Fairfax County is considering hefty cutbacks in bus service in the county in order to hold down its share of Metrobus subsidies.

County staff recommendations included these items for consideration by county officials:

A 10 per cent reduction in peak and off-peak weekday service. Peak periods run from 6 a.m. to 9:30 a.m. and 3 p.m. to 6:30 p.m.

Elimination of all weekday evening service. Evening service runs from 8 p.m. to 11:30 p.m.

Elimination of Saturday service on certain routes. Specific routes were not been named.

Elimination of Sunday bus service.

Metro would be required to hold public hearings before most services could be cut back.

It is likely that only a few, rather than all, of the cutbacks would be made, according to county transportation planners. The specific areas under consideration for curtailment are the most expensive for the county to subsidize.

"What we're talking about is trying to put a level on how much the county is going to pay for bus service," said board member Joseph Alexander (D-Lee). "It's a bad concept to reduce bus service when you're trying to promote public transportation in the whole region, but we've got to control our spending (for transportation)."

By reducing bus service, Fairfax County hopes to limit its share of the 1979 Metrobus subsidy to no more than $383,300 above its 1978 share. Metro had estimated that Fairfax County's share in 1979 would be $700,000 more than its 1978 share, iof bus service in the county remained the same.

Fairfax County's share of the Metrobus subsidy for 1978 is about $6.3 million. The county's estimated share for 1979 - assuming some reductions in current bus service - would be about $6.6 million, according to Richard Wilhelm, a management analyst in the county's office of transportation.

County staff is studying where to make the cuts in service. The supervisors probably will consider in January the staff's suggestions on specific routes and hours to cut, Wilhelm said.

Meanwhile, the county also is studying the possibility of contracting with private bus companies to provide bus service during rush hour. Reston provides rush hour bus service through a private company and Fairfax City will do the same beginning Jan. 1 when it drops all Metrobus service.

In another matter, Fairfax County also is considesring exempting the Northern Virginia Mental Health Institute from the county's no smoking ordinance because some of the institute's patients say it "seems to create undue stress for both patients and staff."

Some patients say that no being allowed to smoke interferes with their participatin in group therapy sessions.

More than half of nearly 200 patients at the institute signed a petition asking that the institute be exempted rom the county ban smoking in public places. Institute director Thomas B. Stage, who forwarded the petition to Fairfax Board Chairman John F. Herrity on behalf of the patients, said he could not support it "philosophically or medically."

The state-owned institute, at 3302 Gallows Road in Fairfax County, is enforcing the county's ban on smoking in areas of the building including hallways and general meeting rooms. Certain limited areas, however, are designated for smoking, according to Dr. Elizabeth R. Strawinsky, director of inpatient services at the institute.