As it has become clear people can contract COVID-19 by inhaling floating virus particles, there’s new scrutiny of mechanical ventilation and air filtration in schools.

Teachers in communities across the state have held protests in recent weeks, expressing concern about the safety of returning to classrooms during a pandemic. The state has offered limited guidance to districts on how to address the issue, and experts say there's no one answer to the challenge — in part because every school has its own issues.

There are two strategies for reducing the indoor spread of coronavirus — filtering out floating infected particles and bringing more fresh air from outside. Ken Wertz, executive director of the Massachusetts Facilities Administrators Association, estimates about one-fifth of Massachusetts classrooms are heated with radiators that don't do either of those things.

Wertz estimates about 60 percent have heaters with air filters, but the filters are not fine enough to pick up tiny floating coronavirus particles. Wertz said only schools built in the last 15 years or so are likely to be able to handle that level of filtering.

Air systems in Massachusetts schools may also not be properly maintained, especially as school budgets run short. In Boston, a 2016 assessment found three-quarters of school ventilation systems were either “functioning minimally” or in need of replacement.

The Student Opportunity Act, which passed late last year, was expected to provide funds to help address these issues in Massachusetts' lower income school districts. But that plan got put on hold when the economy fell apart.

Scientists at the Harvard T.H. Chan School of Public Health say opening windows can sufficiently change out the air in classrooms. But that will become more of an issue as the weather turns cold.

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