The Fairfax County School Board voted last night to install seat belts in the 246 school buses it will buy this year, but await further safety studies before adding the equipment to future purchases.

In keeping with a recommendation from Superintenent Robert R. Spillane, the board agreed to make use of the belts optional.

The buses, costing a total of $7.5 million, are to be delivered later this year. Fairfax County has the nation's largest school bus fleet, with nearly 1,000 vehicles.

Parent groups sought seat belts on school buses, arguing that they improve safety and set a good example for use of seat belts in cars. But opponents of seat belts, including the school bus industry, contend that although the belts lessen the severity of some injuries, they increase the likelihood of others and can make it difficult to extricate children in the event of an accident.

The county supervisors strongly urged installation of seat belts.

In voting 8 to 0, with two members absent, board members conceded that safety studies are not conclusive and said they would reconsider the issue when purchasing buses next year. The National Transportation Safety Board is conducting a study of the use of seat belts on school buses that is expected to be finished within a few months.

"The jury is still out in terms of the future of our fleet," said School Board member Laura McDowall. "We are talking about spending a great deal of money for something that has not been demonstrated to improve the safety of children on buses."

The cost of the seat belts is estimated at $700 for each bus.

The school system already has seat belts on 70 of its school buses, and used them to conduct a pilot study of safety in the county. The results of that study, Spillane told the School Board last month, show that seat belts do not provide a greater degree of safety, may actually cause more serious injuries in some cases, and that it would be difficult to enforce mandatory seat belt use. He said making use of seat belts optional would free the board from legal liability if the belts cause a more serious injury than would have occurred without use of the belt.