Droplets, aerosols, and COVID-19

PAGE – Catching an airborne virus is easy.

It spreads two ways, through droplets and through aerosols. Droplets are larger and sometimes visible. They come from our mouths when we talk.

Generally, they don’t travel more than 6 feet, unless someone is shouting or talking loudly. Aerosols also come from our mouths, in normal breathing and speaking. They’re microscopic droplets. The louder we speak, the further they travel. In poorly ventilated areas, aerosols can linger for a long time. If fans or air conditioning are present, they can spread viruses well beyond 6 feet.

Linsey Marr is a professor of civil and environmental engineering at Virginia Tech. She specializes in airborne transmission of viruses.

Marr likens the spread of the virus to cigarette smoke. In a PPS News Hour interview, she said, “So, imagine you’re interacting with a smoker. You want to stay as far away as possible from them, really, to avoid breathing in the smoke. The smoke doesn’t stop at 6 feet. So, 6 feet is a guideline, and it keeps you kind of farther away from the most concentrated part of the smoke, the plume, but it doesn’t guarantee that you’re not going to be exposed.”

Cigarette smoke is a good way to think about indoor risks. Marr said, “You think about what affects the ventilation in the room. If the room is really well-ventilated and there’s lots of outdoor air coming in, then that smoke will be kind of pushed outside.”

In poorly ventilated areas, smoke builds up. A virus works the same way. If you’re close or downwind from someone, you can breathe in the virus, whether you’re inside or outside.

Masks reduce exposure to both droplets and aerosols.

Proximity is also important. If people are standing close together for a long time, the chances of inhaling their aerosols are greater. It’s best to move around.

With the looming threat of a faster spreading coronavirus mutation on the horizon, it’s important to stay on guard.

The Center for Disease Control said the faster spreading U.K. variant “has not been identified through sequencing efforts in the United States, although viruses have only been sequenced from about 51,000 of the 17 million US cases. Ongoing travel between the United Kingdom and the United States, as well as the high prevalence of this variant among current UK infections, increase the likelihood of importation. Given the small fraction of U.S. infections that have been sequenced, the variant could already be in the United States without having been detected.”

It’s not unusual for a virus to have many variants, and each has potential consequences. The CDC said they could spread more quickly in humans. They could lead to milder or more severe illnesses. They could avoid detection for specific diagnostic tests. They could decrease susceptibility to therapeutic agents like monoclonal antibodies. They could evade vaccine-induced immunity.

The CDC said, “Among these possibilities, the last – the ability to evade vaccine-induced immunity – would likely be the most concerning because once a large proportion of the population is vaccinated, there will be immune pressure that could favor and accelerate emergence of such variants by selecting for ‘escape mutants.’ There is no evidence that this is occurring, and most experts believe escape mutants are unlikely to emerge because of the nature of the virus.”

In November 2020, the CDC launched the National SARS-CoV-2 Strain Surveillance, or the NS3, program. Its purpose is to monitor and identify strains of COVID-19.

The CDC claims it will be “fully implemented in January 2021. Each state will send CDC at least 10 samples biweekly for sequencing and further characterization.” The information will be available in public databases to researchers, public health agencies, and industry.




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