In this study we found that smelter workers had a lifetime prevalence of 28.3% for hand eczema. This is twice as high as that found in the general population in a Norwegian study that is comparable to our study population [14%; 95% CI:12.6–16.5%] [10]. The point prevalence was more than twice as high as the pooled estimate in the general population in the same study population [10 vs. 4.0%; 95% CI: 2.6–5.7] [10]. Another study found a population lifetime prevalence and point prevalence of 11 and 3.4%, respectively [7].

Smelter workers had an over four-fold odds-ratio of hand eczema and an almost two and a half-fold increased odds-ratio of skin complaints at other locations compared to the other occupational groups of our study. However, it was impossible to relate this increased risk of skin complaints to any specific risk factor to which smelters are exposed; instead, the results indicate that all the risk factors combined (showering, hand wash, heat, gloves, clothing, particulate matter exposure etc.) are important and that working at a smelter with all exposure factors combined, increases the risk of hand eczema. Even though we cannot conclude any causality from our cross-sectional data, there are several other data strengthening the suspicion that the higher lifetime and point-prevalence of skin complaints in smelter workers is due to factors at work: Smelter workers were the group where most participants reported onset of eczema while working in the occupation, worsening of skin complaints at work and improvement away from work. It was the only group that had sick leave due to hand eczema.

In contrast to smelter workers, chimney sweepers and firefighters had a lifetime prevalence corresponding to the general population in the above-named Norwegian study [10]. Point prevalence in firefighters and chimney sweepers was half that of the general population [1.9 and 1.4 vs. 4.0%] [6]. Thus, the lifetime and point prevalence in firefighters and chimney sweepers in our study corresponded to that in the general population, and it seemed they did not have a higher or lower risk for hand eczema.

Skin examination revealed more visible skin symptoms and signs in smelter workers. This indicates more skin problems in this group, compared to the other groups. We have not distinguished between allergic and irritant contact dermatitis in this paper, mainly because distinguishing the two clinically are almost impossible, and we had no access to patch testing, except for in one participant. The focus was assessing differences in self-reported skin complaints between the occupational industries, which this study indicates there is.

Particulate matter generated in smelter plants and during firefighting contain high concentrations of UFP, which contain different metals, including nickel (Ni), zinc (Zn), lead (Pb), cadmium (Cd), cobalt (Co), magnesium (Mn), iron (Fe), and silica (Si) [15,16,17, 32, 34,35,36,37, 44]. Chimney sweeping also generates particulate matter of ultrafine fraction (PM0.1) [40], but mainly larger carbon (soot) particles. The concentrations are much lower than the particles generated during smelting or fires [39, 40]. Fine and UFP may penetrate clothing and, thus, the skin [23,24,25]. The skin can also be sensitised to these metals and contact allergy can occur anywhere on the skin [23, 24]. But we do not know if these particles also can act as irritant.

Smelter workers averaged twice the amount of time spent in particle-exposed areas compared with that by chimney sweepers (4.1 vs. 2.3 h/day); firefighters in this study were not exposed to ultra fine particles from fires daily. Chimney sweepers reported particle exposure not only when sweeping but also during vehicle transport from base. We joined chimney sweepers during some of their normal workdays, and an additional study was conducted where their UFP concentrations was measured across a workday during both normal and “high-exposed” week [40]. This study reported an eight hour time weighted average ultrafine (TWA) particle exposure time ranging from 2.0 × 103- 9.6 × 103 particles/cm3, while a study on our smelter workers found a TWA ranging from 1.47 × 104-2.08 × 104 particles/cm3 demonstrating a much higher exposure of UFP for smelter workers than chimney sweepers [32].

There were no differences in hand washing frequencies between the groups, despite sampling comprising time periods before and during the Coronavirus disease 2019 (COVID-19) pandemic. Sampling of firefighters was conducted during the COVID 19-pandemic, and chimney sweepers from November 2022 to June 2022. Despite being sampled before the COVID-19 pandemic, smelter workers had the highest lifetime and point prevalence of hand eczema. There was no information regarding the use of alcohol rubs for hand hygiene. However, during sampling, we found that smelter workers used soap containing microgranules to scrub the skin during hand washing, which further increases mechanical stress on skin and thus may increase their risk of having hand eczema.

Smelter workers reported almost 50% longer mean daily use of protective gloves than the chimney sweepers but reported using primarily leather and fabric gloves. Chimney sweepers reported using airtight rubber gloves underneath fabric gloves. Airtight rubber gloves are associated with a higher risk of hand eczema [5]. However, by observation, the chimney sweepers often changed the rubber gloves between sweeps, giving the skin time to dry from the moisture. Smelter workers reported changing their gloves very seldom, which furthermore may act as a source of particle exposure to their hands because of particle entrapment. Furthermore, two-thirds of the smelter workers reported exposure to heat for over 2 h per day, almost eight times the number of hours exposed for chimney sweepers. Heat in combination with occlusion, such as gloves or clothes may precipitate eczema [3] and may also be a source to why smelters had a higher lifetime and point prevalence. The mediator analysis on glove use, years in occupation and such did not change the OR significantly. This might be due to a healthy worker effect, where workers with hand eczema elicited or worsened by the job, quit their job. Workers with previous hand eczema will most likely experience a worsening of their symptoms after a shorter time of exposure, and therefore quit earlier.

To the best of our knowledge, the assessment of work-related skin complaints in locations other than the hands, is yet to be reported. In this study, there was a good correspondence between the locations of self-reported skin complaints and where participants marked where they were most visibly dirty from particle exposure (Fig. 1). Despite there not being a statistically significant difference between the occupational groups, there is an indication of a higher prevalence among smelter workers (p = 0.057). All groups most frequently marked skin areas not covered by garments, such as the wrists, face, head, and neck. Smelter workers, however, also reported noticeable skin complaints from their shins and behind the knee, with itching and dry, flaky skin as symptoms. They used thin wool as a base layer on their body and coarse wool on their feet throughout the work shift, with occlusive protective shoes covering parts of their legs. Wool can influence the wearer’s perception of garment tolerance, and long fibre-ends coarser than 30–32 μm are more prone to activate itch-neurones [45]. Occlusion, sweat and heat, coarse wool, particle deposition on smelter workers’ skin may give rise to skin complaints, such as itching, as the smelter workers reported on their shins. In all the occupations, thin wool was used underneath their outer protective clothing layer. Chimney sweepers used different types of outer layer clothing depending on their location. For instance, some stations used the same types of outer layers as the firefighters, while some used a thin polyester suit with buttons.

Skin complaints on the head, face, and neck was similar between the groups. This indicates that skin complaints at these locations are not necessarily related to occupational exposure to particulate matter but correspond to the typical locations of common skin diseases such as atopic eczema, rosacea, seborrheic eczema, and psoriasis. This is strengthened by the fact that we did not find any association between skin complaints and years in occupation. This might be due to a possible healthy worker effect, where those with skin complaints change occupation. Moreover, common skin conditions may be worsened by occupational exposures, such as heat, sweat, cold, prolonged use of respiratory protective equipment, particle exposure, or a combination of these [46,47,48,49]. Nonetheless, it is essential to prevent worsening of any skin condition, as this may also be bothersome for workers.

Although we used questions from the NOSQ-2002, we extended the questionnaire with questions to cover skin complaints at other locations. These are not validated but derived from the hand eczema questions. However, the question was formulated in a way that could confuse participants. The questionnaire asked, “Do you have skin complaints other than eczema or urticaria”? A complete figure drawing was then presented to the respondents, in which they could mark the areas of their skin complaints. A more precise question would have been, “Do you have eczema, urticaria or other skin problems at locations other than your hands?” This could mean that the question had an interpretation bias that was dependent on the occupational group. Participants with eczema at other locations may have answered “no”, thus underestimating the prevalence of skin complaints at other locations. Recall bias may be present as workers with current eczema could be more inclined to remember earlier periods with eczema and may also have had a better awareness of risk factors.

The smelter worker group is more heterogeneous regarding their work tasks than chimney sweepers and firefighters. The smelter group comprised electricians, mechanics, oven workers, tappers, energy recovery workers, and storage workers. This implies that there could be different degrees and exposure types at the individual level, which could influence the results. However, the groups were too small to conduct stratified statistical analysis.

Chimney sweepers were recruited from different parts of Norway, which increased the external validity of the findings for this occupational group. In contrast, firefighters and smelter workers were only recruited from one location and type of smelter plant, which limits the generalisation to these or similar settings.

Our study only comprised 186 participants, a relatively low number. In addition, only a small portion of workers were clinically examined, which also is a limitation. On the other hand, the response rate was close to 100% in firefighters and chimney sweepers and two-thirds in smelter workers. Even though the participation rate was lower among smelter workers, we consider the participation rate satisfactory considering the general difficulty in recruiting research participants [50]. We observed genuine interest from workers and employers in participating and contributing to a healthier work environment. Physical information meetings with oral presentations and written information may have contributed to high participation rates. We cannot rule out the possibility of a selection bias associated with the participation of smelter workers. We did not know the skin status of the smelter workers who did not participate (25.6%) or why they did not want to participate. The direction in which this may have influenced the results is unknown since people with existing conditions may or may not have been drawn to participate in the study.

The likelihood ratio test depends on including the major confounders and mediators, and we have tried to identify and include as many as possible, but may have overlooked some factors, resulting in a less effective test.

To the best of our knowledge, no previous study has investigated skin complaints in these occupations to this extent. This cross-sectional study does not allow for conclusions regarding causality, but may be generalised to firefighters, chimney sweepers and smelter workers with similar work-environments. As many of our findings are new, they make a valuable contribution to occupational dermatology, especially regarding skin complaints at locations other than the hands and arms.