Yesterday, Gov. Charlie Baker announced that wearing a mask outdoors will no longer be required after Friday if you can be socially distanced, following CDC guidelines. The governor also announced a timeline for more expansive re-openings, with most restrictions to be lifted by August 1. Dr. Joe Allen, director of the Healthy Buildings Program at Harvard's T.H. Chan School of Public Health and an expert on air ventilation, joined Joe Mathieu on Morning Edition today to talk about the policy change.

Allen wanted to quantify the risk of outdoor time without masks, so he teamed up Marc Lipsitch, a professor of epidemiology at Harvard's T.H. Chan School of Public Health, and used an online risk calculator to simulate outdoor conditions without a mask.

“When we do this, we max out the lowest level of risks,” Allen said, referring to catching COVID-19. “Meaning, [there's] less than one percent trivial risk of any outdoor interaction, even maskless, as long as you have some distance between people."

Although scientists had been recently calling for an end to the outdoor mask mandate, Allen said that the mandate made sense at the time, given the winter COVID-19 surges in Massachusetts.

“I think it was prudent to be really cautious around mask wearing, even outdoor mask wearing,” Allen said about the original mandate. “But, as we have the majority of adults vaccinated at this point, and [that] keeps getting better — Massachusetts is doing a great job — case counts are going down, hospitalizations and deaths are going down, it's time to start adjusting our controls. This is the first step.”

While the CDC relaxed its recommendations on masks outdoors, it still recommended masks for large gatherings and crowds outside — like, a game at Fenway Park, where Allen said you should still expect to wear a mask this summer.

“If we're shoulder to shoulder with a lot of people in these big crowds [or at] a parade, or a ballgame, I think it makes sense in this transition period to continue to wear that mask,” he said.

The harder questions moving forward will be when to remove indoor mask mandates, and what to do about kids who won’t be vaccinated until at least the fall. Allen has been advising the Baker administration on school re-openings and ventilation.

“When the controls are in place that we've been talking about — good air quality, good hand hygiene, we can keep the risk of transmission in schools very low,” Allen said. “And considering the importance of having kids back in school, this is something that should have happened a long time ago. I'm glad it's happening now.”

Allen has been tracking virus levels in wastewater in Boston, which can give an early warnings of surges and declines. The wastewater data unexpectedly leveled off and plateaued in March, mirroring a decline in cases in the area.

“It's a really nice indicator of what we should expect in terms of cases. The most recent data set that dropped two days ago shows that we came off this plateau,” Allen said. “And again, the wastewater data is starting to decrease again, and hopefully headed towards back to the lows from last summer.”

WATCH: Dr. Joe Allen on measuring COVID-19 in wastewater