Aerosols vs Droplets

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By Sonia Fernandez, UC Santa Barbara

Winter is on its way. And in this year of coronavirus, with it comes the potential for a second wave of COVID-19. Add in flu season and our tendency to head inside and close our windows to the cold, wet weather, and it appears the next several months are going to present us with new health challenges.

UC Santa Barbara researchers Yanying Zhu and Lei Zhao hope to arm people with better knowledge of how SARS-CoV-2 spreads as the seasons change. Their new study investigates the secret of this virus’s unusual success: its transmissibility, or how it manages to get from host to host. The dominant mode, it turns out, changes according to environmental conditions.

“Back at the beginning of April a lot of people were wondering if COVID would go away in the summer, in the warmer weather,” said Zhu, a professor of mechanical engineering and one of the authors of a paper that appears in the journal Nano Letters. “And so we started to think about it from a heat transfer point of view, because that’s what our expertise is.”

The virus, of course, did not disappear during the summer as hoped, and in fact COVID cases across the country continued to climb. To understand how the novel coronavirus manages to persist in circumstances in which the flu virus fails, Zhu, Zhao and colleagues modeled different temperatures and relative humidities along a continuum from hot and dry to cold and humid in typical indoor spaces, where the virus is distributed by normal speech and breathing — and, according to the paper, where people “only sneeze or cough into a tissue or their elbows.” To these scenarios they added emerging knowledge about the highly contagious microbe; in particular, how long it remains infectious outside a host.

The results are sobering. For one thing, respiratory droplets — the most common mode of transmission — don’t obey our social distancing guidelines.

“We found that in most situations, respiratory droplets travel longer distances than the 6-foot social distance recommended by the CDC,” Zhu said. This effect is increased in the cooler and more humid environments to distances of up to 6 meters (19.7 feet) before falling to the ground in places such as walk-in refrigerators and coolers, where temperatures are low and humidity is high to keep fresh meat and produce from losing water in storage. In addition to its ability to travel farther, the virus is particularly persistent in cooler temperatures, remaining “infectious from several minutes to longer than a day in various environments,” according to several published studies.

“This is maybe an explanation for those super-spreading events that have been reported at multiple meat processing plants,” she said.

At the opposite extreme, where it is hot and dry, respiratory droplets more easily evaporate. But what they leave behind are tiny virus fragments that join the other aerosolized virus particles that are shed as part of speaking, coughing, sneezing and breathing.

“These are very tiny particles, usually smaller than 10 microns,” said lead author Lei Zhao, who is a postdoctoral researcher in the Zhu Lab. “And they can suspend in the air for hours, so people can take in those particles by simply breathing.

“So in summer, aerosol transmission may be more significant compared to droplet contact, while in winter, droplet contact may be more dangerous,” he continued. “This means that depending on the local environment, people may need to adopt different adaptive measures to prevent the transmission of this disease.” This could mean, for example, greater social distancing if the room is cool and humid, or finer masks and air filters during hot, dry spells.

Hot and humid environments, and cold and dry ones, did not differ significantly between aerosol and droplet distribution, according to the researchers.

The quantitative descriptions of virus propagation under varying local conditions could serve as useful guidance for decision-makers and the general public alike in our efforts to keep the spread to a minimum.

“Combined with our study, we think we can maybe provide design guidelines for the optimal filtering for facial masks,” said Zhao, adding that the research could be used to quantify real exposure to the virus — how much virus could land on one’s body over a certain period of exposure. This knowledge could, in turn, lead to better strategies for airflow and ventilation to prevent virus accumulation. In addition, the insights, according to the study, “may shed light on the course of development of the current pandemic, when combined with systematic epidemiological studies.”

Research on this paper was conducted also by Yuhang Qi and Prof. Paolo Luzzatto-Fegiz at UC Santa Barbara, and Prof. Yi Cui at Stanford University.

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Byzantium Oct 15, 2020 01:55 PM
Aerosols vs Droplets

Now that we have data to protect particularly vulnerable populations and a reasonable array of preventives and therapeutics, it is now time to allow the spread to finally reach the herd immunity option; which has been materially delayed trying to isolate ourselves from this particular corona flu season bug.

a-1602796976 Oct 15, 2020 02:22 PM
Aerosols vs Droplets

I still can't believe we have to keep explaining the scientist's reasoning for this not working. There is no guarantee that by doing this we will reach "herd immunity," especially since there is evidence of people catching this virus more than once. And there is a lot of evidence that young, healthy, non-vulnerable and non at-risk people are dying from this disease without a specific reason as to why. ANYONE could catch this and die, sure vulnerable people at higher rates, but still anyone could. Why not wait until the science is more clear and the vaccine is ready so more innocent people don't have to die.

a-1602801549 Oct 15, 2020 03:39 PM
Aerosols vs Droplets

Too soon to just let the virus spread. Young people without health problems are still being killed by blood clots, and others are left with lingering lung problems. Plus, we can’t reliably and completely isolate vulnerable people. We do need to keep society open, but we’ve got to keep the infected numbers low. This study gives us more knowledge how to do that.

ChemicalSuperFreak Oct 15, 2020 09:47 PM
Aerosols vs Droplets

Hi BYZANTIUM and @2:22 PM: I have to agree and disagree with @2:22 PM. Herd immunity may be mythical in the sense that at least one documented case of a reinfection exists (with a different COVID clade via sequencing), with two negative tests and a few months between the infections. Then again, a few people get chicken pox twice while most of us just get it once. This individual might be one of those rare cases, but we still don't know yet. I disagree with @2.22 PM suggestion that there's no reason why non-risk individuals can get a bad case of COVID and die. There's some suggestion that it's related to cell surface receptors that are differentially expressed in the population. Think blood-types. Some data suggests that O-type blood (lacking certain antigens) have an easier go, while A-type individuals seem to be more reactive to the infection. This makes sense to me, since that's related to the immunological recognition of self, and foreign. Those cell surface proteins and carbohydrates may facilitate the infection in some people, but not others.

PitMix Oct 15, 2020 11:44 PM
Aerosols vs Droplets

2:22, you can't use facts with this crowd. They have heard immunity. See story of Nevada man who got sick twice with different strains and was sicker the 2nd time.

Voice of Reason Oct 16, 2020 08:42 AM
Aerosols vs Droplets

@ 2:22, there are not guarantees anywhere here. Including with the vaccine. So those that think we should keep things closed until their is a vaccine, please stay home, stay safe, until there is a vaccine and let others make their own choices. Washing hands, stay home when sick, don't be sneezing and coughing all over the place. If you're at-risk, stay home, or not, that is your choice.

Voice of Reason Oct 16, 2020 08:44 AM
Aerosols vs Droplets

Pitmix, story of one man out of 7 million positive cases in US. Statistically speaking, much more likely he had a false positive on his first test. Facts cut both ways.

kmarsh8 Oct 16, 2020 09:07 AM
Aerosols vs Droplets

Please name a deadly virus that has been successfully contained through herd immunity. HIV? Smallpox? Ebola? Rabies?

pstarSR Oct 16, 2020 09:27 AM
Aerosols vs Droplets

voice of reason, she did NOT list 2 that were contained due to herd immunity! she listed 3( 1 that is still in the works.... but I dont think we can get herd immunity to AIDS/HIV ) that were contained with vaccines. I dont know about you, but I never got herd immunity to rabbies and neither did anyone else. hence why you have to go get a shot if you get bit by a trash panda

Voice of Reason Oct 16, 2020 09:34 AM
Aerosols vs Droplets

An individual doesn't get herd immunity, a population does. A vaccine is one path to herd immunity.

a-1602887803 Oct 16, 2020 03:36 PM
Aerosols vs Droplets

Trying to achieve herd immunity for a deadly and highly infectious disease without a vaccine is total stupidity. Of course, it's all they had in the medieval world.

Immunity for SARS-CoV-2 looks might be an especially poor approach, even with a vaccine:

1) There is evidence that the immune response to the virus wanes over a period of a month or two. Will a vaccine-induced response last longer? Better hope so!

2) There are several documented cases of repeated infection (not just faulty test results) with SARS-CoV-2, of which this is just one of the latest examples:

Voice of Reason Oct 16, 2020 04:04 PM
Aerosols vs Droplets

@ 3:36, if there was a better option we'd certainly take it. What IS NOT an option: staying locked up inside, keeping schools closed (even this hybrid 2 day per week BS), and restricting the freedoms of many to go out and make a living while we wait and hope that some day, MAYBE, we'll create a safe and effective vaccine. Covid is here (and would have been here no matter who was in the White House), and it isn't going away. It is yet another communicable disease we'll have to learn to live with. Staying at home might be an options for you, but it should not be forced on anyone.

a-1602890497 Oct 16, 2020 04:21 PM
Aerosols vs Droplets

4:04 pm: You say that if there were a better option we'd take it, then proceed to reject the better option. Not very reasonable.

Voice of Reason Oct 16, 2020 04:58 PM
Aerosols vs Droplets

In case I wasn't perfectly clear in that post and others, the better option is everyone at-risk including elderly, and those just feel like it, stay home and stay safe. Everyone else is free to get on with their lives, eat in a resultant, go to school, etc. Just please wash your hands and stay home when sick. This isn't some hairbrained option, it's what we've done in every single other pandemic we've had and it worked.

a-1602894795 Oct 16, 2020 05:33 PM
Aerosols vs Droplets

But just about everyone is at risk, which you fail to acknowledge.

ChemicalSuperFreak Oct 16, 2020 11:08 PM
Aerosols vs Droplets

@8:44 AM: The first test was absolutely not a false positive because they saved his sample and were able to compare the first viral mRNA sequence to that from the second infection, which is why they know he was infected by two different versions of SARS -CoV-2.

a-1603042785 Oct 18, 2020 10:39 AM
Aerosols vs Droplets

@A-1602796976 you sound like a fear mongerer. Evidence and statistics worldwide proof that young people are highly underrepresented in severe cases. Only few young people with chronic diseases or diabetes have problems. Please show your evidence for young healthy people dieing from it.

a-1602796125 Oct 15, 2020 02:08 PM
Aerosols vs Droplets

This looks like a very good paper. Question for the authors- did you compare your results to measurements for validation? If not, that would be a great next step.

Byzantium Oct 15, 2020 04:34 PM
Aerosols vs Droplets

There are experimental model viral droplets/aerosols and real life pathogenic viral load droplets/aerosols. Wake me when art imitates life. What was the death count after the Rose Garden "Super Spreader" event?

PitMix Oct 15, 2020 11:46 PM
Aerosols vs Droplets

Mortality rate has been about 3% given reported deaths and reported infections. But varies depending on age, health, ethnicity, and access to medical care. But you don't really care, do you?

Voice of Reason Oct 16, 2020 08:54 AM
Aerosols vs Droplets

That is completely false information PITMIX.

0-19 years old death rate is .003% or .00003
20-49 years old death rate is .02% or .0002
50-69 years old death rate is .5%% or .005
70 years old or older death rate is 5.4% or .054

Voice of Reason Oct 16, 2020 09:03 AM
Aerosols vs Droplets

For comparison, in 2017 the flu's all population death rate was .017% or .00017. AND THAT IS WITH A FLU VACCINE. If you under 50 it's about as lethal as the flu. I want to be clear on something, the flu is a very dangerous virus. I think people don't appreciate this very important FACT, so when they hear statistical comparisons to the flu they think we're trivializing covid, this is not the case. We need to respond to covid rationally and with perspective, something very difficult to do when the media (a for profit business whose #1 goal is making money) sensationalize this all day every day.

PitMix Oct 16, 2020 10:18 AM
Aerosols vs Droplets

Voice, 218,000 dead. 8,000,000 cases total in US according to your leader's CDC. That's 2.7% according to my HP calculator. Do you dispute the numbers or the math? Since mostly older people are dying, I can see how your 5.4% number gets diluted down to 2.7% for everyone. No charge for the tutoring.

Voice of Reason Oct 16, 2020 10:37 AM
Aerosols vs Droplets

I pulled those stats direct from the CDC website, no calculator needed. At this point in the pandemic if you don't know the difference between IFR and CFR you shouldn't be commenting. Same for the commenter who said herd immunity isn't a thing. You're like the new climate change deniers.

PitMix Oct 16, 2020 03:25 PM
Aerosols vs Droplets

Nice putdown voice. You're so smart to know the difference between IFR and CFR. Clap clap. I just pulled the numbers from the CDC website also, but know how to calculate percents. My 2.7% average is less than your 5.4% for 70 yr olds, so I'm willing to accept your sensational numbers.

kmarsh8 Oct 16, 2020 07:22 AM
Aerosols vs Droplets

Herd immunity isn’t a thing. Think about it. In all of time, there’s not one virus that has been contained or extinguished through herd immunity. Not one.

Voice of Reason Oct 16, 2020 08:39 AM
Aerosols vs Droplets

Every doctor and epidemiologist would say otherwise. That is exactly why we don't have major outbreaks of , measles, chickenpox, mumps, diphtheria, and many others. Small pocks was even eradicated because of it.

a-1602863426 Oct 16, 2020 08:50 AM
Aerosols vs Droplets

Smallpox was eradicated because of vaccination, going back to the 16th century in China ...and because the death rate from it ranged from 30-50%. Herd immunity means that many, many people will die from Covid.

Voice of Reason Oct 16, 2020 09:05 AM
Aerosols vs Droplets

Vaccination is one way to get to herd immunity and the only way with a high death rate.

Ahchooo Oct 16, 2020 04:37 PM
Aerosols vs Droplets

Vaccination is the preferred way to reach herd immunity. Reaching it by letting everybody be exposed, and then suffering through the subsequent deaths and disabilities, is a very sad method.

Byzantium Oct 16, 2020 05:02 PM
Aerosols vs Droplets

Many people over age 70 will die. Even more over age 80 plus will die Many of them even accept this fact of life - law of averages makes one appreciate every extra day if you beat the over 70 numbers game. It is a strange time in life when most of the obituaries are people younger than yourself. Don't destroy life in America on my statistical mortality behalf because chances are I am going to die anyway.

Byzantium Oct 16, 2020 05:04 PM
Aerosols vs Droplets

CDC releases historical numbers showing flu vaccines we are annually encouraged to get average about 40% effectiveness over time, and can be as low as 10% effective some years. Curious that CDC now demands any corona vaccine demonstrate minimum 50% effectiveness before even considering lessening restrictions.

haskelslocal Oct 16, 2020 09:42 AM
Aerosols vs Droplets

Would be interesting to see droplets propelled at high velocity into hot, dry air that are subject to lower air resistance. I'd bet these travel farther and are intact for longer periods of time even while subject to evaporation. Droplet transfer in this study was "polite" to say the least. But don't forget the loud, vicious sneezes where people choose to point their face up into the air replicating a particle canon. Not everyone is handkerchief or elbow savvy!

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