Mayor Art Agnos has signed into law the nation's toughest video display terminal safety legislation to cope with what he calls the worker safety issue of the 1990s.

Union leaders Thursday promised they now would turn the battle for regulation of computers in the workplace to the state and federal arenas.The San Francisco measure, which will take effect in 30 days, affects 56,250 San Francisco workers in private businesses that employ more than 15 people. The figure does not include city workers, who are also affected.

The law is intended to reduce the risk of ailments caused by prolonged use of a computer terminal. Medical problems have included eye strain, muscle fatigue and carpal tunnel syndrome, an injury that can incapacitate the hand and that often requires surgery.

Agnos agreed to sign the measure after prodding labor and business leaders to compromise on amendments that will make the ordinance less costly for businesses and extend to four years, from two, the time they would have to fully comply.

"This legislation, and the accompanying amendments, will ensure that San Francisco's workers are protected from painful injuries in a way that won't drive business out of the city," Agnos said.

VDT safety regulation "is the worker safety issue of the 1990s," Agnos said.

Paul Viracelli, a Service Employees International Union spokesman who pushed for the law, said it is estimated the changes mandated by the ordinance will cost private business $14 million over the next 30 months and $19 million in order to fully comply in 42 months.

The cost to the city is expected to be $231,550, he said.

The legislation protects employees in San Francisco who spend more than 50 percent of their work hours in one week working at a VDT.

It requires employers to provide adjustable workstations and chairs; sets minimum design standards such as lighting and use of anti-glare screens; requires 15-minute alternative work breaks after two hours of continuous VDT work, and requires that workers are trained on health and safety concerns associated with VDT use.

The law has attracted national attention because it is the country's only municipal ordinance regulating the use of computer terminals in private business.

A 1988 law enacted by Suffolk County, N.Y., was struck down by a state court, which ruled such regulation was the business of state or federal governments.