Public Health and physics and psychrometrics

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Dynamics of airborne particles (please feel free to add or suggest – I am not a physicist)

Application of physics and psychrometrics in the field of public health and epidemiology (concepts)

Fact 1: For a sphere, the surface area is S=4*Pi*R*R, where R is the radius of the sphere and Pi is 3.14. The Volume of a sphere is V= 4*Pi*R*R*R/3. So the ratio of surface area to Volume is S/V = 3/R. Thus, the smaller the sphere, the higher the ratio, which means higher the buoyancy.

Fact 2: Air Pressure + Gravity = Buoyancy Air particles are drawn downward by gravity. But the air pressure creates a force in all direction (squeezing it). Air density builds to whatever level balances the gravity force so that this point gravity isn’t strong enough to pull down particles.

Fact 3: Dry air is denser than the humid air (atomic weight of water vapour is less than the air). 

Fact 4. Dry air makes it possible for small particles to float in the air, while in more humid conditions, they bind to the water droplets and fall to surfaces.

Interpretation: Now applying the above facts, can we say smaller particles in the dry air more like to float longer? For instance, think of tiny dust particles that appear to float in the air when viewed against a shaft of sunlight. Do they float more in dry air, than in humid air? Particles absorb some of the humidity from the air and swell and may fall to the ground. In addition, humidity can keep the surface (including the surface of membranes) moist, and particles may not travel as far as possible.

Airborne particles with a diameter <10μm can remain airborne for a long period; in some cases, they can stay in the air for several weeks, while those particles with a diameter >10μm are large enough to deposit quickly under the influence of gravity[1].

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