After only two days of exposure to temperatures like this, the body’s defenses start to break down, and heat prostration strikes.

From Hot Time in the Old Town The Great Heat Wave of 1896 and the Making of Theodore Roosevelt by Edward P. Kohn.  Page 54.

During heat waves, humidity is one of heat’s deadliest accomplices. When a person’s blood is heated above 98.6 degrees Fahrenheit, a body can dissipate the heat in various ways, such as varying the rate of blood circulation, panting, and especially sweating. Sweating cools the body through evaporation, but high humidity retards evaporation, leaving the body unable to cool itself.

As body heat begins to rise, heat-related illnesses and disorders develop. A body’s temperature can rise to 106 degrees in ten or fifteen minutes, but it only takes a rise above 103 degrees to cause hyperthermia, more commonly known as heat stroke. Red, hot, and dry skin, a rapid pulse, throbbing headache, dizziness, nausea, confusion, and unconsciousness are all warning signs. If the body’s temperature is not immediately lowered, namely by placing the victim in a cool bath or shower, permanent disability or death can occur. Even survivors of heatstroke can suffer serious permanent damage, such as loss of independent function and organ failure.

The temperature a body feels when the effects of heat and humidity are combined would later be called the heat index. An 85-degree air temperature with 85 percent humidity will feel like 100 degrees. Small increases in either temperature or humidity will have dramatic effects on the heat index. Only a 5-degree temperature increase will produce a heat index of 118. A 90-degree air temperature with 90 percent humidity will feel like 122 degrees. A temperature of 94 degrees  with 90 percent humidity feels like over 140 degrees. After only two days of exposure to temperatures like this, the body’s defenses start to break down, and heat prostration strikes.

This is what began to occur on Tuesday, as several people in Manhattan and Brooklyn were admitted to hospitals. Although the month began fairly mild, with the official high temperature on Saturday the first of August reaching only 71, temperatures now rose to the high 80s, accompanied by 90 percent humidity.

Based on the official temperatures recorded by the United States Weather Bureau that day, the high temperature for New York City was 87 degrees. With 90 percent humidity this created a heat index of nearly 110 degrees. Yet as would be noted by New Yorkers virtually every day of the heat wave, the official temperatures for the city were recorded high above street level, where a thermometer was free of much of the urban heat island effect and able to catch at least some small amount of breeze. Down on the street, thermometers regularly recorded temperatures ten degrees higher than the official record indicated, while temperatures inside the brick tenements of the Lower East Side easily reached 120 degrees. This would be the general condition for the next ten days.

The elderly are at great risk during heat waves, and they were the first victims in this case. Sixty-five-year-old Annie Kelly fell victim to the heat on the street not far from her home on West Twentieth Street and was taken to New York Hospital. Fifty-nine-year-old Patrick Murray was overcome on the Upper East Side and was taken to Flower Hospital at Sixty-Third Street. Lersen Present, sixty-three, suffered heat stroke downtown on East Broadway.

In addition to the elderly, New York’s many laborers risked serious injury by working and sweating all day in the sun. Even the healthiest could easily fall victim if undertaking strenuous labor  during the heat wave. This was evidenced by an Italian worker named Rolis, who suffered heat stroke and was hospitalized. He was only twenty-two years old.