Extreme weather affects nearly every system in the human body, from your heart and lungs to your sleep, mental health, and ability to think clearly. Heat, cold, storms, floods, and wildfire smoke each carry distinct health risks, and those risks are growing. Heat-related deaths among adults over 65 increased by 167% compared to the 1990s, according to the 2024 Lancet Countdown report, more than double what would be expected from population aging alone.
What Extreme Heat Does to Your Body
Your body cools itself primarily through sweating and redirecting blood flow toward the skin. When the air temperature climbs high enough, or humidity prevents sweat from evaporating, this system starts to fail. The breakdown happens in two stages.
Heat exhaustion comes first. Excessive sweating drains water and electrolytes from your blood, reducing blood volume and dropping blood pressure. You feel dizzy, nauseated, and weak. Muscle cramps are common. At this point, moving to a cool environment, drinking fluids, and resting can reverse the damage.
If cooling doesn’t happen, heat exhaustion can progress to heatstroke, which begins when core body temperature exceeds 40°C (104°F). Heatstroke is a medical emergency. The skin becomes hot and dry because the sweating mechanism has failed. The brain is especially vulnerable: convulsions, delirium, and loss of consciousness can follow. At a systemic level, heatstroke triggers widespread inflammation that can damage multiple organs simultaneously.
Hot nights make things worse. Your body normally drops its core temperature during sleep, which is essential for entering the deepest, most restorative sleep stages. High nighttime temperatures suppress that cooling process, increasing wakefulness while cutting into deep sleep and dream sleep. Humidity amplifies the problem by making it even harder for the body to shed heat. Perhaps most concerning, research shows that sleep disruption from heat does not improve even after five consecutive nights of exposure. Your body simply doesn’t adapt.
Heat Slows Your Thinking
Extreme heat doesn’t just make you uncomfortable. It measurably impairs cognitive performance. A study of young adults during a 2016 heat wave found that people living without air conditioning had reaction times roughly 13% slower than those in cooled buildings. Their ability to process information and complete tasks accurately dropped by about 10%. For every degree Celsius above the optimal indoor range of 22 to 23°C, reaction times increased by 16 to 24 milliseconds per task. That may sound small, but it compounds across a workday and becomes significant for anyone making fast decisions, operating machinery, or studying for exams.
How Cold Weather Strains the Heart
Cold is actually deadlier than heat on a global scale. Cold-related deaths currently exceed heat-related deaths worldwide, though that balance is projected to shift as temperatures rise.
When your body is exposed to cold, it constricts blood vessels near the skin to minimize heat loss. This is a survival mechanism, but it forces the heart to pump against greater resistance. Heart rate increases. Blood pressure rises. At the same time, cold makes blood thicker and more viscous, raising levels of clotting proteins. The combination creates a dangerous mismatch: the heart needs more oxygen to handle the increased workload, but cold also causes the coronary arteries to narrow, reducing blood supply to the heart muscle itself.
A massive study spanning 567 cities found that cold extremes increase the risk of virtually all major cardiovascular disease subtypes, including heart attacks, heart failure, and stroke. The risk is highest for people who already have narrowed arteries or existing heart conditions, but sudden cold exposure can trigger events even in people who considered themselves healthy.
Floods, Storms, and Infectious Disease
Flooding does its most insidious damage after the water arrives. When floodwaters overwhelm sewage systems and contaminate drinking water sources, waterborne diseases surge. Cholera, hepatitis A, typhoid fever, and parasitic infections spread rapidly in densely populated areas where sanitation infrastructure has been destroyed. Following catastrophic floods in southern Brazil in 2024, health authorities confirmed 7,818 cases of leptospirosis (a bacterial infection spread through water contaminated with animal urine) and 10 outbreaks of diarrheal diseases within weeks.
Warmer temperatures and heavier rainfall also expand mosquito habitat. The global abundance of the mosquito species responsible for dengue, Zika, chikungunya, and yellow fever has risen an estimated 9.5% over the past century, with a projected 20 to 30% further increase by 2100. In Rio de Janeiro, researchers observed that increased rainfall was followed by a rise in Zika and chikungunya cases roughly three weeks later, the time it takes for standing water to breed a new generation of mosquitoes.
Wildfire Smoke and Respiratory Damage
Wildfire smoke contains fine particulate matter small enough to penetrate deep into the lungs and cross into the bloodstream. Once there, these particles trigger inflammation and oxidative stress, damaging both lung tissue and blood vessels. The health effects extend well beyond the days when smoke is visible. Long-term exposure to wildfire smoke is associated with increased mortality, and the particles can worsen asthma, chronic obstructive pulmonary disease, and cardiovascular conditions in people hundreds of miles from the fire itself.
The scale of air pollution from burning biomass is staggering. In 2021 alone, outdoor fine particulate matter from fossil fuel and biomass burning contributed to an estimated 3.33 million deaths globally.
Mental Health After Extreme Weather Events
The psychological toll of extreme weather is substantial and persistent. A meta-analysis of people who experienced home flooding found that within 12 months of the event, roughly 30% met criteria for post-traumatic stress disorder, about 21% for depression, and nearly 20% for anxiety. The PTSD rate is particularly striking. The lifetime prevalence of PTSD in the general population is around 7.8%, meaning flood survivors experienced it at nearly four times the background rate.
These aren’t temporary stress reactions. Flooding destroys homes, displaces families, wipes out savings, and disrupts social networks. The uncertainty of rebuilding, combined with the fear of recurrence, sustains psychological distress long after the water recedes. Similar patterns follow hurricanes, wildfires, and other disasters, though flooding has been studied most extensively.
Who Faces the Greatest Risk
Extreme weather does not affect everyone equally. Several groups face disproportionate danger, often for overlapping reasons.
- Older adults are the most vulnerable to extreme heat. The body’s ability to regulate temperature declines with age, and many older people take medications that interfere with sweating or blood flow. Heat-related mortality among people over 65 has reached record levels.
- Infants and young children cannot regulate their body temperature effectively. Their smaller bodies heat up faster, and they depend entirely on caregivers to move them to cooler environments.
- Pregnant women face elevated risk because the body is already working harder to cool both mother and baby. Overheating and dehydration during pregnancy can contribute to complications.
- Low-income households often lack air conditioning or cannot afford to run it. This isn’t just an inconvenience. The cognitive and sleep research makes clear that people without cooling bear a measurable biological burden during heat events, one that compounds day after day when nighttime temperatures stay elevated.
These vulnerabilities stack. An elderly person living alone in a low-income neighborhood without air conditioning faces risks that are orders of magnitude higher than a younger, wealthier person in the same city during the same heat wave. The gap between those two experiences is one of the central public health challenges of a warming climate.