Fairfax County restaurants with 100 seats or more will be required to establish nonsmoking sections under a bill approved yesterday by a sharply divided county Board of Supervisors.

The board adopted the measure by a 5-to-4 vote, after almost two hours of debate that pitted residents and representatives of the American Lung Association against the Northern Virginia Restaurant Association and county restaurant owners.

The ordinance, which goes into effect March 1, excludes lounges and other areas where food is not served. The penalty for violating the law is a fine of $25 a day.

The District already has banned smoking in public gathering places but its law does not cover restaurants. The Montgomery County Council has banned smoking in public and commercial places but it excluded restaurants after the owners promised to voluntarily provide nonsmoking sections.

Supervisor Thomas M. Davis III (R-Mason), who first suggested the Fairfax ordinance in October, said his legislation is not an infringement on anyone's rights.

"This is not an antismoking ordinance," he said. "But for the first time we are recognizing the rights of nonsmokers."

Board Chairman John F. Herrity, a Republican who quit smoking in January after his third heart attack, denounced the measure as an example of excessive government regulation.

"I don't know where government can end in terms of how it's going to regulate the atmosphere," said Herrity. "I happen to believe that . . . no smoking is the way to fly. But I don't believe I should impose that on other people."

Lobbying hard against the bill was the Northern Virginia Restaurant Association. Jim Wordsworth, who represented the organization, said the restaurant business is in a slump and that the law's requirements would put a financial strain on many of those businesses. "Why does it appear that the restaurant is being singled out as the villain in these things?" he asked.

Davis countered that the ordinance was not "unduly burdensome to the restaurant industry." He said it would require only that restaurant cordon off an area with plants or dividers for nonsmokers, not to build a separate dining room.

Supporting the measure was Herbert Rosenblum, representing the American Lung Association. He said cigarette and cigar smoke is just as harmful to nonsmokers as to smokers. The Fairfax County Medical Society also stressed the importance of separating smokers and nonsmokers, saying those inhaling smoke suffer from eye irritation, headaches, coughing and allergies.

Supervisor Nancy K. Falck (R-Dranesville), the only smoker on the board, opposed the measure and suggested that restaurant owners install more powerful ventilation systems.

Voting for the measure were: Davis, Board Vice Chairman Martha V. Pennino (D-Centreville) and Supervisors Elaine N. McConnell (R-Springfield), Audrey Moore (D-Annandale) and James M. Scott (D-Providence).

Opposing the measure were Herrity, Falck and Supervisors Joseph Alexander (D-Lee) and T. Farrell Egge (R-Mount Vernon).