Gov. Charlie Baker on Monday unveiled a four-phase strategy to guide the reopening of businesses in Massachusetts.

The plan, which was formulated with the state's reopening advisory board, will be contingent upon critical public health indicators trending downward.

“The goal is to begin the process around May 18, but it will be gradual and the facts on the ground will obviously determine whether we actually hit that goal,” Baker said.

The four reopening phases are designed to build upon one another, beginning with industries that involve little face-to-face interactions in phase one, or “Start.” Phase two, “Cautious,” will involve resuming industries with increased contact “with conditions,” Baker said. Phase three, called “Vigilant” will involve loosening restrictions if health data merits, and phase four will be “A New Normal.”

Reopening businesses, many of which have been either shut down or operating with steep restrictions since late March, will also need to follow mandatory workplace safety standards when the plan begins. The measures involve keeping 6 feet apart, ensuring provisions for regular hand-washing and sanitization of common areas, and establishing protocols for employees who return to work after recovering from a COVID-19 diagnosis. The new standards were formed to reduce the risk of COVID-19 transmission “as employees and customers begin to return to workplaces during the first phase of reopening,” said Lt. Gov. Karen Polito.

Baker said a forthcoming report from the reopening advisory board will detail activities, industries and key public health metrics associated with each phase. That report is scheduled to be submitted to the governor on May 18.

Baker repeatedly stressed caution about the rollout of the reopening plan, noting that Massachusetts is “one of the hardest hit” states amid the global pandemic. “That means we have to be flexible and honest about the fact that the timing and the details of these phases could change depending upon the track the virus takes as we go forward.”