Heat Waves & Health: A round-up of recent research
The temperature is forecast to break triple digits for many parts of the Northeast this week, just a week after a similar heat dome created a deadly heat wave across much of Europe. Many areas of the world are experiencing record-breaking temperatures more frequently than ever before due to anthropogenic climate change. According to the U.S. EPA, the average rate of extreme heat events increased from two per year in the 1960s to ten per year between 2010 and 2020.
In addition to being uncomfortable, extreme heat — a period of high heat and humidity with temperatures above 90 degrees for at least two to three days — is also extremely dangerous to health. According to Ready.gov, extreme heat is responsible for the highest number of annual deaths among all weather-related hazards.
Research from the Department of Environmental Health at Harvard T.H. Chan School of Public Health has shown an increase in deaths, premature births, hospitalizations, and other serious health impacts from heat waves. These findings help physicians, as well as local and state governments, prepare to support residents before, during, and after heat waves.
Here is a look at some recent findings on the health impacts of extreme heat:
Heat waves are leading to an increase in death rates in older adults. A new nationwide study of more than 73 million older Americans finds that heat waves are driving a substantial increase in deaths each year, according to a recent study co-authored by Harvard Chan researchers James Healy, Edgar Castro, Mahdieh Danesh Yazdi, Mary Rice, and Joel Schwartz. The study also found that this disproportionately affects Black and low‑income communities, while neighborhoods with more green space see lower death rates.
Heat waves pose an escalating threat to the health of pregnant people, infants, and children. This recent commentary in JAMA from researchers and physicians, Caleb Dresser, Shruthi Mahalingaiah, and Kari Nadeau, examines recent research into early and preterm birth rates and heat waves, and the need for more research into the association of extreme heat with time to conception, pregnancy loss, and general health of pregnant individuals and infants, which are less-often discussed and less understood.
A new study from Joel Schwartz and others also found that heat exposure during pregnancy and infancy was associated with slower brain development later in childhood. This study of 3,251 in the Netherlands found that high ambient temperature during pregnancy and infancy was associated with slower thalamic growth later in childhood, suggesting long-term impacts of heat on thalamic development. The thalumus is a key brain structure, acting as a relay station for incoming motor and sensory information from your body. More research needs to be done to understand the extent of climate change impacts on children’s neurodevelopment.
Exposure to heat increases the risk of workplace injuries. Researchers, including Harvard Chan’s Barrak Alahmad, found that workplace injury risk begins to climb when the daily heat index reaches about 85 degrees Fahrenheit and rises steeply past 90 degrees. These heat-related injuries occurred across almost all industry sectors, including indoor jobs. States with existing OSHA heat exposure standards showed lower injury risks on hot days. The study found that heat exposure leads to physiological and cognitive impairments that increase the risk of workplace injuries, and that even moderate heat can increase risk.
Extreme heat can worsen diabetes. Rising temperatures driven by climate change could lead to increased health risks for people with diabetes, according to another study by Dr. Alahmad. “High temperatures lead to increased heat stress, which worsens glucose control and raises the risk of complications such as foot ulcers and fluctuations in blood sugar levels,” explains Dr. Alahmad.
Extreme heat may raise risk of hospitalization for adults with Alzheimer’s disease and related dementias (ADRD), according to a recent study published in JAMA Internal Medicine by Scott Delaney. The study found that one day of extreme heat exposure raised ADRD hospitalization risk not just on the day of exposure but for three additional days—even if the heat subsides. “Compared with cognitively intact adults, [those living with ADRD] may be slower to recognize symptoms of extreme heat exposure, or, if they recognize the symptoms, they may be less able to address them,” the researchers wrote. “Because ADRD affects thermoregulatory processes, adults living with ADRD are more prone than others to heat-related illnesses, thus magnifying the potential impact of extreme heat.”
100⁰ F temps may be linked to irregular heartbeat in those with implanted defibrillators. A recent study by Barrak Alahmad and Petros Koutrakis found that severe temperature spikes may double or triple the risk of irregular heart rhythm in people with implanted defibrillators. The data was presented at the American Heart Association Scientific Sessions in 2024.
Prolonged heat exposure led to an increase risk of psychiatric ER visits. While previous studies have shown the link between extreme heat and mental health impacts, a recent study from Amruta Nori-Sarma, Flannery Black-Ingersoll, and Nadav Sprague sought to better understand how the length of exposure, as well as the timing of symptom onset. This study found that cumulative hourly temperature increases were associated with elevated psychiatric emergency risk over 24 hours, and that sustained heat exposure led to higher risk.
Public health recommendations to reduce these impacts:
Warn the public on the specific risks of heat. Dr. Alahmad recommended that, when extreme heat events are forecast, warning systems be put in place to alert vulnerable patients, including those with diabetes. He also recommended expanding the use of telemedicine during these weather events.
Increase access to greenspace in urban areas. The research by Healy and others found that in ZIP codes with the highest levels of green space, heat waves were associated with a reduction in annual mortality, with 13.51 fewer deaths per 10,000 people per year per heat wave. Greenspace has also been shown to positively impact mental health, cognition, and blood pressure.
Expand access to air conditioning and cooling centers. Joseph Allen and Kari Nadeau wrote this letter to the Boston Globe about the need for cities to plan for extreme heat events by expanding access to cooling centers, and the need for better and more energy efficient air conditioning.
Meanwhile, researchers including Harvard Chan’s Gary Adamkiewicz, Caleb Dresser, Gaurab Basu, Mary Rice, and others, are conducting research on programs that distribute window AC units to older adults in Boston to understand the health impacts of having access to air conditioning, and how energy affordability affects AC use.
Tips that individuals can take to prevent heat-related illness:
The most important thing you can do to stay safe during extreme heat is to be aware just how dangerous hot weather can be, according to Dr. Basu, an ER physician and environmental health researcher.
Dr. Basu recommended taking the following precautions during extreme heat in a recent Boston Globe article:
- Use sunscreen and wear hats.
- Stay hydrated. If your urine is dark, drink 2–3 glasses of water.
- Take baths, showers, use a wet towel on your skin, or spray yourself with water while using a fan.
- Visit a cooling center.
- Avoid alcohol, sugary, or caffeinated drinks.
- Eat easy-to-digest foods like salads or fruits.
- Don’t exert yourself outdoors if possible.
- Avoid dark clothes; wear light, loose-fitting clothes.
The Center for Climate, Health, and the Global Environment (C-CHANGE) has also partnered with Americares to develop this Toolkit for frontline health clinics to help them navigate the impacts of climate change on their clinics and their patients. The Toolkit includes this Heat Action Plan and Tip Sheet.
