Businesses will have up to four years instead of two to provide safer equipment and working conditions for video display terminal users, under a compromise Mayor Art Agnos has agreed to sign, his aides say.

What was hailed as the nation's toughest VDT law won final approval last week from the city Board of Supervisors, but Agnos wouldn't commit to signing it until he talked to business and labor groups about its impact and cost.After a weekend meeting with Agnos, the two sides agreed Saturday to several amendments, one extending the compliance period from two years to as many as four years, the mayor's aides said.

The compromise, which Agnos plans to sign Thursday, must go back to the supervisors for approval.

"This is an example of what happens when both sides come to the table. And we now have something that both sides can live with," said Claude Everhart, deputy mayor for government operations, who mediated the negotiations.

The compromise measure still calls for frequent work breaks, proper lighting, anti-glare screens, adjustable computer furniture and equipment to make VDT use safer.

Among the problems associated with regular work at a computer keyboard are nerve damage to the wrists and fingers, back and neck strain, eye strain, sleep disturbances, chronic headaches, nausea and fatigue.

Labor leaders hope the VDT law will be a model for other cities and counties in California and for the state and other states nationwide.

Business groups have consistently opposed efforts to pass such legislation because of its expense and because some doubt VDT use causes health problems.

City budget analysts estimate businesses will spend as much as $76.5 million and the city as much as $6 million to comply with the law.

At Saturday's eight-hour meeting, business and labor leaders agreed to implement the law in three stages.

First, any new equipment purchased one year or more after the law is adopted must conform to the new standards.

Within 30 months, businesses must bring most workstations into conformance with VDT safety standards.

If doing so costs more than $250, those businesses will have an additional 18 months to comply.

The law applies to companies with 15 or more employees.