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Brian Lehrer: Brian in WNYC, when it comes to reopening schools and just that anything else in the COVID era, Americans are hearing more and more references to the V-word, ventilation. This is an airborne virus. What are the best practices around keeping windows open when you're indoors? When do we need something stronger ventilation equipment in order to keep the virus particles moving along? How effective are different kinds of ventilation machines when it comes to mitigating the risk of infection? How do you tell an HVAC from a HEPA? Anyway, with me now is Dr. Linsey Marr, the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech University. Dr. Marr, thanks so much for coming on with us. Welcome to WNYC.

Dr. Linsey Marr: Hi, Brian. Thanks for having me.

Brian: Can you, as a civil engineering professor, do a little ventilation one oh one for our listeners to start out? What exactly does the word refer to?

Dr. Marr: Ventilation refers to the movement of air through a room or a building. It can be natural as if you open the windows, and even if the windows are closed the air is slowly moving through cracks and other leaky spots in the room. There's also mechanical ventilation. If you have some kind of fan system or HVAC, which stands for heating ventilation and air conditioning system. That moves air through the building too.

Brian: A big headline story right now, just about everywhere, it's about ventilation in schools. This has being a major factor for a lot of parents when it comes to sending their children back into the classrooms, and teachers for whether they'll feel safe. What should parents and school officials consider when thinking about how to use ventilation in order to slow the spread of COVID-19?

Dr. Marr: We're looking to bring in as much outdoor air as possible and to remove indoor air as quickly as possible to keep any virus that might be in the air from building up inside. One of the very simple things you can do is to open windows. You can put a fan in the window to speed things up, but opening windows and doors will definitely help [unintelligible 00:02:32] the ventilation in a classroom.

Another thing you can do is to look at the HVAC system and make sure it is set with the dampers adjusted and any settings to use as much outdoor air as possible compared to recirculated air. They use a lot of recirculated air because it cuts down on energy costs. At this point in time, given our concerns about transmission of COVID-19, we should be using as much outdoor air as possible.

Brian: What are the main questions you would advise parents or teachers to ask the school administrators as they try to assess whether a particular building or classroom is as well ventilated as it could be?

Dr. Marr: You could ask, what is the air exchange rate? That will tell you how often the air has changed over in a house, that might be every one or two hours. In a public building, we are looking for numbers more like six times per hour. That's true in newer buildings, but in older ones, it may not be. If you open windows, you can achieve a much higher air exchange rate.

Another question to ask is, are they upgrading filters in their HVAC system? Because filters come in different qualities, and you can change to better ones, that will remove more virus from the air. A third question to ask is whether they are using air purifiers in rooms, where they cannot increase the ventilation because the air purifier can remove virus from the air.

Brian: Listeners, we can take some ventilation questions for Dr. Linsey Marr, professor of Civil and Environmental Engineering at Virginia Tech, 646-435-7280, or you can tweet a question at Brian. I'll be looking at my Twitter, but 646-435-7280. If you want to do it on the phone, 646-435-7280. You've used the term HVAC a couple of times, and we hear the terms HVAC and HEPA, when talking about ventilation. For listeners who aren't familiar, can you tell us the difference between the two?

Dr. Marr: HVAC stands for heating ventilation and air conditioning system. That may include a fan or blower. It pushes air through vents that are in your ceilings, or floors, or maybe your walls. It can also provide heat in the wintertime if you have some kind of forced air unit, or it can provide you with cooler air during the summer. HEPA, H-E-P-A stands for high-efficiency particulate air. It refers to a type of filtration. Most commonly you would see a HEPA filter in, for example, vacuum cleaners or a standalone air purifier.

Most HVAC systems are not equipped to handle HEPA filters. They can handle other types of filters, but not HEPA. The HEPA filters are very dense, and it's harder to push air through them. You have to have a special type of air handling system that can accommodate those.

Brian: Is there a sort of banned minimum filtration system that you know of to be effective against spreading COVID-19? Whether you need to break that separately into the HVAC category and the HEPA category, I'm not sure. I don't know if you want to ever mention product names or models, but what should people be looking for?

Dr. Marr: Yes, there's in the HVAC system, those almost always have some kind of filter. Often, it's a rectangle, it has a cardboard frame maybe, and then some kind of fabric, or woven filter material. Those come in different ratings. They're usually referred to as MERV ratings, minimum efficient reporting value. Anyway, those numbers run from 1 all the way up to 14 or above.

A typical system might have MERV seven or eight, which could remove maybe half of the viruses of the droplets and aerosols that we're concerned about. If you go up to MERV 13, then you're removing at least 80% of those. You have to be sure that your HVAC system can handle that because it requires more fan or blower capacity to push air through that filter. Then, for a standalone HEPA air purifying unit, you should be looking at the clean air delivery rate, and looking for something that is a higher number, because that indicates that the purifier is running more air through the HEPA filter. It's cleaning more air.

Brian: This is so helpful already. We're going to take a short break. Then, we're going to take some of the listeners' questions that are coming in. You are probably not surprised that our 10 lines filled up like that when we put out the phone number. Your question is about ventilation right after this.

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Brian on WNYC as we are talking about ventilation and the COVID era with Dr. Linsey Marr, the Charles P. Lunsford professor of Civil and Environmental Engineering at Virginia Tech. Pastor Francis in Central Jersey, you're on WNYC. Hello.

Pastor Francis: Hey. Thank you for taking my call. Here's my question. I am very much involved in the church here. It's a semi-rural situation. We're really in a conundrum. We don't know what exactly to do. Our church was built in the late '90s, and it was still fascinating to build airtight buildings. Our windows don't open. We have a very efficient HVAC system, but we don't know what to do.

Should we be running the air conditioning while people are in the church? Is it better to have the air moving around, or is it better to freeze the place first, get it down, get the temperature down, and then have people come in with no air moving around at all? We're just trying to figure out what's the best way to protect our people in a building where there are no open windows.

Brian: Dr. Marr.

Dr. Marr: That's a tough one. The option is that, if people must meet, I hope they are wearing masks and practicing distancing. I think it would be better to run the air conditioning unit because you will get at least some filtration. You should check the filter in the system to make sure it's installed correctly and see if you can upgrade it, as I mentioned earlier, to a higher MERV number. Another thing you could do, even if the windows don't open, is to keep the doors open, possibly. Then, finally, consider adding an air purifier that I mentioned earlier. If it's a large room, you might need several of them.

Brian: I presume outdoor mass would be preferable to any of those.

Dr. Marr: Yes, that was on the premise that people are going to meet indoors and there's no other option, but much safer would be to meet outdoors.

Brian: Pastor Francis, good luck. Fanny, in Brooklyn, you're on WNYC. Hi, Fanny.

Fanny: Hi. Thank you for taking my call. I work in a kitchen that is in the basement. There are no windows, and there is no access to outside air. It is in a hotel that was built in the '30s. We do have adequate air conditioning, but I'm not sure about the ventilation, and how safe is it to go back to work.

Dr. Marr: That is going to depend on a lot of factors. How many people are in that space? Are they wearing masks? It sounds good that there is good air conditioning. Again, you might want to try to check with the building managers to make sure they have upgraded the filters in those. It's all about risk reduction at this point.

Brian: Fanny, good luck. Vicky in Brooklyn, a teacher in a Brooklyn classroom, you're on WNYC. Hi, Vicky. Vicky, listen to your phone, not the radio. Hi.

 Vicky: Oh, sorry. Can you hear me?

Brian: Yes.

Vicky: Hi. I'm a teacher in Brooklyn and I work in a school where the HVAC system has been, according to the custodians, broken since it started, and the windows are only hinged windows. They open about three inches at the bottom, so there's not as much air that comes through. How can I make my classroom ventilation better?

Dr. Marr: That's a good question. I think, with those hinged windows, open them as much as you can. Then, you could put a fan in the windows, some of them blowing-- How many windows are there?

Vicky: I teach in five different classrooms, so it depends. Some have eight, some have four, so it depends.

Dr. Marr: What you can do is open all the windows and put a fan blowing out of one or two of the windows at one end, and then that should help pull in air through the other windows. Then, again, if you have the resources to get a portable air purifier, air cleaner, all you should be looking for is HEPA filtration. You don't need UV or ionization or any other fancy things beyond, just the HEPA filter, that will help.

Brian: What about UV? A couple of callers who we won't have time are asking about UV light.

Dr. Marr: UV can be very effective at killing off the virus, but it's also dangerous for humans to be exposed to UV light, skin cancer. It needs to be installed correctly either in the upper part of the room in a way that people are not exposed to it, or in the HVAC system. It can be quite expensive, so you have to have the resources to do it and make sure you have someone knowledgeable about it installing it.

Brian: Chris, a physical therapist, you're on WNYC. Hi, Chris.

Chris: Hi. I'm working with a group of OTs and PTs from New York City schools to try to prevent an unsafe reopening. My question is, how can either building, people, or even better laypeople, us, who are going to be working in the schools, know that a room's ventilation is adequate? We're being told, "Open the windows, put in fans." How do we know when the air exchange rate is actually adequate? Is there a way that we can test that? Is there a way that that can be easily measured?

Dr. Marr: Yes. There is a way if you can get a hold of a carbon dioxide sensor, that can indicate whether the ventilation is sufficient. When people breathe out, they release carbon dioxide into the air. If there's enough ventilation, then that carbon dioxide in the air will not build up past a certain level. We've generally been targeting something around 800 parts per million. Background air, outdoor air contains 400 parts per million. If you go up to 800, maybe 1000, that is an indicator that the room has good ventilation. Now, you can actually go a little higher than that if there is also good filtration in the room, either through a MERV 13 filter and the HVAC system, or a standalone air purifier.

Brian: I hope that helps, Chris. I'm glad we got some calls in a row there about New York City Schools reopening, because it's just so vague for so many people as they try to figure out whether particular schools and particular rooms within schools are safe. We will leave it there with Dr. Linsey Marr, the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech. Is there a website or some good central information where you would refer people for more on this?

Dr. Marr: For schools, there's a really good website called schools.forhealth.org. It is run by the Harvard Healthy Buildings program. They have a nice report on what schools can do and 20 questions you can ask your school. You can follow me on Twitter, the handle is Linsey Marr. The most up-to-date research is appearing on Twitter right now.

Brian: Thank you very much, Dr. Marr. We appreciate it.

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