Document zejXrdDap6Z2a1OyYB6Mojpa

- 63 - Uptake of asbestos fibres in the human body has two pathways: the respi ratory tract and the digestive tract. Contamination of the skin may give rise to formation of asbestos warts, but this does not lead to the uptake of asbes tos in the body. Parenteral administration of asbestos is used in animal ex periments# and there also exists the possibility of such administration in humans (sec chapter 3X-4). . bptakc_throu<jh ratory tya ct Human airways arc equipped with defence mechanisms preventing the penetration of dust particles to the lungs. This mechanics; consists mainly of the mucous meebrane with ciliary epithelium in the nasopharynx and the upper and the lover respiratory tract. Particles in inhaled aii will be trapr<d and subsequently pro;elled in the direction of the nasopharynx. These mechanisms v.'o: !; very efficiently for particles >f>u. heavy cigarette smoking interferes with the ciliary epithelium activity/. For sr.oiler dimen sions the efficiency decreases rapidly; for the smallest particles the mecha nism has no effect, resulting in deposition in the lung alveoli. Purification of asbestos-dust cent.aininy, respiratory air is unfortunately hampered by the property of asbestos fibres to split up easily in numerous fibres of relatively great length and very small diameters (see chapter 3). Tinbrvll (19C*1, 1973) shov'd that the passage of asbestos fibres through the airways was only related to fibre diameterj the vpr^r limit of this diameter i_s _3u. With regard tc the length of fibres th.-re is no clear limit; lengths > ?00 p however lead to more efficient activity of the defence mechanisms. This means, that exposure of a human being to asbestos dust leads to tin* passage of a substantia) part of the inhaled asbestos to the iuros thc:.:sc!vcs. Subsequently a part of these fibres may be coughed up, either as fib'-^s or in the shape of asbestos bodies (see chapter V13). The clearance of lun^s also depends upon the physical properties of the fibres: the curliness bf th* fibre, greater in chrysotile than in emphiholcs, will influence the deep pene tration: so that there is less penetration in chrysotile, more in amphiboles. The above mentioned physical property of asbestos fibres of splitting up into diameters which arc very small in comparison with diameters of n.dj>t other natural or artificial fibres, leads to the conclusion that fibres found in Bt>0) P75.6 PRODUCED BY FORD