Searching and selecting the studies
A total of 3,790 studies were identified through the electronic databases. Of these, 1,505 were excluded because of duplicating. The titles and abstracts of 2,285 studies were screened to ensure they met the inclusion criteria. Next, 109 full texts were assessed for eligibility. Finally, 25 studies were selected for quality assessment.
Twelve studies possessed high quality using the critical appraisal tool, and 12 exhibited moderate quality. One study showing lower than moderate quality was excluded. Consequently, 24 studies were included in the present review (Table 2).
General characteristics of included studies
The general characteristics of 24 studies are demonstrated in Table 2. Sixteen studies were conducted in Europe [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29], five in Asia [6, 30,31,32,33] two in North America [34, 35] and one in South America . Twenty-two studies were quantitative [6, 14,15,16,17,18,19,20,21,22,23,24, 26,27,28, 30,31,32,33,34,35,36] and two were qualitative [25, 29].
Regarding study design, many applied cross-sectional designs. As to the data collection, five quantitative studies collected data using face to face questionnaires (two study locations were unidentified [19, 36], one study used a piloting training center , and two studies were conducted at a health examination hospital [18, 20]). Five studies distributed questionnaires using online surveys [16, 17, 24, 31, 33]. Five studies distributed questionnaires on board [6, 21, 22, 27, 28]. Two studies trained the captain and crews to serve as a co-researcher for collecting data on board [30, 32]. Two studies included subjects obtained by distributing questionnaire sheets using letters [15, 23]. One study’s data were collected using both face to face administered questionnaires and online method . Four studies used biometrically surveys. (one study determined cortisol level in saliva , two studies used physical activity and heart rate armband monitors [27, 28], and one study used venous blood to assess hormone levels ). Two studies used secondary seafarer’s health data [19, 34], and one study used semi structured interviews cooperating with secondary health data and self-administered questionnaires . Two qualitative studies applied focus group and interviews with seafarers [25, 29].
The number of subjects of quantitative studies ranged from 52 to 1,930. The participants of one study comprised female Chinese seafarers . The others did not specifically identify sex. One study recruited seafarers with or without type 2 diabetics .
Mental health problems and psychological issues among seafarers
Mental health problems and psychological issues among seafarers were explored in 18 quantitative studies. The incidence of mental illness among international seafarers, such as mood swing, and social withdrawal, totaled 3.9 per 100,000 person-years .
Stress among seafarers was reported in seven studies [14,15,16,17, 27, 28, 30]. Two studies revealed seafarers had perceived stress related to physical and mental work environment [27, 28]. One study showed that 65% of total subjects were stressed, that deck officers were the most mentally stressed and that deck rating personnel were the most physically stressed . Another reported that 37.8% of seafarers during port stay, defined as after docking at the port until the departure, experienced physical and mental stress, and that this proportion was larger than those of seafarers involving river and sea passages . Cortisol levels in saliva were the highest among deck officers, followed by deck ratings and engine room personnel, and was highest among seafarers on duty during port stay . One study pointed out that 51.5 to 66.5% of seafarers experienced work-related psychosocial stress reporting worries about family members, financial situation, ship safety, sea piracy, port state control and occupational strain . Similarly, one study reported that 28.3% of seafarers exhibited stress that demotivated seagoing . In addition, two studies reported similar perceived stress scores [16, 17]. One study showed that stress differed by ethnicity on an international ship. East Asian seafarers had higher perceived stress scores than another ethnic seafarers, such as Caucasian, Latino/Hispanic and South Asian seafarers .
Burnout syndrome among seafarers was reported in two studies [6, 18]. The prevalence of burnout syndrome was 10.8% which differed among seafarer’s rank and job: officers rank (10.7%), ratings (4.5%), and galley staff (25%) . However, one study demonstrated that the burnout score of personal issues was higher than that of work-related issues .
Depressive symptoms, anxiety, and loneliness and isolation were reported in five studies [15, 19, 30, 35, 36]. Three studies showed 14.5 to 49% of seafarers reported depressive symptoms [30, 35, 36]. Furthermore, 34 and 46% of seafarers with type 2 diabetes also reported depressive symptoms and anxiety, respectively . One study showed that 30% of Danish seafarers felt isolation or loneliness .
Factors related to mental health and psychological issues among seafarers
The factors related to mental health and psychological issues among seafarers were divided in two: individual and work environmental factors. Work environmental factors were based on the Job Demand-Resources Model .
Experience of seafaring and age were referred to four studies [16, 20, 21, 23]. Those experiencing longer seafaring correlated with lower stress at sea . Those at greater age were associated with psycho-emotional strain , but negatively associated with intention to leave , while younger age was a factor related to low decision latitude .
Health status was documented in four studies [18,19,20, 35]. Poor sleep quality or insomnia predicted psycho-emotional strain, fatigue , burnout , and depressive symptoms . Those presenting type 2 diabetics were more than twice as likely to exhibit depressive symptoms and anxiety . High BMI also was positively related to depressive symptoms among both general seafarers  and presenting type 2 diabetes .
Dispositional resilience and psychological capital work for positive psychology were represented as a protective factor in three studies [16, 17, 22]. Dispositional resilience was associated with lower stress [16, 17] and high job satisfaction levels , and psychological capital including resiliency was related to job satisfaction levels .
Work environmental factors
Pressure from contractors/customers/time and job stress in seafaring were negatively associated with job satisfaction levels  and positively associated with intention to leave [21, 31].
Long working stretches, more than 9 h daily, were related to psycho-emotional strain  and burnout .
Department on the ship was shown in two studies [26, 28]. One study demonstrated different mental and physical stress: the deck officer department introduced more mental stress, and deck ratings and engine personnel departments led to more physical stress . Another showed that department on the ship predicted job satisfaction levels and burn out. Those in the deck department experienced higher burnout and lower job satisfaction levels than those in the engine department .
Job title was shown to be associated with psychological issues in two studies [17, 23]. Job title was classified in two groups: (1) officers such as captains and engineers and (2) nonofficers such as ratings or crew, and caterers. Having a lower job title was related to job decision latitude . Ratings and caterers had higher job satisfaction levels than officers of deck and engine divisions .
Voyage episodes indicated different stress. Staying during port introduced more mental stress than that in river passage and sea passages . The number of ports landed in weekly was related to job decision latitude .
Seafaring duration comprised a specific job demand in the maritime field. Short periods between – two to four months, favorable ratio between working and free days, and regular shifts were associated with high job and life satisfaction levels . Seafaring after 2.6 months from the voyage start was more likely to produce psycho-emotional strain . Among female Chinese seafarers, seafaring – two to three months introduced mental health problems, such as anxiety, somatization . Seafaring over six months was related to depressive symptoms among seafarers with type 2 diabetes .
Ship noise and vibration, meaning a physical environment, were a subjective strain in the seafaring field in two studies [20, 27]. However, they had no effect on objective strain (heart rate and energy expenditure of physical activity) .
Two qualitative studies [25, 29] demonstrated the job demands identified through interviewing and collecting focus group data. Job demand included workload (stress and commercial pressure, and rest), shore leave, signing off and so on . Job demand was characterized as circadian disturbance, workload, difficulty of work and work intrusions, introduced fatigue, poor working climate and increased turnover intention .
Higher instrumental work support was a predictor of high job satisfaction levels  and low perceived stress at sea [16, 17].
Only one study reported team cohesion were a predictor of high job satisfaction levels .
A shipboard caring including laissez-faire leadership style were predictors of high job satisfaction levels . Lack of care by a shipboard superior and/or a shipping company were related to burnout .
Effort-reward imbalance were related to burnout . Reward (high salary, family benefits, and training opportunities) were predictors of high job satisfaction levels .