We found that male seafarers had a higher risk for hospital contact from CVDs, neoplasms and respiratory diseases compared with male farmers. Female seafarers of the 2012 cohort had a higher risk for respiratory diseases than the corresponding female farmers, whereas they had a lower risk for CVDs both in 2002 and 2012 cohorts. The incident rates and differences between seafarers and farmers decreased in CVDs and increased in neoplasms and respiratory diseases, particularly in men, between the 2002 and 2012 cohorts.
Compared to other studies
In our study, an increased risk for CVDs was found in male seafarers, while previous evidence on the risk for CVDs is somewhat conflicting. A Danish study found that non-officers had 1.3 and 1.2-fold higher risk for CVDs in two cohorts of 1994 and 1999 when compared to the national rates among economically active people in the entire Danish population [13], whereas risk among officers was marginal in the two cohorts from 1994 and 1999. In comparison to the general male German population, a 10% lower risk for CVDs was observed among male German seafarers [14]. However, in the differentiated analysis according to occupation on board, the galley staff demonstrated a 10% higher risk for CVDs [14]. The lower risk among German seafarers compared to the general population may be largely attributable to bias arising from the healthy-worker effect. In contrast to our present findings of a lower risk for CVDs among female seafarers, an earlier Danish study based on data from mid–1990’s reported an increased risk [13]. This difference may reflect the overall declining trend in incident of CVDs which could be more pronounced among seafarers, although our study demonstrated a decline of 18% in the incident rates in both occupational groups.
The finding of the elevated risk for respiratory diseases in seafarers is in contrast to a recent study which found that male German seafarers had 8% lower risk for respiratory diseases compared to the general male German population [14], but again, the galley staff had 1.2-fold higher risk for all respiratory diseases and 1.9-fold higher risk for asthma. Our findings are somewhat in line with an earlier Danish study which found 14% higher risk for non-officers but decreased risk for officers [13]. A recent large population-based UK Biobank cohort study including selected occupations found that seafarers had 2.6-fold higher risk for COPD compared with populations not working in this specific job, and that this increased risk did not decrease when only never-smokers were included in the analysis [24]. Our finding that female seafarers had a 60% higher risk for respiratory diseases in the 2012 cohort is much higher than findings from a previous Danish study [13]. It is also notable that the increase in incidence rates was 79% in female seafarers versus 9% in female farmers. The reasons for this increase in incidence of respiratory diseases is unclear but they might involve changes in health-behaviour, especially an increased prevalence of smoking among female seafarers, or/and increased workplace exposure to chemical hazards.
The incidence of neoplasm in our study are close to those reported earlier regardless of the differences in comparison groups and whether neoplasms consist of just malignant or all types of neoplasms. Three previous Danish studies using the general population or economically active people as comparison groups reported an increased risk of between 19% and 30% in malignant as well as all neoplasms among male seafarers (12, 13, 25). A German study comparing seafarers with the general population reported 1.2-fold higher risk for malignant neoplasms at all sites (14). Studies on neoplasms among female seafarers are rare, but an increased risk, in line with our finding, has been reported but the estimate was not statistically significant (13), and another Danish study demonstrated an increased overall incidence of cancer of 14% for female seafarers compared with the general population (25).
Amongst the most plausible explanations for higher risks for CVDs, neoplasms and respiratory diseases among seafarers than among farmers are work-specific occupational hazards and lifestyle factors. Many hazardous chemical substances, present as gases, vapours, dusts and fibres, to which seafarers are exposed are associated with increased risk for CVDs, neoplasms and respiratory diseases. Furthermore, shipboard stress, fatigue, long-time separation from the family, social isolation, reduced sleep quality and quantity and other psychosocial stressors may also impact seafarers’ health (26). Of permanent physical risk factors, noise is strongly associated with hypertension (27), whereas evidence on effects of ship movements and vibration as individual hazards for health outcomes is scarce or nonexistent.
Tobacco smoking is a strong lifestyle-related risk factor for many types of CVDs, cancer and respiratory diseases. Although data on seafarers’ smoking is rare, some evidence is available. Thus, two surveys from Denmark and France found that the prevalence of smoking among seafarers was 44% and 42%, respectively [28, 29], whereas the prevalence of daily smoking in the Danish general population was 32% [28]. The risk of being overweight among Danish male and female seafarers was 1.3 and 1.4 times higher than in the general population (30). A recent study found that 40% of seafarers investigated on Italian-flagged ships were overweight, and more than 10% of them were obese (31). Evidence on dietary intake among seafarers is almost nonexistent. However, a small-scale study among international seafarers recruited from German merchant ships found that the overall supply of meat, fat and eggs was more than double, whereas the proportions of fruits, vegetables, dairy products and cereals were much lower than recommended in the national guidelines (32). While seldom accurately investigated, physical inactivity has been seen as a major problem in seafaring (8), and it is evident that seafaring with shift work, unstructured work time, lack of training space and equipment on board may make regular physical activities challenging both on board and ashore.
Strengths and limitations
Our study benefits from a series of strengths, not at least the data based on registers of high quality with very little loss to follow-up and with two cohorts that improves reliability of the study and enables us to identify changes over time. However, our study is not without its limitations. First, we had no information about health behaviour which is a typical disadvantage in register-based health studies. Second, we had no information on hospital contacts or other health-related outcomes of participants prior to the beginning of the follow-up. It is, however, unlikely that this would have had a different effect on seafarers than on farmers and thereby affected the observed differences on incidence between the two occupational groups, and thus biased results. Third, seafarers’ access to health care is limited, particularly to the Danish land-based health care system. This may lead to an underestimation of hospital contacts among seafarers. In addition, we most probably underestimate the true difference between the two groups as we deliberately introduce a healthy worker effect through the regular health checks of the seafarers but not the farmers. Furthermore, the number of female seafarers was relatively low which affected power to identify associations. Lastly, the health outcomes used in this study were broad categories of diseases, and further studies are needed to address narrower categories of these outcomes, such as specific categories of neoplasms, particularly malignant neoplasms.
Conclusions
This is the first register-based study that compares the risk for CVDs, neoplasms and respiratory diseases among seafarers using another occupational group as a reference group in order to reduce a potential healthy worker effect. This study demonstrates that male Danish seafarers have an increased risk for CVDs, neoplasms and respiratory diseases, female seafarers an increased risk for respiratory diseases and a reduced risk CVDs.