How Might Extreme Hot & Cold Weather Affect Heart Health?

Between April and September 2021, the South Pole experienced its coldest winter on record, dating back to 1957, with an average temperature of -78°F (-61°C).¹ A mere two years later, September 2023 was recorded as the hottest month in recorded history when global temperatures spiked 2.1°F (1.2°C) above the 20th century average summer between 1951 and 1980.²

Today, the relationship between extremes in ambient temperature and human health is one of the most discussed among medical researchers on the topic of climate change.^3,4 Specifically, a growing body of medical research is examining the link between new norms in the earth’s temperature and cardiovascular disease, the leading cause of death worldwide.^5,6 What do population studies tell us about the connection between extreme hot and cold temperatures and cardiovascular disease? How might people prepare for periods of climate change so they can take measures to either avoid exposure or mitigate effects?

In 2022, researchers reporting in the journal Circulation analyzed more than 32 million cardiovascular deaths that occurred in 567 cities in 27 countries on five continents between 1979 and 2019 and compared these deaths on the hottest and the coldest 2.5% of days for each city, with deaths on the days that had optimal temperature (the temperature associated with the least rates of deaths) in the same city.⁵ They found that, for every 1,000 cardiovascular deaths, extremely hot days accounted for 2.2 additional deaths and extremely cold days accounted for 9.1 additional deaths. The greatest number of additional deaths was found for people with heart failure (2.6 additional deaths on extreme hot days and 12.8 on extreme cold days).⁵

Healthcare Disparities

Research suggest that climate change is likely to exacerbate existing racial disparities across a broad range of health outcomes, including pulmonary health.^7-9 Within the United States, Black, Latinx, Native American, Pacific Islander, and Asian communities may be disproportionately affected compared to non-Hispanic white adults.⁷ Children are particularly vulnerable, with infants and children of color experiencing adverse perinatal outcomes, occupational heat stress, and increases in emergency department visits associated with extreme weather.⁷ Older individuals (aged 60 and above), those with disabilities, and women are also at a higher risk of experiencing adverse health effects from a changing climate.⁸

Studies suggest that low-income countries have substantially more impact from severe weather compared to high-income countries.⁸ Approximately 79% of the global poor reside in rural areas and rely on climate-sensitive environmental assets such as lakes, forests, and oceans for their livelihood. South Africa, for example, is one of the most impoverished countries in the world and is experiencing significant environmental impacts caused by higher temperatures and decreased rainfall.⁸

Excessive Heat & Heart Health

In October 2023, National Institutes of Health researchers reported that cardiovascular-related deaths due to extreme heat are expected to increase by 162% by mid-century in the United States.¹⁰ The study, published in the journal Circulation, evaluated county-level data from the contiguous 48 states between May and September of 2008-2019—a time that saw more than 12 million deaths related to cardiovascular disease. Environmental modeling estimates found that the heat index during this time rose to at least 90 degrees about 54 times each summer. (Note: Extreme heat in this analysis was defined by any day in which the maximum heat index (HI) was ≥90°F or 32.2°C).¹⁰

Researchers linked these extreme temperatures to a natural average of 1,651 deaths.¹⁰ Forecasting environmental and population changes, the researchers looked ahead to 2036-2065 and estimated that cardiovascular deaths will increase by 2.6 times for the general population—from 1,651 to 4,320. Deaths could triple to 5,491 if emissions rise significantly, according to the report.¹⁰

Heat-induced cardiovascular damage may result in endothelial dysfunction, impairing the tone and structure of blood vessels, which interferes with a variety of biological factors, including nitric oxide synthase, cytokine production, and systemic inflammation.⁴ Mechanisms linking extreme hot temperatures to different cardiovascular diseases are related to dehydration, hemoconcentration, hypercoagulability, sympathetic activation, and inflammatory mediators.¹² Cardiovascular functioning in hot weather can be decreased, leading to a higher risk of developing heart attack, malignant cardiac arrhythmias, and thromboembolic diseases.⁴

A 2022 systematic review and meta-analysis of 266 studies, published in Lancet Planet Health, suggests that heat exposure may place an increased strain on the heart in healthy populations and may exacerbate symptoms in those with existing cardiovascular conditions.⁶ A consistent positive association was found across all cardiovascular disease diagnoses in mortality, with the strongest effects shown due to stroke (3.8%) and coronary heart diseases (2.8%). Researchers found that high temperatures increased the risk of morbidity due to arrhythmias and cardiac arrest (1.6%) and that the greatest risk of morbidity was out-of-hospital cardiac arrest (2.1%). Heatwaves were also associated with negative cardiovascular disease health outcomes; the risk of heatwaves on cardiovascular disease–related mortality increased significantly by 11.7%, with an increased effect as heatwave intensity increased.⁶

Coping With Cold Weather: Cardiovascular Impacts

Cardiovascular disease mortality is higher in winter than in summer in many countries.¹¹ Exposure to the cold may induce an increase in blood viscosity by elevating blood platelet count and red blood cell count within a few hours, which may increase the risk of ischemic heart disease and stroke. A 2023 systematic review and meta-analysis of 80 studies suggests a high correlation between low temperatures and cardiac arrest morbidity, which the authors hypothesize may be explained by cold-induced autonomic nervous system disruption and inflammation–coagulation cascade activation.¹¹

Long-term, extreme cold exposure was also found to be associated with several risk factors for cardiovascular disease, suggesting that exposure to lower temperatures may be associated with a higher risk of metabolic derangement, including higher plasma glucose, and more insulin resistance.¹¹ Patients with diabetes are more prone to cold-related cardiovascular disease.¹¹ Exposure to the cold increases blood pressure and changes in blood components, which research suggests may induce conditions like hypertension, myocardial infarction, and atherosclerosis.¹¹ The impact of cold temperatures on myocardial infarction hospitalizations is well documented in medical literature.¹²

Lifestyle Interventions & Resources

Prevention is paramount when it comes to cardiovascular–related illness and extreme climate conditions. Primary care providers are on the front lines for educating and preparing patients, especially those who are most at risk, so they can take measures to either avoid exposure or mitigate effects. However, studies suggest that despite solid evidence about the adverse health effects of changing weather patterns, efforts to communicate this to clinicians and patients are lacking.^13,14

In a 2021 global survey published in Lancet Planet Health of 4,654 health professionals regarding climate change, 76% of participants recognized the need for continuing professional education, 72% desired knowledge regarding healthcare sustainability, and 69% felt that effective communication skills were also needed.¹⁵ Effective communication between patient and practitioner can inform treatment decisions and improve health outcomes.¹³

In functional medicine, there are tools that help assess a patient’s exposure to extreme heat or cold and help pinpoint the role these exposures may play—past or present—in that patient’s disease. A wealth of evidence-based resources on extreme weather for patients and clinicians alike can be found in a toolkit provided by Americares, a global health-focused relief and development organization, and the Harvard T.H. Chan School of Public Health’s Center for Climate, Health and the Global Environment. Information from Americare’s Climate Resilience or Frontline Clinics Toolkit is downloadable in both English and Spanish. 

Functional medicine clinicians encourage patients to continuously embrace healthy food, as studies suggest that plant-based diets may reduce cardiovascular disease risk and improve cardiometabolic health.^16,17 IFM’s Cardiometabolic Food Plan is a tool that contains valuable resources and information for clinicians and patients to help them transition to healthier eating patterns that support cardiac and metabolic health. This modified Mediterranean approach was designed for individuals with cardiovascular health conditions or specific cardiometabolic concerns, including cardiovascular disease, metabolic syndrome, type 2 diabetes, high blood pressure, high blood sugar, high cholesterol, elevated blood fats, and increased belly fat.