1 Introduction

The use of work tools and their production has been of central importance to the development of humankind. In the modern manufacture of tools, nickel, chromium and cobalt are often used because of their hardening properties and ability to inhibit corrosion, thereby improving quality [1].

Based on a wide range of studies, a non-sensitizing nickel concentration of 0.5 μg/cm2/week has been suggested for consumer items made of nickel alloys [2]. Elicitation of nickel dermatitis was found to be unlikely for concentrations <0.1–1 μg/cm2 during occluded exposure and 15 μg/cm2 when nonoccluded [2]. Highly sensitized individuals might react to 0.5 ppm nickel (= 0.00005% = 0.5 μg/g ≙ 0.0075 μg/cm2) when exposed to inflamed skin under occlusion [2].

The European Union (EU) nickel directive (Directive 94/27/EC) [3] and subsequently the REACH Regulation (Commission Regulation (EC) No. 552/2009) [4] item 27 of Annex XVII, in force since 1 June 2009, regulates the maximum allowable amount of nickel release from metallic objects intended to come into direct and prolonged contact with the skin: they should not release >0.5 μg nickel/cm2/week [3, 5,6,7]. “Prolonged skin contact” has been defined, according to the European Chemicals Agency, as 3 exposures of 10 min or 1 exposure of 30 min of skin contact within 2 weeks [8].

Handheld tools—despite being regularly in prolonged skin contact according to this definition—are not listed in the list of regulated items (which includes earrings, necklaces, bracelets and chains, anklets, finger rings, wristwatches, cases, watch straps and tighteners, rivet buttons, rivets, zips and metal marks, when these are used in garments) given in the former directive and the current REACH Regulation. This leads to room for interpretation and is interpreted differently in different countries: e.g., in Denmark, there is the official understanding that tools are included in REACH due to the duration of skin contact with tools (personal communication, Jeanne D. Johansen). In contrast, in Germany and most other European countries, there is the understanding that tools are not included in the scope of this directive, since tools are not explicitly listed among the examples.

The greatest and most specific metal exposures still occur in certain occupational settings. This chapter provides an overview of the current knowledge on metals in tools and the workplace, with regard to metal allergy. Work-related airborne exposure to metals and epidemiological evidence for lung cancer are not within the scope of this chapter (recommended reading: [9,10,11]).

2 Metals in Tools

Nickel is ubiquitously present in the earth, water and air. In a recent questionnaire-based survey in nickel-allergic patients, tools were reported to be involved in the initial presentation of nickel dermatitis in 1.4% of women and 5.6% of men, whereas earrings, followed by other jewelry, are still the foremost cause of nickel dermatitis [12].

Only a few studies on metal ion release (nickel, cobalt, chromium(VI)) from tools have been published. The first study, published in 1998, was performed in Sweden. It revealed that 27% of 565 handheld tools with metal parts that came into contact with the skin were found to be dimethylglyoxime (DMG) test (see below) positive, indicating sufficient nickel release to elicit dermatitis [13]. In the same study, the release of other metals (cobalt, chromium, and vanadium) was examined in a subgroup of 30 tools. Analysis of these 30 samples did not detect cobalt release in any of the samples, whereas chromium was detected in 8 samples (27%) and vanadium in 3 samples (10%) [13].

In the second study published in 2011, nickel release was identified from 5% of 200 work tools using the DMG test in Denmark. In eight of ten, positive results were localized to the metal ring located at the end of the grip that acts as a cuff. The positive DMG test results were not related to specific categories of work tools. The cobalt spot test gave no positive test reactions [14].

In a recent study published in 2015, using the DMG test, nickel release was detected in 195 of 600 (32.5%) new handheld tools or small hardware items (nails, screws, screw nuts) purchased in 2013 in Germany [1] (Table 14.1). Nickel release from all parts (grip and functional part of the tool) was found in 10.8% of examined objects, nickel release exclusively from the grip part was found in 12%, and nickel release exclusively from the functional part was found in 9.7% [1]. Nickel release from small hardware items was found in 8.3% (n = 7/84 DMG positive).

Table 14.1 Nickel release from handheld tools and small hardware items in a recent limited market survey from Germany [1]. “Other” tools include cranks, clamps, grips, brackets, scissors and screwdriver electricity testers
Full size table

Positive nickel test results were nearly twice as frequent from tools “made in Germany” than from tools without a mark of origin. Tools made in other European countries did not release nickel. A correlation was found between price level and nickel release: handheld tools from the low price tercile released nickel significantly more frequently compared to intermediately or highly priced tools [1]. Among tool kits, 34.2% were inhomogeneous concerning nickel release [1].

Cobalt release, assessed using the disodium-1-nitroso-2-naphthol-3,6-disulfonatein-based cobalt spot test, was only detected in six tools (1%): five pliers and one saw [1] (Fig. 14.1).

Fig. 14.1
figure 1

Cobalt release from tools is rare: a positive spot test from the functional part of a pair of pliers

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One study published in 2014 investigated skin exposure and metal release from dental tools and alloys [15]. Cobalt-chromium alloys are used as casting alloys by dental technicians when producing dental prostheses and implants. Cobalt and nickel release from tools and alloys was tested in this study with the cobalt spot test and the DMG test for nickel. Also, the release of cobalt, nickel and chromium in artificial sweat (EN1811) [16] at different time points was assessed. Analysis was performed with inductively coupled plasma mass spectrometry. Sixty-one tools were spot tested: 20% released nickel and 23% released cobalt. Twenty-one tools and five dental alloys were immersed in artificial sweat. All tools released cobalt, nickel and chromium. The ranges were 0.0047–820, 0.0051–10, and 0.010–160 μg/cm2/week for cobalt, nickel and chromium, respectively. All dental alloys released cobalt in artificial sweat, with a range of 0.0010–17 μg/cm2/week, and they also released nickel and chromium at low concentrations. The study demonstrated that sensitizing metals are released from tools and alloys used by dental technicians in amounts that may cause contact allergy and hand eczema [15].

In a follow-up study, the same authors quantified cobalt, chromium, and nickel exposure on the skin and in the air, as well as urine levels, in 13 dental technicians working with tools and alloys that could result in skin and respiratory exposure [17]. The metal skin dose was quantified with acid wipe sampling, and air exposure was monitored by personal air sampling (see below). Spot urine samples were collected for 24 h. Metals were analyzed with inductively coupled plasma mass spectrometry. Before work, cobalt was detected on the skin of ten participants (0.00025–0.0039 μg/cm2) and chromium (0.00051–0.011 μg/cm2) and nickel (0.0062–0.15 μg/cm2) on the skin of all participants. After a 2 h period without handwashing, cobalt- and chromium-exposed participants had significantly higher doses of cobalt (median, 0.15 (0.032–1.6) μg/cm2) and chromium (median, 0.022 (0.012–0.19) μg/cm2) on their skin (p = 0.004 and p = 0.003, respectively) than participants who had not been exposed to cobalt or chromium. Cobalt was found in ten air samples (0.22–155 μg/m3), chromium in nine (0.43–71 μg/m3) and nickel in four (0.48–3.7 μg/m3). Metal urine concentrations were considered to be normal. The authors concluded that the cobalt skin doses acquired within a 2 h working interval might potentially elicit allergic contact dermatitis and cause sensitization [17].

A recent market survey using the diphenylcarbazide (DPC) spot test showed no chromium(VI) release from work tools (0/100). However, chromium(VI) release from metal screws (7/60), leather shoes (4/100) and leather gloves (6/11) was observed [18].

From these studies, it can be concluded that the low frequency of nickel release from handheld tools identified in Denmark cannot be taken for granted for the tools of all (European) countries, since in the most recent investigation, an unexpectedly high proportion (23%) of handheld tools currently available in Germany released nickel from the grip. Work tools may therefore still be sources of occupational sensitization and may contribute to the elicitation and maintenance of hand eczema [1]. Cobalt and chromium release from work tools was very rare or absent but was identified to be regularly present from dental tools, in addition to nickel [17].

Occupational metal exposure should be individually assessed in metal sensitized workers and needs to be taken into account for the management of chronic hand eczema [see Chap. 36], as well as for expert medical assessments of work-related hand eczema [1].

3 Other Metal Sources in the Workplace

3.1 Nickel

Industrially, nickel is frequently used as an alloy constituent, along with other metals and noble metals [19]. Nickel is found in products for both occupational and private use, many of which come into contact with the skin [20]. Depending on their composition (pure nickel metal, nickel-containing alloys, coatings), silvery-appearing tools and other objects from the workplace have a varying ability to release nickel ions upon skin contact and to cause sensitization and dermatitis [21]. Neither the exact metal/alloy composition of silvery-appearing objects from the workplace or their nickel ion release upon contact with sweat are usually known or provided on material safety data sheets. There is no relationship between the content of nickel in an alloy and its ability to cause an allergic reaction, while there is a close relationship between the rate of ion formation from nickel in the presence of sweat and the potential to cause a skin reaction [20]. The predicted dose required to elicit an allergic skin reaction in 10% of nickel-allergic individuals was calculated to be 0.78 μg nickel/cm2 in the patch test, whereas the threshold for the repeated open application test (ROAT) in μg nickel/cm2 per application (which better represents repeated workplace exposures) was significantly lower [22]. Notably, the dose-response for the accumulated ROAT dose at 1 week, 2 weeks, and 3 weeks was very similar to the patch test dose-response curve [23].

Work-related nickel exposure (detected by acid wipe sampling after 1–2 h of regular work) exceeding identified elicitation doses [22, 23] has been found among locksmiths [20, 24], metalworkers [25], cashiers [20, 24, 26], sales assistants [26], carpenters [20, 24], electroplaters [26], and, to a much lower degree, secretaries [20, 24]; furthermore, occupational exposure has been shown in nickel-allergic patients with work-related hand eczema [27]. Out of the many occupations that have the potential for nickel exposure [10, 20], the following nickel exposures have been emphasized [19]:

  • Galvanization industry.

  • Assembly of nickel-plated parts.

  • Work-related, nickel-releasing surfaces in contact with the skin, which have to be determined individually.

  • Currently, hairdressers’ scissors do not normally release nickel any longer. However, crochet hooks used by hairdressers have been found to still be a frequent source of excessive nickel release [28].

Occupational nickel dermatitis usually presents as hand dermatitis [20]. A vast number of occupational exposures and work-related cohorts at risk of occupational contact dermatitis due to nickel have been published [10, 11, 20, 29, 30] (Table 14.2); however, due to improved industrial hygiene and technical developments, these cannot be extrapolated without restriction to current work-related exposures.

Table 14.2 Occupational contact dermatitis due to work-related nickel exposure has been reported [20, 29, 30]. Note that, due to improved industrial hygiene and technical developments, this may not be applicable to current workplace exposures, and an individualized assessment is recommended
Full size table

The nickel concentration has been described to be low in unused (<0.1 μg/g) and used (0.1–0.15 μg/g) metalworking fluids [29] including in a recent investigation performed in a socket manufacturing plant on six metals (Cr, Cu, Fe, Mn, Ni and Zn) in unused and used sump metalworking fluid (MWF). In Taiwan, samples of used versus unused MWF did not display significant differences in nickel concentration, whereas samples from thread-cutting machines displayed higher concentrations of chromium, copper, iron, and manganese in sump MWF, and samples from punch press machines displayed higher concentrations of copper, iron, manganese, and zinc in sump MWFs when compared to unused MWFs [31].

In nickel-allergic individuals with a work-related history of hand dermatitis (especially if working in one of the occupations summarized in Table 14.2), a robust individual work exposure assessment is recommended. Exposure reduction is essential [20]. For the diagnosis of occupational nickel sensitization and allergic contact dermatitis, besides contact to a nickel-releasing product, the duration and frequency of exposure, specific circumstances of exposure (e.g., occlusion, pressure), coexisting exposures (irritants, other contact allergens), area of exposure, the degree of individual sensitization and skin barrier integrity at the location of exposure are relevant factors which need to be assessed individually [19,20,21, 23, 26]. Nickel-releasing (DMG positive) surfaces that have been identified to cause work-related hand eczema have included keys (in psychiatric nurses), electrical components (in an industrial worker), sewing needles (in a dressmaker), bread pans and baking trays (in a sandwich maker) and tools (in a carpenter) [27].

A recent limited market survey in Stockholm identified nickel release from 48% of examined electronic devices (e.g., laptop PCs, 13/13 DMG positive; PC mice, 1/5 DMG positive, 4 doubtful) and 54% of examined utensils (e.g., paintbrushes, 17/28 DMG positive; pens, 5/12 positive, 1 doubtful), all of which may be occupationally relevant [32]. Keys from the workplace or home are a frequent source of nickel release: 80% of recently investigated door keys displayed a positive DMG spot test [33].

To gain information on an individual workplace exposure, a DMG test may be indicative and will enable the introduction of exposure-reducing alternatives [27, 28, 34].

3.2 Cobalt

An isolated occupational cobalt allergy is rather rare [35]. Frequently a coexisting contact sensitization to other metals (nickel or chromium) is present [19, 36].

Cobalt is often used in different alloys and hard metals; in orthopedic and dental prosthetics; as a pigment in pottery, glass, and paints; in detergents; in magnets; in cosmetic products; and in many other applications [37, 38]. Occupational contact dermatitis caused by cobalt exposure in hard metalworkers, metalworkers and pottery workers is well known among dermatologists, but cobalt allergy often remains unexplained [39,40,41,42]. A recent multifactorial analysis of risk factors for contact sensitization to cobalt based on long-term data from the Information Network of Departments of Dermatology identified construction workers, metal surface treaters, cashiers and printers as high-risk occupations [36].

In a review on current occupational exposures, the following relevant cobalt exposures were identified [19]:

  • Direct contact with cobalt-containing metals, metal dusts and used cutting fluids occurs in the metal industries when working with steel or hard metals.

  • Cobalt salts (e.g., cobalt chloride, cobalt phosphate, cobalt sulfate or cobalt oxide) are used as blue or green color ingredients in the glass, porcelain, enamel and ceramic industries.

  • Cobalt naphthenate or other cobalt salts of organic acids are used as siccatives in paints or drying accelerators in the hardening of synthetic resins.

  • Cement containing traces of cobalt may lead to cobalt allergy in masons with preexisting chromium allergy.

  • Cases of cobalt sensitization due to exposure to coins [see Chap. 16], cobalt-containing cattle feed, iontophoresis gels and airborne contact dermatitis in diamond grinders due to cobalt-containing grinding discs have been reported.

To prevent sensitization and dermatitis in workers and consumers, legislation limiting the amount of hexavalent chromium in cement and nickel in items intended for prolonged contact with the skin has been enforced and now forms part of REACH [4]. However, no such legislation exists for cobalt [15].

3.3 Chromium

The actual hapten is trivalent chromium (Cr(III)), which penetrates the epidermis poorly and binds to proteins of the stratum corneum [see Chap. 7]. Cr(III)-penetration into the deeper epidermal layers and contact with antigen presenting cells is rare [43, 44]. Consequently, occupational exposure to Cr(III) represents a much lower hazard for sensitization and chromium allergy as compared to hexavalent chromium (Cr(VI)), which is water-soluble and easily penetrates the skin, where it is reduced within the epidermis to the actual hapten Cr(III) [43,44,45].

Chromium-plated surfaces (which have a shiny silver appearance), due to a thin surface layer of insoluble chromium oxide, generally do not release water-soluble Cr(VI) and are thus not regarded as an occupational hazard [46]. Corrosion of chromium-plated items and stainless steel, however, can cause the release of chromium in different oxidation states (mostly chromium(II), Cr(III), and Cr(VI)) [45].

In contrast, handling chromated metal products made from iron or zinc, such as screws, fittings, and other material used in construction and do-it-yourself procedures, must be regarded a hazard to chromium-sensitive individuals, in particular those who are strongly sensitized [46]. The chromating treatment results in a thin surface layer consisting of chromates and hydroxides, providing enhanced corrosion resistance. These chromate layers can appear in various colors, such as yellow, olive, or black [45, 46], and the Cr(VI) released from such chromated products has been shown to vary widely under in vivo and in vitro test conditions: while the Cr(VI) concentration of the supernatant of the yellow and olive items was close to the detection limit, the concentration was about 55 times higher in the supernatant of the black items, which were also capable of eliciting a positive patch test in chromium-sensitive individuals [46].

Elicitation concentrations varied in sensitized individuals: in patch testing, most reacted at 1000 ppm (0.1%) Cr(VI), and a few at concentrations of 5 ppm or less [44, 45].

Allergologically relevant occupational exposure to chromium may mainly occur with [45, 47]:

  • Cement

  • Electroplating

  • Chromium plating

  • Chromate conversion coating

  • Welding

  • Chromated metal parts

  • Leather production and processing

  • Safety gloves or shoes made from chromium-tanned leather

  • Wood impregnation

  • Laboratory work (e.g., chemical analytics)

  • Less frequently, other occupational environments (see below)

In chromium-sensitized patients, the hands and feet are most prone to be involved both in acute and chronic allergic contact dermatitis caused by chromium [48]. Work-related chromium allergy has been frequently reported to be severe, recalcitrant, sometimes widespread, and of relatively poor prognosis [45]. Chromium exposure and allergy have primarily been associated with construction workers, owing to the presence of Cr(VI) in cement (see below) [49, 50]. As a consequence of regulatory interventions concerning the Cr(VI) content in cement, there has been a shift in many European countries in etiology and epidemiology from an occupational contact sensitization of male preponderance toward a sensitization found predominantly in women in the setting of consumer exposure to non-lined leather garments [45, 51]. Despite this shift in primary chromium exposure to leather articles, in a recent survey, chromium-allergic patients still had more severe and more chronic contact dermatitis than control patients with dermatitis but without chromium allergy [48].

3.3.1 Cement

For a long time, contact to cement used to be the most frequent cause of chromium allergy. The addition of iron(II) sulfate during the production of cement reduces Cr(VI) to Cr(III); thus, low-chromate cement produced in this way contains less than 2 ppm Cr(VI) and results in hardly any induction of sensitization and generally no elicitation of contact dermatitis in most sensitized individuals [47]. However, the effect of iron(II) sulfate is limited in time, and therefore low-chromate cement requires an expiration date.

EU directive 2003/53/EC [52] came fully into force in 2005 and regulates the use of cement or cement preparations on the market: cement must contain less than 2 ppm Cr(VI) where there is a possibility of contact with the skin. In controlled, closed, and totally automated processes, skin contact does not occur, and they are exempted. Reducing agents should be used at the earliest possible stage, i.e., at the time of cement production. As a consequence of this directive, a decrease in work-related contact sensitization to chromium in men working in the German building trade (bricklayers, tile setters, etc.) from 43.1 to 29.0% was observed [53]. Logistic regression analysis revealed that patients who had started to work in the building trade after the introduction of low-chromate cement had a significantly decreased risk of chromate sensitization (odds ratio 0.42). In Scandinavia, low-chromate cement had been introduced 20 years earlier, and similar effects were observed [54]. Besides this legislative hapten reduction, increasing mechanization (e.g., the use of rotary machinery for large-scale mixing of cement), education, and implementation of workplace hygiene policies likely contributed to decreased skin contact to chromium. Other occupational groups, such as leatherworkers, metalworkers and cleaners, are also highly exposed to chromium and at risk for contact sensitization (see below) [47, 55]. In contrast to wet cement, during the demolition and handling of aged cement or concrete, no allergologically relevant chromium exposure occurs since the Cr(VI) is bound and water-insoluble.

Aside from the risk for sensitization, chromates have an irritant quality. If in prolonged skin contact, chromates (mostly in wet cement) may induce toxic reactions leading to necrosis (chrome ulcers, cement burns). The majority of cement burns affect the lower limbs (Fig. 14.2). Apart from the alkalinity of cement (pH 12), relevant factors for developing cement burns are abrasion and occlusion: the skin surface is damaged by the abrasive properties of added particulates (e.g., sand), facilitating penetration of the alkaline mostly ready-mixed cement [56]. Exposure is augmented by occlusion due to wet clothes [56]. A few hours after exposure, burning sensations, pain, erythema and vesicles occur as the initial symptoms, and 12–48 h later, partial- to full-thickness burns characterize the clinical picture [56]. Work accidents and do-it-yourself work without adequate protection are the two major risk factors for cement burns [56, 57].

Fig. 14.2
figure 2

Hazards due to hexavalent chromium: (a) Allergic contact dermatitis of the feet due to hexavalent chromium-releasing safety shoes of a kitchen staff member. (b) Full-thickness chemical burns in a do-it-yourself constuction worker due to kneeling in wet cement

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3.3.2 Leather

Leather tanning is performed in most cases with chromium(III) sulfate. A relevant release of Cr(VI) depends on environmental factors such as moisture and pH: with increasing atmospheric humidity, the Cr(VI) release from leather decreases, whereas the Cr(VI) release increases in an alkaline environment (pH 12), e.g., when handling cement [18, 58, 59]. According to the EU Commission Regulation No. 301/2014 [60], in force since 2015, leather articles or leather parts of articles coming into contact with the skin shall not be placed on the market when any of those leather parts contain Cr(VI) in concentrations equal to or greater than 3 mg/kg (3 ppm, 0.0003% by weight) of the total dry weight of that leather part. [For chromate testing in leather, see Chap. 4.] As a consequence of this regulation, a decline in relevance of leather as a source of Cr(VI) sensitization is anticipated. At this point, leather safety gloves exceeding this threshold are still found on the market (author’s own investigations).

3.3.3 Metal Processing and Handling

3.3.3.1 Plating and Chromate Conversion Coating

In electroplating, chromium plating, chromate conversion coating, and electrolytic plating, various chromium compounds/chromium salts are in use [47]. In these occupational fields, allergologically relevant exposure to Cr(VI) exists, easily passing through the skin [18, 44, 61].

3.3.3.2 Metalworking

In contrast to handling chromium-plated surfaces, which do not constitute an allergologically relevant chromium exposure, during welding of chromium-steel alloys, due to oxidizing conditions, an allergologically relevant exposure in welding fumes may occur, leading to reports of airborne allergic contact dermatitis in the older literature [reviewed in 47]. Stainless steel welding process profiling has revealed ways to reduce fume emissions, Cr(VI) emissions, and operating costs in the workplace [62]. Analyses of cutting oils being used during processing of chromium-steel alloys generally demonstrated Cr(VI) concentrations below 1 ppm, with very few exceptions [44, 63].

3.3.3.3 Chromated Metal Products

Many metal products made from iron or zinc, such as screws, fittings, and other material used in construction and do-it-yourself procedures, are chromated in order to prevent rust or surface oxidation [46]. Their surface is not a shiny silver like that of chrome-plated surfaces, but matte black, yellow, or green. Cr(VI) release in allergologically relevant amounts has been found from their surface [18, 45, 46].

3.3.4 Further Occupational Exposures

Currently, chromium exposure is possible by contact to ashes, industrial impregnation of wood, and chrome compounds in laboratory analytics [44, 45, 61, 64]. Chromium exposure due to contact to bleach and cleaning agents, printing, glass polish, wood protection (for topical application), anticorrosion coatings, magnetic tapes, and matches is mostly of historic interest [44, 45, 61, 64]. However, the anticorrosion coatings of metal airplanes nowadays still consist of a chromate-containing paint and primer [47]. Exposure occurs during spray painting and surface grinding. Trivalent chromium salts are used as pigment (e.g., chromium oxide as green pigment) in artist’s paints, ceramics, and tattoo colors; a relevant Cr(VI) exposure is probably not present when in contact with these pigments [47].

4 Practical Approach to Assess Metal Release from Tools and Other Metal Sources in the Workplace

4.1 Nickel Spot Test (Dimethylglyoxime (DMG) Test)

The dimethylglyoxime (DMG) test has been established as a clinically relevant and useful screening method for nickel release and can be used for workplace assessment of nickel release from tools. DMG reacts with nickel salts in the presence of ammonia solution [see Chap. 6]. The detection limit of the DMG test has been estimated to be close to 0.5 μg/cm2/week [34]. This limit indicates the presence of nickel in sufficient concentrations to elicit nickel dermatitis [34]. However, the moderate sensitivity of the DMG test (determined to be 59.3% (CI 95% = 13.1–46.2%)) needs to be kept in mind, whereas its specificity of 97.5% (CI 95% = 92.7–100%) is very good. For calculation of sensitivity and specificity, true-positive reactions were defined as “positive DMG test reactions that were confirmed by nickel release >0.5 μg/cm2/week.” False-positive reactions were defined as “positive DMG test reactions but with a nickel release concentration below 0.5 μg/cm2/week.” True-negative reactions were defined as “negative DMG test confirmed by a nickel release below 0.5 μg/cm2/week.” Finally, false-negative reactions were defined as “negative DMG test reactions but where nickel release was >0.5 μg/cm2/week” [34].

The DMG test has also been shown to be able to detect released nickel on the skin after exposure: positive DMG test reactions occurred in all subjects at the nickel concentrations of 0.50, 0.25, and 0.13 μg/cm2 [65].

4.2 Cobalt Spot Test (Disodium-1-nitroso-2-naphthol-3,6-disulfonate Test)

A color change of disodium-1-nitroso-2-naphthol-3,6-disulfonate from yellow to red–orange indicates a positive test reaction [see Chap. 6]. The spot test detects approximately 8 ppm cobalt in a solution, a limit close to the lowest elicitation threshold concentration in cobalt-allergic patients [66]. The cobalt spot test has proven to be useful for screening purposes, including cobalt release from cobalt-containing powder in the occupational setting [67].

4.3 Chromium (VI) Spot Test (Diphenylcarbazide (DPC) Test)

For the detection of an occupational exposure to Cr(VI), recently a spot test has been established: the diphenylcarbazide (DPC) (1% wt/vol in ethanol)-containing reagent represents a spot test for the identification of Cr(VI) release. It can be used for the detection of chromium on the surface of a solid object, as well as in solutions and powders [see Chap. 6]. It was able to identify Cr(VI) release at 0.5 ppm without interference from other pure metals, alloys, or leather. False-positive test reactions were not found. Confirmatory testing was performed with X-ray fluorescence (XRF) and spectrophotometrically on extraction fluids. The use of DPC as a colorimetric spot test reagent appears to be a valid screening test method for detecting the release of Cr(VI) ions from leather and metal articles [18].

4.4 Acid Wipe Sampling and Chemical Analysis

For the assessment of occupational skin exposure to nickel, chromium, and cobalt, acid wipe sampling, performed via cellulose wipes with 1% nitric acid followed by chemical analysis with plasma mass spectrometry, is a very reliable method that has been shown to recover 93% of nickel, chromium and cobalt deposited on the arms and palms [68]. The technique may be used in studies, in dermatitis patients, in the identification of at risk groups, as well as in developing preventive strategies and following up the results of an intervention [68].

4.5 Immersion in Artificial Sweat and Chemical Analysis

The release of nickel, cobalt and chromium from tools and other workplace contactants can be studied quantitatively by immersing items in artificial sweat (at 30 °C for 1 week) according to the reference test method for the EU nickel regulation (EN 1811: 2011) [16] and performing chemical analysis of the samples for their metal concentration with mass spectrometry. This method is usually limited to academic research and is not routinely available for occupational exposure assessment on a day-to-day basis.

5 Conclusion

Metal release from tools and other workplace materials may pose a risk for occupational contact allergy, especially when surface alteration (cracks due to mechanical use, corrosion, or contact to sweat) occurs. An individualized and timely assessment is required, as identification of the culprit exposure will allow for an appropriate intervention, with a focus on primary and secondary prevention of contact allergy. Spot tests for nickel, cobalt, and chromium may contribute to a simple and feasible occupational exposure assessment.