Heat, Health & Environment

Heat, Health, & Environment

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Compared to rural areas, cities experience higher rates of heat-related illness and death. The heat island effect is one factor among several that can raise summertime temperatures to levels that pose a threat to public health.

Other factors that contribute to heat-related illness and death in urban areas are pre-existing health conditions, access to air conditioning, population age, and within-season temperature variation.

Ozone pollution, which can form in the presence of heat, affects human health.

Under certain conditions, "excessive heat" also can increase the rate of ground-level ozone formation, or smog, presenting an additional threat to health and ecosystems within and downwind of cities.

While researchers are still studying the extent to which heat islands affect temperature-related mortality in a city, implementing heat island reduction strategies – like installing cool roofs, using cool paving, and planting shade trees and vegetation – can minimize vulnerability among vulnerable populations.

Does Vulnerability to Excessive Heat Vary with Location?

Research suggests that the relationship between extreme temperature and mortality in the U.S. varies by location. According to the Program on Health Effects of Global Environmental Change at Johns Hopkins University (JHU), heat is most likely to increase the risk of mortality in cities at mid-latitudes and high latitudes with significant annual temperature variation.

For example, when Chicago and New York experience unusually hot summertime temperatures, elevated levels of illness and death are predicted. In contrast, parts of the country that are mild to hot year-round have a lower public health risk from excessive heat. JHU research shows that residents of southern cities, such as Miami, tend to be acclimated to hot weather conditions and therefore less vulnerable.

Summertime heat can pose a threat indoors as well as outdoors. Residents in homes that have dark-colored, heat-absorbing rooftops and no air conditioning may be at risk for temperature-related health complications. Poor wall and ceiling insulation are other factors that may contribute to increased heat transfer to the indoors.

Fortunately, building owners and residents can take simple steps to reduce heat islands and exposure to excessive heat.

How Does Excessive Heat Affect Human Health?

Extremely hot weather can result in illness – including physiological disruptions and organ damage – and even death. Excessive heat events, or abrupt and dramatic temperature increases, are particularly dangerous and can result in above-average rates of mortality.

Graphic showing how ozone forms when precursor compunds react in the presence of sunlight and high temperatures.

Ozone forms when precursor compounds react in the presence of sunlight and high temperatures.

In fact, the Centers for Disease Control and Prevention (CDC) says that excessive heat claims more lives in the United States each year than hurricanes, lightning, tornadoes, floods, and earthquakes combined. Between 1979-1998, the CDC estimates that 7,421 deaths resulted from exposure to excessive heat in the U.S.

In 1995, a heat wave in Chicago illustrated why excessive temperature and heat islands are of concern. This episode of unusually hot weather resulted in the deaths of over 700 people.

The good news is that communities around the country are beginning to pursue strategies to address excessive heat events. These measures can help cities predict and take steps to decrease the public's exposure to excessive summertime heat. Some municipalities have gone even further and initiated heat-response programs to coordinate a comprehensive, city-wide plan.

What Health Effects Are Associated with Ozone Exposure?

Exposure to ambient ozone, even at low levels, may trigger a variety of health problems, especially in vulnerable populations such as children, the elderly, and those with pre-existing respiratory disease.

Because wind can carry ozone and its precursors hundreds of miles, even residents far away from urban centers and sources of pollution can be at risk. There are specific health effects associated with ozone exposure. The specific health effects include:

Ozone can irritate lung airways and cause inflammation;
Repeated exposure to ozone pollution for several months may cause permanent lung damage;
Even low-level exposure can result in aggravated asthma, reduced lung capacity, and increased susceptibility to respiratory illnesses; and
Studies have linked hospital admissions and emergency room visits to ground-level ozone exposure.

Ozone pollution can harm urban trees and other vegetation.

How Does Ozone Pollution Affect Ecosystems and the Environment?

Ozone pollution can damage vegetation and ecosystems within and downwind of cities. For instance, ground-level ozone interferes with the ability of plants to grow and store food.