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October 2006WASHINGTON, DC-A committee of the Institute of Medicine (IOM) here has concluded that there is enough scientific evidence to state that exposure to asbestos causes cancer of the larynx. Asbestos-exposed smokers may be at even higher risk of laryngeal cancer because of the potential interaction between inhaled asbestos particles and smoking-triggered laryngeal irritation.
The IOM committee, chaired by Jonathan M. Samet, MD, MS, Professor and Chairman of the Department of Epidemiology at Johns Hopkins University’s Bloomberg School of Public Health, found that there is suggestive but insufficient evidence that asbestos exposure causes cancers of the pharynx, stomach, colon, and rectum.
As for esophageal cancer and asbestos exposure, the committee, which included former American Medical Association President Lonnie R. Bristow, MD, concluded in its pre-publication report, Asbestos: Selected Cancers, that the scientific evidence is not sufficient to draw any conclusions. The term asbestos is applied to several mineral species when they occur in a fibrous form. Asbestos particles can do damage when they are inhaled.
The new IOM study was funded by a contract with the National Institutes of Health at the request of Senator Arlen Specter (R-Pa.), a senior member of the Senate Appropriations Committee and a champion of miners’ health and safety. His state, Pennsylvania, has a history of mining.
Fairness in Asbestos Injury Resolution Act
Senate Bill 852, the Fairness in Asbestos Injury Resolution (FAIR) Act, was the impetus behind the IOM committee’s charge of determining whether asbestos was causal in the five cancers studied.
If passed, this bill would establish an industry-funded $140 billion trust fund for monetary compensation of people suffering from the adverse health effects of working with asbestos or of living in Libby, Mont., a high-exposure town. Laryngeal cancer is relatively uncommon: there will be about 9500 new cases this year, according to the American Cancer Society (ACS). About 3740 people will die of laryngeal cancer this year, according to ACS data.
During the 20th century (especially during the years of World War II), asbestos was used in many products, including home insulation and roofing products, and people are still exposed despite a drop in production since the 1980s.
Jonathan M. Samet, MD, MS, chaired the committee that wrote the report. The Institute of Medicine undertook the study to answer questions raised in congressional discussions about compensation for people with ailments associated with asbestos exposure. The study committee assessed the quality, limitations, and applicability of 120 epidemiological studies of asbestos exposure and cancers of the throat and digestive tract, as well as information from 200 experimental studies.In 1973 the U.S. Environmental Protection Agency banned the spraying of asbestos insulation in housing, and other restrictions later followed. Asbestos fibers are known to be carcinogenic. Asbestos has already been shown to be causally linked to lung cancer and mesothelioma, a rare tumor of the pleural and peritoneal mesothelium.
Asbestos has also been linked to asbestosis, which is characterized by fibrosis of the lung and reduced lung function. In the 1950s, epidemiologists documented the link between lung cancer and asbestos exposure, especially in exposed workers who smoked.
U.S. asbestos-using industries have included insulation plants; textile plants; friction products plants; paper, packing, and asphalt products plants; cement pipe plants; and cement shingle, millboard, and gasket plants.
Although exposures in occupational settings have been much higher than those in residential settings, the committee pointed out that some activities in the home-such as shaking out work clothes of an asbestos worker-can approach the exposure levels found in the workplace.
Highest Exposures
The highest exposures have been among workers employed in plants that manufacture asbestos products, or those employed by mining and milling companies. Population exposure can occur not only through inhalation, but also through the consumption of asbestos in drinking water, the IOM committee found.
Asbestos can enter the drinking water supply from erosion of natural deposits, mining operations, or asbestos-containing cement pipes. As the science base has increased, exposure limits considered acceptable for occupational exposure to asbestos have dropped over time, the report found.
In exploring the link between asbestos and other cancers, if any, the committee reviewed 120 epidemiological studies of asbestos exposure and cancers of the throat and digestive tract. It also examined the evidence from about 200 experimental studies.
The base of evidence linking asbestos exposure causally to laryngeal cancer included more case-control studies-18-than were available for other cancer sites reviewed by the committee. The number of cohort populations, 35, was similar to those for stomach or colorectal cancer.
Exposed individuals in the studies reviewed by the committee had come in contact with asbestos in many industries and in many geographical areas, including North America, South America, Europe, and Japan.
For laryngeal cancer, the IOM committee identified consistency of findings among the epidemiologic studies. Further, it found that asbestos exposure was associated with increased risk of laryngeal cancer in all the nine larger cohort studies and in meta-analyses of the cohort and case-control data.
Evidence of Dose-Response Relationship
There was also some evidence of a dose-response relationship in both the cohort and case-control studies of asbestos exposure and cancer of the larynx. It was the consistency of study findings that led the committee to conclude that the evidence is sufficient to infer a causal relationship between asbestos exposure and laryngeal cancer.
Asbestos fibers-as contrasted to other particulate matter in the air-are longer than they are wide. The IOM committee found that an asbestos mineral’s property, such as its particle size, shape, dissolution, and precipitation, can affect its potential relationship to pathogenesis, especially when it is breathed in.
In general, the smaller the particle, the farther it can travel in the human body before it is deposited-a function of gravity. Smaller particles are usually deposited in the lower airways, whereas larger particles are deposited higher in the respiratory tract, the committee found.
In contrast with studies of asbestos fiber deposits in the lower respiratory tract, there are few data on fiber deposition and clearance in the upper respiratory tract. Although data show that a fraction of inhaled fibers would be deposited in the larynx, those that are can be damaging, especially in smokers.
For example, available studies show that tobacco-smoking and other causes of chronic laryngeal irritation might interfere with the clearance of asbestos fibers from laryngeal mucosal surfaces.
Regarding the dose of inhaled asbestos fibers known to be harmful, the data are scant. The IOM committee reached the conclusion that the most relevant dose measure for cancer is probably the cumulative number of fibers that reach and persist in the target organ. The biologically effective dose is related to fibers that interact with target cells.
©2006 The Triological Society