Hospital contacts due to cardiovascular and respiratory diseases and neoplasms among seafarers compared to farmers: a Danish register-based cohort study

Background Seafaring has an occupational environment with many health risks. On the other hand, regular medical examinations are mandatory to get permission for working onboard. The present study examined incidence of neoplasms, cardiovascular and respiratory diseases among Danish seafarers compared with farmers, as a relevant land-based trade. Methods The participants were all seafarers or farmers aged 18-65 years at the baseline and residing in Denmark. Two cohorts with baselines in 2002 (n=56,216) and 2012 (n=42,000) were linked to records from Danish health registers. We calculated incident rates and the hazard ratios of hospital contacts using Cox regression models separately for men and women. Results In all disease categories, the risk for hospital contact was higher among male seafarers compared with male farmers. The adjusted hazard ratios for the 2012 cohort for neoplasms, cardiovascular and respiratory diseases were 1.36 (95% CI: 1.17 to 1.58), 1.16 (95% CI: 1.01 to 1.32), and 1.51 (95% CI: 1.28 to 1.78), respectively. Among female seafarers, the hazard ratio for the 2012 cohort for respiratory diseases was 1.58 (95% CI: 1.18 to 2.10), but for cardiovascular diseases 0.73 (95% CI: 0.54 to 0.99). Between the 2002 and 2012 cohort, the incident rates and differences decreased in cardiovascular diseases and increased in neoplasms and respiratory diseases, particularly in men. Conclusions Findings of this register-based cohort study suggest that, regardless of mandatory and regular medical examinations, seafaring is associated with a substantially increased risk of respiratory diseases in both sexes, and neoplasms and cardiovascular diseases among men.

Introduction relevant external comparison group may be used to avoid problems, such as healthy worker effect, arising when using the general population as the reference group. [15,16].
According to previous research from Nordic countries, agriculture, a firmly land-based trade as opposed to seafaring, can be regarded as an average among industries in terms of health outcomes. Danish farmers had longer expected lifetime in perceived good health than unskilled men but shorter than high-level salaried employees [17]. Farmers in Norway had an increased risk for disability pension than higher grade professionals or non-manual occupations but a lower risk than other manual occupations [18,19]. Danish farmers and self-employed had elevated risk for all-cause mortality and CVD mortality than upper non-manual workers but this risk was lower when compared with manual workers [20].
The aim of this study was, therefore, to examine incidence of CVDs, neoplasms and respiratory diseases among seafarers, and estimate the relative risk of incidence compared with farmers in Denmark, and to identify changes over a decade.

Study and reference population
This is a register-based cohort study. The study population constitutes all seafarers with a permanent address in Denmark aged 18-65 years at the baseline of 2002 and 2012. As employees, including seafarers, are normally characterized by being healthier than the general population [21,22], an external occupation reference group, namely people working in agriculture, was used to reduce a potential healthy worker effect. The two number [23]. The CPR-number is used to register utilization of healthcare services, and it enables Statistics Denmark to carry out the data linkage between various data sources at individual level. Data on the health-related outcomes were obtained from the Danish National Hospital Register and the Cause of Death Register. The Danish National Patient Register contains information on all in-and outpatient, and emergency contacts for all hospitals in Denmark.
All registers have full national coverage, and information from the registers are anonymized when used for research. The study was approved by the Danish Data Protection Agency. Informed formal consent was not required due to the nature of the data.

Health outcomes
Hospital contacts were defined as either an inpatient admission, an outpatient contact, or an emergency contact, and they were used as a proxy measure for disease incidence of interest. Outcomes were identified with the use of codes in the International Classification of Diseases, 10 th Revision and included hospital contacts from diseases of the circulatory system (I00-28, I30-51, I60-99), neoplasms (C00-97, D00-48), and diseases of the respiratory system (J00-99).
The follow-up began on January 1 at the beginning of both cohorts and ended on the day the participant was hospitalized, died or emigrated. For the rest of the participants, the follow-up period ended 5 years after it began, on December 31 (mean follow-up time 4.7 years).

Covariates
Age was used as a continuous variable. Occupational class was divided into two categories. The first category included work requiring skills at the highest level and selfemployed individuals with employees. The second category included self-employed individuals without employees, work requiring skills at the intermediate or basic level, other employees or employees without information on job level. Income was measured as the personal disposable income after taxes and transfers and was used as a continuous variable.

Statistical analysis
We compared the characteristics of participants according to the trade using t-tests or Chi square tests. Age-adjusted incidence rates per 1,000 person-years for the three diagnostic categories of hospital contacts were calculated in both five-year cohorts. For every individual within both cohorts, only the first hospital contact for the given diagnostic category was included and the individual was then removed from the risk population. We did not exclude individuals' subsequent hospital contacts for other diagnostic categories.
Individuals who died or moved out from Denmark were removed from the risk population.
We calculated proportional hazard ratios with 95% confidence intervals (CIs) from Cox regression models to assess relative differences in the three diagnostic categories of hospital contacts between seafarers and farmers. We adjusted these models for age, social class and income. We also performed analyses by using squared age term as covariates. However, the estimates were little changed. All analyses were conducted separately for men and women. Data were analyzed using Stata statistical software, version MP 15.1.

Descriptive results
Descriptive baseline characteristics of the study population of seafarers and farmers in the two cohorts are presented in Table 1. The two cohorts of 2002 and 2012 comprised there were relatively more farmers than seafarers belonging to the age group 50 years or older, whereas there were more seafarers than farmers in the age group of 30 to 39 years.
Seafarers had higher disposable personal income in average. The overall pattern of baseline characteristics and differences between seafarers and farmers did not change much across time. However, a clear shift from lower to higher occupational class occurred among seafarers, and especially among female seafarers. Furthermore, disposable personal income among seafarers increased relatively more than among farmers.  Associations and incidence rates in men Figure 1 shows the age-adjusted incidence rates and  Associations and incidence rates in women Figure 2 and Table 3 show the corresponding age-adjusted incidence rates and results from the Cox regression models among women. Incidence rates of CVDs decreased by 18%   [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.

Availability of data and materials
The data that support the findings of this study are available from The Danish Health Data Authority and Statistics Denmark but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available.
Data are however available from the authors upon reasonable request and with permission of The Danish Health Data Authority and Statistics Denmark.

Funding
The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Author information  Age-adjusted incidence rates with accompanying 95% confidence intervals for hospital contacts from different causes among female farmers (green bar) and seafarers (blue bar) in the 2002 and 2012 cohorts.