Extreme cold kills a lot of people in the U.S.

Thanks to the polar vortex, extreme cold weather blankets large sections of the U.S. Does such extreme cold pose a threat to our health? In a word: Yes.

A 2007 study by Olivier Deschenes and Enrico Moretti (recounted by Ezra Klein) found that "the number of annual deaths attributable to cold temperature is 27,940 or 1.3% of total deaths in the US." That is a lot of deaths:

'Extreme cold turns out to be deadlier than extreme heat. Hot weather kills, but digging deep into the data, Deschenes and Moretti find that it mostly kills people who were already close to death. After the heat wave ends, the death rate drops so sharply that it totally offsets the weather-related spike. "The only effect of the weather shock is to change the timing of mortality, but not the number of deaths," they write.

Periods of extreme cold are also associated with an immediate spike in deaths. But unlike extreme heat, there's no offsetting decline in expected mortality in the weeks following cold snaps. The result is that "the cumulative effect of 1 day of extreme cold temperature during a 30-day window is an increase in daily mortality by as much as 10%." In total, the authors calculate, the cold is responsible for more annual deaths than "leukemia, homicide, and chronic liver disease."'

Extreme cold disproportionately affects the low-income (who do not have adequate heat or shelter) and the elderly. It is known that elderly people frequently have body temperatures below the optimal homeostatic temperature of 98.6 °F. In one study of older people living in nursing homes (average age ~80), the average body temperatures ranged from 97.3 °F in the morning to 97.8 °F at night. Presumably, the aging process reduced the ability to maintain proper temperature regulation and slowed metabolic activity which is necessary to keep the body warm. A lowered body temperature would make the elderly more vulnerable to extreme cold.

The official definition of hypothermia is a body temperature below 95 °F. It is divided into three categories: Mild 32–35 °C (90–95 °F); Moderate, 28–32 °C (82–90 °F); and Severe, 20–28 °C (68–82 °F). With severe hypothermia, critical physiological processes will slow (biochemical reactions occur more slowly at lower temperatures) including the heart rate, breathing rate, blood pressure, metabolic activity, and brain activity. Often the patient will become confused and disoriented and may perform irrational actions such as taking off their clothes, which is counterproductive because adequate clothing is the best way to prevent hypothermia.

Interestingly, from a health policy perspective, the overall migration of people from colder states to warmer states has reduced the number of deaths due to extreme cold, and had a significant impact on lifespan in this country. Indeed according to the Deschenes and Moretti study "our estimates indicate that 8%-15% of the gains in longevity experienced by the US population over the past three decades are due to the secular movement toward warmer states in the West and the South, away from the colder states in the North."

Please stay warm.

Figure 1. The Polar Vortex in Chicago.