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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?

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    $\begingroup$ It goes into the ventilation fan filters. See space.stackexchange.com/a/34579/6944 $\endgroup$ Nov 15, 2023 at 1:04
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    $\begingroup$ From what I heard, the bigger risk at least on older spacecraft was that debris would drift into one of the many hundreds of exposed mechanical switches and either jam them, or start an electrical fire. $\endgroup$
    – Cadence
    Nov 15, 2023 at 4:47
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    $\begingroup$ If a submillimeter fragment of a pencil could kill you if you inhaled it, you'd see it happen on Earth, given the number of pencils in use (a lot) and the number of careless people (approximately all of them). A pencil tip in space is probably only going to be a health risk if you are travellling in a different orbit to it, and you collide at high speeds. $\endgroup$ Nov 16, 2023 at 9:30

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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”

p177 https://ntrs.nasa.gov/api/citations/19670023576/downloads/19670023576.pdf

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

  • Turbinate bones

  • Sneeze reflex

  • Cough reflex

  • 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.

enter image description here https://www.space.com/nasa-international-space-station-full-capacity with photo shopped Roomba

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    $\begingroup$ thanks for the edit - wish I could upvote twice! :-) $\endgroup$
    – uhoh
    Nov 15, 2023 at 12:50

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