Asbestos is used extensively by industry and individual consumers because of its unique combination of physical properties and ready availability at reasonable prices.
Its fire and heat resistance makes it an excellent insulation material. It is also used in the manufacture of textiles, cement, and brake linings. There are, in the United States today, over one-hundred manufacturers of asbestos products.
Several types of asbestos fibers are available commercially: chrysotile, crocidolite, and amosite. In the
The United States, over 90% of the asbestos used by industry is chrysotile. The major producer is Quebec, Canada.
Four diseases have been linked to the inhalation of asbestos. They are pulmonary fibrosis, lung cancer, mesothelioma, and pleural calcification.
Asbestosis, or pulmonary fibrosis, was the first asbestos-related disease to be recognized by medical authorities. It is characterized by the formation of inactive, non-functioning fibrous scar tissue in the lung. Extensive damage can eventually result in respiratory or cardiac failure.
Symptoms include shortness of breath, finger clubbing and basal rales (abnormal breathing sounds). Although the existence of asbestosis as an occupational disease is now established, it was many years before the disease was widely recognized.
In 1907, the first case of extensive lung scarring in a British textile worker was reported.” American
recognition occurred twenty years later with the report of a miner who died of heart failure associated with fibrosis.
These early communications and more recent epidemiologic studies highlight one important characteristic of asbestosis. It occurs mainly among those occupational groups experiencing heavy or moderate exposure.
Extensive documentation in the United States has shown asbestosis to be common in the textile, insulation, and shipbuilding industries. The exposure period is 20-40 years, with death following about two to 10 years later.
Lung Cancer due to Asbestos
The most common complication of asbestosis is cancer of the lung. The first case of lung cancer in a worker suffering from asbestosis was reported in 1935. Later studies showed the incidence of cancer among asbestos occupations to be greater than selected control groups.
A British researcher, in fact, reported that textile workers incurred a 10-times higher risk of having lung cancer than the general population. Cancer has been specifically linked to chrysotile fibers, the most common type used in the United States.
Chrysotile miners and building product workers died of respiratory cancers at a greater rate than would have been anticipated based on general mortality rates. Recent evidence indicates that the dangers of asbestos-related cancers will increase dramatically in the future.
An epidemiologic study of Dresden, Germany, asbestos workers indicated that lung cancers became more common as observation continued. This was due, in part, to the longer latency period required for the development of cancerous tumors compared to pulmonary asbestosis.
Researchers found that death from pulmonary asbestosis occurred at an average of 25.7 years from onset of exposure, whereas 30.7 years was required for the development of cancer.
An additional complication is provided by the recent revelations indicating a co-carcinogenic or synergistic effect with cigarette smoking.
Calculations by New York City scientists indicate that asbestos workers who smoke have about ninety-two times the risk of dying from lung cancer as individuals who neither work with asbestos nor smoke.
A disturbing sidelight is that other synergistic effects may come to light in the future. Malignant tumors of the lung or chest cavity, known as mesothelioma, were recently regarded as very rare.
In 1960, 33 cases of pleural mesothelioma were described in a South African mining town. All but one of the exposures was linked to the inhalation of crocidolite asbestos. Five years later, mesothelioma was linked to chrysotile asbestos in the United States and the Netherlands.
It soon became apparent that mesothelioma also posed a substantial threat to individuals experiencing low exposure levels. In Canada, mesothelioma was found more common in insulation workers than miners.
In London and Capetown, a higher-than-expected incidence of mesotheliomas was found in people living near asbestos factories and fields.
This was verified in New Jersey, where researchers found mesothelioma cases near an asbestos mill. A comparison between cancer and mesothelioma deaths showed people with mesothelioma dying at a younger age, thereby lending support to the low-level exposure theory.
Initial exposure to asbestos occurred 25 to 33 years before deaths from mesothelioma. Thus far, there have been few cases reported among the general population. A review of several thousand deaths in the United States revealed only three mesothelioma cases.
To complicate matters, asbestos is not the only cause of mesothelioma; it has also been produced by silica and polyurethane.
Another disease often associated with low exposure to asbestos is pleural calcification, the depositing of insoluble calcium salts in the lining of the lung.
Epidemiologists found pleural calcification in Finnish farmers living near an asbestos mine and Bulgarian agricultural workers in an asbestos-rich soil region.
New York insulation workers also showed signs of calcification 20 years after their initial exposure. A potential hazard to children has been demonstrated. Aggravation of pulmonary disease has been the major finding among children living near an asbestos plant.
Effects on the General Community
Research concerning the health effects of asbestos on the general public has been focused in two areas. The first has been a determination of the prevalence of “asbestos bodies” in the lungs of urban dwellers. The second has been a measurement of asbestos contamination during and after building construction.
The presence of “asbestos bodies” in the lungs of occupationally exposed groups has been acknowledged for some time.
It was not until 1960, however, that it was realized how widespread the existence was in the general population. Pathologists found 26.4% of lung smears from 500 consecutive autopsies in Cape Town, South Africa, contained these “bodies.”
A more recent study in Pittsburgh found the percentage of “ferruginous bodies” to be as high as 98. These researchers preferred to use the term “ferruginous bodies” because optical microscopy techniques do not permit positive identification of asbestos fibers in these bodies.
They demonstrated that other materials, such as cosmetic talc and glass fibers, can produce ferruginous bodies indistinguishable from those produced by asbestos. Positive identification of an asbestos fiber core can only occur with high magnification electron microscopy.
Using this technique, chrysotile asbestos was identified in 28 consecutive samples of lung tissue in New York City. Most of the asbestos was present as fibrils (thin fibers 200-400 angstroms in diameter). Due to its brittleness, chrysotile tends to split into these fibrils when inhaled.
It is estimated that 200 to 1,000 fibers and fibrils can be seen by electron microscopy for every fiber identified by optical microscopy.
The deleterious effect of these short and thin fibers has not yet been determined. Evidence exists to suggest both a harmful and non-harmful consequence.
J.G. Thomson, in explaining his views on the harmful aspects of short-fibered asbestos, concluded
Asbestos fibers of this diameter as an air contaminant in cities of significant degree now or in the future would constitute a menace, against which man would be relatively powerless.
The source of these fibers are the many asbestos-containing products being used in our society. One of these, asbestos fireproofing, was used extensively in high-rise office buildings from 1958 until its ban in 1973.
Use of Asbestos
In 1970, alone, more than 40,000 tons of asbestos material was used for this purpose. Measurements of asbestos emissions during and after the application of fireproofing material have been conducted.
It was estimated that 15 tons of asbestos were emitted from these materials during building construction in 1968. Asbestos fiber concentrations at the construction sites ranged from 30 to 100 fibers per cubic centimeter.
(The current government standard is two fibers per cubic centimeter.) Concentrations were still high 75 feet away from the operation.
After construction ends, one can still detect asbestos levels. Mass analysis techniques showed the average asbestos levels in 19 United States buildings to be between 2.5 and 200 nanograms per cubic meter. (A nanogram is equivalent to 10^-9grams.)
Concentrations above 100 nanograms/cubic meter were considered indicative of a potential harm because this was the level found in the homes of asbestos – insulation workers (the source is presumed to be the dust brought home in the workers’ overalls).
British Survey Vs American Survey
The fibrous spray-on fireproofing was found to contribute more towards pollution than cementitious fireproofing. In contrast, fiber counting procedures were employed during a British survey of 60 buildings, including schools, offices, and residences, in which asbestos was used for insulation, fireproofing, and/or other application.
In over 90% of the locations sampled, the asbestos dust concentrations did not exceed one-tenth of the level regarded as acceptable for occupational exposure. More than 40% of the concentrations were of the same order as the level in buildings where no asbestos had been used.
It is difficult to compare these two surveys. The methods used are different. The British survey relied on fiber counts; the American study did not. The American scientists considered small particles and fibers in their mass determinations; the British excluded fibers less than five microns in length.
Neither group of researchers provided information on when their asbestos samples were taken. Samples taken during business hours will show more fibers than those taken at night as a result of the movement of people causing air currents that circulate the fibers.
Researchers in New York City found chrysotile concentrations ranging from 10 to 50 nanograms per cubic meter, and European investigators found asbestos fibrils present in all the towns 59 surveyed.
This article was published by the University of Massachusetts in September 1976, prepared by Douglas Gilbert, Research Assistant Massachusetts Asbestos Commission and Science Resource Office, Massachusetts General Court.