Many people do not realize how deadly a heat wave can be. In contrast to the visible, destructive, and violent nature of floods, hurricanes, and tornadoes, a heat wave is a “silent killer”. In 1995 alone, 1021 Americans perished in heat waves, including 633 in Illinois and 57 in Missouri.

Many people do not realize how deadly a heat wave can be. In contrast to the visible, destructive, and violent nature of floods, hurricanes, and tornadoes, a heat wave is a “silent killer”. In 1995 alone, 1021 Americans perished in heat waves, including 633 in Illinois and 57 in Missouri.

The Centers for Disease Control and Prevention (CDC) reports that for the period 1979 - 2002, excessive heat exposure due to weather conditions caused 4780 deaths in the United States. That produces an average of 208 deaths a year from excessive heat. That is more that double the current 30 year average of deaths from flooding (92), the current leader in other weather related deaths.

 

What is a Heat Wave?

A heat wave is a period of excessive heat lasting two days or more that leads to illnesses and other stresses on people with prolonged exposure to these conditions. High humidity, which often accompanies heat in Missouri, can make the effects of heat even more harmful.

While heat related illness and death can occur due to exposure to intense heat in just one afternoon, heat stress on the body has a cumulative effect. Consequently, persistence of a heat wave increases the threat to public health.

 

The Urban Heat Problem

Most heat-related deaths occur in cities. Brick and mortar buildings, asphalt streets, and tar roofs absorb daytime heat and slowly release it at night.

Consequently, temperatures in urban areas can be warmer than rural areas by several degrees both day and night. Some basic comparisons done buy the staff at the NWS St. Louis has found that the temperature in the City of St. Louis often averages about 2 - 5 degrees higher than the temperature at Lambert St. Louis International Airport. higher This is commonly called the urban “heat island” effect.

In addition to the burden of heat, stagnant conditions often develop during heat waves, with pollutants increasing in concentration near the ground and contributing further to public health problems during heat waves.

Socioeconomic factors also place urban residents under extra risk. Some people in cities do not have air conditioning, while people in high crime areas may be afraid to open their windows or venture out to cooler public buildings.

 

Who is most vulnerable during a heat wave?

The elderly population segment is the most vulnerable to the dangers of heat. Of the 522 deaths that occurred in Chicago during the July 12-16, 1995 heat wave, 371 (73 percent) were age 65 or older.

The elderly suffer due to the diminished ability to perspire. Since the function of perspiration is to provide evaporation, which in turn provides cooling, the elderly have a reduced capacity to release heat from the body.

In addition to the elderly, infants, young children, and people with chronic health problems (especially pre-existing heart disease) or disabilities are more vulnerable to the effects of heat waves. People who are not acclimated to hot weather, overexert themselves, are obese, or use alcohol or drugs (including drugs such as antipsychotics, tranquilizers, antidepressants, certain types of sleeping pills, and drugs for Parkinson’s disease) are at great risk. (Source- Centers for Disease Control and Prevention-Morbidity and Mortality Weekly Report)

 

Measuring the combined effects of heat and humidity

The National Weather Service uses the Heat Index (HI) to compute the “apparent temperature,” which is a measure of how hot it feels to people at a certain combination of temperature and humidity.

The heat index values used in forecasts, advisories, and warnings assume an average size adult, with light clothing, in the shade, with a 5 mile per hour wind. Being in full sun, or in an area with little air movement, can increase the apparent temperature, and thus increase the risk for adverse effects from the heat and humidity.

Winds greater than 5 miles per hour usually enhance evaporative cooling and decrease the apparent temperature and the health threat from the heat.

As noted, the impacts of heat are cumulative over time. The greatest number of heat-induced illnesses and fatalities usually peak two days after the maximum heat index values occurred.

 

Heat Index

The Heat Index (Apparent Temperature) can be found by taking the temperature (number on the left) and relative humidity value (number at the top) and matching them on this table.

For example, a temperature of 90 degrees Fahrenheit and a relative humidity of 45 percent gives you a heat index of 93 degrees.