Excessive heat and humidity could result in cancellation -- or at least a shortened distance -- for the Herndon 10-mile footrace this Sunday.

That, say race organizers, is one of the differences in this year's race from last year's, which claimed two lives in the August heat and humidity.

Indeed, the changes resulting from the deaths have reduced the field of runners from 650 to 400 -- and doubled the number of medical experts and volunteer observers who will monitor their progress over the course.

"You evaluate every year whether you run again," said Ed Martin, Herndon town manager. "All of us were at a loss, wished we'd never heard of any races. But if your child fails in school, do you send him to school again? You're not going to succeed every time, but we hope we'll never have any problem again."

Besides, Martin added, "The runners still want to participate."

Last year the deaths of Henry J. Kronlage, an IBM engineer, and Patrick Reiley, an Arlington County schoolteacher, stirred a barrage of criticism from sports medicine experts around the country who said the race should never have been run in the prevailing heat and humidity that day.

According to guidelines of the American College of Sports Medicine, races of 10 miles or more should be run before 9 a.m. or after 4 p.m. whenever temperatures exceed 80 degrees.

Last year's race started at 9:15 a.m. when the temperature was 82 degrees, although it climbed to 88 by noon, when most runners reached the finish line.

Reiley, an inexperienced runner, was found in woods 300 yards from the course, sprawled dead of a heart attack. Kronlage, who was determined to beat his time of the year before, had wandered, apparently deliriously, half a mile past the finish line in heavy underbrush, and collapsed of heat stroke.

"Frankly, I'm almost surprised they're going to run it," said Kronlage's widow, Nancy. "I told the children that in deference to their father they should not run in it. Lynne her daughter said she wouldn't even consider the idea."

Marathon organizers, whom Nancy Kronlage continues to blame for her husband's death, were determined that Herndon's young racing tradition would not end. But last year's mishap has brought numerous changes, among them a more cautious examination of the thermometer readings, which this weekend are predicted to climb near 90.

"We've probably gone overboard, if you look at our race compared to some other races," said the head of Herndon's recreation department, Art Anselene. "But we're in a position where we have to."

For one, the time of the race has been shifted to September, when the weather is expected to be cooler and less humid. That move immediately eliminated many high school cross country runners, who are prohibited by athletic sanctions from running races longer than 3 1/2 miles during their racing season.

Also, the statement runners must sign before entering the race now requires that they have completed a 10-kilometer 6-mile race within 60 days previous of the Herndon event.

"We were trying to avoid inexperienced runners coming out and participating in a long-distance race," Anselene said. "They may not have trained as much as required. And we did receive a surprising number of inquiries from runners who said they hadn't run in a 10-kilometer race."

Last year, the Herndon 10-mile course had been changed for the first time to take runners out of town and into the countryside around Dulles Airport to avoid traffic. As a result, runners, sweating under the midday heat, found less shade than they had been used to, and fewer homeowners who douse runners with water. The course is back in town this year. "We'll have better access to people who'll spray runners," Anselene said.

This year, medical personnel will be positioned at the five-, seven-, eight- and nine-mile points to monitor the health of runners. "At practically every turn," Anselene said, "we'll have sombebody stationed to make sure that runners don't get lost."