Based on infectious dose studies with other coronaviruses, it appears that only small doses may be needed for infection
In order to get infected with Covid-19 one needs to be exposed to an infectious dose of the virus. But how much is an infectious dose?
Based on infectious dose studies with other coronaviruses, it appears that only small doses may be needed for infection.
Some experts estimate that as few as 1,000 Sars-CoV-2 infectious viral particles are all that will be needed, as per an article published by World Economic Forum (WEF) and written by Associate Prof of Biology Erin S Bromage at the University of Massachusetts, Dartmouth.
Bathrooms have a number of high touch surfaces, door handles, faucets, stall doors, etc. Hence, transfer risk in this environment can be high.
It is still unclear whether a person releases infectious material in feces or just fragmented virus, but a flush of a toilet can release many droplets.
Hence it is essential to treat public bathrooms with extra caution (surface and air), until more information about the risk emerges.
Cough or sneeze
A single cough can release about 3,000 droplets traveling at a speed of 80kph. Most droplets are large, and fall quickly due to gravity, but many remain suspended in the air and can travel across a room in just a few seconds.
While, a single sneeze can release about 30,000 droplets, with droplets traveling at up to 321kph. Most droplets are small in this case and travel great distances.
If a person is infected, the droplets in a single cough or sneeze may contain as many as 200 million virus particles which can disperse into the environment.
If one engages in a face-to-face conversation with a person and the person in front of you sneezes or coughs straight at you, it is pretty easy to see how it is possible to inhale 1,000 virus particles and become infected.
But even if that cough or sneeze is not directed at you, some infected droplets can hang in the air for a few minutes, filling every corner of a room. All one has to do is enter that room, within a few minutes of the cough or sneeze, and take a few breaths to get infected.
Spread via breathing
A single breath releases 50-5,000 droplets. Most of these droplets are low velocity and fall to the ground quickly. There are even fewer droplets released via nose-breathing.
Unlike sneezing and coughing which release huge amounts of viral material, the respiratory droplets released from breathing only contain low levels of virus. There isn’t a number for Sars-CoV-2 yet, but influenza can be used as a guide.
Studies have shown that a person infected with influenza can release up to 33 infectious viral particles per minute. Remember the formula: Successful Infection = Exposure to Virus x Time.
But with general breathing, inhaling 20 viral particles per minute from the environment, even if every virus ends up in your lungs -- which is highly unlikely -- one would need 50 minutes of breathing to inhale 1,000 viral particles.
Speaking increases the release of respiratory droplets about 10-fold, up to 200 virus particles per minute. Again, assuming every virus particle is inhaled, it would take five minutes of speaking face-to-face to receive the required dose (1,000 particles).
Hence, a face-to-face conversation higher than 10 minutes would almost certainly lead to an infection. Anyone sharing office space for an extended period is potentially infected.
This is why it is critical for people who are symptomatic to stay at home. Sneezes and coughs expel so much virus that they can infect a whole room of people.