The relationship between asbestos exposure and diseases such as malignant mesothelioma and lung cancer is well established. But now other diseases not typically associated with asbestos may possibly be linked to occupational and non-occupational forms of exposure.
Asbestos refers to a number of naturally occurring minerals that have crystallised to form long thin fibres and fibre bundles. There are three main types that have been used commercially – crocidolite (blue asbestos), amosite (brown asbestos) and chrysotile (white asbestos).
The difference between these types has to do with the shape and size of their fibres. Crocidolite and amosite have long, straight fibres, while chrysotile fibres are short and curly. The shape of these fibres is thought to be central to the damage they do to human health. The long straight fibres, in particular, are thought to easily penetrate into the lungs.
So although all types of asbestos have been found to cause asbestos-related diseases, some types lead to more of these diseases than others. Blue asbestos (the type that was mined at Wittenoom, Western Australia) is the worst for human health, followed by amosite, and then chrysotile.
Asbestos and health
Diseases most commonly attributed to asbestos exposure are malignant mesothelioma, asbestosis and lung cancer. There’s a clear relationship between the amount of asbestos exposure and the risk of developing mesothelioma and asbestosis, with the risk increasing as the level of exposure increases.
Malignant mesothelioma is a diffuse cancer that spreads over the lining of the lung or stomach. It has a long latency period, rarely developing within 15 years of first exposure. And it is universally fatal – the average survival rate, after diagnosis, is nine months.
Malignant mesothelioma is very rare in people who haven’t been exposed to asbestos. In England, cases were established from autopsy reports between 1910 and 1940, and the disease became more frequently diagnosed in the 1950s. The link between asbestos (crocidolite) exposure and mesothelioma was formally established in 1960.
Asbestosis is defined as “fibrosis of the lungs caused by asbestos dust”. Patients with well-established asbestosis usually present with symptoms of shortness of breath and a dry cough. It’s a progressive disease but it’s not necessarily fatal. The first case of asbestosis was described in medical literature in 1906, in a 33-year-old man who had worked in an asbestos textile factory for 14 years.
Other kinds of cancers are also more prevalent among workers exposed to occupational levels of asbestos than the general population. These include brain cancers, blood-related disorders (such as leukaemia), kidney cancer, cancer of the larynx, stomach and colorectal cancer. But the evidence that asbestos causes these other diseases is limited, mainly due to a lack of proof for an exposure-response relationship.
Few studies have found an excessive risk of these other cancers developing in people exposed to asbestos. Fewer still have found or reported a causal link.
Will we ever know for sure?
Former workers and residents of the blue asbestos mining and milling town of Wittenoom, Western Australia have been followed up through cancer and death registries and by regular questionnaires for over 30 years. Our latest study has revealed a relationship between cancers other than mesothelioma among people who, as children, lived in Wittenoom.
Now adults, this group were exposed to blue asbestos before the age of 15. We have found an increased risk of brain cancer among both the males and females of this group.
We also found higher rates of leukaemia, prostate and colorectal cancer among males, and ovarian cancer among females, compared with the general Western Australian population. These are very rare cancers, so it’s very difficult to state with absolute certainty that they’re caused by exposure to asbestos. Despite the significantly increased rates of cancer within this group, we may never know for sure whether asbestos is implicated.
The Wittenoom group also have a high rate of heart disease compared with the Western Australian population. Again, the evidence proving that exposure to asbestos causes heart disease is limited. Although we found an increased risk of heart disease in this group, we didn’t find that the risk increased as the level of asbestos exposure increased.
The Wittenoom children are still young and heart disease is not a major cause of death among them. Perhaps a link between heart disease and asbestos exposure will become apparent as they age. Recent work from the United Kingdom has shown that ischaemic heart disease and cerebrovascular disease were more prevalent among a large group of British workers exposed to occupational levels of asbestos. In that study, longer exposure was associated with a greater risk of ischaemic heart disease.
One of the ways we can prove that a disease is caused by asbestos is to find that the risk of disease increases with the amount of asbestos exposure. Where we do not find that relationship, we have to concede that the disease may be caused by other factors the group have in common. We may not have looked for such factors or perhaps were unable to look for them in our studies.
Following up the former Wittenoom children may reveal links between asbestos exposure and these other diseases. The longer they are followed up, the more information we will learn about the diseases they develop, and whether asbestos exposure is the cause.
This knowledge may help those still being exposed to asbestos, because we would be able to correctly identify diseases they may present with. But the best way of preventing asbestos-related diseases is to avoid exposure altogether.