Following the Baker administration's new proposal to get kids back into classrooms full-time before the school year ends, many people have voiced concern that the move is not safe, especially in older school buildings across the state. GBH Morning Edition host Joe Mathieu spoke with Joseph Allen, the Director of Harvard's Health Buildings Program and Associate Professor at the Chan School of Public Health, to learn more about how schools and other buildings can improve their ventilation and filtration to get more people back indoors safely. The transcript below has been edited for clarity.

Joe Mathieu: Have you been advising the administration on the return to school?

Joseph Allen: I've been advising just about everybody schools, universities, biotech, finance, homeless shelters, police departments, jury trials, the prisons, you name it and stressing the importance of better ventilation and better filtration for everywhere we spend time indoors.

Mathieu: Well, help me understand what the governor was saying there. You open a window a half an inch, he said, it would allow for four times [the] air circulation. Is that what he means by "four turns?"

Allen: Yeah, let me take a big step back first and say we need holistic risk reduction measures in place in schools. First and foremost, we need universal masking with good masks. Ventilation and filtration are also a key strategy. My team has been recommending we get four to six air changes per hour indoors through any combination of better ventilation or filtration. Put some numbers on this, [the] typical home gets half an air change per hour. A school, if it's meeting minimum standards, gets about three air changes per hour. So we want to see this increase. We think it's critically important, and that'll help reduce the threat from far-field transmission, or airborne transmission, beyond six feet.

Mathieu: A lot of school buildings are old [and] a lot of them are different. Some of them have windows, for instance, that as we've seen, don't open. Teachers have put videos up showing painted-over windows and so forth. What do you really need to accomplish that air change? Is he right by cracking a window a couple of inches? Do we need fans in these rooms? Is there more to it?

Listen: "Actually, it can be pretty simple. It doesn't take multimillion dollar fixes."

Allen: Actually, it can be pretty simple. It doesn't take multimillion dollar fixes. And you can do this if you have operable windows, opening up the window even a couple inches. Depending on the airflow and the weather conditions outdoors, you can get two, four, six or even 12 air changes per hour. We also like supplementing that with portable air cleaners with HEPA filters not so dependent on the weather conditions or if you have a mechanical ventilation system, bringing in more outdoor air. Again, I want to take this back to say there's always something you can do. If you have an old building with a rundown mechanical system, you can use these low-cost portable air cleaners as a simple strategy. If you have windows that don't open, you can do the same thing. But ideally, if we have operable windows, you open them up even a couple of inches [and] you can put a fan in that window to facilitate the transfer of air. So I haven't been recommending advanced air cleaning technologies for schools. It can actually be done simply and very cost effectively.

Mathieu: How about social distancing? This is all against the backdrop of masking and social distancing. That's how all of this works together. We're learning this morning that three feet is apparently enough for school children versus six feet for adults in a professional workplace setting. Can you help us understand why?

Allen: Sure. This goes back to an op-ed I wrote with colleague Dr. Sarah Bleich from November or December in The Washington Post arguing that six-foot distancing was one of the key barriers to getting kids back into school. And let's remember that the costs to kids being out of school are escalating rapidly. We can't just think about risk in the classroom, we also have to think about risk outside the classroom. We know how to keep teachers and adults safe, so we have a holistic set of approaches: masking, ventilation, filtration and distancing is part of this. However, we have to realize that kids are different than adults. We recommend kid-to-kid interactions stay at three feet, adult-to-adult stay at six feet, and adults-to-kids stay at three to six feet. I think that's a reasonable solution [and] it's supported by the scientific evidence. Take, for example, what happens in hospitals where they have followed the playbook on keeping health care workers safe good masking, hand hygiene, ventilation and filtration in an environment that's high risk and they can't do physical distancing.

Mathieu: Professor, you wrote in that op-ed in The Washington Post, "six feet is not a magical cutoff." But what does that mean for the rest of us and how should teachers, then, be conducting themselves with young people in the classroom?

Allen: So I recommend the teacher increase distance as much as possible. Definitely get those windows open [and] bring in more outside air. One of the best things adults can do is to wear better masks. All masks are not created equal. A cloth mask might get you 50 percent filtration or removal efficiency, a good surgical mask 70 to 80 percent. But you upgrade to a KF94, KN95 [or] N95, now you're at 95 percent. And remember, with everyone wearing a mask, even if it's everyone in a surgical mask, it's 70 percent, the combined efficiency is greater than 90 percent. That's before any distancing and before the benefits of ventilation and filtration between the masks. So there's a lot we can do and this is now proven. Like I said, the playbook is really well known in all environments. We know how to keep people safe indoors.

Mathieu: A couple of dumb questions, Professor: does ceiling height matter? And how about the plexiglass and some of the plastic sheeting that we're seeing, whether it's in a workplace, in a restaurant. Is it making a difference?

Allen: So on ceiling height, you get a benefit in a large volume room. For example, the four to six air changes for our target, we recommend it for classrooms [with] typical ceiling heights. In a large volume space because of thermal plumes and the way our breath moves indoors, it actually goes up and the large volume helps to dilute it even more. So you get a benefit in a large volume space. Plexiglass [is] not really recommended at all. In fact, not recommended simply because our respiratory aerosols we breathe are small enough and light enough that they just travel around plexiglass. The key thing is source control that's with the mask and then you want good ventilation to dilute any contaminants in the air or good filtration to clean them out of the air. Plexiglass isn't really doing much.

Mathieu: So some of us may have a false sense of security when we walk into a business and you see the glass or in some cases shower curtains set up around tables?

Allen: Yeah, I'd put that under the category of hygiene theater. Looks good, maybe feels reassuring. That said, I think there are some locations where plexiglass makes sense. For example, a cashier in the grocery store, someone who might be facing a couple of hundred people per hour passing by. It's just one extra layer of protection from ballistic droplets, like spit. So that'll stop that, but the aerosols still go around the plexiglass.