Disease or disability in individuals exposed to asbestos has been substantial in number and in human pain and suffering. Future injuries from asbestos and cigarette smoking are projected to continue well into the next century in people with occupational and bystander exposure to asbestos. Present or past cigarette smokers with asbestos exposure are at even greater risk for life-threatening disease than people with either asbestos exposure or cigarette smoking alone because both risks together can multiply the risk. The cancers known to be increased by cigarette smoking - lung, laryngeal, pharyngeal and buccal mucosa, esophageal, stomach, kidney and colon cancers - are also impacted to a greater or lesser degree by inhalation of asbestos fibers.
Are asbestos diseases aggravated by cigarette smoking?
The three major asbestos-related diseases that have been studied are asbestosis (pulmonary fibrosis, or scarring, mostly of the lower lobes of the lung), lung cancer, and mesothelioma, (a cancer of the lining of the lung or abdomen). All three are usually fatal. The last two are rapidly fatal with an average time from diagnosis to death of about twelve months (Cecil et al., 1988).
Pulmonary Asbestosis and Cigarette Smoking
Discovered in the early 20th century, pulmonary fibrosis resulting from asbestos inhalation was well understood by 1930. However, ongoing unprotected worker exposures occurred well into the 1 980s. Animal studies show that cigarette smoking increases pulmonary fibrosis by increasing the penetration of asbestos fibers into the lung (McFadden et al., 1986).
The lungs clean themselves with the help of ciliated cells along the airways that act in a conveyor-belt manner to lift microscopic sized foreign bodies out of the airways. Cigarette smoking decreases the effectiveness of this mechanism. In smokers, asbestos fibers are removed from the lung less efficiently than in non-smokers, so more fibers get deeper into the lungs. Smoking also increases the permeability of the airways to asbestos fibers. Thus, tobacco smoke indirectly increases the asbestos burden in the lungs.
In animals exposed to cigarette smoke and asbestos a 30-fold increase was seen in total numbers of (asbestos) fibers retained in macrophages in smokers compared to non-smokers by one month, and there was an eight-fold increase in retention of short fibers in lung tissue by one month." (Churg). Churg has also shown increased asbestos burden in human cigarette smokers and suggests this may explain one of the ways asbestos fibers cause disease (Churg, et al., 1995; Churg, et al., 1992).
McFadden et al. demonstrated more than ten years ago that cigarette smoking increased the number of asbestos fibers in the lung (McFadden et al., 1986). Weiss et al. (1984) reviewed 21 scientific studies on cigarette smoking and pulmonary fibrosis of different workers exposed to asbestos whose smoking status was also known. He concludes, "A majority of (studies) of asbestos workers with information on smoking habits have shown a positive interaction between the two agents." (cigarettes and asbestos) "The interaction is thought to be additive rather than synergistic. Smoking may exert an effect on the frequency of pulmonary asbestosis by increasing the effective fiber dose retained in the lungs through interference with clearance."
The lung reacts to all inhalation injuries by laying down areas of fibrosis or scar tissue. The fibrosis caused by cigarette smoking is not itself distinguishable from that caused by asbestos. Cigarette smoking has an additive effect on asbestosis seen radiographically, and combined exposure causes more pulmonary fibrosis than either exposure to cigarette smoke or asbestos alone. "We conclude that in the setting of heavy occupational exposure to asbestos, cigarette smoking confers added risk for the development of roentgenographic small opacities." (Barnhardt et al., 1990).
Kilburn et al. (1986) studied smoking and non-smoking shipyard workers in Long Beach, California. He found smoking insulators exposed to asbestos had a 20% incidence of pulmonary fibrosis compared to non-smoking insulators with a 7.2% incidence of fibrosis. He concluded that cigarette smoking and asbestos exposure worked synergistically to cause pulmonary fibrosis.
Blanc et al. (1988) calculated that cigarette smoking increased the likelihood of developing asbestosis (pulmonary fibrosis) thirteen-fold. Other prospective controlled studies drew similar conclusions (Lilis et al., 1991; Schwartz, 1992). The combined effects of cigarette smoking and inhalation of asbestos in causing asbestosis (pulmonary fibrosis) has been confirmed in other countries. A Finnish study of 7095 adults concluded that asbestos-exposed workers who smoked had an increased incidence of pulmonary fibrosis and pleural disease.
Pearle et al. compared smoking asbestos workers to non-smoking asbestos workers. Cigarette smoking increased the presence of radiographic pulmonary fibrosis from asbestos and hastened abnormalities in pulmonary function leading to disability (Pearle, 1982). He concluded that "Smoking contributes to many of the functional and roentgenographic (x-ray) abnormalities in asbestos workers."
Non-Malignant Pleural Disease
There are four types of non-malignant pleural disease caused by inhalation of asbestos:Pleural plaques, diffuse pleural thickening, rounded atelectasis and pleural effusions.
Several studies have found a clear association between pleural thickening, asbestos exposure and cigarette smoking. Harries et al. (1975) studied 19,000 dockyard workers exposed to asbestos. Pleural thickening was found in 2.7% of 5,552 non-smokers and 4.9% of smokers and 6.4% of ex-smokers.
Hedenberg et al. found pleural plaques in 30% of 103 smokers and 19% of 94 non-smokers (1979).
Weiss et al. found a positive correlation between cigarette smoking and the presence of pleural disease (1981).
Findings of a direct correlation between cigarette smoking and radiographic abnormalities such as pleural plaques, thickening and interstitial fibrosis - have also been confirmed by Pearle et al. (1982) and Bader et al. (1970).
In is fair to say that "We conclude that among men exposed to asbestos dust at work the risk of developing an asbestos-related lesion is related to smoking habit in addition to the duration, intensity of exposure and time since first exposure; smokers and ex-smokers may have a significantly higher risk than non-smokers." McMillan et al. (1980).
Asbestos Exposure, Lung Cancer and Cigarette Smoking
Selikoff compared smoking and non-smoking asbestos-exposed workers and discovered a "multiple" or "synergistic effect" (Selikoff, 1968). He found that men not exposed to asbestos or cigarette smoking had a negligible risk of less than one, called X. Smoking non-asbestos-exposed workers had a ten-fold increased risk, of 10x. Asbestos-exposed workers had an eight-fold increased risk, or 8x. But smoking asbestos-exposed workers had roughly an 80-fold greater risk than non-smoking non-asbestos-exposed workers. In other words, the risk was not just additive, i.e., 10+8=18, but multiplicative, i.e., 10x8=80 (The Health Consequences of Smoking: Cancer and Chronic Lung Disease in the Workplace, A Report of the Surgeon General, 1985, p. 218). The risk held regardless of the specific occupation of the asbestos-exposed individual.
In one very well done and respected study, Selikoff stated, "The increased risk of neoplasia (mainly bronchiogenic carcinoma and mesothelioma( among insulation workers reported here should be evaluated in the knowledge that these men have comparatively light exposure as asbestos trades go. Primarily employed in construction work, many of the materials used by such workers contain little or no asbestos and others have only 5% to 15%. Conditions of work vary; these men often work outdoors unlike asbestos operators engaged in factory work. Comparatively few dust exposure surveys have been made in this trade but their results have generally been within the five million particles per cubic foot permissible limits currently accepted by the American Conference of Governmental Industrial Hygienists." (Selikoff, 1968). Nonetheless, he showed that 20% of insulators died on lung cancer. (Selikoff, 1973).
This multiplicative effect has been confirmed by other investigators in the field including Doll, Berry, and Newhouse.
Among physicians there are legitimate differences as to what constitute the necessary criteria to demonstrate that a lung cancer is caused in part by asbestos. Some investigators demand the presence of radiographic or pathologic proof of asbestosis, i.e., pulmonary fibrosis caused by asbestos dust. Others require the presence of an "occupational number" of asbestos fibers in tissue, or other markers of asbestos injury like pleural disease on x-ray (Hughes, 1991; Liddell, et al., 1980).
Asbestos, Cigarette Smoking and Other Cancers Mesothelioma
Mesothelioma is a rapidly fatal cancer of the lining of the lung and abdomen. These cancers have been described since 1870. In 1960 Wager (Wagner et al., 1960) showed a causal relationship between asbestos exposure and these cancers. Experimental work in animals and additional epidemiological work have established that the only known cause of mesothelioma in the United States is asbestos exposure.
In the course of research on mesothelioma and asbestos "burden," studies have been performed identifying the number and type of asbestos fibers present in the lungs of people with mesothelioma. Factors that increase the number of asbestos fibers in the lung, such as cigarette smoking, have also been studied.
A study by McDonald et al. (1980) found that female smokers had a greater incidence of mesothelioma than non-smoking females exposed to asbestos. Mesothelioma is invariably disabling, intensely painful and almost always fatal in less than fourteen months (Cecil, 1988). Mesothelioma exhibits a 'dose response' effect with asbestos. This means that the more asbestos in the lungs, the greater the chance of developing a mesothelioma.
Cigarette smoking plays a key role in increasing the asbestos burden in the lung, because mechanisms used to clear asbestos from the upper respiratory tract are impaired by cigarette smoke, which also increases the amount of asbestos in lungs by facilitating passage of asbestos fibers through deeper airways in the lungs.
McFadden states "Increased penetration of fibers into the airway epithelium and eventually into the interstitium, resulting in functionally increased doses of fibers to the tissues, may be one of the mechanisms whereby smoking potentiates asbestos disease."
Weiss states "However, the effect of cigarette smoking may be exerted through increase in the effective dust dose deposited in the lung tissue. The damaging effects of cigarette smoke on the entire respiratory tract probably produce a vicious cycle leading to serious disruption of defense mechanisms by changing protease-antiprotease balance, compromising immune systems and interfering with clearance processes."
Laryngeal Cancer
Laryngeal cancer is diagnosed in more than 10,000 Americans each year. Cigarette smoking is the greatest promoter of laryngeal malignancy. More than a dozen epidemiological studies have shown that cigarette smoking and asbestos exposure act cumulatively to increase the incidence of laryngeal cancer. Sir Richard Doll and Julian Peto conclude that "On the present evidence we conclude that asbestos should be regarded as one of the causes of laryngeal cancer." (Doll & Peto, 1987).
Gastrointestinal Cancers
In their article "Asbestos Exposure and Neoplasia," in 1964, Selikoff, Churg and Hammond report 29 deaths attributable to gastrointestinal cancers of the esophagus, stomach, colon and rectum. In the age-matched non-asbestos-exposed control group 9.7 gastrointestinal cancers occurred. This increased number (29) may have been artificially lower because a substantial number of men in the asbestos-exposed group died prematurely of mesothelioma and lung cancer. However, other studies have not shown the same high incidence of gastrointestinal cancers. While not all gastrointestinal cancers appear to be increased in asbestos exposed individuals, the cancers that are also promoted by cigarette smoking are increased in asbestos-exposed workers.
Sunday, May 27, 2007
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