Between Extremes: Health Effects of Heat and Cold
Between Extremes: Health Effects of Heat and Cold
Cold and heat can impair the human body and its physiological processes in innumerable ways, while also interacting with pre-existing conditions and chronic diseases. With both exposures, the primary concern is alteration of the body's core temperature beyond a healthy range.
High body temperature is associated with increased heart and respiratory rates and, at extreme levels, damage to the brain, heart, lungs, kidneys, and liver. Blood vessels dilate near the skin, which can be problematic for those with heart issues, while the kidneys become stressed through a variety of pathways and may fail.
"Heat waves exacerbate all sorts of chronic disease issues," says Jeremy Hess, an emergency medicine physician and associate professor at the University of Washington. "The stresses you see on a population level depend on the underlying disease problems in that population."
Heat stroke is a significant factor in the high mortality rates seen with heat waves. It contributes to a cascade of dangerous physical and mental effects and has historically been fatal in one in four people in which it is diagnosed, Hess says. However, he adds, fatalities seem lower in recent years, probably due to improved recognition and early medical treatment. Clinically speaking, the cutoff for heat stroke is a core temperature of 104°F (40°C); below that, the technical term is heat exhaustion, a similar, though far less severe condition.
One uncertainty around the physiology of heat exposure involves the respiratory system, says Meredith McCormack, a pulmonologist at The Johns Hopkins University. "There is still a lot to learn about the physiologic responses that make individuals with underlying lung disease more susceptible to heat exposure," she says. For instance, does the risk come from a systemic reaction to heat that exacerbates chronic respiratory diseases? Or is the effect of heat mediated by airway hyper-responsiveness that results from breathing hot air? More work is needed to tease apart the mechanisms.
In cold weather, the body can lose heat faster than it is produced, which uses up stored energy and can lead to hypothermia, defined as a core temperature below 95°F (35°C). Low temperatures cause veins and arteries to narrow and blood to become more viscous, increasing cardiac workload and leading to many of the same cardiovascular stresses as heat. "In true hypothermia, this extra cardiac workload is coupled with a host of other concerns, including increased cardiac muscle sensitivity that can lead to dysrhythmias," Hess says. In addition to straining the heart and other organs, impaired blood flow and decreased metabolic activity due to low temperatures can affect the brain, making the victim unable to think clearly or move well.
Hypothermia is most likely at extremely low temperatures, but it's also possible well above freezing if a person becomes chilled from rain, sweat, or immersion in cool water. Hess says many hypothermia diagnoses occur in tandem with other illnesses and environmental exposures; some patients, for example, have systemic infections that disrupt thermoregulation and allow sepsis-related hypothermia to occur even in the summer.
The recent CDC study of U.S. temperature-related deaths found that for heat-related deaths, exposure to heat was the most frequently cited underlying cause of death on the death certificates, followed by heart disease and unintentional injuries. For cold-related deaths, the most frequently cited underlying cause of death was exposure to excessive cold, followed by unintentional injuries and heart disease.
Generally speaking, people most at risk of illness or death from exposure to high or low temperatures include those less able to regulate their body temperature due to age, those with pre-existing conditions or chronic diseases, and users (especially heavy users) of alcohol or drugs. Individual vulnerability to heat and cold has also been found to vary with sex and race.
Yet as much as we know about the physiological impacts of temperature exposures, without proper context there are few clear-cut ways of differentiating them in real time from those of other potential causes, and no clear fingerprints left behind for diagnosis after the fact. Both heat stroke and hypothermia can look like severe infection, Hess says, and afterward symptoms become even more contextual. That's particularly true if the patient is deceased—"It's difficult to ascertain a cause of death when you don't have a core body temperature," notes George Luber, associate director for climate change at the CDC.
Cardiorespiratory outcomes, in particular, are nonspecific and have many other causes and triggers besides heat and cold, says Diana Petitti, an independent consulting epidemiologist. "Therefore," she explains, "whatever excess events there are due to temperature can get lost in the general background level." Often, heat and cold are simply overlooked as factors in deaths. As a result, temperature-related health outcomes, which by many measures already surpass those due to all other weather phenomena, are almost certainly under-reported—potentially by a lot.
(Enlarge Image)
Cold, Heat, and the Human Body
Cold and heat can impair the human body and its physiological processes in innumerable ways, while also interacting with pre-existing conditions and chronic diseases. With both exposures, the primary concern is alteration of the body's core temperature beyond a healthy range.
High body temperature is associated with increased heart and respiratory rates and, at extreme levels, damage to the brain, heart, lungs, kidneys, and liver. Blood vessels dilate near the skin, which can be problematic for those with heart issues, while the kidneys become stressed through a variety of pathways and may fail.
"Heat waves exacerbate all sorts of chronic disease issues," says Jeremy Hess, an emergency medicine physician and associate professor at the University of Washington. "The stresses you see on a population level depend on the underlying disease problems in that population."
Heat stroke is a significant factor in the high mortality rates seen with heat waves. It contributes to a cascade of dangerous physical and mental effects and has historically been fatal in one in four people in which it is diagnosed, Hess says. However, he adds, fatalities seem lower in recent years, probably due to improved recognition and early medical treatment. Clinically speaking, the cutoff for heat stroke is a core temperature of 104°F (40°C); below that, the technical term is heat exhaustion, a similar, though far less severe condition.
One uncertainty around the physiology of heat exposure involves the respiratory system, says Meredith McCormack, a pulmonologist at The Johns Hopkins University. "There is still a lot to learn about the physiologic responses that make individuals with underlying lung disease more susceptible to heat exposure," she says. For instance, does the risk come from a systemic reaction to heat that exacerbates chronic respiratory diseases? Or is the effect of heat mediated by airway hyper-responsiveness that results from breathing hot air? More work is needed to tease apart the mechanisms.
In cold weather, the body can lose heat faster than it is produced, which uses up stored energy and can lead to hypothermia, defined as a core temperature below 95°F (35°C). Low temperatures cause veins and arteries to narrow and blood to become more viscous, increasing cardiac workload and leading to many of the same cardiovascular stresses as heat. "In true hypothermia, this extra cardiac workload is coupled with a host of other concerns, including increased cardiac muscle sensitivity that can lead to dysrhythmias," Hess says. In addition to straining the heart and other organs, impaired blood flow and decreased metabolic activity due to low temperatures can affect the brain, making the victim unable to think clearly or move well.
Hypothermia is most likely at extremely low temperatures, but it's also possible well above freezing if a person becomes chilled from rain, sweat, or immersion in cool water. Hess says many hypothermia diagnoses occur in tandem with other illnesses and environmental exposures; some patients, for example, have systemic infections that disrupt thermoregulation and allow sepsis-related hypothermia to occur even in the summer.
The recent CDC study of U.S. temperature-related deaths found that for heat-related deaths, exposure to heat was the most frequently cited underlying cause of death on the death certificates, followed by heart disease and unintentional injuries. For cold-related deaths, the most frequently cited underlying cause of death was exposure to excessive cold, followed by unintentional injuries and heart disease.
Generally speaking, people most at risk of illness or death from exposure to high or low temperatures include those less able to regulate their body temperature due to age, those with pre-existing conditions or chronic diseases, and users (especially heavy users) of alcohol or drugs. Individual vulnerability to heat and cold has also been found to vary with sex and race.
Yet as much as we know about the physiological impacts of temperature exposures, without proper context there are few clear-cut ways of differentiating them in real time from those of other potential causes, and no clear fingerprints left behind for diagnosis after the fact. Both heat stroke and hypothermia can look like severe infection, Hess says, and afterward symptoms become even more contextual. That's particularly true if the patient is deceased—"It's difficult to ascertain a cause of death when you don't have a core body temperature," notes George Luber, associate director for climate change at the CDC.
Cardiorespiratory outcomes, in particular, are nonspecific and have many other causes and triggers besides heat and cold, says Diana Petitti, an independent consulting epidemiologist. "Therefore," she explains, "whatever excess events there are due to temperature can get lost in the general background level." Often, heat and cold are simply overlooked as factors in deaths. As a result, temperature-related health outcomes, which by many measures already surpass those due to all other weather phenomena, are almost certainly under-reported—potentially by a lot.
(Enlarge Image)
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