Potential Interactions with Human Health

Although the overall susceptibility of Americans to environmental health concerns dropped dramatically during the 20th century, certain health outcomes are still recognized to be associated with the prevailing environmental conditions. These adverse outcomes include illnesses and deaths associated with temperature extremes; storms and other heavy precipitation events; air pollution; water contamination; and diseases carried by mosquitoes, ticks, and rodents. As a result of the potential consequences of these stresses acting individually or in combination, it is possible that projected climate change will have measurable beneficial and adverse impacts on health (see NHAG 2000, 2001).

Adaptation offers the potential to reduce the vulnerability of the U.S. population to adverse health outcomes -- including possible outcomes of projected climate change -- primarily by ensuring strong public health systems, improving their responsiveness to changing weather and climate conditions, and expanding attention given to vulnerable subpopulations. Although the costs, benefits, and availability of resources for such adaptation must be found, and further research into key knowledge gaps on the relationships between climate/weather and health is needed, to the extent that the U.S. population can keep from putting itself at greater risk by where it lives and what it does, the potential impacts of climate change on human health can likely be addressed as a component of efforts to address current vulnerabilities.

Projections of the extent and direction of potential impacts of climate variability and change on health are extremely difficult to make with confidence because of the many confounding and poorly understood factors associated with potential health outcomes. These factors include the sensitivity of human health to aspects of weather and climate, differing vulnerability of various demographic and geographic segments of the population, the international movement of disease vectors, and how effectively prospective problems can be dealt with. For example, uncertainties remain about how climate and associated environmental conditions may change. Even in the absence of improving medical care and treatment, while some positive health outcomes -- notably, reduced cold-weather mortality -- are possible, the balance between increased risk of heat-related illnesses and death and changes in winter illnesses and death cannot yet be confidently assessed. In addition to uncertainties about health outcomes, it is very difficult to anticipate what future adaptive measures (e.g., vaccines, improved use of weather forecasting to further reduce exposure to severe conditions) might be taken to reduce the risks of adverse health outcomes.

Effects on Temperature-Related Illnesses and Deaths

Episodes of extreme heat cause more deaths in the United States than any other category of deaths associated with extreme weather. In one of the most severe examples of such an event, the number of deaths rose by 85 percent during a five-day heat wave in 1995 in which maximum temperatures in Chicago, Illinois, ranged from 34 to 40°C (93 to 104°F) and minimum temperatures were nearly as high. At least 700 excess deaths (deaths in that population beyond those expected for that period) were recorded, most of which were directly attributable to heat.

For particular years, studies in certain urban areas show a strong association between increases in mortality and increases in heat, measured by maximum or minimum daily temperature and by heat index (a measure of temperature and humidity). Over longer periods, determination of trends is often difficult due to the episodic nature of such events and the presence of complicating health conditions, as well as because many areas are taking steps to reduce exposure to extreme heat. Recognizing these complications, no nationwide trend in deaths directly attributed to extreme heat is evident over the past two decades, even though some warming has occurred.

Based on available studies, heat stroke and other health effects associated with exposure to extreme and prolonged heat appear to be related to environmental temperatures above those to which the population is accustomed. Thus, the regions expected to be most sensitive to projected increases in severity and frequency of heat waves are likely to be those in which extremely high temperatures occur only irregularly. Within heat-sensitive regions, experience indicates that populations in urban areas are most vulnerable to adverse heat-related health outcomes. Daily average heat indices and heat-related mortality rates are higher in these urban core areas than in surrounding areas, because urban areas remain warmer throughout the night compared to outlying suburban and rural areas. The absence of nighttime relief from heat for urban residents has been identified as a factor in excessive heat-related deaths. The elderly, young children, the poor, and people who are bedridden, who are on certain medications, or who have certain underlying medical conditions are at particular risk.

Plausible climate scenarios project significant increases in average summer temperatures, leading to new record highs. Model results also indicate that the frequency and severity of heat waves would be very likely to increase along with the increase in average temperatures. The size of U.S. cities and the proportion of U.S. residents living in them are also projected to increase through the 21st century. Because cities tend to retain daytime heat and so are warmer than surrounding areas, climate change is very likely to lead to an increase in the population potentially at risk from heat events. While the potential risk may increase, heat-related illnesses and deaths are largely preventable through behavioral adaptations, including use of air conditioning, increased fluid intake, and community warning and support systems. The degree to which these adaptations can be even more broadly made available and adopted than in the 20th century, especially for sensitive populations, will determine if the long-term trend toward fewer deaths from extreme heat can be maintained.

Death rates not only vary with summertime temperature, but also show a seasonal dependence, with more deaths in winter than in summer. This relationship suggests that the relatively large increases in average winter temperature could reduce deaths in winter months. However, the relationship between winter weather and mortality is not as clear as for summertime extremes. While there should be fewer deaths from shoveling snow and slipping on ice, many winter deaths are due to respiratory infections, such as influenza, and it is not clear how influenza transmission would be affected by higher winter temperatures. As a result, the net effect on winter mortality from milder winters remains uncertain.

Influences on Health Effects Related to Extreme Weather Events

Injury and death also result from natural disasters, such as floods and hurricanes. Such outcomes can result both from direct bodily harm and from secondary influences, such as those mediated by changes in ecological systems (such as bacterial and fungal proliferation) and in public health infrastructures (such as reduced availability of safe drinking water).

Projections of climate change for the 21st century suggest a continuation of the 20th-century trend toward increasing intensity of heavy precipitation events, including precipitation during hurricanes. Such events, in addition to the potential consequences listed above, pose an increased risk of floods and associated health impacts. However, much can be done to prepare for powerful storms and heavy precipitation events, both through community design and through warning systems. As a result of such efforts, the loss of life and the relative amounts of damage have been decreasing. For the future, therefore, the net health impacts of extreme weather events hinge on continuing efforts to reduce societal vulnerabilities. For example, FEMA's Safe Communities program is promoting implementation of stronger building codes and improved warning systems, as well as enhancing the recovery capacities of the natural environment and the local population, which are also being addressed through disaster assistance programs.

Influences on Health Effects Related to Air Pollution

Current exposures to air pollution exceed health-based standards in many parts of the country. Health assessments indicate that ground-level ozone can exacerbate respiratory diseases and cause short-term reductions in lung function. Such studies also indicate that exposure to particulate matter can aggravate existing respiratory and cardiovascular diseases, alter the body's defense systems against foreign materials, damage lung tissue, lead to premature death, and possibly contribute to cancer. Health effects of exposure to carbon monoxide, sulfur dioxide, and nitrogen dioxide have also been related to reduced work capacity, aggravation of existing cardiovascular diseases, effects on breathing, respiratory illnesses, lung irritation, and alterations in the lung's defense systems.

Projected changes in climate would be likely to affect air quality in several ways, some of which are likely to be dealt with by ongoing changes in technology, and some of which can be dealt with, if necessary, through changes in regulations. For example, changes in the weather that affect regional pollution emissions and concentrations can be dealt with by controlling sources of emissions. However, adaptation will be needed in response to changes in natural sources of air pollution that result from changes in weather. Analyses show that hotter, sunnier days tend to increase the formation of ground-level ozone, other conditions being the same. This creates a risk of higher concentrations of ground-level ozone in the future, especially because higher temperatures are frequently accompanied by stagnating circulation patterns. However, more specific projections of exposure to air pollutants cannot be made with confidence without more accurate projections of changes in local and regional weather and projections of the amounts and locations of future emissions, which will in turn be affected by the implementation and success of air pollution control policies designed to ensure air quality. Also, more extensive health-warning systems could help to reduce exposures, decreasing any potential adverse consequences.

In addition to affecting exposure to air pollutants, there is some chance that climate change will play a role in exposure to airborne allergens. For example, it is possible that climate change will alter pollen production in some plants and change the geographic distribution of plant species. Consequently, there is some chance that climate change will affect the timing or duration of seasonal allergies. The impact of pollen and of pollen changes on the occurrence and severity of asthma, the most common chronic disease among children, is currently very uncertain.

Effects on Water- and Food-borne Diseases

In the United States, the incidence of and deaths due to waterborne diseases declined dramatically during the 20th century. While much less frequent or lethal nowadays, exposure to water-borne disease can still result from drinking contaminated water, eating seafood from contaminated water, eating fresh produce irrigated or processed with contaminated water, and participating in such activities as fishing or swimming in contaminated water. Water-borne pathogens of current concern include viruses, bacteria (such as Vibrio vulnificus, a naturally occurring estuarine bacterium responsible for a high percentage of the deaths associated with shellfish consumption), and protozoa (such as Cryptosporidium, associated with gastrointestinal illnesses).

Because changes in precipitation, temperature, humidity, salinity, and wind have a measurable effect on water quality, future changes in climate have the potential to increase exposure to water-borne pathogens. In 1993, for example, Cryptosporidium contaminated the Milwaukee, Wisconsin, drinking-water supply. As a result, 400,000 people became ill. Of the 54 individuals who died, most had compromised immune systems because of HIV infection or other illness. A contributing factor in the contamination, in addition to treatment system malfunctions, was heavy rainfall and runoff that resulted in a decline in the quality of raw surface water arriving at the Milwaukee drinking-water plants.

In another example, during the strong El Niño winter of 1997 -- 98, heavy precipitation and runoff greatly elevated the counts of fecal bacteria and infectious viruses in Florida's coastal waters. In addition, toxic red tides proliferate as sea-water temperatures increase. Reports of marine-related illnesses have risen over the past two and a half decades along the East Coast, in correlation with El Niño events. Therefore, climate changes projected to occur in the next several decades -- in particular, the likely increase in heavy precipitation events -- raise the risk of contamination events.

Effects on Insect-, Tick-, and Rodent-borne Diseases

Malaria, yellow fever, dengue fever, and other diseases transmitted between humans by blood-feeding insects, ticks, and mites were once common in the United States. The incidence of many of these diseases has been significantly reduced, mainly because of changes in land use, agricultural methods, residential patterns, human behavior, vector control, and public health systems. However, diseases that may be transmitted to humans from wild animals continue to circulate in nature in many parts of the country. Humans may become infected with the pathogens that cause these diseases through transmission by insects or ticks (such as Lyme disease, which is tick-borne) or by direct contact with the host animals or their body fluids (such as hantaviruses, which are carried by numerous rodent species and transmitted to humans through contact with rodent urine, droppings, and saliva). The organisms that directly transmit these diseases are known as vectors.

The ecology and transmission dynamics of vector-borne infections are complex, and the factors that influence transmission are unique for each pathogen. Most vector-borne diseases exhibit a distinct seasonal pattern, which clearly suggests that they are weather-sensitive. Rainfall, temperature, and other weather variables affect both vectors and the pathogens they transmit in many ways. For example, epidemics of malaria are associated with rainy periods in some parts of the world, but with drought in others. Higher temperatures may increase or reduce vector survival rate, depending on each specific vector, its behavior, ecology, and many other factors. In some cases, specific weather patterns over several seasons appear to be associated with increased transmission rates. For example, in the Midwest, outbreaks of St. Louis encephalitis (a viral infection of birds that can also infect and cause disease in humans) appear to be associated with the sequence of warm, wet winters, cold springs, and hot, dry summers. Although the potential for such diseases seems likely to increase, both the U.S. National Assessment (NHAG 2000, 2001) and a special report prepared by the National Research Council (NRC 2001b) agree that significant outbreaks of these diseases as a result of climate change are unlikely because of U.S. health and community standards and systems. However, even with actions to limit breeding habitats of mosquitoes and other disease vectors and to carefully monitor for infectious diseases, the continued occurrence of local, isolated incidences of such diseases probably cannot be fully eliminated.

Although the United States has been able to reduce the incidence of such climatically related diseases as dengue and malaria, these diseases continue to extract a heavy toll elsewhere (Figure 6-11). Accordingly, the U.S. government and other governmental and nongovernmental organizations are actively supporting efforts to reduce the incidence and impacts of such diseases. For instance, U.S. agencies and philanthropies are in the forefront of malaria research, including the search for vaccines and genome sequencing of the anopheles mosquitoes and the malaria parasite Plasmodium falciparum. Efforts such as these should help to reduce global vulnerability to malaria and other vector-borne diseases, and need to be considered in global adaptation strategies.

The results from this work will serve the world in the event that human-induced climate change, through whatever mechanism, increases the potential for malaria. This work will also be beneficial for U.S. residents because our nation cannot be isolated from diseases occurring elsewhere in the world. Of significant importance, the potential for disease vectors to spread into the United States via travel and trade is likely to increase just as the natural, cold-winter conditions that have helped to protect U.S. residents are moderating.

Potential Adaptation Options to Ensure Public Health

The future vulnerability of the U.S. population to the health impacts of climate change will largely depend on maintaining -- if not enhancing -- the nation's capacity to adapt to potential adverse changes through legislative, administrative, institutional, technological, educational, and research-related measures. Examples include basic research into climate-sensitive diseases, building codes and zoning to prevent storm or flood damage, severe weather warning systems to allow evacuation, improved disease surveillance and prevention programs, improved sanitation systems, education of health professionals and the public, and research addressing key knowledge gaps in climate -- health relationships.

Many of these adaptive responses are desirable from a public health perspective, irrespective of climate change. For example, reducing air pollution obviously has both short- and long-term health benefits. Improving warning systems for extreme weather events and eliminating existing combined sewer and storm-water drainage systems are other measures that can ameliorate some of the potential adverse impacts of current climate extremes and of the possible impacts of climate change. Improved disease surveillance, prevention systems, and other public health infrastructure at the state and local levels are already needed. Because of this, we expect awareness of the potential health consequences of climate change to allow adaptation to proceed in the normal course of social and economic development.