On Heat Wave Trends, Risks, Illness and more
In a July 2024 report, the U.S. Environmental Protection Agency said:
“Global average surface temperature has risen at an average rate of 0.17°(F) per decade since 1901, similar to the rate of warming within the contiguous 48 states. Since the late 1970s, however, the United States has warmed more quickly than the planet as a whole.
"For the contiguous United States, nine of the 10 warmest years on record have occurred since 1998, and 2012 and 2016 were the two warmest years(Figure 1).
"Worldwide, 2023 was the warmest year on record, 2016 was the second warmest, and 2014–2023 was the warmest decade on record since thermometer-based observations began (Figure 2)."
Figure 1. Temperatures in the Contiguous 48 States, 1901–2023
Figure 2. Temperatures Worldwide, 1901–2023
The Importance of Understanding the Heat Index
The heat index plays a crucial role in public health by providing a more accurate representation of how environmental conditions affect the human body.
As the heat index rises, so does the risk of various heat-related illnesses, ranging from mild heat cramps to potentially fatal heat stroke. This relationship is particularly significant because high humidity impairs the body's natural cooling mechanism of sweat evaporation, leading to an increased core temperature even when air temperatures alone might not seem extreme.
Vulnerable populations, such as the elderly, young children, and those with pre-existing health conditions, are especially susceptible to these effects.
Figure 3 presents heat index levels and potential effects on the body.
Figure 3: Heat Index - Effect on the body
Figure 4 presents the relationship of temperature and relative humidity to heat index levels.
Figure 4: Heat Index Matrix: Temperature and Relative Humidity
Heat- related Illnesses, according to the Center for Disease Control
Heat stroke: The most serious heat-related illness. It occurs when the body can no longer control its temperature: the body’s temperature rises rapidly, the sweating mechanism fails, and the body is unable to cool down. When heat stroke occurs, the body temperature can rise to 106°F or higher within 10 to 15 minutes. Heat stroke can cause permanent disability or death if the person does not receive emergency treatment.
Heat exhaustion: It is the body’s response to an excessive loss of water and salt, usually through excessive sweating. Heat exhaustion is most likely to affect: the elderly, people with high blood pressure, those working in a hot environment. Symptoms of heat exhaustion include: headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating, elevated body temperature, decreased urine output
Rhabdomyolysis (rhabdo): Associated with heat stress and prolonged physical exertion. Rhabdo causes the rapid breakdown, rupture, and death of muscle. When muscle tissue dies, electrolytes and large proteins are released into the bloodstream. This can cause irregular heart rhythms, seizures, and damage to the kidneys.
Factors that Increase Heat Illness Risk
Environmental factors that can increase risk include exposure to high temperatures, high humidity, direct sunlight, and too much strenuous activity.
Personal risk factors include 1) age (infants, young children, and older adults, are more at risk), 2) health conditions such as heart disease, high blood pressure, and diabetes, 3) medications such as diuretics and certain heart medications, 4) obesity, 5) dehydration, 6) alcohol use, 7) drug use such as cocaine and amphetamines, 8) sugary drinks.