Last weekend America celebrated Memorial Day. Usually, this is comprised of barbecues, family gatherings, parties and paying respect to those who have fallen while serving our country. It's normally a day that unites us all, coast to coast. However, Chicago was not celebrating.

The Windy City suffered a very, very bloody Memorial Day weekend. Police Superintendent Garry McCarthy confirmed that 11 people were shot and killed, and more than 40 were wounded in shootings across the city. Standing by his side at the news conference was Chicago Mayor Rahm Emanuel.

McCarthy insisted that the majority of the shootings in Chicago were gang-related and retaliatory. But, Chicago was experiencing a record setting heat wave over the weekend. With a high of 95 degrees, it was the city's hottest Memorial Day since 1871. Could external factors, including the sweltering heat, have been to blame?

Psychological stress is cumulative. It builds over time, growing in strength, causing frustration, anger, feelings of powerlessness and the inability to relax. While we know divorce, the death of a loved one, and the loss of a job are very real psychological stressors, so are physical illness and environmental stress.

There are three environment stressors that have been found to increase aggression: (1) Temperature (2) Crowding and (3) Noise. Stress is cumulative. It builds over time, growing in strength, causing frustration, anger, feelings of powerlessness and the inability to relax. Since all large cities are already crowded and noisy, heat becomes the wild card.

There have been several studies on crowding, from subjects in cities to housing projects to rooms. Where there is overcrowding, there will be not only increased aggression, but also depression. In other words, as living space shrinks, tempers, aggression and depression are likely to follow.

In Crowding Stress, by Peter Csermely, it is noted that stress increases in both animals and people when crowding occurs. Interestingly, it doesn’t matter if a space is small or large. Limitation of personal space was the triggering factor, leading to feelings of confinement, frustration and irritability. Not only that, but in dense populations, not only was aggressive behavior greater, but immunities to illnesses decreased. Infections, ulcers and certain cancers were more common AND Csermely states, "in human populations crowding stress is suggested as an important factor in the development of increased urban insanity/schizophrenia."

Studies on noise and its effect on social behavior are also telling. It's not that noise in and of itself causes aggression, but studies illustrate if you are already irritable, angry or annoyed, uncontrolled noise that cannot be escaped can be the tipping point for violence. An article in the Washington Post, Noise Pollution Takes Toll on Health and Happiness, suggests that noise may well activate stress hormone release. It should be noted that schools located near airports have significantly higher numbers of children with high blood pressure than children in quieter neighborhoods.

Heat is another example of environmental stress and this is even more exaggerated in large cities, which develop into urban heat islands. The temperature in a city with a million or more people can be 5˚F higher than the surrounding area. This adds to pollution, greenhouse gas emissions and heat related illnesses, which includes stress.

Sweltering temperatures put physical and psychological demands on the body, which increases irritability and aggression. Also, in physical terms, extreme heat affects body chemistry, via electrolyte imbalance. Electrolytes in the body are sodium, potassium, chloride, calcium and phosphate, and they are maintained in a strict range for optimal functioning. In hot weather we lose electrolytes through sweat and evaporation, and this imbalance can alter moods, increase anxiety and decrease our ability to deal with other stressors.

There is, of course, no way to definitively blame the heat for the 40 shootings in Chicago last weekend. But without a doubt there is certainly the possibility it played a role. The population of our planet continues to gravitate towards large cities. Only 3 percent of people lived in urban areas in the 1800; by 1900 it was 14 percent. In 1950 it had grown to 30 percent, and in 2008 it was split 50/50. It is projected that by 2050, a whopping 70 percent of the world's population will live in an urban areas.

As this urban shift continues, environmental stressors will come under more scrutiny as a cause for not only violence, but a host of psychological and physical ailments.

About the Author

Dr. Dale Archer

Dale Archer, M.D., is a clinical psychiatrist and the author of Better Than Normal: How What Makes You Different Can Make You Exceptional.

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