Seasickness among Icelandic seamen.

INTRODUCTION: The working environment abroad a ship is unique, with constant stimuli such as rolling of the vessel, noise, and vibration. Fishing industry is important for Icelandic economy, still the effect of seasickness-related symptoms on seamen´s health is not fully understood. Thus, the objective of this study is to explore the impact of seasickness-related symptoms, i.e., seasickness, seasickness symptoms and mal de débarquement on seaman´s health, and how their working environment may affect those factors.
METHODS: Cross-sectional data was collected from 262 seamen answering questionnaire. Majority of the seamen participated while attending a compulsory course held by the Maritime Safety and Survival Training Centre. The majority of participants were men. A chi-square test was used to detect the difference between variables.
RESULTS: The majority of seamen had experienced seasickness (87.8%) or mal de débarquement (85.8%). Having a history of tension headache (38.1%) and tinnitus (37.9%) was quite common. A total of 30.6% of the participants had been admitted to hospital once or more due to mishaps or accidents on land. DISCUSSION: Seasickness and seasickness symptoms together with mal de débarquement are common in Icelandic seamen. Working conditions at sea are demanding and seam to affect the seamen´s health both at sea and ashore, making further research needed.

Introduction

Motion sickness is not a disease, but manifestation of symptoms that appear when an individual finds himself in moving environment, most often due to a passive stay in a vehicle, travelling in different media of which sea is the best known [1]. Seasickness symptoms are various and include nausea, headache, cold sweats, fatigue, and vomiting [2, 3]. Seasickness is common among seamen [1, 2], as is mal de débarquement, which tends to accompany seasickness [2]. Mal de débarquement is a subjective perception of self-motion after exposure to passive motion, in most cases sea travel [4]. It may also be experienced after other forms of travel, for instance in a plane or motor vehicle [5]. An Icelandic study carried out on seamen in 2012 [2] revealed that 80% of Icelandic seamen experienced seasickness in their work on board ship, and a similar proportion (79%) experienced mal de débarquement after disembarking. Studies from other countries have reported variable figures for the incidence of seasickness, up to 60% of seamen [3]. The incidence of mal de débarquement in the same group has been 72–80% [6, 7]. Both seasickness and mal de débarquement vary in their impact on individuals; it is sometimes suggested that an individual may be susceptible to motion sickness, i.e., unusually sensitive to all movement that may cause symptoms of motion sickness [8-12]. All healthy individuals become motion sick, owing to sufficient motion stimuli, although up to 20% of people are believed to be more susceptible to motion sickness [9, 11]. As a rule, women report motion sickness and motion sickness susceptibility more often than men [13, 14]. Age is also a factor; motion sickness symptoms become familiar in children aged 6–12 years old and rare in people aged over 50 [3, 12, 15]. Genetics are known factor in motion sickness, as studies on twins [12] and direct genetic mapping confirm [16], the same applies to motion sickness susceptibility [17]. In addition, it has been demonstrated that patients who experience migraine are more susceptible to motion sickness [18, 19]. Motion sickness is also a synergistic disruption in many vestibular disorders, such as benign paroxysmal positional vertigo, Menière’s disease, and vestibular migraine [20].

The fisheries are a major economic sector in Iceland, in which 4,700 seamen were employed in 2019 [21]. Their working conditions at sea are demanding, with a combination of difficult weather and sea conditions creating a constantly moving environment, in which the body must adapt to. The size and design of a vessel, as well as the seamen´s workstation on board, appear to play a part in how these weather forces act on the ship, and hence on the seamen [3, 13]. The incidence of accidents at sea among seamen is high, as is incidence of accidents on land by comparison with other professions [22]. Loss of lives at sea has decreased considerably in recent years in Iceland; factors such as enhanced training of seamen at the Maritime Safety and Survival Training Centre, are likely to have contributed greatly to this development [23]. Despite their demanding working conditions, seamen are generally satisfied with their profession [24].

On fishing vessels, seamen generally work on shifts around the clock; shift patterns vary on ships of different kinds, from six-hour shifts (six hours on, six hours off) to sixteen-hour shifts (sixteen hours on, eight hours off); while seamen on small fishing boats (daytrips) work throughout the trip, up to 14 hours, but stay ashore during the night [25]. Studies from Iceland and other countries indicate that seamen often feel that they are tired [24], and their sleep is often disrupted, due to both noise and the movements of the vessel [26]. Inadequate sleep increases the likelihood of headache and can also be a cause of headache [27, 28]. Various environmental factors at sea are believed to have a negative impact on seamen´s health. These factors include vibration, noise, and heat [26]. Vibration and noise are known to induce headache [29] and have a negative impact upon sleep [30].

It is clear, that environmental and working conditions out at sea have a considerable effect upon seamen´s health. The fisheries are an important sector of the Icelandic economy, but few studies have been carried out on the interaction of health, working conditions and aspects relating to seasickness-related factors. The objective of this study is to explore the impact of seasickness and mal de débarquement on seamen´s health, and how their working environment may affect those factors.

Methods

Study design and selection of participants

The study is cross-sectional, and the sample was chosen by convenience sampling. Collection of data took place from April to June 2019. All participants attending compulsory courses held by the Maritime Safety and Survival Training Centre [31], a total of 376 seamen, were invited to participate in the study. Seamen in the Eyjafjörður region of north Iceland were also invited by email, in consultation with their employers; these totalled 180. The study was introduced via an electronic introductory letter which explained the purpose of the study and how it was to be carried out. If a seaman was interested, he/she would answer the introductory letter, and was provided with online access to the questionnaire. It was also possible to scan a QR code and answer the question on various smart devices. A total of 262 participants answered the questionnaires (47% participation; >90% participants came from the Maritime Safety and Survival Training Centre). Informed consent was elicited, and the participants were informed how the data would be used; the study was not personally identifiable, nor traceable in any way. The study was approved by the National Bioethics Committee (project no. 18–178), and notification was sent to the Data Protection Authority.

Questionnaires

Participation entailed answering a questionnaire, which was an improved version of a questionnaire submitted to a similar group of participants in 2000 to 2002 [2]. The questionnaire comprised four parts; 1) demographic background variables, history of accidents at sea and on land, general health, and family history of diseases; 2) working conditions at sea; 3) seasickness experience (seasickness, mal de débarquement, and seasickness symptoms) (see S1 Appendix); 4) MSSQ short [32]. The questionnaire was in digital form (SoGoSurvey) and hosted by the University of Akureyri Research Centre.

Statistics

Statistical processing was carried out using IBM SPSS Statistics version 25 and Microsoft Excel 2016. A Chi-square test was used to detect if variables were associated. Phi and Cramer‘s V coefficients were used to explore correlation between variables. Confidence limits of 5% (p<0.05) were applied.

Descriptive statistics were also applied to the data. In certain statistical calculations participants were divided into two age groups, <50 years (referred to as under 50) and ≥ 50 years (referred to as over 50), as age-related degeneration has an impact upon the inner ear, and hence on the experience of seasickness [33-35].

Results

Demographic information

A total of 262 seamen took part in the study; most of the answers were received from participants in the Maritime Safety and Survival Training Centre courses (91.6%). The average age of participants was 43.8 years (age range 17 to 84 years). The majority were males (95.8%; n = 250). All the participants were Icelandic citizens. Descriptive characteristics of seamen´s health and working conditions are shown in Table 1.

Table 1

Descriptive characteristics on health and work conditions.

Age and working experience	n (%)
Age
≤ 25	31 (12.2)
26–35	48 (18.9)
36–45	56 (22.0)
46–55	64 (25.2)
56–65	43 (16.9)
≥ 66	12 (4.7)
Working experience
< 1 year	30 (11.6)
1–5 years	27 (10.5)
> 5 years	201 (77.9)
Working conditions
Length of sea journey
Day tour	36 (13.8)
1–7 days	86 (33.1)
7–30 days	88 (33.8)
>30 days	50 (19.2)
Type of vessel
<15 meters	30 (11.6)
15–24 meters	16 (6.2)
25–45 meters	47 (18.2)
>45 meters	165 (64.0)
Age of vessel
<5 years	44 (17.0)
5–10 years	29 (11.2)
>10 years	182 (71.8)
Condition of vessel*
Good	136 (53.3)
Average	101 (39.6)
Bad	18 (7.1)
Workplace at vessel
Deck	66 (25.9)
Lower deck (closed from environmental visual cues)	79 (31.0)
Engine room	30 (11.8)
Bridge	80 (31.4)
General health
BMI categories (kg * m -2 )
Normal weighta (%)	51 (20.2)
Overweightb (%)	116 (45.8)
Obesityc (%)	86 (34.0)
Hospital admission due to accidents on land
Never	181 (69.3)
1–3 times	76 (29.1)
>3 times	4 (1.5)
Hospital admission due to accidents at sea
Never	215 (82.4)
1–3 times	45 (17.2)
>3 times	1 (0.4)

BMI = Body mass index.

a Normal weight BMI 18.5–24.9 kg*m-2

b Overweight BMI 25–29.9 kg*m-2

c Obesity BMI ≥30 kg*m-2.

*Individual subjective vessel assessment.

Health, age, working conditions and experience

Participants were asked to specify health conditions for which they had had a diagnosis or symptoms. The findings are shown in Fig 1. According to self-reports, a total of 38.1% had a history of tension headache and 37.9% had experienced tinnitus. Hypertension had been diagnosed by a physician or symptoms of hypertension noticed among 33.0% of the seamen, and 18.4% had a history of migraine. It also turns out that 22% of seamen took over-the-counter medications for seasickness, while little use was made of prescription medications for seasickness.

Fig 1

Proportion of participants that had experienced specific health conditions.

Seasickness, seasickness symptoms and mal de débarquement

The majority of the seamen (87.8%) had experienced seasickness at some time in their lives. Of those who had experienced seasickness, most experienced it after spending a long time on land, or after their first sea journey (Fig 2). The majority (85.8%) had also experienced mal de débarquement; of those who had experienced mal de débarquement, most experienced it after a prolonged period at sea, or after their first sea journey (Fig 2). A total of 77.3% had experienced nausea, dizziness, sweats, or other symptoms relating to the movement of the ship at sea (seasickness symptoms). No difference was discerned in seasickness-related symptoms (seasickness, mal de débarquement or seasickness symptoms) according to where on the vessel the seaman was working, nor when the vessels were divided into upper (deck and bridge) and lower (hold and engine room) sections (p > 0.05).

Fig 2

Conditions causing mal de débarquement and/or seasickness.

Age, length of sea journey and working experience

Seamen aged under 50 were more likely to experience seasickness symptoms at sea, compared with those aged over 50, χ2 (1, n = 254) = 7.87, p = 0.005. Significant association was found between having experienced seasickness and having experienced seasickness symptoms at sea χ2 (1, n = 262) = 11.67, p = 0.001, phi = 0.21. Seamen on smaller vessels (<45 m) were more likely to have experienced seasickness at some time in their lives χ2 (1, n = 227) = 4.26, p = 0.039 compared to those working on larger vessels. Seamen over 50 were less likely to be working on day-tour boats than those aged under 50, χ2 (1, n = 252) = 6.20, p = 0.013. Those who were out at sea for a week or less were more likely to experience seasickness symptoms than those who were out for more than a week χ2 (1, n = 259) = 6.81, p = 0.009. Pulled together, length of sea journey was not associated with seasickness, mal de débarquement, and experience of seasickness-symptoms. Seamen under 50 were more likely to experience tinnitus than those aged over 50 χ2 (1, n = 253) = 8.27, p = 0.004. Seamen over 50 experienced hearing loss more than those aged under 50 χ2 (1, n = 251) = 8.75, p = 0.003. Work experience at sea was associated with tinnitus χ2 (1, n = 257) = 12.72, p < 0.001, as well as hearing loss χ2 (3, n = 255) = 10.91, p = 0.012. Both tinnitus and hearing loss appear to increase in accord with length of sea journey, although no significant difference was found (p > 0.05; see Fig 3). When pulling tension headache and migraine variables, a significant association was identified between having experienced symptoms of seasickness χ2 (1, n = 257) = 7.08, p = 0.008 and mal de débarquement χ2 (1, n = 257) = 4.82, p = 0.028.

Fig 3

Participants who had experienced or been diagnosed with loss of hearing or tinnitus according to length of sea journey.

Accidents at land and sea

A total of 30.6% had been admitted to hospital once or more due to accidents on land, while 17.6% had been admitted to hospital once or more due to accidents at sea (see Table 1). No difference was found between these factors and seasickness-related symptoms (all p>0.05). Seamen who had experienced tenson headache were more likely to experience accidents at sea χ2 (1, n = 256) = 7.56, p = 0.023. Scrutiny of seamen who had experienced migraine or tension headache revealed that these individuals were more likely to suffer accidents on land than those who had not experienced such symptoms χ2 (2, n = 258) = 6.09, p = 0.048. Seamen who regularly experienced tinnitus χ2 (2, n = 260) = 12.0, p = 0.002 and hearing loss χ2 (2, n = 258) = 6.75, p = 0.034, were more likely to suffer accidents at sea compared to those who did not have such symptoms.

Discussion

The objective of the study was to explore the impact of seasickness and mal de débarquement, on seamen´s well-being and health, and how their working environment may affect these factors. It is distinct that seasickness and mal de débarquement are commonly experienced by Icelandic seamen. The incidence of accidents among seamen in Iceland is high, and headache is common. The findings of the study indicated that seasickness is as much as 8% more common that has previously been indicated for this group [2], and the same is true for mal de débarquement. A comparison with studies from other countries on seasickness and mal de débarquement indicated that the incidence is relatively high among Icelandic seamen [6, 13, 36]. Our findings indicate an increase in seasickness-related events, i.e., seasickness, mal de débarquement or seasickness symptoms. This may be attributable to increased knowledge of the symptoms and a broader discourse in society about seasickness; also, people today are more willing to discuss their health and well-being and working conditions than in the past. Finally, the presumption may be made that Icelandic seamen work in very demanding weather and sea conditions, even though vessels are designed to withstand environmental conditions out at sea, i.e., changing weather, temperature, and sea conditions [25]. Seamen on smaller vessels were more likely to experience seasickness symptoms; this is consistent with the study of Schutz et al., which indicates that the design of vessels is believed to have an impact on symptoms of seasickness, as seasickness is more likely on smaller vessels [13]. Smaller vessels have more agiler movements compared to larger ones, which gives rise to more energic acceleration detected by the inner ear, which in turn increase the symptoms of seasickness [37]. Our study shows that younger seamen commonly work on smaller vessels, i.e., they tolerate better the swifter moving and laborious environment experienced on board the smaller boats. Their endurance on these smaller vessels is limited as they recruit to bigger ones with increased age and time at sea. Our findings indicate that shorter tours at sea also exacerbate seasickness among seamen. These findings are not surprising, as the need for the body to adapt to frequent changes between a high-motion and a static environment is stressful and thus increases the likelihood of experiencing seasickness symptoms [38]. It transpired that seasickness declined with age, which is consistent with previous studies [9, 15]; with age the sensitivity of the inner ear declines, and this leads to decreased incidence of seasickness [33-35]. Hence it was not surprising that younger seamen, who in addition were both more likely to work on smaller vessels and taking shorter sea journeys were more likely to experience seasickness.

The incidence of accidents among the seamen who participated in the study was high, whether at sea or on land. While work-related accidents at sea are common [23], it is particularly interesting to note that 30.6% of the seamen have been admitted to hospital after accidents on land. Comparable findings have not been published before. A comparison with Iceland’s Accident Register reveals that the incidence of accidents among males in general was 11.1% in 2005–2019 [39]. Our findings also show an association between migraine/tension headache and increased likelihood of accidents, although no comparable studies have been found that indicate such an association. Migraine alone has also been associated with fatigue and drowsiness, and driving performance [28]. The incidence of migraine among seamen in the study was rather high (18.4%), but due to a lack of reports on seamen and their physical health, a comparison is difficult. Population-based surveys in Europe [40] and the USA [41] do though report similar findings. However, account being taken of the fact that the majority of the participants in our study were males, the inference may be drawn that our sample displays a higher proportion of migraine than in the European study, according to which only 8% of males experience migraine, as against 14.7% of females [40]. Comparable figures are seen in the US sample [41]. However, the incidence of tension headache in our study is comparable to the figures for men in Europe (45%) [40]. While seamen´s work involves strenuous physical activity and is often repetitive [24] it is known to give rise to increase incidence of musculoskeletal pain and associated tension headache [42].

Accidents at sea were more common among participants who had tinnitus or hearing loss. Seamen aged under 50 were more likely to experience tinnitus, while those over 50 were more likely to report hearing loss. Our findings are consistent with the known fact that increasing age, and length of service, entail increasing incidence of tinnitus. Tinnitus and hearing loss have been associated with a 25% increase in accident risk for individuals working in a noisy environment [43], and it has been demonstrated that individuals with hearing loss are more likely to sustain injuries [44].

Finally, it is important to discuss the seamen´s BMI as 80% of the participants were overweight or obese. High BMI is not a new subject, whether with respect to Icelanders in general [45] or Icelandic seamen [24]. Increased BMI does have a negative impact upon health, as it, for example, increase the likelihood of headache [46], hypertension and need for blood-pressure medication [47]. Increased BMI is also linked with poorer sleep [48].

Many of the diseases and symptoms diagnosed in the seamen or experienced by them can have a direct effect upon the experience of headache. In general, these findings lead us to consider the possible causes of this high incidence of headache. Work aboard ship is known to be demanding, with vibration, noise, poor sleep, and shift work, in addition to seasickness. It is worth considering whether the symptoms may be intensified in seamen, dwelling as they do in a constantly moving environment with constant stress-inducing noise in the environment, vibration, long shifts, and brief rest periods. It would be interesting to explore whether the motion of the vessel has an impact on seamen´s headache experience, or the working conditions as such, or a combination of the two. According to our findings, the incidence of accidents is high, and more research is required regarding its association with mal de débarquement. Studies are needed regarding how long after disembarking the seamen sustain accidents. In addition, it would be interesting to make a special study regarding health of female seamen, in view of their greater susceptibility to seasickness [3] and the possible impact of vibration and environmental motion on premature births [49]. The working environment abroad a ship at sea is unique. It may be viewed in a sense as a laboratory where we can research seamen´s response to stimuli found there, such as rolling of the vessel (impact of acceleration changes on the body), noise and vibration, mental challenges (stress, confined space, isolation, tedium), and finally infections in confined spaces, such as have been seen during the COVID-19 pandemic.

The strength of the study is that it was carried out using a questionnaire that has previously been used for Icelandic seamen that validate and strength our results. The cohort examined consisted of experienced seamen that had been working at sea for a relatively long time. The weakness is that the sample included few women; but the sample accurately reflect the proportion of men and women working at sea [21]. Another factor is the possibility that those who experience seasickness were more likely to participate in the study than those who do not. Participants were asked about length of service as <5 years or >5 years. It would have been interesting to be able to distinguish between seamen who had 10 to 20 years’ experience at sea, or even longer i.e., 30 to 40 years. It may also be deemed a weakness of the study that the participants themselves had to define their health conditions such as type of headache, instead of seeking permission to access medical records. Regarding seamen accidents, it is important to study the exact time of incidents on land as mal de débarquement might be a causative factor the first four days ashore. Understanding and attending seamen´s working environment, their health, well-being, and physical conditions is fundamental to be able to improve their working conditions and safety.

To conclude, the study indicated an increase in seasickness-related factors among Icelandic seamen compared to other studies. Icelandic seamen have a quite high admission rate to hospitals after accidents, and also report a high rate of headache, but both these factors indicate a hostile working environment where longing for safety is mandatory.

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28 Apr 2022

Seasickness among Icelandic seamen

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Reviewer #1: 1. The research is a simple questionnaire based cross-sectional study whose conclusions are based on responses to a 90-question questionnaire of subjects who have volunteered to participate in the project. Thus, assessment may be considered subjective depending on the state of mind and condition of individuals who have consented to provide data. To bring in objectivity, the study could have included physical examination of subjects as well as covered the assessed conclusions by validating data over a period of time.

2. The questionnaire used has not been published along with the manuscript for the reviewer to objectively assess the interpretation of data. Also, the questionnaire has been validated not by a pilot study but by the mere improvement of an already existing questionnaire which was used in earlier research. Thus, objective validation of the questionnaire needs to be done along with a prior pilot assessment of a fixed number of subjects before undertaking the actual research. The technical soundness of the research may not be possible to comment upon since the questionnaire is not available at the present period.

3. Although cross-sectional in nature, lack of controls in the study renders the comparative analysis of data impossible.

4. Conclusions drawn based on the data collated reflects earlier studies in the same arena, with only a few aspects coming out as unique in the current study.

5. Increased risk of accidents due to Mal de Debarquement within first four days after coming ashore (as declared by the authors) requires corroboration and statistical evidence which is not brought out in the current paper.

6. Objective assessment of tension headache and tinnitus has not been done in the study and is totally based on patient description which may lead to erroneous conclusions bringing in subjectivity to the research.

7. Having included women in the study may lead to errors in assessment, and separate criteria for assessment could have been included for women and a descriptive data analysis of impact of being at sea for women needs to be furnished. A comparative data analysis between men and women who have been at sea for the same period of time with constants of type of vessel, age profile and experience at sea may throw more light on the impact of seasickness among Icelandic ‘sea-persons’.

8. Lines 229 to 232 are contradictory in nature. It’s a known fact that tinnitus is more common under age 50 while hearing loss is common in seamen over 50 years of age. However, the authors state the opposite. This needs to be addressed.

9. The study concludes a higher proportion of subjects having migraine compared to a European study, however, the nature of the cited European study is obscure, viz., sample size, type of study, controls etc. Thus, this comparison is fallacious till such time both the current and European studies have been done under similar if not exact circumstances.

10. The study purports in line 160-161 that those who were out at sea for a week or less were more likely to experience seasickness symptoms than those who were out for more than a week (p=0.009) however a contrary statement is made in line 164-167 that both tinnitus and hearing loss appear to increase in accord with length of sea journey, although no significant difference was found (p>0.05). Nor was any significant difference found between length of sea journey and seasickness-related symptoms (p>0.05). This needs deliberation.

11. The data analysis does support the conclusion that seasickness and seasickness like symptoms along with migraine, tinnitus and tension headache are all common features in individuals who venture at sea. However, nothing new has emerged out of this questionnaire-based study except for the fact that further research needs to be done to corroborate various evidence of accidents ashore, types and sizes of vessels, duration of sea, gender differences, age variations among other factors to validate the correlation between trysts at sea with sea sickness.

12. Statistical analysis is basic with use of SPSS ver 25. It would be prudent to bring out correlational study graphs, pie charts and a comprehensive correlational analysis to establish the soundness of the questionnaire. Further, results of Chi Square, Phi and Cramer’s V coefficient may be brought out in detail to solidify the statistical claims. Although the statistical analysis has been performed appropriately, the reviewer needs to be presented with detailed results.

13. Authors have not made all the data underlying the findings available in the manuscript. Questionnaire is not attached and pilot survey if conducted is not described.

Reviewer #2: This paper successfully conducted a questionnaire survey on 262 of the 376 male crew members, and investigated their age, BMI, disease history, seasickness, etc., to understand the working environment and current situation of Icelandic crew members, which has certain research significance, but there are also some shortcomings. :

1. This article is not innovative enough.

2.The reliability of the questionnaire is not high, the subjective factors are mixed, and the individual assessment is inaccurate.

3. 30% of citations are from 10 years ago, which is relatively old.

4. The picture is not clear and the figure lenged is missing.

5. The citation format of literature 32,39 is different from other literatures.

Reviewer #3: The present manuscript describes the results of a survey that was done among Icelandic seamen with regards to their level of seasickness, health-related factors, and their work environment. More than 260 people completed the survey and offered an interesting insight into the effect of seasickness among this group. The rationale of the study is clear and the procedure sound. However, I have some concerns regarding the presentation of the methods and results that need to be addressed by the authors.

Major concerns:

1. Methods: The structure of the questionnaire and the inquired items remain unclear. The authos say that “Questions were asked about working conditions at sea, history of seasickness (seasickness, mal de debarquement and seasickness symptoms), health, demographic background variables…” (p6). This is not enough detail to really understand what was measured in this survey and how it was structured. As this is the main component of the study, the questionnaire needs to be described with mor details. For instance, what is the difference between seasickness and seasickness symptoms? What was the scale to measure seasickness, simply binary “yes/no” or with more details such as “never/sometimes/often”? What do health measures include and why were these items specifically chosen? In addition to describing the main components of the questionnaire, the authors could consider adding the full questionnaire as additional material to the manuscript.

2. Results:

a. The objective of the study was to “…explore the impact of seasickness and mal de debarquement on seamen’s health…” (p5). This makes sense, but the way this objective was tackled in the results section does not really address this objective. The authors chose to run a large number of chi-squared tests to compare different groups, but these tests simply compare different groups (sometimes based on age, sometimes based on experience, sometimes based on position on the vessel etc.). There is no clear rationale offered why these groups were compared (except for age). In order to explore the impact of seasickness on health, it seems more logical to run regression analysis with sickness measures (and other factors such as age, duration on sea etc.) as predictive factors and health-related factors as outcome variables.

b. Only p-values are provided as statistical outcome. This is not sufficient; full statistics (degrees of freedom, effect size) should be included as well in order to better understand how meaningful these group differences are. With a large sample size, even small effects will become significant, but this does not mean that these differences are meaningful.

c. The authors mention that correlations were performed between the groups. I cannot find a summary of these correlations in the manuscript, except for the ones that were explicitly mentioned. I would prefer to see a summary of the correlations, even if they failed significance. Also, why were more common correlation coefficients such as Spearman correlations not used?

d. The survey offers a lot of information. However, the results are not very well structured in the manuscript; instead, the authors list a lot of findings and p-values for a variety of group comparisons (see comment above). I suggest to structure these findings more clearly and present them in subsections that address each factor of interest separately (e.g., seasickness symptoms, health etc.).

e. The figures should be optimized. At least, the SD or Sem should be included. More recently, it has become more common to present the data in a more informative way that indicates the variability and/or shows the individual data points (violin plots, box plots). A bar graph with no further information only provides little information.

Other comments:

1. What was the exact number of male and female seamen in the survey?

2. With regards to terminology, is there a more gender-neutral term instead of seamen (this is not my area of expertise, just wondering). According to a quick google search is seems that some countries (e.g., Canada) have decided to drop the term seamen and now generally refer to them as sailors to be more gender inclusive. Might be worth considering.

3. A large number of tests between two groups were calculated, where the p values adjusted accordingly?

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

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Submitted filename: PONE_review_renu.docx

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27 Jul 2022

Thank you for your valuable comments. Responses to all comments are attached in a file (answers to all reviewers), our answers are written in italic.

Submitted filename: Answers to all reviewers2._july.docx

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1 Aug 2022

9 Aug 2022

Academic Editor, Mr. Andreas C. Bryhn:

(August 4th): We have had an additional native English speaker reading through the manuscript again. We hope that the manuscript is now in proper English. In our submission today we have updated “Revised Manuscript with Track Changes” which indicates changes made in the manuscripts, “Manuscript” referring to a clean version of the revised manuscript as well as this file “Response to Reviewers”. Other files are the same since our earlier resubmission.

Reviewer #1:

Thank you for your comments. All changes in the manuscript are highlighted in yellow to make more visible for the reviewers.

1. The research is a simple questionnaire based cross-sectional study whose conclusions are based on responses to a 90-question questionnaire of subjects who have volunteered to participate in the project. Thus, assessment may be considered subjective depending on the state of mind and condition of individuals who have consented to provide data. To bring in objectivity, the study could have included physical examination of subjects as well as covered the assessed conclusions by validating data over a period of time.

Thank you for an excellent suggestion, we have actually planned to do a follow up study on seamen and preventative procedures for seasickness, where physical examination will be conducted.

We have included more details about the participants, see line 106-107 in yellow.

2. The questionnaire used has not been published along with the manuscript for the reviewer to objectively assess the interpretation of data. Also, the questionnaire has been validated not by a pilot study but by the mere improvement of an already existing questionnaire which was used in earlier research. Thus, objective validation of the questionnaire needs to be done along with a prior pilot assessment of a fixed number of subjects before undertaking the actual research. The technical soundness of the research may not be possible to comment upon since the questionnaire is not available at the present period.

More detailed description on the questionnaire has been added in lines 114-117 in yellow. An English version of the questionnaire (questions used) has been added as an appendix (referred to in line 116-117). Further questions were based on MSSQ short.

3. Although cross-sectional in nature, lack of controls in the study renders the comparative analysis of data impossible.

We agree that that is the golden standard, but the purpose of this study was to explore the impact of seasickness and mal de débarquement (MdD) on seamen´s health, rather that comparing them with other professions. We also questioned how valid comparison between two different professions would be.

4. Conclusions drawn based on the data collated reflects earlier studies in the same arena, with only a few aspects coming out as unique in the current study.

Even though sea sickness and MdD has been reported high in Icelandic seamen before, as far as we know, 1) symptoms of seasickness is current in majority of seamen that still deny that they are seasick, but clearly suffer from seasickness symptoms, 2) It is correct that migraine and tension headache are common symptoms in individuals who venture at sea, but in the same way important in sailors working at sea. Seamen, that work at sea are unfortunately not immune to those symptoms that occur in their working environment. We found increase proportion of headache in seamen compared to general cohort which gives us important information to look closer into e.g. in relation to their working environment. 3) no study has before addressed the increased admission to hospital ashore within seamen (see line 227-232). We believe that these facts increase the novelty of this paper.

5. Increased risk of accidents due to Mal de débarquement within first four days after coming ashore (as declared by the authors) requires corroboration and statistical evidence which is not brought out in the current paper.

Here we have come across a misunderstanding from the reviewer which was really important to have the opportunity to clarify. Our results did not mention any certain days. We only mention these four days as a suggestion for future studies concerning accidents and MdD. This is now part of “Regarding seamen…” lines (287). Time frame of 4 days is used as this is the time needed to gain (adapt) sea legs when going at sea and loosing them when coming ashore (adapt to fixed terrestrial environment) see Stoffregen, T. A., et al. (2013). "Getting Your Sea Legs." PLoS One 8(6): e66949.

6. Objective assessment of tension headache and tinnitus has not been done in the study and is totally based on patient description which may lead to erroneous conclusions bringing in subjectivity to the research.

Thank you for your comment, we totally agree. We have now added this to the discussion of the study weakness, to the section we addressed their subjective analyze of their health condition (see line 278-286). We have also added some extra clarifications in lines 147 to 149 to make this clearer.

7. Having included women in the study may lead to errors in assessment, and separate criteria for assessment could have been included for women and a descriptive data analysis of impact of being at sea for women needs to be furnished. A comparative data analysis between men and women who have been at sea for the same period of time with constants of type of vessel, age profile and experience at sea may throw more light on the impact of seasickness among Icelandic ‘sea-persons’.

Thank you for your suggestion. We wanted to look at seamen as a group, women included (sea-persons �  great thank you), even though women are a minority group of seamen. We ran the analysis excluding women, but that did not result in any significant difference. We also added some extra details about gender ratio in lines 131-132.

8. Lines 229 to 232 are contradictory in nature. It’s a known fact that tinnitus is more common under age 50 while hearing loss is common in seamen over 50 years of age. However, the authors state the opposite. This needs to be addressed.

We believe that there has been a misunderstanding. We say (see line 248-249 in the revised manuscript): “Seamen aged under 50 were more likely to experience tinnitus, while those over 50 were more likely to report hearing loss” This also point to the known fact that tinnitus is often the first symptom of noise related hearing loss.

9. The study concludes a higher proportion of subjects having migraine compared to a European study, however, the nature of the cited European study is obscure, viz., sample size, type of study, controls etc. Thus, this comparison is fallacious till such time both the current and European studies have been done under similar if not exact circumstances.

We are aware of this weakness and address this in the weakness chapter (lines 286-296) as well as changing wording in lines regarding those two studies mentioned.

10. The study purports in line 160-161 that those who were out at sea for a week or less were more likely to experience seasickness symptoms than those who were out for more than a week (p=0.009) however a contrary statement is made in line 164-167 that both tinnitus and hearing loss appear to increase in accord with length of sea journey, although no significant difference was found (p>0.05). Nor was any significant difference found between length of sea journey and seasickness-related symptoms (p>0.05). This needs deliberation.

Thank you for the comment. This has now been clarified, see e.g. lines 173-175.

11. The data analysis does support the conclusion that seasickness and seasickness like symptoms along with migraine, tinnitus and tension headache are all common features in individuals who venture at sea. However, nothing new has emerged out of this questionnaire-based study except for the fact that further research needs to be done to corroborate various evidence of accidents ashore, types and sizes of vessels, duration of sea, gender differences, age variations among other factors to validate the correlation between trysts at sea with sea sickness.

Thank you for your comment. It is correct that migraine and tension headache are common symptoms in individuals who venture at sea, but in the same way important in sailors working at sea. Seamen, that work at sea are unfortunately not immune to those symptoms that occur in their working environment. We found increase proportion of headache in seamen compared to general cohort which gives us important information to look closer into e.g. in relation to their working environment. Tinnitus is an injury on the cochlea and predecessor of hearing loss and is important to study further in relation to seamen´s working environment. Former studies that have studied seasickness have usually used subjective methods and asked “have you ever felt seasickness”, but when sailors are pressed further and asked about symptoms that classifies as symptoms of seasickness at sea, more sailors are in fact seasick. This is a new approach in determine seasickness. We have reworded the end of the paper and conclusion to address this better.

12. Statistical analysis is basic with use of SPSS ver 25. It would be prudent to bring out correlational study graphs, pie charts and a comprehensive correlational analysis to establish the soundness of the questionnaire. Further, results of Chi Square, Phi and Cramer’s V coefficient may be brought out in detail to solidify the statistical claims. Although the statistical analysis has been performed appropriately, the reviewer needs to be presented with detailed results.

Thank you for this suggestion. We have improved our result chapter in accordance to your comment and added appropriated coefficients. Se changes in the result chapter in yellow.

13. Authors have not made all the data underlying the findings available in the manuscript. Questionnaire is not attached and pilot survey if conducted is not described.

We have provided an English version of the questionnaire (see Appendix 1), to clarify how data was collected and which questions were asked. Data has as well been made available to repeat analysis if wished (see attached SPSS file).

Reviewer #2:

This paper successfully conducted a questionnaire survey on 262 of the 376 male crew members, and investigated their age, BMI, disease history, seasickness, etc., to understand the working environment and current situation of Icelandic crew members, which has certain research significance, but there are also some shortcomings. :

Thank you for your comments. All changes in the manuscript are highlighted in yellow to make more visible for the reviewers.

1. This article is not innovative enough:

Thank you for your comment. Even though sea sickness and MdD has been reported high in Icelandic seamen before, as far as we know, 1) symptoms of seasickness is current in majority of seamen that still deny that they are seasick, but clearly suffer from seasickness symptoms, 2) It is correct that migraine and tension headache are common symptoms in individuals who venture at sea, but in the same way important in sailors working at sea. Seamen, that work at sea are unfortunately not immune to those symptoms that occur in their working environment. We found increase proportion of headache in seamen compared to general cohort which gives us important information to look closer into e.g. in relation to their working environment. 3) no study has before addressed the increased admission to hospital ashore within seamen (see line 228-231). We believe that these facts increase the novelty of this paper.

2. The reliability of the questionnaire is not high, the subjective factors are mixed, and the individual assessment is inaccurate.

Thank you for your comment. Motion sickness and it´s important part seasickness is and has been validated through questionnaires in all studies. Our questionnaire is well known and used in Iceland and has been tested earlier and in accordance to other studies on seasickness. We find the subjective factors clear and fixed and individual assessment accurate.

3. 30% of citations are from 10 years ago, which is relatively old.

Seasickness has not been examined sufficiently in recent years, which gives the reason for the age of the references. We did though add one reference (ref 33). The novelty of this study is that we have subjects that have worked at sea for several years and such results has been missing in studies on seasickness. We have made this clearer in lines 278-279.

4. The picture is not clear and the figure lenged is missing.

Thank you for your suggestion. Figure legends have been expanded and the figures made clearer.

5. The citation format of literature 32,39 is different from other lituratures.

These are Icelandic web pages. We have now put English titles with the Icelandic one´s in the reference list (see references 32 and 40 in yellow).

Reviewer #3: The present manuscript describes the results of a survey that was done among Icelandic seamen with regards to their level of seasickness, health-related factors, and their work environment. More than 260 people completed the survey and offered an interesting insight into the effect of seasickness among this group. The rationale of the study is clear and the procedure sound. However, I have some concerns regarding the presentation of the methods and results that need to be addressed by the authors.

Major concerns:

Thank you for your comments. All changes in the manuscript are highlighted in yellow to make more visible for the reviewers.

1. Methods: The structure of the questionnaire and the inquired items remain unclear. The authos say that “Questions were asked about working conditions at sea, history of seasickness (seasickness, mal de debarquement and seasickness symptoms), health, demographic background variables…” (p6). This is not enough detail to really understand what was measured in this survey and how it was structured. As this is the main component of the study, the questionnaire needs to be described with mor details. For instance, what is the difference between seasickness and seasickness symptoms? What was the scale to measure seasickness, simply binary “yes/no” or with more details such as “never/sometimes/often”? What do health measures include and why were these items specifically chosen? In addition to describing the main components of the questionnaire, the authors could consider adding the full questionnaire as additional material to the manuscript.

Thank you for the suggestion. More detailed description on the questionnaire has been added in lines 114-117 in yellow. An English version of the questionnaire (questions used) has been added as an appendix (referred to in line 116-117). Further questions were based on MSSQ short.

2. Results:

a. The objective of the study was to “…explore the impact of seasickness and mal de debarquement on seamen’s health…” (p5). This makes sense, but the way this objective was tackled in the results section does not really address this objective. The authors chose to run a large number of chi-squared tests to compare different groups, but these tests simply compare different groups (sometimes based on age, sometimes based on experience, sometimes based on position on the vessel etc.). There is no clear rationale offered why these groups were compared (except for age). In order to explore the impact of seasickness on health, it seems more logical to run regression analysis with sickness measures (and other factors such as age, duration on sea etc.) as predictive factors and health-related factors as outcome variables.

Thank you for a good and valid comment. Because of the nature of the questionnaire, nominal variables were most common type variables obtained. After consulting a statistician, it was decided that it would be best to describe the data with Chi-square test (in most cases), but after further consulting we have improved our result chapter in accordance to your comment and added appropriated coefficients. Se changes in the result chapter in yellow.

We can´t say that seasickness is causing this or that, but we can draw the conclusion that the frequency of headache is high. The questionnaire suggests that health problems such as headache (tension and migraine), tinnitus and seasickness symptoms are clearly factors that are closely related to the working environment at sea. The duration of how long the seaman been at sea gives us a reason to interpret that this hostile working environment is contributing to the incidence of these health problems.

b. Only p-values are provided as statistical outcome. This is not sufficient; full statistics (degrees of freedom, effect size) should be included as well in order to better understand how meaningful these group differences are. With a large sample size, even small effects will become significant, but this does not mean that these differences are meaningful.

Thank you for an excellent suggestion. We have improved our result chapter in accordance to your comment and added appropriated coefficients. Se changes in the result chapter in yellow.

c. The authors mention that correlations were performed between the groups. I cannot find a summary of these correlations in the manuscript, except for the ones that were explicitly mentioned. I would prefer to see a summary of the correlations, even if they failed significance. Also, why were more common correlation coefficients such as Spearman correlations not used?

Thank you for your comment. We have added phi coefficient in line 168 to make the result clearer. We chose Phi and Cramer´s as we had nominal data (yes/no answers in both variables).

d. The survey offers a lot of information. However, the results are not very well structured in the manuscript; instead, the authors list a lot of findings and p-values for a variety of group comparisons (see comment above). I suggest to structure these findings more clearly and present them in subsections that address each factor of interest separately (e.g., seasickness symptoms, health etc.).

The result chapter has been reorganized according to your suggestions, see subtitles in result chapter in yellow.

e. The figures should be optimized. At least, the SD or Sem should be included. More recently, it has become more common to present the data in a more informative way that indicates the variability and/or shows the individual data points (violin plots, box plots). A bar graph with no further information only provides little information.

We have improved the presentation of the figures. Figure 2 has been put in a table as a suggestion for reviewers (could be an appendix if needed). Because of the nature of the data, SD can´t be calculated sufficiently (percentage of yes/no answers – SD would not be descriptive, but would apply if we had e.g. mean/average).

Other comments:

1. What was the exact number of male and female seamen in the survey?

Information added in results, line 132-133.

2. With regards to terminology, is there a more gender-neutral term instead of seamen (this is not my area of expertise, just wondering). According to a quick google search is seems that some countries (e.g., Canada) have decided to drop the term seamen and now generally refer to them as sailors to be more gender inclusive. Might be worth considering.

Thank you for your interesting comment. The term “madur” (sjomadur = seamen) in Icelandic is not gender specific, has to do with both men and women. We considered “sailor” but refers more to those who sail as a part of a hobby. Our future studies aim to study women only – and maybe we will find a good term for them in that study.

3. A large number of tests between two groups were calculated, where the p values adjusted accordingly?

We have improved the presentation of the result chapter and hope that it will answer your question.

Submitted filename: Responce to Reviewers.docx

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10 Aug 2022

Seasickness among Icelandic seamen

PONE-D-21-29503R2

Dear Dr. Arnardottir ,

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Reviewers' comments:

15 Aug 2022

PONE-D-21-29503R2

Seasickness among Icelandic seamen

Dear Dr. Arnardottir:

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