102

KEY ISSUES

ertain health outcomes are known to be associated with weather and/or climate,

including:illnesses and deaths associated with temperature; extreme precipitation

events;air pollution; water contamination;and diseases carried by mosquitoes,ticks,and

rodents. Because human health is intricately bound to weather and the many complex

natural systems it affects,it is possible that projected climate change will have measurable

impacts,both beneficial and adverse,on health. Projections of the extent and direction of

potential impacts of climate variability and change on health are extremely difficult to

make because of many confounding and poorly understood factors associated with potential health outcomes,population vulnerability, and adaptation. For example,not enough is

yet known about particulate matter to project how levels of this air pollutant might

change in projected future climate scenarios. Basic information on the sensitivity of

human health to aspects of weather and climate is limited,and it is difficult to anticipate

what adaptive measures might be taken in the future to mitigate risks of adverse health

outcomes,such as vaccines or improved use of weather forecasting.

Health outcomes in response to climate change are highly uncertain. Currently available

information suggests that a range of negative health impacts is possible. These have been

the focus of much of the public health research on climate change to date. Some positive

health outcomes,notably reduced cold-weather mortality, are possible,although the balance between increased risk of heat-related illnesses and death and changes in winter illnesses and death cannot yet be confidently assessed. At present, much of the US population is protected against adverse health outcomes associated with weather and/or climate,

although certain demographic and geographic populations are at greater risk. Adaptation,

primarily through the maintenance and improvement of public health systems and their

responsiveness to changing climate conditions and to identified vulnerable subpopulations should help to protect the US population from adverse health outcomes of projected climate change. The costs,benefits,and availability of resources for such adaptation

must be considered,and further research into key knowledge gaps on the relationships

between climate/weather and health is needed.

Temperature-related Illnesses and Deaths

pisodes of extreme heat already pose a health threat in parts of the US. For example,

following a five-day heat wave in 1995 in which maximum temperatures in Chicago,

• Temperature-related

Illnesses and Deaths

• Health Effects Related to

Extreme Weather Events

• Air Pollution-related

Health Effects

• Water- and Food-borne

Diseases

• Insect-, Tick- and Rodentborne Diseases

HEALTH SECTOR

C

E

Canadian Model Hadley Model

Both models project substantial increases in the July heat index (which combines heat and humidity) over the 21st Century. These maps

show the projected increase in average daily July heat index relative to the present. The largest increases are in the southeastern states,

where the Canadian model projects increases of more than 25 ̊F. For example, a July day in Atlanta that now reaches a heat index of 105 ̊F

would reach a heat index of 115 ̊F in the Hadley model, and 130 ̊F in the Canadian model.

+25oF

+20oF

+15oF

+10o

+5oF

0o

July Heat Index Change - 21st century
Average Summer Mortality Rates 

Attributed to hot weather episodes

Estimates for 2020 and 2050 are based on the Max

6

7

8

9

1964-1991

2020

2050

1964 - 1991

2020

2050

9

8

7

6

5

4

3

2

1

0

Illinois ranged from 93 to 104oF, the number of deaths increased 85% over the

number recorded during the same period of the preceding year. At least 700

excess deaths (deaths in that population beyond those expected for that period

of time) were recorded,most of which were directly attributable to heat.

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 heat index (a measure of temperature and humidity).Some of

these studies adjust for other weather conditions.

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 most sensitive

to projected increases in severity and frequency of heatwaves are likely to be

those in which extremely high temperatures occur only irregularly.

Within heat-sensitive regions,populations in urban areas are most vulnerable to

adverse heat-related health outcomes.Heat indices and heat-related mortality

rates are higher in the urban core than in surrounding areas. Urban areas

remain warmer throughout the night compared to outlying suburban and rural

areas. The absence of nighttime relief from heat for urban residents is a factor

in excessive heat-related deaths. The elderly, young children,the poor, and people who are bedridden,on certain medications,or who have certain underlying

medical conditions are at particular risk.

Overall death rates are higher in winter than in summer, and it is possible that

milder winters could reduce deaths in winter months. However, the relationship between winter weather and mortality is difficult to interpret. For example,many winter deaths are due to respiratory infections such as influenza,and

it is unclear how influenza transmission would be affected by higher winter

temperatures. The net effect on winter mortality from climate change is therefore extremely uncertain.

103

...following a five-day heat

wave in 1995 in which maximum temperatures in Chicago,

Illinois ranged from 93 ̊ to 104 ̊F,

the number of deaths increased

85% over the number recorded

during the same period of the

preceding year.

180

160

140

120

100

80

60

40

20

0

120

115

110

105

100

95

90

85

80

75

HI

Tmax

Heat Related Deaths - Chicago, July 1995

Maximum Temperature and Heat Index

This graph tracks maximum temperature (Tmax), heat index

(HI), and heat-related deaths in Chicago each day from July

11 to 23, 1995. The gray line shows maximum daily temperature, the blue line shows the heat index, and the bars indicate number of deaths for the day.

Deaths due to summer heat are projected to

increase in US cities, in a study using several climate models. Mortality rates (number of

deaths per 100,000 population) are shown for

the Max-Planck Institute model, the results

from which lie roughly in the middle of the

models examined. Because heat-related illness and death appear to be related to temperatures much hotter than those to which

the population is accustomed, cities that

experience extreme heat only infrequently

appear to be at greatest risk. For example,

Philadelphia, New York, Chicago, and St.

Louis have experienced heat waves that

resulted in a large number of heat-related

deaths, while heat-related deaths in Atlanta

and Los Angeles are much lower. In this

study, statistical relationships between heat

waves and increased death rates are constructed for each city based on historical

experience. Deaths under a city's future climate are then projected by applying that

city's projected incidence of extreme heat

waves to the statistical relationship that was

estimated for the city whose present climate

is most similar to the projected future climate

for the city in question. This approach

attempts to represent how people will acclimate to the new average climate they experience. 
104

Heat and heat waves are very likely to increase in severity and frequency with

increasing global average temperatures. The climate scenarios used in this

Assessment show increases in average summer temperatures and relatively larger

increases in average winter temperatures,leading to new record high temperatures,

both in summer and winter. The size of US cities and the proportion of US residents living in them are also projected to increase over the next century,so it is

possible that the population at risk from heat events will increase.

Heat-related illnesses and deaths are largely preventable through behavioral adaptations,including use of air conditioning and increased fluid intake. However, the

degree to which these adaptations might be broadly adopted or economically available to sensitive populations has not been assessed.

Health Effects Related to Extreme Weather Events

njury and death are the direct health impacts most often associated with natural

disasters such as floods and hurricanes.Secondary health effects have also been

observed. These effects are mediated by changes in ecological systems (such as

bacterial and fungal proliferation) and in public health infrastructures (such as the

availability of safe drinking water). The health impacts of extreme weather events

such as floods and storms therefore hinge on the vulnerabilities and recovery

capacities of the natural environment and the local population. There is controversy about the incidence and continuation of significant mental problems,such as

post traumatic stress disorder, following disasters. However, a rise in mental disorders has been observed following several natural disasters in the US.

HEALTH KEY ISSUES

These graphs illustrate the

observed association between

ground-level ozone concentrations and temperature in Atlanta

and New York City (May to

October 1988-1990). The projected higher temperatures

across the US in the 21st century are likely to increase the

occurence of high ozone concentrations, especially since

extremely hot days frequently

have stagnant air circulation

patterns, although this will also

depend on emissions of ozone

precursors and meteorological

factors. Ground-level ozone can

exacerbate respiratory diseases

and cause short-term reductions in lung function.

Maximum Daily Ozone Concentrations

and Maximum Daily Temperature - Atlanta & New York

Atlanta

Changes in precipitation,

temperature, humidity,

salinity, and wind have a

measurable effect on

water quality. In 1993, the

Milwaukee, Wisconsin

drinking water supply

became contaminated by

Cryptosporidium, and as a

result 400,000 people

became ill and 54 died. 

I
105

Increases in heavy precipitation have occurred in the US over the past century.

Future climate scenarios show likely increases in the frequency of extreme precipitation events,including precipitation during hurricanes. This poses an increased risk of

floods and associated health impacts.

Air Pollution-related Health Effects

urrent exposures to air pollution have serious public health consequences.

Ground-level ozone can exacerbate respiratory diseases and cause short-term

reductions in lung function. 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 can include reduced work capacity, aggravation of existing cardiovascular

diseases,effects on breathing, respiratory illnesses,lung irritation,and alterations in

the lung’s defense systems.

The mechanisms by which climate change affects exposures to air pollutants include

1) affecting weather and thereby local and regional pollution concentrations;2)

affecting human-caused emissions,including adaptive responses involving increased

fuel combustion for power generation;3) affecting natural sources of air pollutant

emissions;and 4) changing the distribution and types of airborne allergens. Analyses

show that higher surface air temperatures are conducive to increased concentrations

of ground-level ozone. Since it is very likely that temperatures will increase significantly across the US by the end of the 21st century,this creates a risk of higher concentrations of ground-level ozone,especially because higher temperatures are frequently accompanied by stagnating circulation patterns. However, without knowledge of future emissions in specific places,the success of air pollution policies,and

local and regional meteorological scenarios,more specific predictions of exposure to

air pollutants and health effects cannot be made with confidence.

In addition to affecting exposure to air pollutants,there is some chance that climate

change will play a role in exposure to airborne allergens. Climate change will possibly alter pollen production in some plants and 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 of childhood,is currently very uncertain.

Water- and Food-borne Diseases

xposure to water-borne disease can result from drinking contaminated water, eating seafood from contaminated water, eating fresh produce irrigated or

processed with contaminated water, or from activities such 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).Changes

in precipitation,temperature,humidity,salinity, and wind have a measurable effect on

water quality. In 1993,the Milwaukee,Wisconsin drinking water supply became contaminated by Cryptosporidium, and 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 treatC

E

Injury and death are the

direct health impacts most

often associated with natural

disasters such as floods and

hurricanes. Future climate

scenarios show likely

increases in the frequency of

extreme precipitation events,

including precipitation during

hurricanes. This poses an

increased risk of floods and

associated health impacts.

Since it is very likely that

temperatures will increase

significantly across the US

by the end of the 21st century, this creates a risk of higher concentrations of groundlevel ozone, especially since

higher temperatures are frequently accompanied by

stagnating circulation

patterns.
106

ment 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 Florida during the strong El Niño winter of

1997-1998,heavy precipitation and

runoff greatly elevated the counts of

fecal bacteria and infectious viruses

in local coastal waters. In Gulf Coast

waters, Vibrio vulnificus bacteria are

especially sensitive to water temperature,which dictates their seasonality

and geographic distribution. In addition,toxic red tides proliferate as seawater 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.

Climate changes projected to occur

in the next several decades,in particular the likely increase in extreme

precipitation events,will probably

raise the risk of contamination

events.

Insect-, Tick-, and Rodentborne Diseases 

alaria, yellow fever, dengue

fever, and other diseases transmitted between humans by bloodfeeding insects,ticks,and mites were

once common in the US. Many of

these diseases are no longer present,

mainly because of changes in land

use, agricultural methods,residential

patterns,human behavior, and vector

control. 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 these vector-borne infections

are complex,and the factors that

influence transmission are unique for

each pathogen. Most vector-borne

diseases exhibit a distinct seasonal

HEALTH KEY ISSUES

M

Mexico

Texas:

43 Cases

Border States

50,333 Cases

Combined Wastewater Systems Reported Cases of Dengue 1980-1996

Dengue along the US-Mexico border. Dengue, a mosquito-borne viral disease,

was once common in Texas (where there were an estimated 500,000 cases in

1922), and the mosquito that transmits it remains abundant. The striking contrast in the incidence of dengue in Texas versus three Mexican states that

border Texas (43 cases vs. 50,333) in the period from 1980-1996 provides a

graphic illustration of the importance of factors other than temperature, such

as public health infrastructure, use of air conditioning and window screens, in

the transmission of vector-borne diseases.

Wastewater systems that combine storm drains,

sewage and industrial waste are still used in about

950 communities, mostly in the Northeast and Great

Lakes regions. During rainstorms or spring

snowmelt, when the volume of water being discharged can exceed the capacity of the sewage

treatment system, these systems are designed to

overflow and discharge untreated sewage into surface waters. In 1994, EPA developed a framework to

control such combined-sewer overflows under the

federal Clean Water Act's water discharge permit

program. If combined sewer systems remain in

place and continue to discharge untreated wastewater during storms, they will very likely pose an

increased health risk under projected increases in

intense precipitation events.
107

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

midwestern US,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. The factors underlying this

association are complex and require

more investigation.

Adaptation Strategies

he future vulnerability of the US

population to the health

impacts of climate change largely

depends on the magnitude of the

increase in potential health impacts

and on our capacity to adapt to

potential adverse changes through

legislative,administrative,institutional,technological,educational,and

research-related measures. Examples

include building codes and zoning to

prevent storm or flood damage,

severe weather warning systems,

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 shortand 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. Adaptation is a complex

undertaking,as demonstrated by the

varying degrees of effectiveness of

current efforts to cope with climate

variability. Considerable work still

needs to be done to assess the feasibility (for example,the ability of a

community to incur the costs) and

the effectiveness of alternative adaptive responses,and to develop

improved mechanisms for coping

with climate variability and change.

T

Regional

Weather

Changes

• Heat Waves

• Extreme

 Weather

• Temperature

• Precipitation

CLIMATE

CHANGE

(Natural and

HumanCaused)

Research

Moderating

Influences*

Health Effects

Air Pollution

Levels

Contamination

Pathways

Transmission

Dynamics

Heat-related Illnesses and Deaths

Extreme Weather Events

- related Health Effects

Air Pollution-related

Health Effects

Water- and Food-borne

Diseases

Vector- and Rodentborne Diseases

Adaptation

Measures**

Potential Health Effects of

Climate Variability and Change

*Moderating influences

include non-climate factors

that affect climate-related

health outcomes, such as

population growth and

demographic change, standards of living, access to

health care, improvements

in health care, and public

health infrastructure.

**Adaptation measures

include actions to reduce

risks of adverse health outcomes, such as vaccination

programs, disease surveillance, monitoring, use of

protective technologies,

such as air conditioning,

pesticides, water

filtration/treatment, use of

climate forecasts and development of weather warning

systems, emergency management and disaster preparedness programs, and

public education.