(CBS News) NEW YORK --The country is suffering through the most widespread heat wave of the season.
The combination of heat and humidity made it feel like the temperature was in the triple digits in many big cities: 100 degrees in New York City; 102 in Washington, D.C.; 103 in Baltimore; and 105 in Detroit.
(Map of the United States shows temperatures for major cities on Wednesday, July 17, 2013)
Wednesday was the fourth day of a heat wave in New York City. At St. Luke's-Roosevelt hospital and others, they're starting to see more people who have succumbed to the heat.
CBS News was with an ambulance crew for just two minutes before they got their first heat-related call.
"As an EMT, we really have to watch out for heat stroke, heat exhaustion," said Rocky Robinson, who heads an ambulance crew in Brooklyn. "As a matter of fact, they are dropping like flies even as we talk. My units are very, very busy."
In Philadelphia, where it was the hottest day of the year so far, the city has activated its heat emergency hotline.
St. Luke's-Roosevelt Hospital in New York City is treating patients for dehydration and heat stroke. Dr. Jeffrey Rabrich said they're averaging about a dozen a day. He expects more.
"In this heat wave it's been staying hot at night -- and we've been seeing problems with that, particularly respiratory problems," he said.
"The risk gets higher. Each day of added heat increases the risk we're going to see more people with both heat stroke and the respiratory problems."
(EMT workers with St. Luke's-Roosevelt hospital attend to a man during a brutal heat wave in New York City July 17, 2013)
St. Luke's-Roosevelt Hospital has added EMS units and doctors to help deal with the expected increase in heat-related cases. The heat wave is expected to last until Saturday.
In Chicago, the windy city, there was barely a puff of wind to cool the air. CBS News correspondent Dean Reynolds reports that with temperatures stuck in the nineties for the next few days, air quality is becoming a concern.
"Almost like shooting through milk. It is really thick out here," said a helicopter pilot for WBBM, a CBS News affiliate.
(A child find relief from triple-digit weather in Chicago by running through water on July 17, 2013)
Meteorologists told Reynolds it feels like 100 degrees there.
Eighteen years ago, more than 400 people died in a heat wave in Chicago. A subsequent study of went wrong, by the New England Journal of Medicine, urged health care workers to be more attentive to the sick.
Air quality in nearby Indiana is classified as unhealthy now. As hot as it is Chicago, Reynolds reports, the heat and humidity will make it feel even hotter -- like 105 -- for the next couple of days.