An article in Tuesday's editions on the use of sodium fluoride to prevent tooth decay reported that "large parts of Fairfax and Prince William counties" do not fluoridate their water. That information was provided by an official of the National Institute for Dental Research. Fairfax County officials said yesterday that 90 percent of the county's drinking water - all but water from wells - is fluoridated. In Prince William County, just over half is fluoridated - that for the 11.6 percent of the county's residents who live in Manassas and for the 40 percent whose water is bought from Fairfax County.

A weekly one-minute mouthwash with mildly fluoridated water to prevent tooth decay was recommended yesterday to the half the nation that does not fluoridate its drinking water.

The National Institute for Dental Research - a unit of the National Institutes of Health - urged schools and health departments to start regular programs as the result of several new demonstrations showing that the rinsing can reduce children's tooth decay by 35 percent, on the average.

Meanwhile, dentists or doctors can prescribe pills to make the required mouthwash to parents who want to have their children use it once a week - or for any adults who want to try it.

Tooth decay is the leading chronic disease of childhood, and in all, costs the nation $5 billion a year, Dr. James P. Carlos, associate director of the government research institute, estimated. Ideally, all decay is preventable, so wider use of fluoride could save the country billions, he said.

The dental institute recommended the rinsings primarily for children partly because they are in school and can be easily reached, and partly because decay usually tapers off after young adulthood.

"But many adults can probably benefit too, just as they benefit by using fluoridated toothpaste," Carlos said. "They just won't benefit as much, and we can't put a specific number on adult benefit."

Most cities, including Washington, fluoridate their dirinking water. But in the Washington area, large parts of Fairfax and Prince William counties do not. And, nationally, Los Angeles, San Diego, San Antonio, Portland, Ore., Phoenix, Birmingham, Newark and Honolulu are among cities that don't.

Some areas, mainly suburban or rural, don't fluoridate because they lack suitable central water treatment plants at which to add the antidecay chemical. But in many communities there has been successful opposition so by people who think fluoridation may cause cancer or other illness or destroy their liberty to drink unmedicated water.

"There is absolutely nothing to the claims that fluoridation causes any illness - those claims have been throughly refuted by the National Cancer Institute," Carlos said.

Where water is fluoridated, decay is reduced by 50 to 65 percent by the early teens for youths who have drunk the water since birth, according to Carlos, and by less for those who drank it part of that time.

Periodic application of a fluoride solution or gel by dentists typically cuts childhood decay by 35 percent, but it costly, he said. But only half the population goes to a dentist each year.

Use of fluoride toothpast cuts childhood decay by 15 to 20 percent, Carlos estimated. "And we think over 80 percent of the population now uses it," he added.

But the weekly rinsing can effectively be added to daily fluoride brushing, he said. Children in 17 federal projects demonstrating the rinsing included both fluoride toothpaste users and nonusers.

For two years, 75,000 children through sixth grade in Alabama, California, Colorado, Connecticut, Guam, Massachusetts, Michigan, Missouri, Montana, New York, Ohio, Texas and Wisconsin - and Charles City County, Va. - have rinsed for a minute a week with a 0.2 percent sodium fluoride solution.

Various groups have so far shown reductions in decay from 46 percent to nothing, for the 35 percent average benefit.

"If everyone got enough fluoride and got rid of sugary nacks and drinks and chewing gum between meals, decay in this country would approach zero," Carlos said. "I say 'between meals' because you can tolerate sugar ocassionally during the day wihtout damage. It's the repeated use that does the harm."

He also advises thorough enough brushing to remove plaque or tartar deposits on teeth, but "you should be taught how to do it by your dentist."