I am hoping we have good science behind the “1.5 metre” distance rule...
Does anyone have links to videos that show how breath swirls around, especially in outdoor areas? I guess I could get some pollen and try and model it myself!
Does anyone have links to videos that show how breath swirls around, especially in outdoor areas? I guess I could get some pollen and try and model it myself!
1) Exposure to influenza virus aerosols during routine patient care. https://academic.oup.com/jid/article/207/7/1037/2192312
>HCPs within 1.829 m of patients with influenza could be exposed to infectious doses of influenza virus, primarily in small-particle aerosols.
2) Short-range airborne transmission of expiratory droplets between two people. https://www.ncbi.nlm.nih.gov/pubmed/27287598
>The threshold distance of about 1.5 m distinguishes the two basic transmission processes of droplets and droplet nuclei, that is, short-range modes and the long-range airborne route.
It seems that depending on conditions < 2m is a good practical threshold for droplet transmission. Probably more if person is really sick.
The goal is to get as small dose viruses at once as possible. Immune system can usually fight off small exposure.
Sometimes even healthy people who inhale huge amount of viruses into their lungs at at once can get very seriously sick because their immune system struggles with the large sudden attack and goes into overdrive.
This is interesting, i had always thought it was binary, either you get infected or you don't
To be clear, that's not an antibody-mediated immune response, which takes longer. Before then, cells penetrated by virus particles often undergo apoptosis, self-destructing to defend the larger organism by preventing the virus from hijacking the cell's machinery to reproduce itself. Sometimes the virus wins, though. So the more particles you're exposed to, the higher the odds that one or more will establish itself and start making you sick.
For more, see e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2517702/ - but if you don't want that level of detail, you can just think of it as "flattening the curve" on an individual scale, and not really go far wrong.
edit: More accurate use of "immune" now that I'm not commenting from a phone with a small screen. Apoptosis isn't initiated by the immune system, but rather within the affected cell itself, and doesn't only occur in response to viral invasion; many types of cellular injury or damage can cause it, including most of the ways that a cell can become precancerous. (Which happens a lot more often than you might think! But most such cells self-destruct before they can give rise to a tumor.)
IIRC, some vaccines work by exposing the person to a tiny amount of virus so that their immune system can "learn" how to fight that particular type of virus.
A relevant graph is https://academic.oup.com/view-large/figure/90149845/jis77302...
This shows that large droplets don’t travel, but that small (< 4.7nm - accurate?) do travel e.g. ~10% particles measured at 6 feet.
Issue 1: drop off rates look to rude approximation to be r^2. The were measuring a point source. If you are standing in a line of people, you need to integrate along the line, and inter-person distance needs to be increased. A field of people (supermarket) needs something different...
Issue 2: large droplets should travel further if outdoors? (turbulent air)
Issue 3: cigarette smoke is a familiar visual indicator: “cigarette smoke particle mainly distributed in the size range 0.1 to 1.0 micron”. Think about how the smoke exhaled by someone travels (ignore how smoke travels from end of cigarette due to heated air).
As I understand it - the virus combined with droplets is much denser and doesn't float nearly as much.
It's key to remember that the 1.5m (or whatever rule) isn't about making you 100% safe, it's about sufficiently reducing risk. For example, if standing at normal (2-3 feet) distance has a 1% chance getting you infected, and at 5-6 feet it has a 0.1% chance of getting you infected, that's a huge win.
(it doesn't support such simplistic advice of 1.5 meters distance, that advice seems pretty bad and dangerous)
The way I see it, the advice we're given is not so much geared towards safety of the individual as to slowing the spread of the infection, basically lowering R0. They could say five meters, but that is impractical, so would be universally ignored. So the idea would be to settle for a recommended distance that seems practical, while lowering the risk of infection to a reasonable degree.
Let's face it: Whether you or I get through this unharmed, is of little or no significance compared to what will happen on a larger scale.
Also note that it describes aerosol concentrations as "undetectable" in the patient-use areas where samples were collected.
* https://arstechnica.com/science/2020/03/dont-panic-the-compr...
Fog also has very high humidity (otherwise it would evaporate), so any study that shows lower transmission of the virus in high relative humidity environments has relevance to anyone that assumes droplet evaporation matters? https://www.accuweather.com/en/health-wellness/new-study-say...
So fog is very relevant, thanks.
If evaporation matters, then transmission rates should depend strongly upon humidity. However, there are studies saying transmission rates decrease with relative humidity. Maybe drying is irrelevant, or maybe there is another factor, or maybe the studies are wrong.
If we assume the same volume of air is expelled in the examples, then that video just looks sciency, and really doesn’t show us anything actionable (apart from don’t sneeze directly at people, duhhh!).
Also, why cough onto the back of the hand? What cultures is that normal?