He recommended that those who venture outdoors drink plenty of water, avoid intense exercise, wear light-colored clothing and stay in air-conditioned areas.
"It's just common sense," he said. "You have to take it easy."
The heat index, designed to reflect how hot it seems or feels outside, is calculated from a scale of air temperatures and relative humidity. It measures the amount of physical stress the two weather factors have on an individual's body.
The heat index can range from 64 in lukewarm, dry weather to 151 in sizzling, humid weather.
A temperature of 94 degrees and a heat index close to 99 didn't bother Robert Forrest, 71, of Williamsport on Tuesday.
"This is my kind of weather," said Forrest, who while serving in the U.S. Navy during World War II was stationed in New Guinea, where the temperatures reached 140 degrees. "We didn't wear anything but dungarees ... and life jackets. I guess I got used to it back then."
"I can't do anything that requires exertion," said Paul Beard, 72, of Hagerstown, who has emphysema and breathes with the help of a portable oxygen pump.
Kris Decker, of Hagerstown, was outside riding her bike, which she said is "the only thing that keeps me from suffering from heat-exhaustion."
Because of the heat, Marchita Armstrong, who moved to Hagerstown from Baltimore in May, bought an air conditioner for her home last week. "I thought it would be cooler here in the mountains," she said.
"It must be 200 degrees," said Mark Webb, of Hagerstown, who was kneeling on the sidewalk under the fierce sun repairing a handicapped ramp for a downtown business. "We're sweating to death."
The sweltering weather led several passers-by into the Scoops and Slices ice cream shop and deli at 4 E. Franklin St., raising ice cream and milkshake sales by about 35 percent, said owner Dominic Coluzzi.
"It does make us happy," said store manager Faye Benjamin.