There are four main diseases associated with inhalation of asbestos fibres. These are asbestosis (a scarring of the lung tissue caused by asbestos), two kinds of cancer (mesothelioma and asbestos related lung cancer), and diffuse pleural thickening (a non-malignant disease affecting the lung lining).
- Lung cancer
- Pleural thickening
Current evidence suggests that asbestos can also cause laryngeal cancer and may be implicated in causing phayngeal, stomach and colorectal cancers.
Asbestosis is defined as lung fibrosis caused by the inhalation of asbestos fibres. Diagnosis is made on the basis of clinical features, X-ray appearances and a history of heavy asbestos exposure. It is generally recognised that heavy asbestos exposures are required in order to produce clinically significant asbestosis within the lifetime of an individual. Current trends therefore still largely reflect the results of heavy exposures in the past.
Mesothelioma is a formerly rare form of cancer in which malignant (cancerous) cells are found in the mesothelium, a protective sac that covers most of the body’s internal organs. It principally affects the pleura (lining of the lungs) and peritoneum (surrounding the lower digestive tract). Most of the people who develop have worked on jobs where they inhaled asbestos particles.
Working with asbestos is the major risk factor for mesothelioma. The typically long delay between first exposure to asbestos and death from mesothelioma (seldom less than 15 years, but possibly as long as 60 years) means that deaths occurring now and most of those expected to occur in the future reflect industrial conditions of the past rather than current work practices. This latency period means that the effectiveness of current controls cannot yet be assessed from the mesothelioma mortality figures.
A history of asbestos exposure at work is associated with about 80 percent of all cases. However, mesothelioma has been reported in some individuals without any known exposure to asbestos. mesothelioma
Updated statistical modelling of mesothelioma deaths during the period 1968-2001 in males aged 20-89 was used to produce an estimate of the future peak number of deaths to males and females of all ages:
- The annual total number of mesothelioma deaths in Great Britain is currently predicted to peak at a level of 1950 to 2450 deaths during the period 2011 to 2015.
- The updated modelling suggests that annual total number of mesothelioma deaths to males aged 20-89 in Great Britain will peak at a level of 1650 to 2100 deaths during the period 2011 to 2015.
- The annual total number of mesothelioma deaths to females in Great Britain is predicted to peak at a level of 250 to 310 deaths, though this estimate is more uncertain since it is derived by simply applying the results for males to the average proportion of deaths among females.
Although these projections rest on a number of uncertain (and largely unverifiable) assumptions, the timing and scale of the maximum annual death toll is not highly sensitive to these uncertainties. What is highly uncertain, is the rate at which the numbers will fall after this peak is reached.
Asbestos has been recognised as an important risk factor for lung cancer for many years. However, although lung cancer death statistics for Great Britain are readily available, the number of deaths attributable to asbestos cannot be determined directly. This is because there are a number of agents that can cause the disease – most importantly, tobacco smoke – and lung cancers resulting from asbestos exposure are clinically indistinguishable from those caused by these other agents
In populations with heavy asbestos exposures there have typically been at least as many, sometimes up to ten times as many, excess lung cancers as there have been. The ratio depends on a range of factors – the most important of which are type of asbestos, level of exposure, age at exposure and smoking – and so one cannot be too precise about the overall ratio.
Two lines of argument suggest that the ratio of asbestos related lung cancers to mesotheliomas in the British population as a whole is towards the lower end of the range of 1-10 estimated from the epidemiological studies.
Firstly a study of lung cancer mortality in relation to indices of asbestos exposure and smoking habits in the west of Scotland suggested a ratio of around two asbestos lung cancers per mesothelioma for this region – which is known to be associated with fairly high asbestos exposures.
Secondly, analyses of mesothelioma deaths in Great Britain by occupation and geographical area suggest that substantial numbers of deaths may have arisen in workers other than those that were most heavily exposed.
On this basis HSE has for some years operated a rule of thumb that there are 1-2 asbestos related lung cancers for every mesothelioma death.
Another disease associated with asbestos exposure is diffuse pleural thickening. The pleura is a two-layered membrane which surrounds the lungs and lines the inside of the rib cage. Some asbestos fibres inhaled into lungs work their way out to the pleura and may cause fibrosis or scarring to develop there. This causes the pleura to thicken and this may show up on a chest X-ray or CT scan.
Pleural thickening occurs in two forms:
- Diffuse pleural thickening extends over a large area and may restrict expansion of the lungs, leading to breathlessness; and
- Pleural plaques are localised areas of pleural thickening, that don’t usually interfere with breathing.