Scientists, citizens and citizen scientists — as well as dozens more Islanders on both sides of the Martha’s Vineyard Regional high school fields project — provided emotional testimony before the Martha’s Vineyard Commission on Thursday night, with the development continuing to draw praise and criticism from every corner of the Vineyard, and beyond.

“Unknown” became the word of the evening — as project opponents noted the scientific and safety unknowns regarding synthetic turf, and project proponents remarked on the unknowns for students and athletes if the project isn’t approved.

The public hearing Thursday marked the fifth three-hour session on the high school’s $7 million project to revamp its athletic complex, which includes, as its centerpiece, a synthetic turf field that has garnered considerable controversy throughout the community. The project also includes plans for a new grandstand, and five natural grass fields.

As of Thursday, more than 340 letters have been submitted into the public record, and approximately 80 Islanders have provided public testimony.

“It’s difficult to keep count,” commission Joan Malkin noted at the start of the hearing.

Thursday’s hearing kicked off with a presentation from Dr. Jeff Gearhart, a scientist and research director for the Ecology Center. Mr. Gearhart was brought on to give testimony by The Field Fund, an organization that has advocated for natural grass fields throughout the school’s public hearing and planning process.

Previous toxicology reports have stated that chemicals in synthetic turf plastic, like per and polyfluoroalkyl substances (PFAS), exist, but are present below state or federal risk standards.

In his presentation, Mr. Gearhart did not question the findings of previous toxicology reports, but instead focused on what he said were the substantial unknowns in the scientific community surrounding the chemicals, their impacts, how they breakdown and their life cycles.

“My concern is that what we’ve missed a lot with, in the world of environmental health and environmental chemistry, is what I would call early warnings,” Mr. Gearhart said. “And, those are cases where we don’t entirely understand how a chemical can break down in the environment, and this is just one example.”

Dr. Laura Green, a toxicologist brought on by project advocates, countered that the field would have no negative impact on the aquifer and that septic systems would cause 1,000 to 10,000 times as much PFAS leaching than the proposed field. She added that the chemicals would not break down when exposed to UV light.

“That’s why they are called forever chemicals,” Ms. Green said.

Mr. Gearhart and Brian Massa, a consultant involved with an independent toxicology report conducted by the commission, questioned those claims.

“There’s a lot we don’t know,” Mr. Massa said. “And I wouldn’t necessarily want to jump in saying that it’s great, it’s fine, everything’s great.”

Ron Myrick, who conducted the commission’s toxicology report, did add that the field likely would not impact nearby wells.

The hearing then moved on to public testimony, with a spattering of coaches, athletes, parents, grandparents and Islanders of all stripes weighing in on the project.

Joe Mikos, Christopher Greene and John Wilson, all lacrosse coaches at the high school, advocated for the project, saying that athletes needed the fields to compete and to provide a quality opportunity for Vineyard youth. Mr. Wilson noted that professional sports organizations had issues with grass turf installations, and all said that a quality turf field would be the best option for the health of students.

Tim Creato, a parent, agreed, noting that promoting outdoor opportunity for kids was in its own right an issue of health and safety.

“I see this as a tremendous opportunity that we should not waste,” Mr. Creato said. “Anything you can do to provide things for these kids to do is an asset.”

Gabriel Bellebuono, a former soccer player at the high school, spoke passionately about how his own experience as a student athlete helped get him through high school on the Island. He advocated strongly for the project, saying that the Vineyard’s fields were the worst quality he had seen in his entire soccer career

“Playing soccer was sometimes a reason that I got up in the morning to go to school,” Mr. Bellebuono said. “Our athletes just need a better playing surface.”

Strong opposition to the project was voiced during the hearing as well. Emily Solarazza, a parent, said that the turf field’s environmental impact created its own safety concerns, while Susan Desmaris said that the Island had the resources to properly maintain grass fields, which were more in line with the Island’s character. Jana Berktau said that the ubiquity of turf elsewhere in the state did not mean the Island should follow suit.

“We’re Martha’s Vineyard, we’re different than everyone else,” Ms. Berktau said. “We care about our Island’s unique unspoiled environment. We want to keep it that way.”

Carole Vandal, an Aquinnah Wampanoag elder, also spoke passionately against the project. She said it was vital to protect the health and safety of Island waters, particularly for tribal members who rely on them for sustenance.

“We don’t know enough about where these chemicals will end up,” Ms. Vandal said. “I don’t want our children to look up and say, you could have prevented this.”

After a lengthy testimony, Ms. Vandal was asked by public hearing chairman Doug Sederholm if she had any final concerns.

“We need to consider the unknowns,” she said.

Commissioners continued the public hearing until April 15, which will be the sixth in the project’s public review process. Mr. Sederholm concluded the meeting by saying that there was sound testimony on both sides.

“I have been deeply, deeply impressed by the dialogue I have heard over the course of these five sessions,” Mr. Sederholm said.