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Your hot-weather guide to coronavirus, air conditioning and airflow - Boston.com

Your hot-weather guide to coronavirus, air conditioning and airflow - Boston.com

Your hot-weather guide to coronavirus, air conditioning and airflow - Boston.com
Aug 09, 2020 5 mins, 27 secs

Outdoor

Open Houses

It’s that easy to take for granted

If some escapes from the room, more will find its way back in, whether we open the door or not

“If you are comfortable, you ignore it,” said Wade Conlan, a mechanical engineer who evaluates ventilation systems on behalf of Hanson Professional Services

But like so many little luxuries we once took for granted, our days of blissfully ignoring air may be numbered, because a growing number of scientists are convinced that a significant amount of coronavirus transmission occurs through the air in indoor spaces and that poor ventilation magnifies the risk

But scientists and engineers say that it’s worth trying to understand the basics of how airflow works — in case there is a relatively easy tweak that could keep you a bit safer

When in doubt, open the windows

And remember that outdoor air is good

The precise way that viral particles flow through a room when an infected person talks, sings, exhales or eats is something that scientists are continuing to investigate

If there is one easy-to-understand principle that aerosol scientists and engineers have come to agree on, though, it’s this: The more outdoor air coming into a room, the better for dispersing that cloud of viral particles that might be lingering

And one of the most reliable and cost-effective ways to get outdoor air into a room is to open a window

“If you don’t know if the place is well ventilated, but you have the ability to open a window, I would do it,” said Shelly Miller, a professor of mechanical engineering at the University of Colorado Boulder

That, she said, or get out quickly if you’re swinging by an indoor location with other people in it

The outdoor air that comes in will eventually replace the indoor air, according to Jose-Luis Jimenez, an aerosol scientist at the University of Colorado Boulder

“The more outside air you have, the more you dilute the virus,” said Jimenez, who was among the scientists and engineers who sent a letter that pushed the World Health Organization to acknowledge that airborne transmission of the novel coronavirus is a threat in indoor spaces

If you want to speed up the flow of outdoor air into a room, you could also take a box fan, place it in a window and blast it outward, Jimenez said

When any amount of air leaves, that same amount of air returns — it’s a fixed volume

Therefore, the fan should help pull in the same amount of outdoor air

Some pull in outdoor air

Others simply recirculate indoor air

When it’s sweltering out, air conditioning can be essential not only to help you function but also to avoid heatstroke

But if you are going to spend time in a cooled space with other people, it may be worth understanding a bit more about the cool air you are breathing

— The unit cools both indoor and outdoor air

— The unit cools and recirculates only indoor air

— The unit relies entirely on pulling in outdoor air

Jimenez and other building scientists involved in coronavirus prevention are currently advising owners of businesses and buildings with category one systems to adjust the ratio to pull in more outdoor air, an enterprise that can be costly

Take a casino in Las Vegas, which is kept cool enough to keep people gambling inside while it feels like 120 degrees Fahrenheit outside

Cooling that hot outdoor air will be more expensive than recirculating the already cool inside air

But given that keeping customers healthy is also a priority, more are willing to revisit their approach, Jimenez said

Most window units sitting with their rears facing the outdoors, for example, fall into category two

Instead of pulling in outdoor air, they are dumping heat from the room outdoors, said William Bahnfleth, a professor of architectural engineering at Penn State’s Institutes of Energy and the Environment

If you live alone or with people you’re sure aren’t infectious, those units are fine

But if you give in to throwing that birthday dinner for your parents, or if your teenager has been less than strict about staying home, it’s worth remembering that “any virus that’s present will be mixed in” to the recirculating indoor air, Jimenez said

So if you have to have people over, it may be preferable to revert to rule one: When in doubt, open the windows

But a good filter can be just as effective as pulling in outside air

So, what do you do if you’re stuck with a unit that primarily recirculates indoor air, and it’s unrealistic to open the window

The right filter is just as effective as pulling in outside air, said Dr

ASHRAE, a professional society of air conditioning, heating and refrigerating engineers, recommends MERV 13 and above for filtering out the coronavirus, said Bahnfleth, who leads the group’s epidemic task force

Any HEPA filter is even more efficient than the highest-rated MERV filter, he added, so either should effectively capture coronavirus particles

Lower-rated filters still could be helpful, Conlan said: It’s not that they won’t ever catch smaller particles; they just won’t do it as frequently

Window units are typically designed for comfort, not health, and have even more filter limitations

For those who can afford them — or push their employers or landlords to buy them — a stand-alone HEPA filter is a good option, Bahnfleth said

The key, Jimenez added, is picking one that will filter all the air in the room at least twice an hour

Some of them appear to rely on questionable marketing and science, Jimenez said

There’s no ‘good spot’ in a room

Now that you’re an air expert, it may be tempting to think that you know how to pick the safest position in a restaurant or other indoor space you might find you have a reason to be in

But even experts cannot easily eyeball the lowest-risk location, said Andrew Persily, who oversaw the development of an online tool to estimate exposure to infectious aerosols in rooms and buildings as chief of the Energy & Environment Division at the National Institute of Standards and Technology

“Depending on the airflow pattern and where the aerosols are released, there may be regions in the room that result in higher exposure than others,” he said

It’s also hard to gauge how many is too many people in a given space

After all, it only takes one infected person to get other people sick

If carbon dioxide levels are above 1,000 parts per million, you’d be wise to decrease the number of people in the indoor space, increase the amount of outdoor air or both, she says

Do you see other people

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