Our study shows that there was an increased mortality from CHD and cerebrovascular disease in relatively younger male seafarers having worked on different types of vessels, especially those with several years of service. We could not clearly show any significantly increased mortality from CHD or cerebrovascular disease among women. Our results demonstrate that Swedish seafarers on different types of vessels, both men and women, had a significantly but modestly increased total mortality. Our results corroborate earlier international studies regarding increased general mortality among seafarers (Brandt et al. 1994; Rafnsson & Gunnarsdottir 1994; Jensen 1996).
We had no information regarding location of death. It is possible that the increased mortality from CVD could partially be attributed to long distance to qualified health care if the illness occurs at sea. In a Polish study, pre-hospital mortality from myocardial infarction was higher, but the incidence was not increased, in seafarers compared with the general working population (Jaremin & Kotulak 2003). The authors concluded that work-related factors reduce survival at sea in the case of a myocardial infarction. However, the long distance is not likely to explain the entire increase as in our study the high mortality figures were also related to seafarer age, type of vessel, and length of registration as a seafarer.
The mortality from CHD was not significantly increased for seafarers who started their sea service after 1985, or for the observation period 2000–2013, but this may partly have been due to lack of power. The modest increase in total mortality decreased over time. There could be several reasons for that; hopefully, the work environment has improved, smoking has probably decreased especially among men, and the length of time as seafarer has overall decreased. In a Danish study analysing mortality in seafarers 1986–2009, the total mortality diminished over time; however, there was no clear decrease in mortality from heart diseases (Borch et al. 2012).
Seafarers often serve on several types of vessels during their professional career, and the occupational exposure is likely different on different types of vessels. Taking this into account, although it was not possible in this study to separately analyse mortality for each type of vessel, we divided the seafarers into two groups; those who had worked on different types of vessels and those who had worked solely on passenger ferries. The seafarers who had worked on passenger ferries only, had no increased mortality, but they had worked shorter time as seafarers and our study population had few seafarers who had both served before 1985 and worked solely on passenger ships, which may have affected the internal validity of this result. It should, however, be noted that the exposure patterns of passenger ferries and other types of vessels differ substantially, including more regular work, and less chemical exposure on passenger ferries. A Norwegian register study detected an increased overall mortality, but no significantly increased mortality from CVD, for seafarers working on tankers compared with seafarers working on other ships. The increased risk was related to work as a mate; there was no increased risk for those working as captains (Moen et al. 1994).
In our study, mortality from CHD was increased among subjects who had worked 10–20 years at sea. After 20 years of registered seamanship, the mortality from CHD declined. This could have been due to a healthy worker effect and to the earlier increased mortality in the cohort causing diminished cohort mortality later in life. The earlier increase in mortality is likely also to have been caused by competing risks such as lethal accidents and cancer diseases, in addition to CVD. Competing risks and earlier increased mortality can also be applied to male seafarers > 65 years of age who had no increased mortality. In an Icelandic study analysing mortality among seafarers, the mortality from ischaemic heart disease was the highest among seafarers who had worked for 8–10 years, SMR 1.56 (95% CI 1.14–2.7); but thereafter the mortality decreased (Rafnsson & Gunnarsdottir 1994).
In our study it was the categories younger male deck crew and male engine officer/crew (ever) that had significantly increased mortality from CHD. In a Danish retrospective cohort study on seafarers from 1970 to 1985, a significantly increased mortality rate from CHD was reported only for the engine crew, compared with the general population (Brandt et al. 1994). The occupational exposure is probably different among different positions at sea due to different work tasks in different locations with varying occupational hazards.
For the women in our study, there was an insignificant tendency towards increased mortality from CHD at a later age compared with the men. Women generally develop CHD later in life compared with men (Bello & Mosca 2004). The insignificant result for the females in our study may be due to lack of statistical power; there are fewer female seafarers and hence, there were fewer females in the cohort.
This study has several strengths, such as high validity, it being a large longitudinal cohort study with a long follow-up time and including information on type of work and type of vessel for each service for each individual. Also, we had access to data on both total mortality and mortality from CVD from the Swedish national registers which have high coverage and good quality.
Our study has several limitations. It was not possible to obtain individual smoking data or other individual medical data regarding risk factors for CVD since this was a register study. International studies have shown increased risk of obesity and the metabolic syndrome among seafarers (Moller Pedersen & Jepsen 2013; Nittari et al. 2019), but we do not know if this could be applied to Swedish seafarers. In 1977 in Sweden, 67% of male deck officers, 51% of male engine officers and 35% of male engine crew were smokers, compared with 44% of the general male population (Nilsson 1998). The increased risk for CHD associated with cigarette smoking, however, decreases relatively fast after smoking cessation (Teo et al. 2006). In a Swedish questionnaire survey from 2014 answered by 1972 seafarers, 11% of the respondents were current daily smokers, with no marked difference between men and women. Among the service personnel, the proportion of smokers was higher, 23%. In the Swedish general population, 9% of men and 11% of women were daily smokers in 2015, according to the Public Health Agency of Sweden. One of our inclusion criteria, a minimum of 30 days of work on a vessel, could be considered short and a limitation of the study.
Our results show an increased mortality from CVD at a relatively young age for male seafarers who had served on different types of vessels, particularly those who had served as seafarers for several years. Our results also show a disparity in mortality between subjects working on passenger ferries and subjects working on other types of vessels, which indicates a difference in the work environment and calls for improvements such as optimization of the shift work schedules and reducing noise, stress and chemical exposure.
The modestly increased mortality among Swedish seafarers seems to have decreased over time, but further studies are needed on the specific causes of the increased total mortality among seafarers.