Hot, humid air is blanketing wide areas of the nation this week, and Missouri health experts have been urging people to stay in air-conditioned buildings and take it easy.

Temperatures in much of Missouri are expected to reach well into the upper 90s — and in many cases above 100 degrees — through much of this week. The heat index, calculated from a combination of temperature and humidity, is predicted to climb above 105.

Highs in the 90s were expected from the western Plains to the East Coast.

"People don't realize it but heat is generally the number-one killer" among weather-related causes, said Ben Miller, a meteorologist for the National Weather Service office in the St. Louis area. "We've gone all year without a serious heat wave so we want people to be aware of what to do to keep themselves safe."

"It's just that time of year," he said.

The weather forced the St. Louis Rams inside for daytime workouts this week.

"You don't want anybody overheating and passing out," rookie defensive tackle Keith Jackson said.

The Arkansas Health Department said Monday it had recorded the state's second heat-related death of the year. The agency did not say when the death occurred, identifying the victim only as an elderly person. The state's first heat death of the year came last month.

Last summer, Arkansas had seven heat-related deaths, and in 2005 the toll was 11.

Authorities in parts of Tennessee issued warnings over the weekend and Monday to those with heart or lung disease, older adults and children to avoid prolonged exertion outdoors.

On the East Coast, Tuesday's horse races were canceled at Delaware Park because the temperature was expected to hit the 90s with a heat index above 100, a threat to horses and jockeys, track officials said.

"Safety is far more important than monetary gains that are always secondary," said John F. Wayne, Delaware's thoroughbred racing administrator.