Extreme Heat and Its Ripple Effects on Health in Colorado
When people discuss the ties between extreme heat and health, the conversation usually stops at dehydration and heat stroke. But in a place like Denver and the Colorado Front Range, where temperatures are climbing, wildfire seasons are intensifying, and water systems are already stretched thin, the most significant impacts of extreme heat are often the ones we don’t see coming. In the following article, I’ll discuss the less visible pathways through which extreme heat may be affecting the health of Coloradans. These "indirect effects" ripple outward across entire populations, quietly shaping the health of our communities long before patients arrive at a clinic or emergency department. As a current medical student, I’ve come to accept that the patients I'll spend my career caring for will be living through a very different Colorado than the one my training was built around. Recognizing these connections is becoming an essential part of protecting the health of Colorado's communities in a warming climate.
The Obvious Risks Are Just the Surface
Let's start with what most people already know. Extreme heat causes a spectrum of direct illnesses, ranging from heat rash and heat exhaustion to heat stroke, a life-threatening emergency marked by a core body temperature exceeding 40°C, altered mental status, and the potential for multiorgan failure and death. [1] But these acute conditions truly only represent a fraction of the total health burden attributable to heat. Large epidemiologic studies have found that on the hottest days of the year (those in the top 5% of local warm-season temperatures) emergency department visits for all causes increase by 7.8%, amounting to roughly 24 additional emergency department visits per 100,000 people each day. [1] The conditions driving those visits extend far beyond these classic heat illnesses. Cardiovascular events, respiratory exacerbations, kidney disease, adverse pregnancy outcomes, and even mental health crises all spike during extreme heat.
Air Quality Degradation: Ozone, Smoke, and the Double Hit
Ozone. Denver already faces ongoing challenges with ground-level ozone pollution. Extreme heat accelerates the photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides that generate ozone, a pollutant that triggers asthma exacerbations, worsens COPD, and increases cardiovascular risk. But in Colorado, the problem compounds. Wildfire smoke transports additional VOCs and particulate matter into the Front Range urban corridor. Research from the Colorado Department of Public Health and Environment done in Boulder, Colorado during the 2020 wildfire season demonstrated that both local and long-range smoke events increased carbon monoxide, background ozone, and total VOC concentrations in the urban atmosphere. [2]
Compounding. The result is a "double hit": heat-driven ozone formation layered on top of wildfire smoke carrying fine particulate matter (PM2.5). For health professionals, this means that bad air days during summer heat waves are not just about one pollutant. They represent compound exposures with synergistic health effects, particularly for patients with cardiopulmonary disease.
Water Systems Strain
Pathogens. Extreme heat also stresses water systems in a way that directly affects our health. As temperatures rise, drinking water distribution systems heat up. At temperatures above 40°C, residual disinfectants like chlorine decay faster, creating conditions favorable to thermotolerant opportunistic pathogens like Legionella pneumophila (Legionnaires’ Disease) and Naegleria fowleri (brain-eating amoeba). [3]
Scarcity. At the same time, higher temperatures increase evaporation, reduce snowpack, and alter streamflow, contributing to reduced water availability throughout the Front Range. [4] Reduced water availability affects agricultural output, food prices, and nutritional access, all of which are critical upstream determinants of health, particularly for lower-income communities.
Physical Activity When the Outdoors Becomes Off-Limits
Inactivity. In a state where outdoor recreation is woven into everyday life, it’s disheartening to drive along I-25 and see signs advising residents to "exercise indoors." Rising temperatures and unsafe air quality has increased the number of hours during which air quality conditions exceed safe thresholds for outdoor physical activity. This matters because physical inactivity is itself a major risk factor for cardiovascular disease, diabetes, cancer, cognitive decline, and poor mental health. [5] When heat forces people indoors, or discourages physical activity altogether, it creates a secondary health burden that compounds over weeks and months. The American Heart Association has noted that increasing exposure to nonoptimal temperatures is associated with lower physical activity levels, and has called for guidelines to encourage safe physical activity in the context of the climate crisis, particularly for individuals with cardiovascular disease. [6]
For community health professionals, this creates a difficult dilemma: How can you promote physical activity when the environment itself becomes a barrier? Though the answer isn’t clear, shifting exercise to early morning or evening hours, identifying indoor alternatives, and ensuring access to shaded public areas may be some ways to begin.
The Cooling Paradox
Air Conditioning. When temperatures soar, our instinct is to crank up the air conditioning. Undoubtedly, it works. Globally it has been estimated to reduce heatwave-related mortality by approximately 23% compared with the complete absence of air conditioning, and has averted an estimated 195,000 heat-related deaths among people aged 65 and older in 2019 alone. [7] However, air conditioning is truly a double-edged sword. On hot days in areas with high air conditioning prevalence, cooling can account for more than half of peak electricity demand. When that electricity comes from fossil fuels, it generates CO2 and PM2.5 emissions. An estimated 21,000 deaths globally in 2019 were attributable to PM2.5 from fossil-fuel-powered electricity used for air conditioning. [7]
Open Windows. Knowing that air conditioning may be contributing to these harmful effects, our next move on a hot afternoon may be to open up the windows. But still this paradox persists. Opening the windows to cool off is an invitation inside for the harmful ozone and fine particulate matter that accompany this summer heat. In fact the American Heart Association directly advises against opening your windows during the hot, smoky, ozone-heavy part of the day, and waiting until night when the Front Range cools off and the air clears. [8]
Heat Pumps. The way out of this isn’t to abandon cooling completely, but rather to change how we power it. Heat pumps are essentially two-way air conditioners. In the summer they move heat out of your house, and in the winter they run in reverse to move heat in. That single detail matters, because it means one appliance can replace both the AC unit and the gas furnace. Life cycle analyses comparing heat pumps with conventional natural gas furnaces have found reductions in global warming potential exceeding 85%, with respiratory health impacts reduced by nearly tenfold. [9] The catch, of course, is money. Heat pumps are expensive to install, and older homes often require additional investment in insulation, electrical and/or ductwork upgrades to accommodate heat pumps. Unfortunately, renters and low-income homeowners least able to access or afford these upgrades are often those at greatest risk for the health impacts of heat and air pollution.
Mental Health: The Hidden Casualty
Perhaps the least discussed consequence of extreme heat is its impact on mental health. A large U.S. case-crossover study of over 3.4 million emergency department visits found that days of extreme heat were associated with an 8% increase in ED visits for any mental health condition. [10] The associations were broad, with substance use disorders, anxiety and stress-related disorders, mood disorders, schizophrenia spectrum disorders, and self-harm all increased on extreme heat days.
The mechanisms behind this are likely multifactorial, with factors such as disrupted sleep from hot nights, social isolation when outdoor activities become unsafe, economic stress from energy costs, and direct neurobiological effects of heat and dehydration on mood regulation and impulse control all playing a part. For clinicians and mental health professionals, this pattern of increased demand of services during heat events may be worth paying attention to when anticipating staffing and resource allocation.
Who Bears the Greatest Burden?
The Age Factor. Heat is not distributed evenly, and neither is the ability to escape it. Aging blunts the body’s thermoregulatory response, so it’s unsurprising that older adults sit at the top of nearly every risk list. Layer on the medications that so many older patients take and this picture worsens. Diuretics, beta-blockers, SSRIs, calcium-channel blockers, anticholinergics, and antipsychotics all interfere with either heat dissipation or volume status.
In a study exposing young and older adults to nine hours at 40°C older participants stored 88 kJ more heat over the first three hours and ran core temperatures 0.3°C higher by hour six, with the effect amplified in those with hypertension and type 2 diabetes. Additionally, those same older adults reported no more heat symptoms and less mood disturbance than the younger group, meaning their perception of strain underestimated the strain itself. [11]
Outdoor workers face an overwhelming heat load from exertion stacked on ambient and radiant heat. A case-control study in Maricopa county, Arizona found more than triple the odds of heat-associated death among men in agriculture and more than double among those in construction. [12] The most concerning number though is about timing rather than occupation. 73% of U.S. worker heat deaths from 2011 to 2016 occurred during the first week on the job, before acclimatization which can take up to two weeks to develop and is lost after just a week away. [13]
Pregnant people are another important group that are counseled far too rarely. A systematic review found that each 1°C increase in heat exposure raised the odds of preterm birth by 4%, rising to 26% during heat waves, alongside elevated risks of stillbirth, congenital anomalies, and gestational diabetes. [14]
Income, Race & Class. But most notably, the burden of heat falls on low-income communities and communities of color, and it does so for reasons that have more to do with policy than with weather. Historically redlined neighborhoods remain those with the most pavement and fewest trees. Within a single city, low-income neighborhoods can be up to 5°C hotter than wealthier ones, due in large part to discriminatory mortgage lending that dates back to the 1930s.
But heat exposure is not the same thing as heat risk, and heat becomes dangerous when people lack the capacity to respond. The same temperature can be an inconvenience for one person and a medical emergency for another. Health outcomes depend on access to cooling, stable housing, preventive healthcare, and management of chronic disease. So the disadvantages stack: hotter environment, fewer means of escaping it, more underlying disease, and less access to the care that could have made a disease survivable. Importantly, this is not just a poverty problem. The excess risk in marginalized racial and ethnic communities remains even after accounting for income and air conditioning access. [1]
That is the part worth sitting with as someone entering medicine. These are not background conditions to be documented in a social history. They are the accumulated result of decisions about where to lend, what to pave, and which neighborhoods get shade, and these decisions can be made differently.
What Can We Do? Action Items for This Summer
Individuals. Individual protective measures remain the first line of defense: limit exposure during peak heat, stay hydrated, wear loose and light-colored clothing, and identify a cool place to go if home cooling fails. [15]
Clinicians. For clinicians, the most useful intervention happens before the heat arrives, not during it. Health professionals are positioned to fold heat counseling into routine health education, and this matters most for the patients already at high risk. A practical checklist may include:
· Reviewing the medication list through a heat lens. Flag prescriptions like diuretics, laxatives, beta-blockers, calcium channel blockers, anticholinergics, antihistamines, antipsychotics, lithium, topiramate, SSRIs, and tricyclics, and tell the patient why it matters, rather than just noting it.
· Asking about cooling access directly. Does the home have air conditioning? Does it work? Can the electric bill be paid?
· Screening for isolation and arranging check-ins for patients who live alone.
· Counseling pregnant patients specifically, including work modification and heat awareness during late gestation.
· Talking about air quality alongside heat. Recommend closing windows during peak ozone and smoke hours of the day, opening them at night, and shifting outdoor physical activity to early morning.
Communities. Community-level interventions with evidence of benefit include heat action plans, cooling centers, and organized health checks for isolated individuals. [1] Coordinated efforts like these, which pair early-warning systems with active outreach to at-risk residents, consistently reduce heat-related illness and death, narrowing the disparities that leave older adults and lower-income neighborhoods most exposed. After Montreal implemented a heat action plan in 2004, there were an estimated 2.5 fewer deaths per hot day, with significant reductions in disparities between older and younger residents and between low- and high-socioeconomic-status neighborhoods. [1]
Infrastructure. Longer-term, Colorado communities can invest in green infrastructure, like increased tree canopy, green roofs, and vegetated corridors, which can reduce urban heat island temperatures by 2°C or more. [1] These investments yield benefits across every indirect pathway described in this article. Cooler ambient temperatures curb ozone formation, ease water system demands, reduce wildfire-prone conditions, and lessen physiologic stress on those most at risk.
Extreme heat is not just a weather event, it's a public health challenge that demands the same kind of coordinated, evidence-based response we bring to any other threat to our patients and communities. The ripple effects are real, but so are the tools to blunt them.
Works Cited
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2. Rickly PS, Coggon MM, Aikin KC, et al. Influence of Wildfire on Urban Ozone: An Observationally Constrained Box Modeling Study at a Site in the Colorado Front Range. Environ Sci Technol. 2023;57(3):1257-1267. doi:10.1021/acs.est.2c06157
3. Furst KE, Graham KE, Weisman RJ, Adusei KB. It’s getting hot in here: Effects of heat on temperature, disinfection, and opportunistic pathogens in drinking water distribution systems. Water Research. 2024;260:121913. doi:10.1016/j.watres.2024.121913
4. Aliyari F, Bailey RT, Arabi M. Appraising climate change impacts on future water resources and agricultural productivity in agro-urban river basins. Science of The Total Environment. 2021;788:147717. doi:10.1016/j.scitotenv.2021.147717
5. Romanello M, McGushin A, Di Napoli C, et al. The 2021 report of the Lancet Countdown on health and climate change: code red for a healthy future. The Lancet. 2021;398(10311):1619-1662. doi:10.1016/S0140-6736(21)01787-6
6. Hanneman K, Alahmad B, Ghosh A, et al. Nonoptimal Temperature and Cardiovascular Health: A Scientific Statement From the American Heart Association. Circulation. 2026;153(16). doi:10.1161/CIR.0000000000001419
7. Watts N, Amann M, Arnell N, et al. The 2019 report of The Lancet Countdown on health and climate change: ensuring that the health of a child born today is not defined by a changing climate. The Lancet. 2019;394(10211):1836-1878. doi:10.1016/S0140-6736(19)32596-6
8. Rajagopalan S, Brauer M, Bhatnagar A, et al. Personal-Level Protective Actions Against Particulate Matter Air Pollution Exposure: A Scientific Statement From the American Heart Association. Circulation. 2020;142(23). doi:10.1161/CIR.0000000000000931
9. Addo-Binney B, Agelin-Chaab M, Bamfo E, Koohi-Fayegh S. A comparative life cycle assessment of a cascade heat pump and a natural gas furnace for residential heating purposes. Integrated Environmental Assessment and Management. 2021;18(2):572-580. doi:10.1002/ieam.4494
10. McGarr GW, Meade RD, Notley SR, et al. Physiological responses to 9 hours of heat exposure in young and older adults. Part III: Association with self-reported symptoms and mood state. J Appl Physiol (1985). 2024;136(2):408-420. doi:10.1152/japplphysiol.00740.2023
11. Nori-Sarma A, Sun S, Sun Y, et al. Association Between Ambient Heat and Risk of Emergency Department Visits for Mental Health Among US Adults, 2010 to 2019. JAMA Psychiatry. 2022;79(4):341. doi:10.1001/jamapsychiatry.2021.4369
12. Petitti DB, Harlan SL, Chowell-Puente G, Ruddell D. Occupation and Environmental Heat-Associated Deaths in Maricopa County, Arizona: A Case-Control Study. Nishiura H, ed. PLoS ONE. 2013;8(5):e62596. doi:10.1371/journal.pone.0062596
13. Spector JT, Sack CS, Bonauto DK. Occupational Heat-Related Illness. JAMA. 2025;334(3):267. doi:10.1001/jama.2025.7629
14. Lakhoo DP, Brink N, Radebe L, et al. A systematic review and meta-analysis of heat exposure impacts on maternal, fetal and neonatal health. Nat Med. 2025;31(2):684-694. doi:10.1038/s41591-024-03395-8
15. Sorensen C, Hess J. Treatment and Prevention of Heat-Related Illness. Solomon CG, ed. N Engl J Med. 2022;387(15):1404-1413. doi:10.1056/NEJMcp2210623