MATERIALS AND METHODS

All male seafarers (n = 30 940) who had worked on board Finnish ships for any time during the period 1960–80 and who had not died before 1 January 1967 were identified from the files of the Seamen's Pension Fund, as published earlier.¹⁹ All residents of Finland since 1 January 1967 have a unique personal identifier (PID) which is used in all the main registers in Finland. The search concerning work history before the year 1967 was done according to the name and the date of birth which in Finland give almost perfect match.

Follow up of cancer through the files of the population based countrywide Finnish Cancer Registry was done automatically using the PID as a key. Primary cancers diagnosed after the date of first employment registered in the Seamen's Pension Fund or on 1 January 1967, whichever was later, and registered in the Cancer Registry file by June 1995 were eligible as cases. The following six sites were studied: lung, bronchus (International Classification of Diseases, ninth revision (ICD-9) code 162), brain/nervous system (191–192), kidney (189), leukaemia (204–208), pancreas (157), and lymphoma and multiple melanoma (201–203). In addition, all histologically defined mesotheliomas were included as cases.

For each cancer case, three (for lung cancer only two) controls were randomly selected from the same seafarer cohort. The controls had to have the same year of birth and to be alive and free of the six cancer types mentioned above when the respective cancer was diagnosed.

The numbers of years at sea in various occupational categories on board ship since 1 January 1938 were collected from the computerised files of the Seamen's Pension Fund. The post on board ship was classified as follows: deck officer, engine officer, radio officer, deck crew member, engine crew member, or catering department crew member. If the occupational category was not available in this register it was searched for in the manual files of the National Board of Navigation in Finland (Seamen's Register and Shipping Register).

The work history was collected according to the type of ship (dry cargo ship, tanker, passenger vessel, icebreaker, other vessel). Information was first collected from the files of the Seamen's Pension Fund and the National Board of Navigation. If the information from these registers was insufficient, the search was continued in Lloyd's Shipping Register, the National Board of Antiquities of the Maritime Museum of Finland, The Finnish Shipowners' Association, Rettig Oy, and finally in the files of certain individuals interested in old ships as a hobby. The occupational vacancies and types of ship were traced for virtually all; only 377 of the 35 934 work years could not be accounted for.

The length of service in the occupational category and on board various types of ship was calculated separately for each person. Deck and engine officers often have service years as crew members during their earlier sailing years. Thus, one person may have work years in more than one occupational category (table 1).

The connection between work history/exposure and cancer incidence was studied by odds ratios (OR). The exposure times were studied both as a dichotomic (thresholds: one day, one month, one year) and as a linear variable. The linear correlation of exposure time with cancer morbidity was statistically tested with logistic regression analysis. The analyses were performed using the LOGISTIC procedure in the SAS 6.12 program.

The results were analysed dichotomically: whether the seafarer had spent at least a given time in service on board a certain type of ship or not, or whether he had worked in a certain post on board ship or not. The results for the cut off points of one day and one month were almost identical, and therefore only the latter ones are shown in this report. The latency time, the time between the exposure and the identification of cancer, was basically set at 10 years, but ORs for 5 and 20 year latency times were also calculated. In the analyses by type of ship, a further stratification into deck personnel (officers and crew) and engine room personnel was made to find possible interactions attributable to different exposure conditions.

It was planned that information on life habits would be collected for cases and controls. This would have been especially important for lung cancer where smoking is such a strong confounder. The vital status of the 299 cases with lung cancer on 1 July 1996 was obtained from the National Population Register Center. This register also includes information on the closest relatives and the contact information for those who have been living in the same household since 1974. Only 18 persons with lung cancer were alive. It was possible to find a living next of kin for 174 (64%) of the deceased seafarers with lung cancer. From similar earlier studies we know that only about 50% of those asked about the smoking and other life habits of their deceased relatives answer the questions. As it would have been possible to get any information for less than one third of the cases, and taking into account the recall bias involved in this kind of survey, the planned interview was abandoned.

RESULTS

The OR between cases and controls of pancreatic cancer for deck officers was 2.00 (95% CI 1.02 to 3.93); the OR did not, however, grow with the increase in latency time or with employment time as a deck officer (table 2). The OR for engine officers was significantly low (0.32 to 0.27 depending on the length of the exposure time). The changes in latency time had no effects on the results.

The OR of lung cancer for deck officers was 0.42 (95% CI 0.29 to 0.61; table 3). Among engine crew, the OR of lung cancer increased with the increase in employment time; after three years it was 1.68 (95% CI 1.17 to 2.41). The OR per linear increase in 10 years was 1.22 (95% CI 1.00 to 1.63). Deck personnel who had been working more than one month on icebreakers had a significantly increased OR with latency times of one year (2.21, 95% CI 1.03 to 4.77), five years (2.22, 95% CI 1.00 to 4.93), and 20 years (3.41, 95% CI 1.23 to 9.49), but not with a latency of 10 years, a priori defined as the most likely latency.

The OR in mesothelioma among engine crew was 7.50 (95% CI 1.53 to 36.7) when a latency time of 10 years was used (table 4). With a latency time of 20 years, the OR increased to 9.75 (1.88 to 50.6). The OR also tended to be increased among engine officers.

The OR for kidney cancer among deck officers after three years employment was 2.15 (95% CI 1.14 to 4.08), and there was an increase by exposure time. The OR per linear increase in 10 years was 1.34 (95% CI 1.00 to 1.79; table 5). The OR per linear increase in 10 years exposure time among deck personnel on tankers was 11.8 (95% CI 1.34 to 97.0).

The OR in brain and nervous system cancer was close to 1.0 in all categories of seafarers (table 6). A trend by exposure time was seen among engine officers. The OR per linear increase in 10 years exposure was 1.62 (95% CI 1.00 to 2.59).

The only category of seafarers showing a (borderline) significant OR for lymphoma/myeloma was the deck personnel on tankers; their risk also increased with increasing time at work (table 7).

The risk pattern for leukaemia was similar to that for lymphoma, the deck personnel on tankers having an even clearer excess of leukaemia and a clearer increase with increasing exposure time. The OR per linear increase in 10 years exposure was 4.41 (95% CI 1.00 to 20.1; table 8).

DISCUSSION

The present study concerned the eight types of cancer which have been reported to be over represented among seafarers as possible work related diseases. Cancers considered to be connected entirely with sunburn (skin melanoma) and life habits (certain alcohol related cancers) were excluded. Because most female seafarers had been working in the catering department, only male sailors were included in this study.

The sailor cohort collection was unselected in terms of cancer incidence or general mortality—that is, possible later cancer diagnosis and death neither decreased nor increased the likelihood of a person being included in the cohort.¹⁹

The working history of each cancer case and the controls was clarified from the files of the Seamen's Pension Fund and from the registers of the Finnish Maritime Administration. Shipowners have been obliged to report every worker's days on board ship to this central register since 1938. These registers are used for official, legal, and insurance purposes, and also for official national statistics in the maritime industry; they are therefore very accurate. Additional details were collected from historical documents, for example, Lloyd's Shipping Register, which is one of the most reliable registers in the shipping industry. Information on some of the very old working years and the years during the Second World War were difficult to find, but less than 1% of all working histories remained unsolved.

The OR of lung cancer among engine crew increased borderline significantly with increasing years at work. This suggests that engine room conditions may include factors that add to the risk of lung cancer, such as polyaromatic hydrocarbons (PAHs) and asbestos.²⁰ Old steamers were coal driven, and crew members shovelling coal into ovens were exposed to coal dust, combustion products, heat, and the strain of heavy physical work. Combustion products have been reported to increase the risk of pulmonary cancer.²¹ Unfortunately, on the basis of the information available it was not possible to analyse work on steamers separately from that on board other ships.

The risk of mesothelioma was especially high among engine crew, the OR approaching 10 after 20 years of latency. The OR was also increased among engine crew officers. Asbestos is the only known cause of malignant mesothelioma and it was widely used earlier on board ship. As a result of the vibration of the ship and various repair works of asbestos containing constructions, asbestos fibres can be loosened and spread into the indoor air on board ship.^6,10–12 There should not be any notable dose-response between asbestos exposure and mesothelioma and we did not see such an effect. The long latency also observed in the present study is typical of the asbestos-mesothelioma relation.²²

An increased risk of brain cancer has in some studies been associated with exposure at work, for example, with electric and magnetic field exposure.²³ A fourfold risk of cancer of the brain and nervous system was found previously among Finnish deck and engine officers whose first employment was at an older age.¹⁹ However, no signs of work related risk were observed for cancers of the brain or nervous system in the present study.

The excess of kidney cancer was significantly high (twofold) among deck officers on any ship, and the point estimate was highest (threefold, although not significant) among deck crew on tankers. Exposure to the fumes of gasoline and other cargo chemicals is known to be enormous during the filling of the tanks.^16–18,24 Younger deck officers in particular should take precautions during this procedure. A similarly high relative risk of kidney cancer (SIR after five years of employment 2.8, 95% CI 1.6 to 4.7) reported earlier for Finnish oil refinery workers,²⁵ was suggested to be attributable to the mixture of chemicals in the gasoline, which is also classified as possibly being carcinogenic for humans (group 2B) by the International Agency for Cancer Research.²⁶

The OR of leukaemia was high and statistically significant for tanker personnel; it increased with years at work, strongly suggesting work related factors similar to those for kidney cancer. A similar but weaker trend was seen for lymphoma. It was not possible to find exact data about the contents of cargo or to analyse crude oil and chemical tankers separately. Seamen may be exposed to cargo vapours from gasoline and other light petroleum products on chemical or product tankers and—in lower concentrations—on crude oil tankers.^16–18,24 It has been suggested that these exposures may be a possible reason for the increased incidence of lymphatic and haematopoetic malignancies among mates and able seamen on board ships of these types.⁶ The ORs for dry cargo ships were not increased for any cancers.

There are certain important potential confounders about which we did not have data, such as smoking habits and occupational exposures on shore. We attempted a postal inquiry to obtain answers to these issues, but it did not prove successful. The effect of general smoking prevalence on the ORs of lung cancer is crucial—causing, for example, the artefact that the work as a deck officer would be protective—but it hardly affects the increase of risk with increasing working years as seen among engine crew personnel. For mesothelioma, lymphoma, or leukaemia there are no such strong lifestyle related aetiological factors that could seriously bias our results.

In conclusion, this study indicates that work of deck crew on tankers involved exposure to occupational factors which add to their risk of kidney cancer, leukaemia, and possibly lymphoma. Engine crews evidently had an asbestos related risk of mesothelioma, and asbestos or other agents in the engine room air also seemed to increase their risk of lung cancer.