Your hot weather for coronavirus, air conditioning and airflow

Despite its critical role in our daily lives, air is not something most of us spend a lot of time thinking about. It’s so easy to take for granted. Unlike water, we do not have to fill a bag to consume it. When some escape from the room, more will find their way back, whether we open the door or not.

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

But like so many small luxuries we once took for granted, our days of blissful neglect of air can be counted as a growing number of scientists are convinced that a significant amount of coronavirus transmission through the air takes place in indoor spaces and that poor ventilation the risk.

Not everyone has the ability or resources to make the changes to a home or workplace to improve air circulation. But scientists and engineers say it’s worth trying to understand the basics of how airflow works – in case there’s a relatively easy tweak that could keep you a little safer.

If in doubt, open the windows. And remember that outdoors is good.

The exact way viral particles flow through a room when an infected person talks, sings, exhales or eats is what scientists continue to investigate. Previous case studies have shown that it is complicated. If there is one easy-to-understand principle that aerosol scientists and engineers have agreed upon, though, it is this: The more outdoor air enters a room, the better for dispersal of that cloud of viral particles that may be attracted. And one of the most reliable and cost effective ways to get outdoor air into a room is to open a window.

“If you do not 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 of there quickly if you swing through an indoor location with other people there.

The outside air will eventually replace the indoor air, according to Jose-Luis Jimenez, an aerosol scientist at the University of Colorado Boulder.

“The more outdoors you have, the more you dilute the virus,” said Jimenez, who was among the scientists and engineers who sent a letter urging the World Health Organization to recognize that airborne transmission of the new coronavirus a threat is within spaces.

If you want to accelerate the flow of outdoor air to a room, you could also take a box of it, put it in a window and blow it out, Jimenez said. When any amount of air is released, that same amount of air returns – it is a fixed volume. Therefore, the fan should help to draw in the same amount of outdoor air.

Your type of air conditioner is important. Some draw in outdoor air. Others simply recycle indoor air.

If you have air conditioning in your home, no one is saying you should give it up completely. When it rains, air conditioning can be essential not only to keep you functioning but also to prevent heat stroke.

But if you are going to spend time in a cool room with other people, it may be worthwhile to understand a little more about the cool air you breathe. In principle, all air conditioning falls into one of three categories.

– The unit cools both indoor and outdoor air.

– The unit cools and recirculates indoor air only.

– The unit is completely dependent on drawing outside air. (These are uncommon outside hospitals and labs.)

Centralized air systems, such as those commonly found in office buildings, bedrooms and some large apartment buildings, often fall into category one. 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 attract more open air, a business that can be costly. Take a casino in Las Vegas that is kept cool enough to keep people gambling inside while feeling outside the 120 degree Fahrenheit. Cooling that hot outdoor air will be more expensive than recycling the already cool indoor air. But given that keeping customers healthy is also a priority, they are more willing to consider their approach, Jimenez said.

Some of us have the ability to adjust our air conditioning in this way. For example, most window units that sit with their backs to the front door fall into category two. Instead of drawing in the air, they dump heat from the room outside, said William Bahnfleth, a professor of architectural engineering at Penn State’s Institutes of Energy and the Environment.
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If you live alone or with people, then you are sure they are not infected, these units are fine. But if you’ve given up on throwing that birthday party to your parents, or if your teen has been less than strict about staying home, it’s worth remembering that “any virus that is present will be mixed in” to the recirculating indoor air, said Jimenez.

So if you need to get people over with, it may be preferable to go back to rule: When in doubt, open the windows. Or better yet, go outside.

Not all filters are created equal. But a good filter can be just as effective as inhaling indoor air.

So, what do you do when you are sitting with a unit that primarily recycles indoor air, and it is unrealistic to open the window? This is where filters come in. The right filter is just as effective as pulling outside air, said Dr. Edward Nardell, a professor at Harvard Medical School who has written about the role that air conditioning plays in spreading diseases in the air.

Along with removing dust, pollen, cooking odors, tobacco smoke and chemicals, filters can remove viral particles from the air. Some filters go directly into air-conditioning units and central air systems. Others are designed to stand alone. MERV and HEPA are two widely trusted, certified types.

MERV filters are evaluated on how efficiently they remove particles in a specific large range of air. ASHRAE, a professional company of engineers for air conditioning, heating and refrigeration, recommends MERV 13 and above for filtering the coronavirus, said Bahnfleth, who heads the group’s epidemic task force. It’s what Bahnfleth has in his own house. Each HEPA filter is even more efficient than the highly rated MERV filter, he added, so both must effectively capture coronavirus particles.

Many central air systems are designed to accommodate specialized filters. But not all can handle the most advanced filters. Filters with lower ratings may still be useful, Conlan said: It’s not that they’ll ever capture smaller particles; they just won’t do it that often. Window units are typically designed for comfort, not health, and have even more filter restrictions.

For those who can afford them – or pressure their employers or landlords to buy them – a stand-alone HEPA filter is a good option, Bahnfleth said. Some are designed for larger spaces than others. The key, Jimenez added, is to choose one that will filter all the air in the room at least twice an hour.

Be aware that if an air filtration system sounds too good to be true, your instincts may be good. Some of them appear to rely on dubious marketing and science, Jimenez said.

There is no ‘good place’ in a room. Instead, keep your distance, wear a mask and walk away if you can.

Now that you are an aerospace expert, it may be tempting to think that you know how to choose the safest position in a restaurant or other indoor space that you might find you have a reason to be in.

But even experts at the lowest risk location cannot easily spot it, said Andrew Persily, who oversees the development of an online tool to estimate exposure to infected aerosols in rooms and buildings as head of the Energy and Environment Department 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 hard to predict.”

It is also difficult to measure how many too many people are in a given space. After all, it only takes one infected person to make other people sick. If you are a carbon dioxide detector, you could try a technique that was previously used to control the spread of tuberculosis and use that to tip you, Miller suggests. If carbon dioxide levels are above 1,000 parts per million, you would be wise to reduce the number of people in the indoor space, increase the amount of outdoor air or both, she says.

An alternative approach is to see. Do you see other people? If so, leave.