Climate change is projected to harm human health by increasing ground-level ozone and/or particulate matter air pollution in some locations. Ground-level ozone (a key component of smog) is associated with many health problems, such as diminished lung function, increased hospital admissions and emergency room visits for asthma, and increases in premature deaths.
Factors that affect ozone formation include heat, concentrations of precursor chemicals, and methane emissions. Particulate matter concentrations are affected by wildfire emissions and air stagnation episodes, among other factors. By increasing these different factors, climate change is projected to lead to increased concentrations of ozone and particulate matter in some regions. Increases in global temperatures could cause associated increases in premature deaths related to worsened ozone and particle pollution.
Estimates that assume no change in regulatory controls or population characteristics have ranged from 1,000 to 4,300 additional premature deaths nationally per year by 2050 from combined ozone and particle health effects. Less certainty exists about the responses of airborne particles to climate change than the response of ozone. Health-related costs of the current effects of ozone air pollution exceeding national standards have been estimated at $6.5 billion (in 2008 U.S. dollars) nationwide, based on a U.S. assessment of health impacts from ozone levels during 2000–2002.
Asthma prevalence (the percentage of people who have ever been diagnosed with asthma and still have asthma) increased in the U.S. from 7.3% in 2001 to 8.4% in 2010. Asthma visits in primary care settings, emergency room visits, and hospitalizations were all stable from 2001 through 2009. Asthma death rates per 1,000 persons with asthma declined from 2001 through 2009. As pollen exposures increase, patients and their physicians will face increased challenges in maintaining adequate asthma control.
Diseases Carried by Vectors
Climate change effects on the geographical distribution and incidence of vector-borne diseases in other countries where these diseases are already found can also impact North Americans, especially as a result of increasing trade with, and travel to, tropical and subtropical areas. Whether climate change in the U.S. will increase the chances of domestically acquiring diseases such as dengue fever is uncertain due to vector-control efforts and lifestyle factors, such as time spent indoors, that reduce human-insect contact.
Infectious disease transmission is sensitive to local, small-scale differences in weather, human modification of the landscape, the diversity of animal hosts, and human behavior that affects vector-human contact, among other factors. Finer-scale, long-term studies are needed to help quantify the relationships among weather variables, vector range, and vector-borne pathogen occurrence; the consequences of shifting distributions of vectors and pathogens; and the impacts on human behavior. Enhanced vector surveillance and human disease tracking are needed to address these concerns.
Mental Health and Stress-Related Disorders
In addition, some patients with mental illness are especially susceptible to heat. Suicide rates vary with weather, rising with high temperatures, suggesting potential climate change impacts on depression and other mental illnesses. Dementia is a risk factor for hospitalization and death during heat waves. Patients with severe mental illness, such as schizophrenia, are at risk during hot weather because their medications may interfere with temperature regulation or even directly cause hyperthermia. Additional potential mental health impacts, less well understood, include the possible distress associated with environmental degradation and displacement and the anxiety and despair that knowledge of climate change might elicit in some people.
Globally, climate change is expected to threaten food production and certain aspects of food quality, as well as food prices and distribution systems. Many crop yields are predicted to decline because of the combined effects of changes in rainfall, severe weather events, and increasing competition from weeds and pests on crop plants. Livestock and fish production are also projected to decline. Prices are expected to rise in response to declining food production and associated trends such as increasingly expensive petroleum (used for agricultural inputs such as pesticides and fertilizers).
While the U.S. will be less affected than some countries, the nation will not be immune. Health can be affected in several ways. First, Americans with particular dietary patterns, such as Alaska Natives, will confront shortages of key foods. Second, food insecurity increases with rising food prices. In such situations, people cope by turning to nutrient-poor but calorie-rich foods and/or they endure hunger, with consequences ranging from micronutrient malnutrition to obesity. Third, the nutritional value of some foods is projected to decline. Elevated atmospheric CO2 is associated with decreased plant nitrogen concentration, and therefore decreased protein, in many crops, such as barley, sorghum, and soy. The nutrient content of crops is also projected to decline if soil nitrogen levels are suboptimal, with reduced levels of nutrients such as calcium, iron, zinc, vitamins, and sugars; this this effect can be alleviated if sufficient nitrogen is supplied. Fourth, farmers are expected to need to use more herbicides and pesticides because of increased growth of pests and weeds, as well as decreased effectiveness and duration of some chemicals. Farmers, farmworkers, and consumers will be increasingly exposed to these substances and their residues, which can be toxic. These climate change impacts on the nutritional value of food exist within a larger context. Other factors, such as agricultural practices, food distribution systems, and consumer food choices, also play key roles. Adaptation activities can reduce the health-related impacts of some of the anticipated food security challenges.
Food and Waterborne Diarrheal Disease
In general, diarrheal diseases, including salmonellosis and campylobacteriosis, are more common when temperatures are higher, though patterns differ by place and pathogen. Diarrheal diseases have also been found to occur more frequently in conjunction with both unusually high and low precipitation. Sporadic increases in stream flow rates, often preceded by rapid snowmelt and changes in water treatment, have also been shown to precede outbreaks. Risks of waterborne illness and beach closures resulting from changes in the magnitude of recent precipitation (within the previous 24 hours) and in lake temperature are expected to increase in the Great Lakes region as a result of projected climate change.
Precipitation Extremes: Heavy Rainfall, Flooding, and Droughts
In addition to the immediate health hazards associated with extreme precipitation events when flooding occurs, other hazards can often appear once a storm has passed. Elevated waterborne disease outbreaks have been reported in the weeks following heavy rainfall, although other variables may affect these associations. Water intrusion into buildings can result in mold contamination that manifests later, leading to indoor air quality problems. Buildings damaged during hurricanes are especially susceptible to water intrusion. Populations living in damp indoor environments experience increased prevalence of asthma and other upper respiratory tract symptoms, such as coughing and wheezing, as well as lower respiratory tract infections such as pneumonia, respiratory syncytial virus (RSV), and RSV pneumonia.
At the opposite end of precipitation extremes, drought also poses risks to public health and safety. Drought conditions may increase the environmental exposure to a broad set of health hazards including wildfires, dust storms, extreme heat events, flash flooding, degraded water quality, and reduced water quantity. Dust storms associated with drought conditions contribute to degraded air quality due to particulates and have been associated with increased incidence of coccidioidomycosis (valley fever), a fungal pathogen, in Arizona and California.
Extreme heat events have long threatened public health in the United States. Many cities, including St. Louis, Philadelphia, Chicago, and Cincinnati, have suffered dramatic increases in death rates during heat waves. Deaths result from heat stroke and related conditions, but also from cardiovascular disease, respiratory disease, and cerebrovascular disease. Heat waves are also associated with increased hospital admissions for cardiovascular, kidney, and respiratory disorders. Extreme summer heat is increasing in the United States, and climate projections indicate that extreme heat events will be more frequent and intense in coming decades.
Some heat-related sickness and death risks have diminished in recent decades, possibly due to better forecasting, heat-health early warning systems, and/or increased access to air conditioning for the U.S. population. However, extreme heat events remain a cause of preventable death nationwide. Urban heat islands, combined with an aging population and increased urbanization, are projected to increase the vulnerability of urban populations to heat-related health impacts in the future.
Milder winters resulting from a warming climate can reduce illness, injuries, and deaths associated with cold and snow. Vulnerability to winter weather depends on many non-climate factors, including housing, age, and baseline health. While deaths and injuries related to extreme cold events are projected to decline due to climate change, these reductions are not expected to compensate for the increase in heat-related deaths.
Climate change is increasing the vulnerability of many forests to wildfires. Climate change is also projected to increase the frequency of wildfires in certain regions of the United States. Long periods of record high temperatures are associated with droughts that contribute to dry conditions and drive wildfires in some areas. Wildfire smoke contains particulate matter, carbon monoxide, nitrogen oxides, and various volatile organic compounds (which are ozone precursors) and can significantly reduce air quality, both locally and in areas downwind of fires.
Smoke exposure increases respiratory and cardiovascular hospitalizations; emergency department visits; medication dispensations for asthma, bronchitis, chest pain, chronic obstructive pulmonary disease (commonly known by its acronym, COPD), and respiratory infections; and medical visits for lung illnesses. It has been associated with hundreds of thousands of deaths annually, based on an assessment of the global health risks from landscape fire smoke. Future climate change is projected to increase wildfire risks and associated emissions, with harmful impacts on health.