I heard a claim that a piece of graphite, of half a millimeter size which broke from a pencil can kill a person who inhales it. Is this a major problem or an exaggeration? In a gravitational environment this would fall to the floor but where does such garbage accumulate naturally in a microgravitational environment?
Microgravity actually has a protective effect from inhaled particles
… weightlessness might offer some protection to an astronaut from certain contaminants which, if inhaled in a unit gravity environment, would be irritating to or damage alveoli and respiratory bronchioles”
Humans have very effective filter mechanisms to prevent debris (of all sizes) from getting into the alveoli of the lungs. These mechanisms are not dependent on gravity and, in fact, work to counter the effect of gravity.
Some of the mechanisms (in anatomical order, but non-coincidentally in order of the size of particles removed)
Size of nostrils
Spacing of nasal hairs
Cilia and mucous lining trachea and bronchi
Particles down to 2.5 microns are filtered by the above mechanisms. The mucous sheet lining the bronchioles is constantly transported up the airways (against gravity) to the pharynx where it is swallowed (yuk!)
Fine particles (generally 2.5 μm in diameter or smaller) and ultrafine particles (diameters less than 0.1 µm) are primarily deposited in the small peripheral airways and the alveoli (the pulmonary region). A large proportion of fine and ultrafine particles that reach the small airways and alveoli remain suspended in the airways and are subsequently exhaled. As illustrated in Figure 5, for ultrafine particles of around 0.03 µm and less, the pattern of deposition begins shifting toward the mouth.
Source: EPA's Particle Pollution Exposure
The half millimeter pencil point has no hope of getting down into the lungs. Even if it did, the graphite is biologically non-reactive and would kill no one.
In the ISS, the Roomba would snap it up anyway.
https://www.space.com/nasa-international-space-station-full-capacity with photo shopped Roomba