Understanding the way the body deals with inhaled fibres is important in understanding how cancer in the lung and pleura is caused by asbestos. Asbestos fibres do occur naturally in the environment and may contaminate water supplies. However, there is no definitive evidence that they are carcinogenic unless they are inhaled.
The body has a number of mechanisms to prevent the passage of inhaled fibres into the respiratory tract. The amount of asbestos fibres that remain in the lung, and which are not cleared by the body’s defence mechanism, depends on the physical properties (size, shape, density) and chemical properties of the fibres. Straight fibres with high length to width ratios, such as those normally seen in amphibole asbestos, (blue or brown), are very good at beating the body’s defence and clearance systems. Amphibole fibres cause particular problems for lung clearance mechanisms. Their aerodynamic shape enables them to pass deep into the airways where they are too large to be removed by the body’s immune system. This results in an accumulation of long fibres, with the short fibres being preferentially cleared.
Furthermore, asbestos fibres are good at resisting the acid attack within macrophages. Macrophages are the body’s defence system for surrounding, engulfing and neutralising alien fibres.
In summary, the physical properties of asbestos fibres facilitate its passage into the deep lung and pleura, and its chemical properties resist its removal by the body’s defence systems.
A carcinogen is a substance that induces a cancerous change in a cell. The latency period (the time between exposure to asbestos and onset of the cancer) in asbestos cases can be anything from 20 to 50 years or more. Over this long period the asbestos fibre will aggravate and irritate with the cells to ultimately cause cancer. The length and width of an asbestos fibre is thought to be critical to causing mesothelioma. Some studies have shown that fibres more than 8mm long and less than 0.25mm wide are the most toxic.
The biology of mesothelioma is complex and incompletely understood even now. The mechanism by which asbestos induces malignant change in the mesothelium (the linings of the lung or abdomen) are not entirely understood.