Dust may be breathed in through the nose (nasal route) or mouth (oral route). It may then be exhaled, or deposited in the respiratory system. The key deposition mechanisms for dust are sedimentation and impaction, both of which are governed by the aerodynamic diameter of the dust particles. Note that there are big differences between individuals in the amount deposited in different regions.
The majority of the largest particles, with an aerodynamic diameter of 30𝜇m or more, will be deposited in or above the larynx. In the nose, particles are filtered by the nasal hairs, and deposited by impaction where there is a change in direction of the airway, and then retained by mucus. The mouth has no hairs to filter out dust, so about twice as muc dust is inhaled if breathing through the mouth than through the nose.
The Inhalable particulate fraction is the proportion of dust that can be breathed into the nose or mouth.
Medium sized particles are most likely to be trapped in the trachea and bronchii. There, insoluble particles will be held in mucus until expelled, whereas soluble material may enter the body by dissolving.
The Thoracic particulate fraction is the proportion of dust that can penetrate the head airways ad enter the trachea and bronchii.
Small particles can enter the alveolii. Only about 1% of 10𝜇m particles get as far as the alveolii, so this is normally taken as the upper size limit for small particles. In fact, most particles greater than 2𝜇m will have been deposited higher up in the lungs. Absorption is a maximum at about 2𝜇m, and at about 0.5𝜇m 85-90% of particles are simply exhaled. Below this point, deposition increases again, as particles are small enough for diffusion effects.
The Respirable particulate fraction is the proportion of dust that can penetrate beyond the bronchii and enter the alveolii.
Nanoparticles are taken to be less than 100nm (or 0.1𝜇m). It isn't clear what proportion of our usual materials are of nanoparticle size. It is known that particles of this size are emitted during kiln firings (including electric kilns, so not just as products of combustion).
Nanoparticles are potentially extremely damaging, as they are small enough to pass through cell membranes, without needing to be soluble or for the cells to have an absorption mechanism.
Note that comparatively long fibres may be deposited in the lungs, as their aerodynamic diameter is primarily a function of its diameter, and not its length. Fibres as long as 100𝜇m have been found in the alveolii.
Dust that has not been deposited in the respiratory system is simply exhaled. For dust that has been deposited, there are a number of mechanisms for expelling the particles. Note that soluble particles may dissolve before they are expelled.
In the alveolii, particles are engulfed by macrophage cells (phagocytes) - though some particles, such as silica, are toxic to the phagocytes and kill them off. Once engulfed, one of three things may happesn:
- they may remainin the alveolii
- they may be propelled up to the bronchii
- they may enter the lymphatic system
Clearance may take anything between 10 and 200 days.
In the trachea and bronchii, several mechanisms may come into play, all moving particles up towards the epiglottis:
- the cells have a mucus layer and cilia (hairs), and movement of the cilia push the particles up, at speeds of up to 10mm/minute
- the bronchioles have intermittent peristalsis to push particles up
- coughing and sneezing takes the particles to the larynx and beyond
If not coughed or sneezed out, particles may be swallowed or spat out. It takes about half a day to clear particles from the bronchii and trachea.
Particles in the nose and mouth are propelled to the throat, where they are swallowedor spat out.
If the level of dust builds up to an excessive level, the clearance mechanisms become less efficient. This can occur when there is a long term exposure to low levels of dust, and can lead to the development of tumours even if the dust particles are not inherently harmful. If too many particles enter the alveolii, the macrophages may become overwhelmed. They may allow particles to gather in the lungs, or they may die before the particles have been transported to larger passageways. Either of these result in scar tissue, fibrosis, which restricts the ability of the lungs to contract and expand, reducing the efficiency of gas exchange in the lungs.