A little Shakespeare came to mind during a recent shift in the Boston emergency room where I work.
"Good Mercutio, let's retire," Romeo's cousin Benvolio says. "The day is hot, the Capulets abroad, and, if we meet, we shall not 'scape a brawl."
It was hot in Boston, too, and people were brawling. The steamy summer months always seem to bring more than their fair share of violence.
But the ER was full of more than just brawlers. Heart attacks, strokes, respiratory problems — the heat appeared to make everything worse.
I wasn't the first to notice this effect. In 1938, a statistician named Mary Gover found a surprising association between heat waves and increased mortality from all causes. Only about a quarter of deaths during these periods could be attributed to heatstroke, a dangerous form of heat illness that occurs when temperatures outstrip the body's ability to shed heat.
In heatstroke, proteins begin to unravel once the core temperature exceeds 104 degrees. Enzymes become inert. Cells' ability to produce energy fails near 106, ultimately causing multiple organ failure, shock and death.
Heatstroke is an important cause of heat-related deaths and tends to get the most attention during extreme heat waves. But other diseases are affected by the heat as well.
Gover found that a majority of the excess deaths occurring during heat waves were from "diseases of the heart, cerebral hemorrhage ... and pneumonia." That observation seemed right to me, and subsequent research has shown that she was on to something.
Three September heat waves in Los Angeles — 1939, 1955 and 1963 — were associated with increased total mortality of 271%, 445%, and 172% more than usual. Death from all causes doubled during a heat wave in New York City in August 1975, with heart attacks and strokes accounting for a majority of the excess deaths.
More recently, total mortality rose 26% in Philadelphia during 10 scorching days in July 1993, with mortality from cardiovascular disease nearly doubling. A 2018 analysis of 23 studies confirmed the association between cardiovascular mortality and heat waves. And in 2013, a study of 12 million Medicare patients found a strong association between heat and exacerbation of chronic respiratory diseases.
Other scientists began looking beyond heat waves. British researchers compared mortality rates with average temperatures throughout the year and found that death rates from heart attack, stroke and pneumonia increased steadily with temperatures over 70.
In 2002, another British team found that total mortality in London increased by roughly 3% for every 1 degree Celsius above 21.5 Celsius, or about 70 Fahrenheit. That same year, a team from Johns Hopkins confirmed that total mortality increases linearly above 70 degrees in a study of 11 U.S. cities.
Why would heat have such a profound effect on cardiovascular disease in particular? It may relate to the body's own adaptive response to high temperatures.
When body temperature rises, blood becomes a critical means of ditching heat. Vessels near the skin dilate to increase peripheral blood flow. Heat is circulated from the core to the skin, where sweating helps to transfer heat to the environment.
The heart is the engine that drives this adaptation, and the added stress could prove fatal to a damaged one.
Heat also causes dehydration, which could in turn increase the risk of clotting by concentrating coagulation factors within the blood and by triggering the release of molecules that spur inflammation. Any predisposition to clotting could contribute to a heart attack or stroke.
Heat may also exacerbate mental illness. In 2014, Canadian researchers found that ER visits for mental illness increased 29% during periods of extreme heat in Toronto. Vietnamese scientists demonstrated in 2016 that risk of admission to a mental health facility increased 36% during weeklong heat waves. The following year, a team of Korean researchers estimated that nearly 15% of emergency admissions for mental illness over an 11-year period could be attributed to extreme heat.
And what about violence and aggression? Was Benvolio right to warn Mercutio against meeting any Capulets in the heat?
Associations between heat and violent crime have been noted since official crime statistics became available around 1900. In 1997, researchers used modern statistics to confirm that violent crime occurs more often in warmer years. Scientists used FBI crime data in 2004 to show that crime rose 5% for every 10-degree increase in weekly average temperatures. And a 2013 study found that violent crime increased 1% for every degree above average monthly temperatures in St. Louis.
Field experiments have also supported an association between heat and aggression. In 1986, researchers found that drivers were more likely to use their horns when it was hot outside. And in 1994, police officers exposed to elevated temperatures during firearms training were found to exhibit increased aggression and were more likely to discharge their weapons.
These are worrying trends, especially with temperatures projected to rise ever higher in coming years. Benvolio may have put it best. "For now, these hot days," he says to Mercutio, "is the mad blood stirring."
Clayton Dalton is a resident physician at Massachusetts General Hospital in Boston.
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