Posted By Adrian Watson
Dear All,
There are a number of issues and misconceptions. Firstly, the main problems with epidemiology is that you cannot detect small changes above background and most asbestos studies were done on highly exposed populations. It is a simple rule you should compare like with like. Peto's studies relate to smoking populations that were exposed to high levels of asbestos.
Secondly, cancer in simple terms is a process where the cells can grow in an uncontrolled manner. This process is caused by mutations within the cell. As a molecule, fibre etc can cause a mutation in a cell, the idea went that this one cell could develop, proliferate and become cancer ... so, far so good. However what the one fibre theory omitted was, was the fact that cell mutations are normal and occurs on a regular basis. So much so that the body has an effective defence mechanism against them. The accepted theory now is that cancer is a multifactorial and multistage process with at least three stages. These stages are mutation, initiation, proliferation and growth. The first two stages, mutation and initiation, are in some cases combined.
The most common cause of cell mutation in the body is cell reproduction; therefore anything that increases cell reproduction, such as cell repair, increases the risk of cancer. This is supported by the fact that surgery, breaks etc are known to increase the risk of cancer. In fact one of the known causes of mesothelioma is a thoracotomy to removed tubercular nodules.
Going back to asbestos, with amphibole asbestos fibres, the fibres are small, sharp and have a long half-life. This means that they will reach the respiratory regions of the lung. On the way there they will impact into the lung tissue and once there they will impact into the lung. This means that they should increase the risk of lung cancer, which they do. They are 10-50 times more likely to cause lung cancer than white asbestos. Once in the respiratory region there is impaction into the lining of the lung as well as transfer of the fibres into and through the pleural cavity by the macrophages and the lymphatic system. The pleural cavity is in fact the normal dumping ground for the lymphatic system.
Once in the pleural cavity the macrophages and the immulological system will attempt to deal with the asbestos fibres. Unfortunately this process causes cellular injury and cellular repair, as well as the release of cytokines by macrophages. Cytokines are chemicals that the macrophages use to increase cell division in bacteria as a means of killing the bacteria.
These processes increase the risk of mutations. In some people this process will result in mesothelioma. The risk being a result of the type of asbestos, the number of fibres and some factors relating due to the individual such as genetic susceptibility to cellular mutation, immune status as well as the responsiveness and effectiveness of the immune system as well as cofactors much as disease and smoking. Because of the nature of the process and the interaction between the agent and the individual the risk becomes stochastic; i.e. the more you are exposed the greater the risk but in no individual does exposure equal disease.
However, in white, serpentine, asbestos the fibres are long, curly with a short half-life. This means that they do not reach the parts that the other asbestoses reach in such large numbers, and do not stick in lung tissue as readily on the way down. This means that they should pose a reduced risk of lung cancer, when compared against other types of asbestos, as they do. Once in the respiratory region they do not as readily breach the barrier into the pleural cavity and they do not remain as long. All these factors make it less likely that they cause mesothelioma in man. In fact there is strong evidence that they do not cause mesothelioma in man. However, we know that they can cause mesothelioma in rats through implantation studies. Unfortunately, white asbestos is frequently contaminated with 1% tremolite that will cross the barrier and has a longer half-life. This is believed to cause the mesothelioma in the cohorts exposed to white asbestos.
Thirdly, in respect of wives having low exposure top asbestos, you have to remember back to 30-40 years ago. In the 1960's coveralls were mainly made of heavy-duty cotton, which accumulated dust and held onto it, so there was substantial dust exposure from it. Then prior to washing machines being generally available how many people remember the copper, mangle and the general washing processes. This would have caused substantial exposure. So just because a person was at home did not mean that they had low exposures.
I hope that this helps.
Regards Adrian Watson