During the summer months, New Yorkers are especially vulnerable to heat-related hazards. On warm summer days, the city can be as much as 10 degrees warmer than its surrounding areas. The city's infrastructure — largely made up of asphalt, concrete and metal — traps the heat. This is known as the "urban heat island" effect.

Heat waves are particularly dangerous for children, seniors, people with cardiovascular disease, and people taking psychotropic and other medications. In July and August of 2006, extreme heat waves gripped New York City, claiming 46 lives and disrupting power throughout pockets of the city. In France, a blistering heat wave in August 2003 left an estimated 15,000 dead, many of whom were seniors. In July 1995, a two-week long Chicago heat wave — which saw the heat index peak at 119°F — claimed 465 lives. More than half of the victims were 75 or older. According to the Centers for Disease Control, social isolation and the inability to care for oneself put a person at greater risk for heat-related illness.

A heat wave's duration plays an important role in how people are affected. Studies show that a significant rise in heat-related illnesses occurs when excessive heat lasts for more than two days. Spending even two hours per day in air-conditioned spaces can significantly reduce the number of heat-related illnesses.