« on: 27/03/2020 21:53:40 »
Quote from: Bored Chemist
It's also plausible (if simplistic) to assume an inverse square law for infection.We are familiar with the inverse-square law in physics because phenomena like light and gravity radiate out as a sphere in 3 dimensions.
- If a light ray travels 2 x Distance, it spreads out over 4 x Area.
- At 2 x Distance, the intensity is Intensity/4
- This is an inverse-square law
- The inverse-square law still works if you emit light from a point on a flat surface, so it only fills a hemisphere, rather than a full sphere (eg an FM radio transmitter on "flat" ground).
However, if you constrain that third dimension with floor and ceiling, the spread is not like a 3D sphere (Area α Radius2), but like a 2D circle (Circumference α Radius).
- This suggests an Inverse Distance law, rather than an Inverse Distance2 law.
Of course, the range of light and gravity is infinite (in a vacuum), but other physics is at play with virus-laden water droplets...
Quote from: David Cooper
If you spray tiny droplets of water into the air, they just float about for ages without falling much at all.I heard about some tests done with lifetime of virus-laden water droplets. Apparently, the lifetime depends on the size of the droplets.
- Large droplets(as big as raindrops) fall down immediately
- Small droplets (such as you might see in a backlit sneeze) fall down in seconds
- Very tiny droplets can linger for an hour in the air
The size of these small droplets depends critically on the humidity
- These droplets have large surface area for their volume
- So they exchange water molecules easily with the surrounding air
- If the humidity is around 40%, these droplets will stay small, and linger longer in the air. This is a problem for dry, air-conditioned air in the winter months.
- If the humidity is 60% or above, these droplets will grow, gravity will dominate over random Brownian motion, and they will fall out of the air more quickly.
Perhaps humidity is a useful factor to consider in air conditioning for hospitals, supermarkets and cruise ships?
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