Extreme heat is increasingly being recognized as a threat to the heart, lungs, kidneys, and brain, with new data linking heat waves to higher rates of hospital admission, preterm birth, and sick leave.

New Normal

At the end of May, Europe experienced its first heat wave of the year. Currently, more hot days are on the horizon. Climate experts predict another warmer-than-average summer in 2026, whereas the United Nations projects a further rise in temperature due to climate change over the next 5 years.

As temperatures rise, heat waves become more frequent and last longer. Researchers have warned that these events pose a growing threat to public health and place increasing pressure on healthcare systems. The health consequences of heat exposure are reflected by both higher heat-related mortality and increased morbidity.

However, the actual effect is difficult to quantify because heat is rarely identified as the direct cause of an illness or death. Therefore, researchers rely on statistical modeling. For example, the Robert Koch Institute estimated that approximately 2500 deaths in Germany in 2025 were attributable to heat.

Data on heat-related morbidities are also emerging. After a single day with temperatures above 30 °C, the number of sick leave reports increased by 3.5%. After 5 consecutive days of extreme heat, the increase reached 5%, rising to 10.8% after 7 consecutive days.

Growing Burden

Speaking with Science Media Center, Alexandra Schneider, MD, PhD, deputy director of the Institute of Epidemiology and head of the Research Group “Environmental Risks” at Helmholtz Munich, said, “There are now relatively reliable epidemiological estimates of heat-related morbidity and mortality in German.”

“Overall, heat is now one of the most significant environment-related health risks in Germany and worldwide, alongside air pollutants,” Schneider said.

Without effective adaptation measures, temperature-related mortality is expected to increase substantially, according to Martin Röösli, PhD, head of the Environmental Exposures and Health Unit at the Swiss Tropical and Public Health Institute in Basel, Switzerland. He cited findings from a large study published in January 2025.

Research on heat-related morbidity remains less extensive than that on heat-related mortality.

Veronika Huber, PhD, of the Department of Conservation Biology and Global Change at Estación Biológica de Doñana in Seville, Spain, said that “Heat-related morbidity has been studied far less in Germany than heat-related mortality. To my knowledge, there are no published statistical estimates to date of the total number of heat-related hospital admissions in Germany.”

Nevertheless, several indicators can be used to assess the health impact of extreme heat, including hospital admissions, emergency department visits, emergency medical service calls, outpatient consultations, sick leave data, and registries tracking myocardial infarction and stroke.

Multiple Organs

Schneider noted that extreme heat places a major burden on the cardiovascular system because blood pressure falls as blood vessels dilate, forcing the heart to work harder.

In individuals with underlying diseases, this increases the risk for angina, myocardial infarction, heart failure, and arrhythmia.

Simultaneously, excessive sweating and fluid loss can lead to dehydration and electrolyte disturbances, increasing the risk for circulatory collapse and thrombosis. Certain medications, including diuretics and beta-blockers, may further increase these risks.

Individuals with kidney disease, lung disease, diabetes, or neurologic disorders are also particularly vulnerable.

Heat often coincides with elevated concentrations of ozone and particulate matter, which can irritate the respiratory tract and trigger the exacerbation of chronic obstructive pulmonary disease and asthma.

Vulnerable Groups

Older adults remain the population to be most affected by heat stress and heat waves, according to Kilian Rapp, MD, medical director of acute geriatrics and head of the Department of Clinical Gerontology at Robert Bosch Hospital in Stuttgart, Germany.

“Heat poses a concrete threat of harm to older people,” Rapp said.

Risk factors for heat-related mortality include advanced age, low socioeconomic status, substance use disorders, limited mobility, pulmonary disease, cardiovascular disease, geriatric psychiatric conditions, and the use of sedative medications.

Living alone or residing on higher floors also increases the risk. Protective factors include access to air conditioning, transportation, and the ability to maintain independent self-care.

Brain Effects

Heat also affects the brain and nervous system in multiple ways.

High temperatures can alter neuronal function, promote oxidative stress and inflammation, and impair the blood-brain barrier. According to Ameli Breuer, MD, a neurologist and researcher at Charité – Universitätsmedizin Berlin in Berlin, Germany, is a spokesperson for the Neurology Working Group of the German Alliance for Climate Change and Health (KLUG).

High temperatures increase the risk for stroke. Symptoms may worsen in individuals with Parkinson’s disease or multiple sclerosis, and evidence suggests that heat may trigger seizures or exacerbate symptoms in individuals with epilepsy or migraine.

Among individuals with dementia, extreme heat is associated with increased hospital admissions and mortality.

According to Breuer, from a neurologic perspective, heat is no longer a problem of the future. These effects are already visible in everyday clinical practice and are expected to increase as climate change progresses.

Heat can also affect mental health. Individuals may experience reduced concentration, poor sleep, increased irritability, and, in some cases, aggressive behavior, according to Sebastian Karl, MD, from the Department of Psychiatry and Psychotherapy, Central Institute of Mental Health, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany.

From a psychological perspective, heat acts as a stressor, thereby reducing the capacity to cope with additional demands. Biologic mechanisms may also contribute, including inflammatory changes in the brain and alterations in neurotransmitter balance.

Children’s Risks

Children are particularly sensitive to environmental stressors such as heat, air pollution, and extreme weather events. Their bodies are still developing, they spend more time outdoors, and they are generally more physically active than adults, according to Marie Standl, PhD, head of the Research Group “Allergic Disease Epidemiology” and “Lung Epidemiology” at Helmholtz Munich.

“Children are not small adults when it comes to heat effects. Their thermoregulation is still developing; they have a larger body surface area relative to volume and are less able to sweat. Young children and infants are particularly dependent on their caregivers to protect them from heat,” Standl said.

Children and adolescents are particularly vulnerable because their thermoregulatory systems are not yet fully developed. They generate more metabolic heat relative to body weight, have a larger body surface area relative to body mass, and therefore absorb more heat from the environment.

At the same time, they have a lower blood volume and reduce cardiac output than adults.

Dirk Holzinger, MD, lead physician for pediatric rheumatology at University Hospital Essen in Essen, Germany, and a member of KLUG’s extended executive board, noted that evidence points to higher hospitalization rates among children and adolescents on extremely hot days.

Pregnancy Risks

Petra Arck, MD, PhD, professor from the Department of Experimental Feto-Maternal Medicine at the University Medical Center Hamburg-Eppendorf (UKE) in Hamburg, Germany, said extreme heat during pregnancy places considerable strain on the body and is associated with serious risks.

High temperatures can lead to dehydration, cardiovascular stress, and reduced uterine blood flow, potentially affecting pregnancy outcomes.

Anke Diemert, MD, from the Department of Obstetrics and Fetal Medicine at the same institution, noted that numerous studies have linked heat stress to preterm birth.

Research conducted at the center found that temperatures between 30 °C and 35 °C, sustained over several consecutive days, can increase the relative risk for preterm birth.

Protective Measures

By the end of 2024, only 20 German municipalities (out of several thousand) had a heat-health action plan. Most German states now require municipalities to develop such plans, generally by 2030, according to Clemens Becker, MD, head of geriatrics at UKE.

Becker also noted that hospitals and long-term care facilities are legally required to maintain and regularly update heat warning systems and heat action plans.

Older individuals living alone. They must be protected by their families, neighbors, and volunteers.

Kerstin Kammerer, PhD, of the Institute for Gerontological Research in Berlin, highlighted existing heat protection models, including plans developed by the Berlin Heat Protection Action Alliance for healthcare staff and vulnerable groups such as older adults, people who are sick, and homeless people — from rising temperatures and dangerous heat wave. Comprehensive information on dealing with heat is also provided by the LMU Medical Center’s Heat Action Plan for Inpatient Long-Term Care Facilities.

Schneider, Röösli, Huber, Standl, Kammerer, Rapp, Breuer, Arck, Diemert, and Becker reported having no relevant conflicts of interest. Karl reported authorship of publications related to environmental change and mental health and serving as a city council member in Stuttgart. Holzinger reported serving as a member of KLUG’s extended executive board.

This story was translated from Medscape’s German edition.