BOSTON (CBS) – Massachusetts Gov. Charlie Baker said businesses who fall in Phase 1 of the state’s coronavirus reopening plan won’t know in advance of the announcement, adding they should study up on global safety standards to prepare in the meantime.

The governor also declined to say if his stay-at-home advisory will expire on May 18.

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Baker made the remarks during his daily press briefing, saying that while he understands companies are eager to get back to work, doing so safely is critical.

“I get the fact that everybody would like everything to be open sooner,” said Baker. “The downside to that – and it’s an important downside – is, you want to do this in a way where you can sustain the opening.”

On Monday, Baker announced the state will begin to reopen with a 4-phased plan, though he did not offer specifics about what industries would be allowed to begin operating in Phase 1. Baker was asked Tuesday when people will know they qualify for Phase 1, the governor said “I think it would probably be on (May) 18.”

“The folks that are most likely to be able to operate successfully on the 18th are going to be many of the folks that are currently operating,” said Baker. “They’re essential businesses, but they are going to have to operate under the statewide guidelines and the industry specific guidelines that will be issued as part of issuing the report on the 18th.”

Baker said the second group will be people who work in ways that do not have a lot of face-to-face interaction with customers.

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“And then as you move into the second and third phases, or intermediate steps along the way, we’ll start to bring the folks who have direct face-to-face contact with customers in, and make sure we do it in a way that gives them the time that they need to actually create the infrastructure that’s necessary to preserve and protect their employees and their customers,” said Baker.

For the time being, Baker said businesses can prepare themselves by looking at global safety standards to see how they think they perform against those.

“Part of the reason we put those out was to make clear that there’s going to be a standard that everybody has to abide by and comply to,” said Baker.

The governor was asked if the state’s stay-at-home advisory will expire on Monday as scheduled.

“I think all of those various issues that people have questions about, we’re going to talk about all of those on Monday,” said Baker.

Greater Boston Chamber of Commerce President Jim Rooney said businesses want more details. “So the more clarity that we can have for the types of businesses that are in each phase, what the restrictions or guidance are to go with each phase, then businesses can begin to plan,” Rooney said.

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The Chamber is recommending a seven to 10 heads up for businesses to begin to reopen.