Several examples of workplace hazards often are quoted to exemplify not only the possible adverse health effects associated with workplace exposures, but also to reveal how a systematic approach to the study of worker populations can uncover important exposure-disease relationships. One such example is that of asbestos. The simple elegance with which the late Dr. Irving J. Selikoff demonstrated the elevated cancer risk among asbestos workers has been documented in an article by Lawrence Garfinkel. It is reprinted here with only slight modification and with the permission of CA-A Cancer Journal for Clinicians (Garfinkel 1984). The tables came from the original article by Dr. Selikoff and co-workers (1964).
Asbestos exposure has become a public health problem of considerable magnitude, with ramifications that extend beyond the immediate field of health professionals to areas served by legislators, judges, lawyers, educators, and other concerned community leaders. As a result, asbestos-related diseases are of increasing concern to clinicians and health authorities, as well as to consumers and the public at large.
Historical Background
Asbestos is a highly useful mineral that has been utilized in diverse ways for many centuries. Archaeological studies in Finland have shown evidence of asbestos fibres incorporated in pottery as far back as 2500 BC. In the 5th century BC, it was used as a wick for lamps. Herodotus commented on the use of asbestos cloth for cremation about 456 BC. Asbestos was used in body armour in the 15th century, and in the manufacture of textiles, gloves, socksand handbags in Russia c. 1720. Although it is uncertain when the art of weaving asbestos was developed, we know that the ancients often wove asbestos with linen. Commercial asbestos production began in Italy about 1850, in the making of paper and cloth.
The development of asbestos mining in Canada and South Africa about 1880 reduced costs and spurred the manufacture of asbestos products. Mining and production of asbestos in the United States, Italy and Russia followed soon after. In the United States, the development of asbestos as pipe insulation increased production and was followed shortly thereafter by other varied uses including brake linings, cement pipes, protective clothing and so forth.
Production in the US increased from about 6,000 tons in 1900 to 650,000 tons in 1975, although by 1982, it was about 300,000 tons and by 1994, production had dropped to 33,000 tons.
It is reported that Pliny the Younger (61-113 AD) commented on the sickness of slaves who worked with asbestos. Reference to occupational disease associated with mining appeared in the 16th century, but it was not until 1906 in England that the first reference to pulmonary fibrosis in an asbestos worker appeared. Excess deaths in workers involved with asbestos manufacturing applications were reported shortly thereafter in France and Italy, but major recognition of asbestos-induced disease began in England in 1924. By 1930, Wood and Gloyne had reported on 37 cases of pulmonary fibrosis.
The first reference to carcinoma of the lung in a patient with “asbestos-silicosis” appeared in 1935. Several other case reports followed. Reports of high percentages of lung cancer in patients who died of asbestosis appeared in 1947, 1949 and 1951. In 1955 Richard Doll in England reported an excess risk of lung cancer in persons who had worked in an asbestos plant since 1935, with an especially high risk in those who were employed more than 20 years.
Clinical Observations
It was against this background that Dr. Irving Selikoff’s clinical observations of asbestos-related disease began. Dr. Selikoff was at that time already a distinguished scientist. His prior accomplishments included the development and first use of isoniazid in the treatment of tuberculosis, for which he received a Lasker Award in 1952.
In the early 1960s, as a chest physician practising in Paterson, New Jersey, he had observed many cases of lung cancer among workers in an asbestos factory in the area. He decided to extend his observations to include two locals of the asbestos insulator workers union, whose members also had been exposed to asbestos fibres. He recognized that there were still many people who did not believe that lung cancer was related to asbestos exposure and that only a thorough study of a total exposed population could convince them. There was the possibility that asbestos exposure in the population could be related to other types of cancer, such as pleural and peritoneal mesothelioma, as had been suggested in some studies, and perhaps other sites as well. Most of the studies of the health effects of asbestos in the past had been concerned with workers exposed in the mining and production of asbestos. It was important to know if asbestos inhalation also affected other asbestos-exposed groups.
Dr. Selikoff had heard of the accomplishments of Dr. E. Cuyler Hammond, then Director of the Statistical Research Section of the American Cancer Society (ACS), and decided to ask him to collaborate in the design and analysis of a study. It was Dr. Hammond who had written the landmark prospective study on smoking and health published a few years earlier.
Dr. Hammond immediately saw the potential importance of a study of asbestos workers. Although he was busily engaged in analysing data from the then new ACS prospective study, Cancer Prevention Study I (CPS I), which he had begun a few years earlier, he readily agreed to a collaboration in his “spare time”. He suggested confining the analysis to those workers with at least 20 years’ work experience, who thus would have had the greatest amount of asbestos exposure.
The team was joined by Mrs. Janet Kaffenburgh, a research associate of Dr. Selikoff’s at Mount Sinai Hospital, who worked with Dr. Hammond in preparing the lists of the men in the study, including their ages and dates of employment and obtaining the data on facts of death and causes from union headquarters records. This information was subsequently transferred to file cards that were sorted literally on the living room floor of Dr. Hammond’s house by Dr. Hammond and Mrs. Kaffenburgh.
Dr. Jacob Churg, a pathologist at Barnert Memorial Hospital Center in Paterson, New Jersey, provided pathologic verification of the cause of death.
Tabe 1. Man-years of experience of 632 asbestos workers exposed to asbestos dust 20 years or longer
|
Age |
Time period |
|||
|
1943-47 |
1948-52 |
1953-57 |
1958-62 |
|
|
35–39 |
85.0 |
185.0 |
7.0 |
11.0 |
|
40–44 |
230.5 |
486.5 |
291.5 |
70.0 |
|
45–49 |
339.5 |
324.0 |
530.0 |
314.5 |
|
50–54 |
391.5 |
364.0 |
308.0 |
502.5 |
|
55–59 |
382.0 |
390.0 |
316.0 |
268.5 |
|
60–64 |
221.0 |
341.5 |
344.0 |
255.0 |
|
65–69 |
139.0 |
181.0 |
286.0 |
280.0 |
|
70–74 |
83.0 |
115.5 |
137.0 |
197.5 |
|
75–79 |
31.5 |
70.0 |
70.5 |
75.0 |
|
80–84 |
5.5 |
18.5 |
38.5 |
23.5 |
|
85+ |
3.5 |
2.0 |
8.0 |
13.5 |
|
Total |
1,912.0 |
2,478.0 |
2,336.5 |
2,011.0 |
The resulting study was of the type classified as a “prospective study retrospectively carried out”. The nature of the union records made it possible to accomplish an analysis of a long-range study in a relatively short period of time. Although only 632 men were involved in the study, there were 8,737 man-years of exposure to risk (see table 1); 255 deaths occurred during the 20-year period of observation from 1943 through 1962 (see table 2). It is in table 28.17 where the observed number of deaths can be seen invariably to exceed the number expected, demonstrating the association between workplace asbestos exposure and an elevated cancer death rate.
Table 2. Observed and expected number of deaths among 632 asbestos workers exposed to asbestos dust 20 years or longer
|
Cause of death |
Time period |
Total |
|||
|
1943-47 |
1948-52 |
1953-57 |
1958-62 |
1943-62 |
|
|
Total, all causes |
|||||
|
Observed (asbestos workers) |
28.0 |
54.0 |
85.0 |
88.0 |
255.0 |
|
Expected (US White males) |
39.7 |
50.8 |
56.6 |
54.4 |
203.5 |
|
Total cancer, all sites |
|||||
|
Observed (asbestos workers) |
13.0 |
17.0 |
26.0 |
39.0 |
95.0 |
|
Expected (US White males) |
5.7 |
8.1 |
13.0 |
9.7 |
36.5 |
|
Cancer of lung and pleura |
|||||
|
Observed (asbestos workers) |
6.0 |
8.0 |
13.0 |
18.0 |
45.0 |
|
Expected (US White males) |
0.8 |
1.4 |
2.0 |
2.4 |
6.6 |
|
Cancer of stomach, colon and rectum |
|||||
|
Observed (asbestos workers) |
4.0 |
4.0 |
7.0 |
14.0 |
29.0 |
|
Expected (US White males) |
2.0 |
2.5 |
2.6 |
2.3 |
9.4 |
|
Cancer of all other sites combined |
|||||
|
Observed (asbestos workers) |
3.0 |
5.0 |
6.0 |
7.0 |
21.0 |
|
Expected (US White males) |
2.9 |
4.2 |
8.4 |
5.0 |
20.5 |
Significance of the Work
This paper constituted a turning point in our knowledge of asbestos-related disease and set the direction of future research. The article has been cited in scientific publications at least 261 times since it was originally published. With financial support from the ACS and the National Institutes of Health, Dr. Selikoff and Dr. Hammond and their growing team of mineralogists, chest physicians, radiologists, pathologists, hygienists and epidemiologists continued to explore various facets of asbestos disease.
A major paper in 1968 reported the synergistic effect of cigarette smoking on asbestos exposure (Selikoff, Hammond and Churg 1968). The studies were expanded to include asbestos production workers, persons indirectly exposed to asbestos in their work (shipyard workers, for example) and those with family exposure to asbestos.
In a later analysis, in which the team was joined by Herbert Seidman, MBA, Assistant Vice President for Epidemiology and Statistics of the American Cancer Society, the group demonstrated that even short-term exposure to asbestos resulted in a significant increased risk of cancer up to 30 years later (Seidman, Selikoff and Hammond 1979). There were only three cases of mesothelioma in this first study of 632 insulators, but later investigations showed that 8% of all deaths among asbestos workers were due to pleural and peritoneal mesothelioma.
As Dr. Selikoff’s scientific investigations expanded, he and his co-workers made noteworthy contributions toward reducing exposure to asbestos through innovations in industrial hygiene techniques; by persuading legislators about the urgency of the asbestos problem; in evaluating the problems of disability payments in connection with asbestos disease; and in investigating the general distribution of asbestos particles in water supplies and in the ambient air.
Dr. Selikoff also called the medical and scientific community’s attention to the asbestos problem by organizing conferences on the subject and participating in many scientific meetings. Many of his orientation meetings on the problem of asbestos disease were structured particularly for lawyers, judges, presidents of large corporations and insurance executives.