This study examined cold exposure and thermoregulatory responses among shift-working seafood handlers, providing a comprehensive analysis of subjective reports and objective measurements. The results show significant variations in exposure levels and thermoregulatory responses based on specific tasks, and shifts, pointing out the complex interaction between work demands and organisational factors, and thermal comfort and recovery. The findings also shed light on how circadian rhythms and shift work influence inflammatory markers and metabolic markers, contributing to our understanding of how cold exposure may impact workers’ health.

Most seafood handlers reported experiencing both cold and warmth during work, confirming the presence of thermal strain in this occupation. These findings are consistent with previously documented symptoms associated with cold environments, such as white fingers and skin-related issues [26]. Subjective assessments further indicated that cold exposure particularly affected the hands, highlighting the extremities as a primary site of thermal discomfort in this occupational group. Seafood handlers tended to report experiencing both colder and warmer sensations at work compared to administrative personnel. This apparent paradox may be explained by the nature of their working conditions: while seafood handlers are exposed to a cold environment, they typically wear insulated clothing and engage in physically demanding tasks. The combination of external cold exposure and internal heat generation may contribute to subjective fluctuations between feeling excessively cold and overly warm throughout the workday.

Longitudinal temperature measurements of the work environment revealed an association between specific roles and tasks with varying levels of cold exposure, which was most severe during morning and evening shifts compared to night shifts. Our data show that the subdivisions Packers, Thawers, and Operators were exposed to ambient temperature below 10 ℃ during their work shift, whereas Controllers were not. Brachial skin temperature, measured under clothing, partially mirrored the ambient temperature of specific tasks, suggesting that the clothing did not fully counterbalance the low temperatures. Packers, that frequently move in and out of cold storage while handling the finished product, experienced the widest range of ambient temperatures and were more exposed to temperatures below 10 ℃ than any other work task group.

Our study also revealed that, in these factories, time of day influenced the extent of cold exposure, with ambient temperatures being lowest during the morning and evening shifts. Nevertheless, brachial skin temperature measurements were lowest during the evening shift alone. This disparity may indicate a difference in the physiological response to cold exposure depending on the time of day. Further investigation is warranted to clarify this relationship.

FGF21, a potential metabolic plasma marker for cold exposure, decreased from pre- to post-morning shift in seafood handlers, while remaining unchanged across other shifts. On the contrary, earlier studies have shown increased plasma FGF21 with cold exposure in both humans [40, 45] and rodents [34, 46, 47]. It is known that circulating FGF21 has been shown to peak in the morning, possibly due to increased levels of circulating fatty acids during nocturnal fasting, and then gradually decrease until noon [48], which is in line with our data. However, experimental studies have shown that cold exposure selectively increases FGF21 levels in the evening [40], a finding not supported by our study. This discrepancy may be attributed to differences in the population studied, as participants in the earlier studies were younger and leaner than those in our study [40, 45]. Given that FGF21 is recognised as a metabolic regulator, and elevated FGF21 levels are associated with obesity [49, 50], we stratified our FGF21 data by BMI and age. However, as our data suggest, no significant differences in FGF21 plasma levels were observed pre-and post-works across these groups.

Plasma GDF15 levels remained unchanged across all shift types in our study, a finding consistent with previous research in both mice [51] and in humans [40]. Campderros et al. demonstrated that the exposure of mice to 4 °C for 1 day dramatically increased GDF15 transcripts levels in BAT, reaching levels like those in the liver, but did not translate into altered plasma levels [51]. Furthermore, Hoekx et al. showed that neither cold exposure in the morning nor in the evening affects plasma GDF15 levels in humans [40]. It is worth noting that GDF15 has a short half-life of 3 h, which could make it difficult to detect any effects in our study population [52]. Although low temperature does not seem to impact plasma levels, we observed a potential age-related influence on circulating GDF15. Several earlier studies have presented evidence indicating that the expression of GDF15 is significantly lower in healthy and young individuals [53, 54]. The mechanisms responsible for the increase in circulating GDF15 levels with age are not fully understood. GDF15 has been shown to be increased in obese mice and humans [55, 56], but was not associated with BMI in our study.

Regarding inflammatory cytokines, IL6 increased significantly during the night shift but remained stable during morning and evening shifts. This is noteworthy because both circadian rhythm and environmental conditions influence IL6 production. A similar effect on IL6 has been reported in studies of mice [57] and humans [58]. Cold exposure of humans induced an increase in plasma IL6 after 1–2 h [58]. Hence, the increase in IL6 in night-shift workers could be potentially explained due to the cold exposure and circadian disruption. In contrast, the other cytokines measured did not change across the shifts. Processing of seafood can release bioaerosols into the air. Thus, bioaerosols and other airborne allergens may present a potential bias, as these can affect the levels of cytokines such as IL1β, IL6, IL10, and TNF [59].

In summary, our data provide preliminary evidence that certain biomarkers may reflect physiological responses to occupational cold exposure and shift work among seafood handlers. FGF21, a proposed marker of cold-induced metabolic regulation, decreased from pre- to post-morning shift, while remaining stable across other shifts. This may indicate a shift-specific thermoregulatory or metabolic adaptation, possibly related to the timing and intensity of cold exposure during early-day operations. In parallel, plasma IL-6 levels increased during night shifts Together, these findings suggest a selective inflammatory and metabolic response to night work and cold exposure, highlighting IL-6 and FGF21 as potential biomarkers of physiological strain in this occupational context. Further studies are needed to determine whether these responses reflect acute adaptation or contribute to longer-term health risks associated with shift work and cold environments.

Thermal imaging pre- and post-breaks provided valuable insight into peripheral thermoregulation at different shifts. We captured thermographic images of the seafood workers’ hands pre- and post- breaks to assess hand temperature and evaluate how breaks affect temperature recovery, serving as a proxy for thermoregulatory efficiency. The results showed that shift type—whether morning, evening, or night—does not seem to significantly affect hand temperature change during breaks. This aligns with the biomarker data, where only selective responses, such as decreased FGF21 during morning shifts and elevated IL-6 during night shifts were observed. Together, these findings indicate that while systemic markers may reflect shift- and context-specific physiological strain, peripheral thermoregulation remains largely robust across shifts in seafood handlers.

Break duration had a significant impact on hand temperature recovery; this suggests that implementing tailored break duration emerges as a critical factor for thermal recovery. Different countries have various regulations that typically entitle workers to 20–30 min breaks, but our results indicated that longer breaks may be more beneficial, particularly for tasks with higher cold exposure, such as Packers. Our data were limited, and further investigations are needed to reveal how work task may influence hand temperature recovery during breaks. As only smaller parts of the hands may become cold and stay cold during recovery, we compared developments of the lowest (5th percentile) and median temperature during breaks. The two methods of hand temperature analysis yielded similar results. Thermoregulatory responses, such as vasodilation and vasoconstriction, play crucial roles in maintaining skin temperature, and these responses are potentially influenced by circadian rhythm [60, 61] and anthropometric parameters such as age, BMI and sex [62, 63]. For example, it has been suggested that those who are obese or have a high BMI have higher hand skin temperature compared to those with normal weight or BMI [64].

This study has notable strengths and limitations. A key limitation involves the "healthy worker effect" (HWE), which may influence the findings [65]. Cold exposure can cause discomfort, prompting individuals to adjust their occupational or workplace settings to mitigate adverse effects. Consequently, our study population may exclude individuals particularly susceptible to shift work or cold exposure. Secondly, the relatively young age of participants may limit generalizability to older population, and it is plausible that workers struggling with either shift work or cold exposure may transition to alternative employment as they age. This focus on younger workers could influence health outcomes and affect the interpretation of results [65]. Over the past decade, considerable efforts have been made to understand sex-based differences in cold response. While variations in body shape and composition result in minimal disparities in thermoregulatory responses between men and women at the whole-body level [66,67,68], the limited number of women in our study precludes a comprehensive analysis of potential sex-based differences. This represents an important area for future research.

Despite being situated in the Arctic, the factories benefit from a rather stable climate due to the influence of the North Atlantic Drift. In this area, July is the warmest month, with an average temperature of 14.3 °C, while January experiences the coldest temperatures of the year, with an average minimum temperature of −4 °C [69]. Consequently, the workers in these factories encountered cold weather not just while working but also during their leisure activities. Research has shown that repeated cold exposure allows the body to acclimate or acclimatize in various ways depending on the duration and intensity of the cold exposure [70,71,72,73], which could reduce the effects of a cold working environment on the workers. In addition to ambient air temperature, several factors, such as the quantity and quality of protective clothing worn, can influence a worker's thermal balance, as hand temperature also would be influenced by the insulation properties of gloves used [74].

This study examines cold exposure among shift-working seafood handlers, assessed through both subjective reports and objective measurements. The findings indicate that exposure levels are strongly associated with specific tasks and shifts, with thermoregulatory responses differing based on time of day and with the type of task. This variation is evident in the thermal recovery of hands during breaks, highlighting the interplay between work demands and recovery dynamics. Although FGF21 and IL-6 exhibited shift-specific changes, these did not appear to reflect overlapping physiological responses. The findings of this study underscore the importance of tailoring workplace policies to address the specific needs of different worker groups. For seafood handlers, improving thermal conditions, providing appropriate clothing, optimizing break duration, and ensuring access to warm or cool areas, as needed, can help mitigate the negative impacts of temperature extremes [75]. Future research should explore the interplay between shift work, circadian biology, and thermal strain using longitudinal and mechanistic designs.