The Changing Climate and Human Health

In this paper, RMC’s Intelligence & Analysis Division will examine some of the ways in which the changing climate has impacted human health. The hazards discussed in this paper will be extreme heat, wildfires, insect-borne disease, and drought. While there are many other environmental factors that impact human health, these four were chosen to highlight the interrelated nature of hazards tied to increasing global temperatures. Analysis of both the history, current status, and likely future of these hazards will provide insight into the importance of these environmental factors on human health.

Extreme Heat
Extreme temperatures associated with heat waves and extreme heat events can impact human health in several ways. Extremely high temperatures can compromise the body’s natural ability to regulate internal temperatures. Illnesses such as heatstroke, heat exhaustion, heat cramps, and hyperthermia are the result of high internal temperatures. Chronic conditions such as respiratory disease, cardiovascular disease, diabetes-related conditions, and cerebrovascular disease can be exacerbated by extreme heat. Most individuals can adapt biologically and physically to incremental increases in average normal temperatures. However, children, older adults, and pregnant women are less able to regulate their body temperature, making them more susceptible to the adverse effects of extreme heat. Athletes as well as outdoor and manual laborers are more likely to experience heat injury due to the amount of time they spend outdoors exposed to high temperatures, and the level of exertion expected of them.1

The effects of extreme heat on human health are more apparent now due to advances in modern technology. Evidence suggests that although there is an increase in climate-related hazards including extreme heat, there is a decline in global vulnerability. Advances in technology and the availability of information mean that mankind is better prepared for environmental disasters than ever before. With respect to extreme heat, the ability to go or remain indoors with air conditioning, education on proper hydration, the recognition of signs of heat injury, and modern medical treatment of heat injury help reduce the effects of extreme heat on human health. While these mitigation actions may be readily available in developed countries, less developed countries may have greater challenges related to infrastructure, water availability, and medical care.2,3,4

Heat waves are projected to become longer and hotter in the future, increasing the need for adaptation and mitigation measures. Effects of extreme heat should be considered when planning future urban design and development. Urban environments can create heat islands due to the amount of land covered by concrete and asphalt. This heat island effect causes cities to be an average of 1-7°F warmer during the daytime. City development plans that increase the amount of tree canopies and greens space can help mitigate heat island effects. Localities should have heat action plans that include essential services such as fresh water, air conditioning, electricity, and healthcare. Healthcare systems should be designed to handle a potential increase in heat injury casualties. Emerging research is also beginning to link the increase in extreme heat events with observed declines in mental health. Rising temperatures appear to coincide with societal declines in cardiovascular health, homelessness, and drug abuse as well. Considering that an estimated 60% of the world’s population is expected to be living in cities by 2030, urban planning will become more important.4,5,6

Large-scale wildfires are becoming more frequent. The U.S. has experienced a yearly average of 70,072 wildfires burning an average of 7 million acres since the year 2000. That is more than double the yearly average of 3.3 million acres burned from an average of 78,600 wildfires per year in the 1990’s. The U.S. is not alone. In 2020, Australia and Russia saw one of the worst wildfire seasons ever recorded in those countries. European and African nations are grappling with the problem as well. Extreme heat events and drought reduce moisture and increase the amount of dry vegetation, the fuel that drives these larger wildfires.7,8,9

Wildfire smoke contains a mix of solid and liquid particles suspended in the air known as particle pollution. These particles are no larger than one third the diameter of a hair follicle. The miniscule size of these particles allows them to enter and lodge deep in the lungs. Children, the elderly, and those with chronic conditions such as COPD, asthma, bronchitis, heart disease, and diabetes are more likely to have significant adverse effects from breathing wildfire smoke. Studies after California wildfire indicate that children exposed to wildfire smoke exhibited more coughing, wheezing, bronchitis, colds, and were more likely to receive medical treatment for respiratory causes. Particle pollution can trigger asthma attacks, heart attacks and strokes which can be deadly. The tiny particles in wildfire smoke can stay suspended in the air and travel long distances depending on wind conditions. Plumes of smoke from wildfires can rise to 14 miles, well into the stratosphere and spread across the globe. The 2020 Siberia Wildfires spread across the Pacific Ocean and caused air quality issues in Alaska and Washington State.10

As wildfires increase in frequency and size, so will the impacts to human health. Cities in British Columbia experienced particulate levels twenty times higher than would be expected on an average day during recent Canadian wildfires. Studies on the long-term impacts of exposure to wildfire particle pollution are ongoing, but the work suggests that some people may never fully recover after experiencing just one severe wildfire season. This is especially true with children because their lungs are still developing. Researchers are also finding that smoke particles become more toxic the farther they move away from the fire. The particles oxidize over time and convert into highly reactive compounds that have a greater capacity to damage cells and tissue. One study in Greece indicated that toxicity of smoke compounds can double in just a few hours after the smoke was emitted. Wildfire emission mixing with existing air pollution in populated cities further exacerbates the problem.8

Insect-Borne Diseases
Mosquitoes, ticks, fleas, flies, and other insects can spread diseases that range from mild to severe. Some are untreatable with vaccines or medication. Mosquitoes spread malaria, dengue, West Nile virus, yellow fever, Zika, chikungunya, and lymphatic filariasis. They cause more deaths than any other animal worldwide. In 2021, 619,000 people died from malaria and over 247 million others became ill in 84 countries, with nearly 3.2 billion worldwide still at risk. However, West Nile is the most common disease spread by mosquitoes in the continental United States (CONUS), with an average of 2,400 cases each year. Dengue outbreaks occur worldwide (OCONUS), including elsewhere in the Americas, Africa, the Middle East, Asia, and the Pacific Islands. In 2019, the U.S. experienced the highest number of travel-associated dengue cases since 2010. Dengue is still common in some territories of the U.S., including Puerto Rico and the U.S. Virgin Islands. Chikungunya occurs in tropical and subtropical areas. It can cause long-term joint issues and poses a hazard to infants and older adults. In 2023, there was an Chikungunya outbreak in Paraguay of over 115,000 cases. Additionally, lymphatic filariasis is transmitted via repeated mosquito bites over several months. The World Health Organization estimates that 51 million people in 44 countries are infected with the disease.11,12

Tickborne diseases range in severity and symptoms. Lyme disease is transmitted to humans through blacklegged ticks (deer ticks). Anaplasmosis and ehrlichiosis are closely related bacterial diseases that are also transmitted by infected ticks. Babesiosis is a parasite that infects red blood cells and causes flu-like symptoms. It can be fatal in the elderly and the immunocompromised. Rocky Mountain spotted fever is a bacterial disease that can be transmitted via the American dog tick, the Rocky Mountain wood tick, and the brown dog tick. Powassan virus is transmitted to humans via groundhog ticks, squirrel ticks, and deer ticks, which are found primarily in the Northeast and upper Midwest of the U.S. It can also be transmitted via blood transfusions. Other tickborne diseases in the U.S. include southern tick-associated rash illness, tickborne relapsing fever, tularemia, and Powassan virus. However, tick-borne diseases are not only found in CONUS, but worldwide, including. Canada, Europe, Africa, and Asia.13,14

Some fleas carry pathogens, including those that cause the bubonic plague. The pathogens are commonly transmitted in CONUS by ground squirrel fleas and in OCONUS by infected rat fleas. Pathogens are sometimes transmitted by the handling of infected animals. Most CONUS cases occur in rural areas in the West, including Texas, California, and even Hawaii. Flea-borne (murine) typhus can be transmitted via cat fleas, rat fleas, and their feces (“flea dirt”). Cat scratch disease is often transmitted by the Ctenocephalides Felis flea, or, again, through flea feces. Flea-borne parasites, such as tapeworms, can also spread by the accidental ingestion of an infected specimen. Most fleas with public health and veterinary relevance spread via human travel and the movement of livestock, pets, and rodents.15,16

Houseflies are found worldwide. They can transmit over 100 pathogens including bacteria, viruses, fungi, and parasites. The evidence supporting the role of flies in transmitting diseases is mostly circumstantial. There is a correlation between a rise in diarrhea within a geographic area and an increase in the fly population. The pathogens carried by flies depend on the area. In hospitals or on farms, they often carry antimicrobial resistant bacteria and fungi. They may also transmit nosocomial infections, which are illnesses acquired by being hospitalized itself. Flies mechanically transmit pathogens, in which a host transmits an illness without being infected. House flies feed and reproduce in feces, carrion, and other decaying substances. They then carry microorganisms on the surface of their bodies from breeding sites to humans and animals.17,18

Insect-borne illnesses will continue to pose a hazard in various countries and regions throughout the world. The hazard is exacerbated by climate change, human travel, and wildlife migration. There is evidence that temperate regions, such as the United Kingdom, are now receiving invasive mosquito species and their associated illnesses. This short-distance spread has occurred in Western Europe and throughout the Mediterranean basin. Over the past three (3) decades, insect-borne diseases that were once controlled have emerged in new geographic locations, causing new and reemergent outbreaks.19,20

Dehydration is the primary risk associated with droughts. Dehydration occurs when the body uses or loses more water than it takes in, negatively affecting health. This is especially dangerous for young children and older adults. The most common cause in young children is diarrhea and vomiting. Older adults already have a lower volume of water in their bodies, increasing the risk from minor illnesses and certain medications. However, droughts pose other hazards. Some effects are short-term, but others are long-term. These effects can include unsafe drinking water; poor air quality, sanitation, hygiene, food, and nutrition; and diseases, including those carried by mosquitoes from stagnant water.21,22

Reduced precipitation and increased evaporation of surface water can result in less groundwater over time. Many areas within the U.S. use groundwater as their primary water source. Other drought-related factors affect air quality, including airborne toxins originating from cyanobacteria in fresh water, which can become airborne and cause lung irritation. Drought can also limit the agricultural growing seasons and lead to insect and disease infestation in crops. Low crop yields cause food shortages and increase food prices. This can contribute to malnutrition, particularly in populations dependent on subsistence farming. Drought can also affect livestock, which can become dehydrated, malnourished, and diseased, further reducing food availability.22

The frequency and intensity of droughts are expected to increase as evaporation rates and average temperatures continue to rise. Trends in precipitation rates across the globe vary, with some regions projected to see greater rates of precipitation as other regions see a reduction in overall precipitation. Areas with increased precipitation are at risk of flooding, while areas with less precipitation have an increased risk of drought. However, higher temperatures may affect the frequency and magnitude of drought elsewhere due to increased evapotranspiration, which is the combination of evaporation from the ground surface and transpiration by vegetation.23,24

August 2023 was the warmest August in the 1910-2023 National Oceanic and Atmospheric Administration’s records for Africa, North and South America, and Asia. It was also notably dry over parts of the Americas, Africa, Australia, India, the western Mediterranean, and Russia. Climate change is expected to increase the risk of drought conditions worldwide. These consequences are more pronounced for countries in the developing world. Sub-Saharan Africa and South Asia will likely face the greatest risk of more frequent and more intense droughts in the future. As global temperatures rise, those areas will also experience not only dehydration, but the secondary effects of drought, including disease and negative impacts on plants and animals, which can result in food shortages.23,25,26

Weather and climate are closely intertwined with human health. As climate patterns change across the globe, so too will extreme heat, wildfires, insect-borne disease, and drought impacts on humans. As these hazards are all being driven by increases in global temperatures, it is important to keep in mind that average temperatures have been increasing in recent years and are projected to continue to do so. While the severity of the increase cannot be precisely quantified, the observed trends and their impact on health will become continue to become more apparent and impactful. The potential climate-related causes of illnesses and danger to individual human health are imperative to track to enable appropriate mitigation efforts. RMC’s Intelligence & Analysis Division will continue to monitor relevant developments and research in order to inform potential impacts to RMC’s government and commercial clients.


1. National Institute of Health. (n.d.) Temperature-related Death and Illness. NIH. Retrieved from

2. Formetta, G and Feyen, L. (2019, June) Empirical evidence of declining global vulnerability to climate-related hazards. Global Environmental Change Volume 57. Retrieved from

3. Hanna, E and Tait, P. (2015, July 15). Limitations to Thermoregulation and Acclimatization Challenge Human Adaptation to Global Warming. National Library of Medicine. Retrieved from

4. Khatana, S. (2023, May 22). Extreme Heat and Health: Understanding the Scope of the Problem. University of Pennsylvania Leonard Davis Institute of Health Economics. Retrieved from

5. 2023 Global Shifts Colloquium Report. (2023 March 21-22). Living with Extreme Heat: Our Shared Future. University of Pennsylvania Perry World House. Retrieved from

6. Gregory, J., Azarijafari, H. n.d.). Urban Heat Islands. MIT Climate Portal. Retrieved from

7. Hirscchlag, A. (2023, June 7). The long-distance harm to health caused by wildfires. BBC. Retrieved from

8. Moore, A. (2022, August 29). Climate Change is Making Wildfires Worse — Here’s How. North Carolina State University College of Natural Resources. Retrieved from

9. Congressional Research Service. (2023, June 1). Wildfire Statistics. Retrieved from

10. Editorial Staff, American Lung Association. (2016, January 1). How Wildfires Affect Our Health. American Lung Association. Retrieved from

11. U.S. Dept. of Health and Human Services. (2022, May 11). Avoid bug bites. Center for Disease Control and Prevention. Retrieved from

12. U.S. Dept. of Health and Human Services. (2023, August 17). Fighting the world’s deadliest animal. Center for Disease Control and Prevention. Retrieved from

13. Yale Medicine. (n.d.). Tick-borne illnesses. Yale Medicine. Retrieved from

14. John Hopkins Bloomberg School of Public Health. (n.d.). Geography, ticks, and you. John Hopkins Bloomberg School of Public Health. Retrieved from geography-ticks-and-you/.

15. U.S. Dept. of Health and Human Services. (2020, August 13). Fleaborne diseases of the United States. Center for Disease Control and Prevention. Retrieved from diseases.html.

16. European Centre for Disease Prevention and Control. (2021, November 9). Fleas (Siphonaptera) – factsheet for health professionals. European Centre for Disease Prevention and Control. Retrieved from

17. Honarvar, B., Khamesipour, F., Kwenti, T.E., & Lankarani, K.B. (2018, August 22). A systematic review of human pathogens carried by the housefly (Musca domestica L.). BMC Public Health. Retrieved from

18. WebMD Editorial Contributors. (2021, November 27). What is a nosocomial infection? WebMD. Retrieved from

19. Hernandez-Triana, L. (2023, July 6). Exploring the rise of vector-borne diseases. APHA Science Blog. Retrieved from

20. Mack, A. (2016). Global health impacts of vector-borne diseases. National Academies of Science-Engineering-Medicine. Retrieved from

21. Mayo Clinic. (2021, October 14). Dehydration. Mayo Clinic. Retrieved from

22. U.S. Dept. of Health and Human Services. (2020, January 16). Health implications of drought. Center for Disease Control and Prevention. Retrieved from implications.htm.

23. NOAA National Integrated Drought Information System. (n.d.). Historical drought. NOAA National Integrated Drought Information System. Retrieved from historical-drought.

24. USACE Hydrologic Engineering Center. (n.d.). Evapotranspiration. HEC-HMS Users Manual. Retrieved from

25. NOAA National Centers for Environmental Information. (2023, August). August 2023 global drought narrative. Global Drought Information System. Retrieved from access/monitoring/monthly-report/global-drought/202308.

26. Elkouk, A., Lhoussaine, B., Pokhrel, Y, & Satoh, Y. (2022, September 1). Implications of changes in climate and human development on 21st-century global drought risk. Journal of Environmental Management. Retrieved from S0301479722009513?via%3Dihub.