the impact of extreme heat on health outcomes has several challenges. Firstly, with no universal definition of extreme heat, comparing the incidence of extreme heat across regions is challenging. In general, extreme heat refers to temperatures that are significantly hotter than typical for a given area. Given differences in infrastructure and lifestyle, a relative threshold—i.e., one that compares temperatures on a given day in a given area to previous temperatures that occurred during the same season or day in that same area in previous years— would be most useful. Deciding on a baseline period will influence the threshold above which extreme heat is defined. However, in many localities a fixed threshold (e.g., above 90°F or 32.2°C) is often used. Other sources of variation include use of temperature versus heat index or the wet bulb globe temperature. This lack of standardization in defining extreme heat makes it challenging to quantify the health impact of the issue and identify the regions and communities that are most significantly affected.

In addition to the definition of extreme heat, identifying which deaths, or other adverse health issues, are directly related to extreme heat can be difficult. In the US, the Centers for Disease Control and Prevention, in a report of heat-related deaths in the US, identified a total of 10,547 deaths between 2004 and 2018 (an average of 702 deaths per year). This analysis identified heat-related deaths based on data from death paperwork that directly listed heat exposure as either a cause of death or contributing to the death. However, it is not always clear, or listed on death paperwork, whether a particular death is related to heat exposure. This lack of clarity is also the case for other adverse health events, such as hospitalizations or exacerbations of pre-existing medical conditions.

An alternative approach to directly measuring deaths related to extreme heat based on death paperwork is to use a statistical approach to estimate the relationship between heat exposure and deaths (or other adverse health events). In two studies published in 2022, our group–including experts from Penn’s Perelman School of Medicine and Leonard Davis Institute of Health Economics, as well as the Center for Health Equity Research and Promotion at the Michael J. Crescenz Veterans Affairs Medical Center–attempted to estimate the number of deaths from any cause, and from cardiovascular disease specifically, associated with extreme heat in the contiguous United States. By examining monthly variation in the number of extreme heat days (identified based on a combination of absolute and relative thresholds), we estimated that between 2008 and 2017, there were an average of approximately 1,400 deaths per year associated with extreme heat. Another analysis, using a different approach and examining deaths associated with “non-optimal” hot temperatures, estimated a total of approximately 20,000 deaths per year related to heat in Canada and the United States from 2008 to 2017.

Data quality and availability is also an important factor impacting the understanding of the impact of extreme heat on health outcomes. Death paperwork, even in industrialized countries, can be challenging to work with, especially if a particular cause of death is being identified. Miscoding of the underlying cause of death or use of less-than-fully informative “garbage codes” is an issue, related in part to local coding practices and physician experience. Many lower-income countries have additional issues in availability and accuracy of vital statistics and health data in general      that can make studying this issue challenging. Statistical methodologies have been developed to account for the uncertainty in these data sources, but a true understanding of the scope of the issue will require improvements in health data collection and dissemination.

In our analysis of extreme heat-related deaths, we also identified particular populations that may be especially vulnerable to the impacts of heat on health. Elderly adults, compared to the non-elderly, are at a higher risk of adverse health events, in part due to the higher burden of pre-existing medical conditions. In a study of deaths occurring during a heatwave in Chicago in 1995, other factors that were found to significantly increase the risk of heat-related adverse health effects included social isolation or lack of access to transportation. This highlights the need to ensure that elderly individuals, particularly those who may have fewer social contacts, are provided access to cooling in a timely manner.

Our studies also noted that in the US, there was a greater increase in mortality among Black adults, compared to non-Hispanic white adults, related to extreme heat. Although there are differences in the prevalence of medical comorbidities between different populations, this is also related to the history of underinvestment in the infrastructure in minority neighborhoods in the US. Other studies have demonstrated that neighborhoods with a higher proportion of non-white residents experience the urban heat island effect—the phenomenon of built environments being hotter than surrounding rural areas—to a greater degree. Similarly, there are significant disparities in access to air conditioning, with poorer households having the lowest access in the US.

Similar disparities likely exist within low-income countries as well, just as they do across the income distribution between countries. However, there is currently little data exploring disparities within these countries to help identify the communities and populations most at risk for the adverse health effects of extreme heat.

The statements made and views expressed in this article are solely the responsibility of the author.