Climate change affects human health in nearly every system of the body, from the heart and kidneys to mental well-being. Between 2000 and 2019, roughly 489,000 people died each year from heat alone, and the World Health Organization projects that climate change will cause an additional 250,000 deaths per year between 2030 and 2050 from just four causes: undernutrition, malaria, diarrheal disease, and heat stress. The full picture is broader than that, spanning infectious disease, respiratory illness, waterborne pathogens, and a growing mental health burden.
How Extreme Heat Strains the Heart and Kidneys
When your body overheats, it redirects blood flow away from your internal organs and toward the skin to shed heat. That redistribution forces the heart to work significantly harder to maintain blood pressure while simultaneously keeping the brain, muscles, and skin supplied. For people with existing heart disease or older adults whose cardiovascular systems are less adaptable, this extra demand can push the heart past its limits. Heat-related mortality among people over 65 increased by roughly 85% between the early 2000s and the period from 2017 to 2021.
The kidneys take a direct hit too. During severe heat stress, blood flow to the kidneys drops as the body prioritizes cooling. Repeated episodes of dehydration and reduced kidney perfusion, common in outdoor workers and people without air conditioning, can cause cumulative damage over time. When the body’s cooling mechanisms fail entirely, a cascade of widespread inflammation begins that can injure multiple organs at once, cause dangerous blood clotting problems, and in severe cases prove fatal.
Mosquito-Borne Diseases Are Spreading
Warmer temperatures expand the geographic range of mosquitoes that carry diseases like dengue fever. Dengue transmission peaks at about 28°C (82°F), with the two main mosquito species transmitting most efficiently between 26°C and 29°C. As average temperatures rise in regions that were previously too cool for these mosquitoes, new populations become exposed to a disease they have little immunity against. Dengue burden is already expanding across the Americas and parts of Asia.
Tick-borne illnesses are shifting too. Modeling studies in the eastern United States show that moderate warming (around 2.5°C above current averages) would push Lyme disease hotspots into new areas. In Maryland, for example, western counties like Garrett and Allegany that historically had low Lyme risk are projected to become new hotspots, while some currently high-risk areas in the south and east may see declines. The net effect is not simply “more disease everywhere” but a geographic reshuffling that catches communities off guard.
Warming Oceans and Waterborne Illness
Rising sea surface temperatures are creating favorable conditions for dangerous bacteria in coastal waters. Vibrio vulnificus, a flesh-eating bacterium found in warm, brackish water, thrives when water temperatures climb. During the July and August 2023 heat waves, three eastern U.S. states reported clusters of severe Vibrio infections. The CDC notes that as coastal waters continue to warm, these infections are expected to become more common, particularly along coastlines where they were once rare. People with open wounds who wade into warm seawater or who eat raw shellfish harvested from warm waters face the highest risk.
Air Quality and Respiratory Health
Hotter temperatures accelerate the formation of ground-level ozone, the primary ingredient in smog. Longer, more intense wildfire seasons fill the air with fine particulate matter that penetrates deep into the lungs. These particles are small enough to enter the bloodstream and trigger inflammation throughout the body. For people with asthma or chronic lung disease, worsening air quality translates into more frequent flare-ups, more emergency room visits, and reduced lung function over time. Even healthy adults experience measurable drops in respiratory performance during prolonged wildfire smoke events.
Warmer seasons also extend pollen production. Plants release pollen earlier in the spring and continue later into the fall, and higher carbon dioxide levels cause plants to produce more pollen per season. The result is longer, more intense allergy seasons that affect the roughly one in four adults who are sensitized to airborne allergens.
Mental Health After Disasters and Beyond
The psychological toll of climate change operates on two levels. The first is acute: people who survive hurricanes, floods, and wildfires frequently develop anxiety, depression, and post-traumatic stress. Research following Hurricane Katrina documented high rates of PTSD and anxiety among survivors, and similar patterns have been observed after major floods and prolonged heat waves. These reactions affect both people with pre-existing mental health conditions and those with no prior history of illness.
The second level is slower and harder to measure. A growing number of people, particularly younger adults, experience what researchers describe as distress tied to awareness of environmental degradation. This isn’t a clinical diagnosis but a persistent sense of dread or helplessness about the future of the planet. Displacement from a home or community due to repeated flooding or wildfire compounds this distress, severing social connections that are protective for mental health.
The Cost to Healthcare Systems
Climate-related health effects carry an enormous financial burden. In the United States, extreme heat alone drives an estimated $177 million in emergency department visits and $835 million in hospital admissions each summer. A 2019 analysis found that just 10 major climate events generated $10 billion in combined costs from hospitalizations, emergency visits, and lost wages. Since 1980, U.S. climate disasters have cost nearly $2 trillion in total.
Individual disaster events illustrate how quickly costs escalate. After the 2016 Baton Rouge flood, Medicaid spending on healthcare visits in the affected area nearly tripled, jumping from $7.3 million before the flood to $21 million in the ten months afterward. The per-visit cost also rose, reflecting the greater complexity of post-disaster health needs. After Hurricane Sandy in 2012, emergency departments in affected areas saw an average of 1,700 additional cases in a single month, with trauma care alone adding $4 million in costs.
Who Faces the Greatest Risk
Climate change does not distribute its health effects evenly. Older adults are disproportionately vulnerable to heat because aging reduces the body’s ability to regulate temperature and increases the likelihood of cardiovascular disease. Children breathe faster and spend more time outdoors, making them more susceptible to air pollution and heat illness. Outdoor workers, from agricultural laborers to construction crews, face repeated heat exposure that compounds kidney and heart strain over years.
Low-income communities often lack air conditioning, live in urban heat islands where pavement and concrete amplify temperatures, and have less access to healthcare when climate-related illness strikes. These same communities are more likely to live in flood-prone areas or near industrial zones where air quality is already poor. The health effects of climate change layer onto existing inequalities, widening gaps that were already significant.