3. How do fibres enter the lung?
The speed that fibres settle in air is mainly dependent on their diameter. Table 1 shows the falling speed of cylindrical fibres of different diameters.
Table 1 Falling speed of fibres in air
Diameter (μm) |
Falling speed (mm/s) |
100 |
3,000 |
30 |
250 |
10 |
30 |
3 |
2.5 |
1 |
0.3 |
Fibres with diameter less than 3 μm will remain suspended in the air for long enough to reach deep into the lung. Long thin fibres are just as likely to penetrate into the lungs as short thin ones.
When someone is working with asbestos some of the airborne fibres are inhaled into the lungs, which comprise a series of branching tubes. The first of these tubes is called the trachea, which is attached to the larynx. The trachea is about 2 cm in diameter and approximately 12 cm long. The trachea divides into the left and right bronchi. After a few centimetres these in turn divide into segmental bronchi… and so on with about 25 further sets of branches.
Broader fibres continue to impact in the upper sections of the tracheobronchial tree falling out of the air stream onto the walls of the bronchial tubes. However, most fibres with diameter less than 3 μm regardless of length, penetrate to the alveolus. This is the part of the lung where gas exchange between the air and the blood occurs. There are over 200 million alveoli with a total surface area of more than 100 m2.
Once a fibre has deposited in the lung there is a possibility that it may cause damage to the lung tissue. This is a condition in which the lung becomes scarred as a result of prolonged inhalation of asbestos fibres. It only occurs in people exposed to relatively large amounts of asbestos, normally over many years, such as in milling, weaving, lagging or asbestos removal operations. The scarring is more properly known as fibrosis. The part of the lung that is damaged is at the far end of the smallest bronchial tubes, the place where the lung transfers oxygen to the blood stream.
As we have seen, only the finest fibres, less than 3 μm in diameter, are able to reach this part of the lung. This is why when we come to evaluate the fibre concentration in the air we count only fibres of this diameter or less.
« previous | 03 of 11 | next »