November 3, 2020-The coronavirus is spreading across the country, and colder weather forces more Americans to stay indoors, which increases the time most of us spend in homes with severe coldness.
Manufacturers of household air purifiers have noticed that product publicity is suddenly as common as autumn leaves.
But can a portable air purifier really protect you from the coronavirus?
According to experts such as Joseph Allen, Assistant Professor of Exposure Assessment Science and Director of the Healthy Building Program at Harvard University, the short answer is yes.
"Portable air purifiers with HEPA filters can definitely help reduce the risk of airborne COVID-19," Allen said.
For a few hundred dollars, you can buy a high-quality household unit that can remove 99.97% of the pollutants in the air, including respiratory droplets that spread the virus.
But before you rush to buy, you need to know the following:
"Air purifiers must be part of a multi-level defense strategy," Allen pointed out. "It must be combined with other strategies and overall risk reduction methods."
As infectious disease experts warn that coronavirus cases may continue to surge in the coming months, portable air purifiers are receiving increasing attention.
In recent weeks, as students return to classes in many parts of the country, the reopening of schools and colleges has caused multiple outbreaks.
As the temperature drops, more and more Americans withdraw indoors, close doors and windows, and reduce the time spent visiting family and friends on outdoor porches or patios.
As the winter vacation is approaching, health experts worry that the increase in travel, family gatherings, and holiday gatherings will bring more people into close contact—with the seasonal cold and flu virus outbreaks, the number of COVID-19 cases will increase.
One thing that people have paid more attention to portable air purifiers in recent weeks is that people pay more attention to the role of tiny aerosolized respiratory droplets in the spread of the virus.
Dr. Linsey Marr, a scientist at Virginia Tech who specializes in the airborne transmission of viruses, pointed out that these micro-droplets are smaller, stay in the air longer, and can spread more than 6 feet indoors.
Experts say that, therefore, these tiny aerosols have a greater risk of transmission than respiratory droplets.
Marr said that when we breathe, sing, sneeze or cough, we release respiratory droplets, ranging in size from about 100 microns (the thickness of a human hair) to the microscopic level (1 micron, or one millionth of a meter). Wait. .
Usually, we only release some larger respiratory droplets, which can only travel about 6 feet before falling to the ground. But at the same time, studies have shown that we emit hundreds of smaller tiny droplets-aerosols-which can float in the air like smoke or fog for a few minutes to a few hours, and can travel up to 16 feet. Or farther.
Allen said that aerosolized respiratory droplets are usually in the range of 1 to 10 microns. Although virus particles are much smaller than this-about 0.1 to 0.3 microns-they usually "hitchhiker" and spread on larger dust or water droplets, "so the virus is never naked," he said.
Marr and Allen agree that the good news is that portable air purifiers are very good at filtering out these smaller and more dangerous aerosols.
"In fact, they are more efficient at capturing smaller particles than larger particles—for 0.3 micron particles, the capture rate is 99.97%," Allen said, "but they are capturing smaller particles. It’s even more effective when it’s granular."
Since the beginning of the pandemic, the important role played by droplet aerosols in spreading the coronavirus has changed.
Since February, Allen, Marr, and dozens of other experts have been arguing that aerosols can spread the virus on a large scale. But the World Health Organization (WHO) and the Centers for Disease Control and Prevention initially opposed this idea.
In July, 239 scientists urged WHO to change its position in comments published in the journal Clinical Infectious Diseases.
The WHO has since changed its position. Earlier this month, the CDC also changed its position.
Allen said that despite the earlier support of Anthony Fauci, MD, the chief infectious disease expert in the United States, it took so long for the CDC to acknowledge the scientific evidence of aerosol airborne transmission, which is "astounding."
But he was very happy that the agency finally appeared.
"This is something we have been pushing since the beginning of February, and it is shocking that they (earlier) did not recognize science," he said.
Dr. Shelly Miller, an environmental engineer at the University of Colorado Boulder, agrees that portable air purifiers are very good at removing virus particles in indoor air.
But before deciding whether such a device is right for you, she recommends figuring out whether you really need one.
"The most important thing to consider when buying an air purifier is whether the space where you want to use it is already well ventilated-then, the air purifier will not increase too much," Miller said. The focus of his work is About airborne diseases.
But for rooms with insufficient fresh air, such as basements or bedrooms, portable devices "will really improve air cleanliness," she said.
If you decide that you need an air purifier at home, she recommends that you choose an air purifier equipped with a high-efficiency particulate air (HEPA) filter.
You also need to make sure that the equipment you buy can fit the size of the room you will be using.
Most equipment on the market relies on mechanical filtration to clean the air and is rated by the so-called CADR system (abbreviation for Clean Air Delivery Rate). Their working principle is to use a fan to suck air in from the room, pass it through the filter to remove particles, and then exhaust the filtered air back to the room.
It is worth noting that some devices use electrostatic precipitators or ionization to trap particles. But for household use, experts say this is not necessary, and they advise against using these types of devices because they produce ozone, which is a harmful respiratory pollutant.
The CADR rating system for portable air purifiers measures the efficiency of the filter used by the device and the amount of air passing through it.
According to data from the US Environmental Protection Agency, cleaners using HEPA filters (a foldable mechanical air filter) can remove at least 99.97% of dust, pollen, mold, bacteria, pathogens, and other 0.3 micron-sized particles.
This 0.3-micron size is the worst-case scenario; Allen explained that larger or smaller particles are better captured.
He also said that the filter does not destroy the virus like ultraviolet light, but it is not important for removing COVID-19 particles from the air.
"You don't need ultraviolet light or some other process because the virus is captured by the filter media," he said. "Once the [virus] particles are captured, they will not grow and cannot be released. Therefore, you don't need anything other than a portable air purifier with a HEPA filter.
"It's really simple, just like washing your hands is very simple and basic."
When choosing a portable air purifier, the second key factor to consider is the size of the room where you plan to use it.
"If you buy a portable air purifier and stick it in the corner of the gymnasium, it won't be very effective," Allen said. "So you need to adjust its size according to the living room or other rooms."
The higher the CADR, the larger the area it will serve. Generally speaking, product packaging will tell you the largest area or room that the cleaner can handle.
Environmental engineers measure indoor air quality by air exchange per hour (ACH). This represents the number of times the air in the room is filtered every 60 minutes.
Based on experience, Allen said experts recommend four to six air changes per hour in the indoor space. This means that all the air in the room is cleaned or replaced every 10 to 15 minutes. This is sufficient to remove the COVID-19 aerosol from the air and reduce the risk of transmission.
If you are willing to do some mathematical calculations, the following is the formula for matching the equipment with the room to be used according to the CADR of the equipment:
Suppose you want to buy an air purifier for a 500-square-foot room (with a ceiling of 8 feet high) and want to know whether a unit with a CADR of 300 can meet the requirements.
First, you multiply the CADR (300) by the number of minutes in an hour (60)-the sum is 18,000.
Then you divide 18,000 by the volume of cubic feet of space in the room (500 square feet, multiplied by 8)—4,000 in total.
Finally, divide 18,000 by 4,000 to get 4.5—this is the ACH that the air purifier will provide, which is between the four to six ratios recommended for indoor spaces.
For reference, Allen provides this rough guide: "Remember, we recommend changing the air four to six times per hour, so a quick rule of thumb is to find a CADR of at least 300 per 500 square feet of space."
The Environmental Protection Agency also provides guidelines on the minimum CADR levels for rooms of various sizes in its online "Guide to Home Air Purifiers." For an 8-foot-tall room, it recommends:
In addition, the Harvard-CU Boulder School Portable Air Purifier Calculator, an online tool created by Allen and Miller, can help teachers choose effective air purifiers for classrooms, and can give a rough estimate of the different sizes of rooms in the home.
Remember: you need to clean or replace the filter regularly for the air filter to work properly. Therefore, if you buy a device with a HEPA filter, please follow the manufacturer's recommendations for maintenance and replacement.
The last point Allen and Miller emphasized: Portable air purifiers are not a single solution to fight COVID-19 at home.
"This is not a panacea," Miller said, noting that it should be used in conjunction with other virus prevention measures recommended by health experts.
"We must strengthen our defenses," Allen said. "We should wear masks indoors, open windows, introduce more outdoor air, and improve air filtration levels, whether through mechanical systems... or portable air purifiers with HEPA filters. We should keep our social circle small, we Hands should be washed."
In fact, if you are already doing all these things and staying at home without many visitors, you may not even need an air purifier.
Allen said the same is true for people living in apartment buildings or apartment buildings.
"However, if someone comes over, adding a portable air purifier with a HEPA filter is a good strategy," he pointed out. "Even if it's only a few inches, it makes sense to open the window, and you have to wear a mask."
Allen bought two air purifiers for his family this year. But he doesn't think they are a panacea.
"We don't use them now because the weather has always been good. We open the windows as much as possible. We don't have too many people. If someone comes over, we will be outdoors and separate, wearing masks," he said.
Joseph Allen, PhD, Assistant Professor of Exposure Assessment Science and Director of the Healthy Building Project at Harvard University.
Shelly Miller, PhD, environmental engineer at the University of Colorado Boulder.
Linsey Marr, PhD, Professor of Civil and Environmental Engineering at Virginia Tech.
Shelly Miller's blog: "A short report on air purifiers that I hope will be useful."
Clinical Infectious Diseases: "It's time to solve the problem of airborne transmission of Coronavirus Disease (COVID-19) in 2019."
Indoor air: "Ventilation and health in non-industrial indoor environments: a report from the European Multidisciplinary Scientific Consensus Conference (EUROVEN)".
Environmental Protection Agency: "Air Purifiers and Air Filters in the Home", "Indoor Air Quality: What is a HEPA Filter?" A Guide to Home Air Purifiers, "Air Purifiers, HVAC Filters and Coronavirus ( COVID-19)".
Home Appliance Manufacturers Association: "Facts about Clean Air".
California Air Resources Board: "California Certified Air Purification Equipment".
Consumer Reports: "The Best and Worst Air Purifiers of 2020", "What You Need to Know About Air Purifiers and Coronavirus".
CNET: "The best air purifiers of 2020: Dyson, Coway, Molekule, etc.", "Can air purifiers protect against COVID-19? What about wildfire smoke?"
Harvard-CU Boulder portable school air purifier calculator.
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CNN: "Fauci:'There are good enough data to show that aerosol transmission has indeed occurred.'"
NBC News: "Fauci called the Amy Coney Barrett ceremony at the Rose Garden a'super communicator event.'"
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