Four Microworlds

Each medium sorts particles by its own rules. In air, settling competes with wind, turbulence and rain, and shape changes how fast a particle falls. In water, buoyancy decides first and living films may change the outcome. In soil, narrow pores and charged mineral surfaces can hold particles back. In the lung, size decides where an inhaled particle lands. In breathing experiments, particles about 0.5 µm across deposited least of all (level B)¹.

That size is too large to diffuse quickly and too small to settle quickly. The four panels on the home page illustrate these processes. They do not record real particles.

What the four panels are

The home page shows four panels: air, water, soil and lung tissue. Each combines a still background made with an AI image generator and particles drawn live by the page's code, which drift, sink, lodge or move with a breathing rhythm. The particles are enlarged far beyond true scale so that the motion is visible.

Beside the air, water and lung panels a counter adds up mass from a named study value chosen in settings (for the human body, an intake estimate); it is an estimate, not a measurement, explained in Flows & Model. The soil panel has no counter: we found no sourced global estimate of plastic entering soil. The panels show which process dominates in each medium, so that the numbers on the system pages make physical sense.

What controls particles in each microworld

Data table

label	value	unit	conditions	level	source
air	settling, wind and turbulence, and dry and wet deposition decide how long particles stay aloft		shape: model fibers settled up to 76% slower than same-volume spheres; the lung's 0.5 µm minimum is not a rule for open air	B (processes); C (fiber result)	S-283, S-049
water	buoyancy first: polyethylene and polypropylene float, yet reach deep sediment traps and sediments		biofouling may sink particles (model); disputed for particles of 100 µm or less, where marine snow and ingestion are proposed instead	B (deep finds); C (model); D (small-particle sinking)	S-278, S-277
soil	entrapment in narrow pore channels and adsorption to grain surfaces		charged iron and aluminum sites may impede migration; in one cited study smaller particles stayed better dispersed yet moved less; controls: particle, flow, porous medium, co-transported substances	C (mechanisms); B (groups of controls)	S-279, S-280
lung tissue	size-dependent deposition: total deposition lowest near 0.5 µm; diffusion dominates below 0.5 µm, sedimentation at 1–8 µm, impaction mainly above 5 µm		therapeutic aerosols: above about 6 µm mainly upper airway, below about 2 µm mainly alveolar; shifts with breathing, density and shape; fibers also deposit by interception	B (mechanisms, minimum); C (regional cutoffs)	S-281

Air: shape decides how far a particle drifts

Particles leave the air by settling, by wind and turbulence carrying them to surfaces, and in rain or snow; transport models and field samplers track this dry and wet deposition (level B)^2,8. The lung's 0.5 µm minimum was measured in breathing experiments and is not a rule for open air (level P)¹.

In laboratory tests, 3D-printed model fibers 0.5–2 mm long settled up to 76% slower than same-volume spheres; a transport model then found 1 mm fibers more likely than spheres to reach the high Arctic (level C, model estimate)². In western US protected areas, dry-deposited plastic was smaller than plastic washed out by rain (level C)⁸.

Illustration

Water: floating, sinking and the biofilm dispute

Polyethylene (PE) and polypropylene (PP) float in seawater, yet both turn up in deep sediment traps and deep-sea sediments (level B)⁵. One explanation is biofouling. A 2017 model by Kooi and colleagues added biofilm growth to settling: depending on size and density, particles floated, sank to the seafloor or oscillated up and down, with small particles collecting at intermediate depths (level C, model estimate)⁴.

The small-particle case is disputed. Benner and Passow (2024) argue that particles of 100 µm or less cannot host the heavy organisms needed to sink, and point to marine snow and ingestion instead (level D)⁵.

Illustration

Soil: pores trap and charges may hold

A 2024 review of porous media notes that particles can enter fine pores but can also be trapped in narrow channels (straining), and that attraction between positively charged iron and aluminum sites and negatively charged plastic may impede migration (level C)⁶. Size alone gives no simple rule: the review reports a study in which smaller particles stayed better dispersed yet moved less (level C)⁶.

A second review sorts the controls into particle, flow and porous-medium properties, plus co-transported substances (level B)⁷. In a greenhouse experiment, earthworms carried polyethylene beads down the soil profile, and smaller beads were moved down more (level C)⁹.

Illustration

Lung tissue: a thin wall and size-sorted deposition

The panel shows alveoli, the lung's gas-exchange sacs. In young healthy adults the air-blood tissue barrier has a harmonic mean thickness of about 0.6 µm (level C)³; thinness is not a size cutoff for passage. Inhaled therapeutic aerosols above about 6 µm deposit mainly in the upper airway, those below about 2 µm mainly in the alveolar region (level C)¹.

These are tendencies, not limits for plastic fragments or fibers. The review does not say whether its diameters are aerodynamic; impaction also depends on density, and fibers deposit by interception, negligible for spheres (level B)¹. Deposition is exposure only; uptake is covered on Human Body.

Illustration

Where to go next

Amounts and evidence for each medium are on Atmosphere, Oceans, Soil and Human Body. The same scales in one continuous zoom are in From Planet to Cell.