Chromium and leather: a review on the chemistry of relevance for allergic contact dermatitis to chromium

As other causes decline in importance, chromium-tanned leather has become a more important source for chromium allergy, which affects around 1% of the general population. The aim of this review is to give suggestions on how to minimize the risk of leather-related allergic contact dermatitis, which can be elicited in chromium-allergic persons by hexavalent and trivalent chromium released from leather. Hexavalent chromium is the more potent chromium form and requires a lower skin dose to elicit allergic reactions. It is formed on the surface of some, antioxidant-free, leathers at dry conditions (< 35% relative humidity) and is influenced by the tanning process and other conditions, such as UV irradiation, contact with alkaline solutions, and leather age. Trivalent chromium is the dominant form released from chromium-tanned leather and its released amount is sufficient to elicit allergic reactions in some chromium-allergic individuals when they are exposed repetitively and over longer time (days – months). A low initial test result (< 3 mg/kg) for hexavalent chromium with the current standard test (ISO 17075) does not guarantee a low release of chromium from the leather or a low release of hexavalent chromium under typical exposure conditions during the service life of the leather. Information, labels, and certificates regarding leather products are often insufficient to protect chromium-allergic individuals. Correct labelling and information on the possible content of different allergens, as well as different tanning alternatives for certain leather products, are crucial.


Chromium speciationtoxicological considerations
It is widely accepted that the chemical speciation of chromium matters for different toxicological outcomes in man and other species. However, its importance is different for different diseases and different species. For example, while hexavalent chromium is known to be by far more toxic and carcinogenic for man as compared to trivalent chromium species [1], this might be reversed for other species, such as bacteria and water organisms.
Trivalent chromium is more toxic to freshwater algae as compared to hexavalent chromium [2]. Predicted no effect concentrations of 3.4 μg/L for hexavalent chromium and of 4.7 μg/L for trivalent chromium for aquatic organisms have been estimated within the framework of environmental risk assessment in the European Union in 2005 [3]. This is in line with a recent biotoxicity study on Photobacterium phosphoreum of leachates from chromium-tanned leathers, that contained trivalent chromium bound to different organic ligands along with traces of other organic leachates [4]. The leachates were significantly more toxic as compared to control solutions of trivalent and hexavalent chromium, although the exact speciation and pH of these control solutions was not reported [4]. In the following, however, this review will only focus on man and skin allergies to chromium.

Chromium tanning
Tanning is a necessary process to preserve animal skins and hides and convert them into useful leather, which is a stable material resistant to microbial attack and with enhanced resistance to wet and dry heat [5]. Chromiumtanning is the most common tanning process worldwide, using a basic trivalent chromium sulfate salt, Cr(OH)SO 4 [5]. There are several advantages of chromium tanning as compared to other mineral tanning types, oil tanning, vegetable tanning, and aldehyde tanning: i) it results in higher heat resistance, ii) it is one of the cheapest and fastest tanning processes, iii) it is considered to be one of the environmentally most friendly and least toxic options, and iv) the raw material is readily available [5]. Trivalent chromium binds to the proteins, primarily collagen, of the skin/hide forming a complex [5,6].

Chromium allergy and allergic contact dermatitis (ACD)
Chemically, the binding of a molecule to a protein is a common prerequisite for successful tanning, for skin allergy, and for one type of allergic asthma. Allergic asthma to chromium is relatively scarcely reported [7][8][9][10][11][12]. Respiratory illness, coupled with high immunoglobulin E (IgE) and chromium levels in the blood have been reported for tannery workers [13,14], although it is not possible to rule out from these studies whether this was due to allergic asthma caused by chromium. This review will focus on skin allergy. The skin allergy discussed herein is a hypersensitivity of type IV, which is a delayed and T-cell-mediated type of skin allergy, and which can elicit allergic contact dermatitis (ACD) [15]. The word "contact" refers to the observation that ACD occurs at that skin area that has been in physical contact with the sensitizing substance. Type IV hypersensitivity consists of the sensitization step and the elicitation step, which take up to 30 days and 1-7 days, respectively, after skin contact with the sensitizing substance, to which the sensitization is specific. Once sensitized, the allergy is life-long, but contact eczema are only elicited after contact to the sensitizing substance. The reason for the long time between skin contact with the substance and visible skin reactions is a complicated cascade of immune reactions that involve dendritic cells (in the skin), T-cells, and lymph nodes [16]. To be sensitizing, chromium must bind to a protein. The chromium species/ion is in this context denoted hapten, and the protein to which it binds is the carrier, and together, as a conjugate, they form the antigen [17,18]. The chromium species changes the protein structure upon binding. The antigen is then processed and presented to T-cells by cutaneous dendritic cells (Langerhans cells) [18] during the sensitization step or recognized by circulating haptenspecific T-cells during the elicitation step [17][18][19][20]. Several common skin proteins, such as albumin and heparin, have been found to be able to be the carriers for the chromium antigen, and chromium binding as well as structural changes of these proteins have been confirmed [7,[21][22][23][24][25][26][27][28][29].

Relevant chemical speciation of chromium for ACD
Since hexavalent chromium forms exclusively negatively charged mono-or dichromates under the conditions of relevance for skin exposure and since these negatively charged chromates are not able to bind to proteins, it is only trivalent chromium that binds to proteins and forms the antigen [7,[30][31][32]. This means that hexavalent chromium first needs to be reduced prior to forming the antigen. Still, hexavalent chromium is considered to be the more potent skin sensitizer as compared to trivalent chromium species, since it i) penetrates the skin much more efficiently [7], ii) penetrates cell membranes at much higher rates [30,32,33], and iii) seems able, due to its oxidative properties, to induce some immunological responses that trivalent chromium is unable to cause [34,35].
The extent of skin penetration and doses required in the sensitization and elicitation steps of chromium allergy are affected by the chemical speciation of chromium, in particular its size and charge, and the status of the skin (intact or damaged skin, state of skin barrier) [7,36]. The tanning agent Cr(OH)SO 4 belongs to those chromium species of lowest skin penetration rates and lowest potency, which means that relatively high doses are required to elicit ACD to chromium [7,37]. Still, a dose of 0.1 M (16.5 g/L) Cr(OH)SO 4 was able to elicit ACD in 70 of 94 tested chromium-allergic individuals [37]. In general, hexavalent chromium is found to be the most potent species, requiring lowest doses for elicitation (as low as 0.03 μg per cm 2 skin area or 1 μg/L) [38], followed by negatively charged trivalent chromium species, such as trivalent chromium oxalate [7,36,39]. Most importantly, the chromium speciation during leather tanning and that of relevance for consumer exposure to leather articles varies as a function of environmental parameters, such as pH, presence of complexing agents and antioxidants, relative humidity, temperature, and time [40][41][42][43][44]. Due to the acidic pH during chromium tanning and desired chemistry during the tanning process, exposure to Cr(OH)SO 4 and related trivalent chromium species seem most relevant for tannery workers. During production of Cr(OH)SO 4 , however, there is exposure possible to the highly potent hexavalent chromic acid, from which the spray dried powder Cr(OH)SO 4 is obtained by reduction by sulfur dioxide [5]. Tannery workers have been found to have a higher risk of ACD to chromium [13,14,45,46] as compared to control groups. This review will however mostly focus on chromium allergy of relevance for consumers.

Prevalence of chromium allergy
Chromium allergy is relatively common. Chromium allergy becomes only clinically relevant upon sufficient skin exposure to chromium, which elicits ACD. Whether a person is chromium-allergic, can be tested by patch testing (further explained in section 2.2.). Estimates are approximately 1% of the general population in Europe, which is the region that has been studied most and has the lowest prevalence of chromium allergy among those countries and regions that have been studied, Table 1. Among clinical groups, which are mainly patients of specialized dermatology clinics, the prevalence of chromium allergy is about 4.2-4.8% in Europe and North America, while it is higher (6-10%) in other countries and continents, Table 1. Compared to other allergens, chromium is among the most frequent, usually among the top 1-20 most frequent allergens in different studies, see references in Table 1 and [91]. Over time, the prevalence of chromium allergy has declined in European and North American countries, but the fraction of leather related chromium allergy has increased. Most recent clinical data from Austria, Switzerland, and Germany suggests however an increase of the prevalence of chromium allergy after 2015 (from 4.2% in 2007-2010, to 3.3% in 2011-2014, to 4.5% in 2015-2018) [49], which requires further monitoring. Clinically, the leather related fraction of chromium allergy may be determined by asking the patient about the cause of his/her dermatitis, e.g. due to contact with certain shoes or a certain item, confirming the localization of the dermatitis related to a certain exposure, patch-testing the patient with that source confirming its relevance for the dermatitis, testing the leather for different allergens, and patch-testing the patient with different allergen series, e.g. the shoe series containing a number of shoe allergens covering, among others, glues, plastic chemicals, metal allergens, dyes, and preservatives. For example, Danish studies suggest that the leather related fraction of chromium allergy increased from about 24% in 1989-1994 [92] to 55% in 2010-2017 [50]. In other countries, workrelated exposures and other consumer exposures, such as to cement, detergents, cleaning agents, chemicals, metal work, metal fumes, and chromated surfaces, remain dominant as cause of chromium allergy [7].

Severity and persistence of chromium allergy and ACD
Once sensitized, ACD to chromium can be elicited by relatively low doses of chromium [93]. Chromium allergy is characterized by that it more often causes chronic and severe eczema as compared to other type-IV allergies [94,95], resulting in lower quality of life and higher unemployment rates. Also, the contact dermatitis to chromium is more often located on the hands and feet [96,97]. ACD to chromium has been found to be very persistent [98][99][100] and to have a poor prognosis [94][95][96][101][102][103].
This review focuses on chemical questions regarding chromium allergy of relevance for the leather industry, regulators, chromium-allergic persons, and consumers of leather articles. The aim of this review is to give suggestions on how to minimize the risk of chromium allergy related to leather articles. 2 Trivalent versus hexavalent chromium from an allergic perspective

Release of trivalent and hexavalent chromium from leather
Due to the different potency and skin permeability of trivalent and hexavalent chromium, it is important to understand the chemistry behind the release of trivalent and hexavalent chromium from leather articles in skin contact or under relevant exposure conditions. Figure 1 summarizes some data of studies investigating trivalent and hexavalent chromium release from different leathers under different pre-exposure and exposure conditions (for detailed information, see the supplementary file Table S2). For most leathers and exposure conditions, trivalent chromium is the predominant form of chromium that is released. The release of trivalent and hexavalent chromium from leather, even for the same leather, are strongly affected by pre-exposure conditions and exposure conditions. To date, a number of factors has been investigated both during the tanning and manufacturing steps and during pre-exposure and exposure from a consumer perspective. These factors and their influence on trivalent and hexavalent chromium release from leather are summarized in Table 2.
The release of total (trivalent and hexavalent) chromium is highest for new leather since excess chromium is released. Further, it is highest at acidic and alkaline conditions and after exposure to dry (< 35% relative humidity) air. The release of hexavalent chromium from leathers is affected by a larger number of parameters. The initial leather chemistry, which is largely determined by the tanning process, plays an important role as well as pre-exposure and exposure conditions. In the absence of antioxidants, which is the case for some leathers and/ or for old/used leathers, the most important factor is exposure to dry air (< 35% relative humidity) prior to exposure to any solution or the skin. For antioxidant-free and old/used leather, hexavalent chromium can even be detected in artificial sweat (pH 5-6.5) [133], while it is otherwise only detected at neutral and alkaline pH. This agrees with early studies on extraction of hexavalent chromium by human sweat [40]. The risk for release of hexavalent chromium from leathers increases for: -Purely chromium-tanned leathers without any antioxidants, vegetable tannins, or reducing acids -Tanning or manufacturing at too high pH or with oxidative species -Exposure to dry air (< 35% relative humidity) prior to skin contact or exposure -Exposure to UV irradiation and dry air prior to skin contact or testing / exposure -Pre-exposure or exposure to alkaline media (pH 12 or higher), such as contact with cement -Age / usage of leather.

Allergic reactions to trivalent and hexavalent chromium, and chromium-tanned leather
When comparing clinical studies on ACD to chromium as a function of trivalent and hexavalent chromium skin doses, it is important to consider the chemical speciation of the applied chemical substance. Many trivalent chromium salts induce a low pH that is not suitable for skin patch testing. Patch testing means that a small amount Fig. 1 Overview on some published data of the release of trivalent and hexavalent chromium from different leathers under different conditions. The inset shows a magnified graph. The same leather name, e.g. L1, indicates the same leather. All available information on the leather, preexposure, exposure conditions, and all data are given in Table S2 (supplementary file) Hedberg Journal of Leather Science and Engineering (2020) 2:20 Leather factors during exposure Surface area to mass ratio If increased, more release Surface area that has been in contact with dry (< 35% relative humidity) air prior to exposure: if increased, more release. Increased surface area at > 35% relative humidity may result in decreased release due to co-released antioxidants/acids. [41,132] Relative humidity and temperature prior to testing Most release at 35% relative humidity. Slightly less release at lower humidity, and strongly reduced release at higher humidity. Minor importance of temperature.
The lower relative humidity (and longer storage time), the higher release. Relative humidity above 35% results in very low / non-detectable release.
Minor importance of temperature. [41,43] UV irradiation (at dry conditions) prior to exposure No effect Increase possible for certain exposure conditions [41,123] Temperature during exposure (immersion or skin contact) Increased release for higher temperature (even 5°C higher) - [132] Solution pH Lowest release at neutral pH. Increased release for acidic (< 5) and alkaline (> 10) pH Increased release for more alkaline pH. Very low release for pH < 7. of the chemical substance is applied on the skin in an occluded chamber and kept there for usually 2 days [135]. The allergic skin reaction is then read by a dermatologist after additionally 1-5 days. The pH of the chemical substance applied should be in the range between 4 and 9. Most trivalent chromium salts are not stable in solution above pH 4 and would precipitate, which hence would reduce the actual, bioaccessible, skin dose during patch testing. However, trivalent chromium species bound to organic ligands can be stable under conditions relevant for leather and skin contact. Relevant chemical speciation of clinically important chromium species is illustrated as a function of pH in Fig. 2a. When the line of a certain species is on the top of that figure, all of it is in solution (in its aqueous phase) and when it is at the bottom, it is precipitated from solution (in a solid phase). From Fig. 2a, it can be deduced that it is not possible to design a clinical study that investigates stable trivalent and hexavalent chromium species at the same pH. The pH is an important factor for skin permeability [136] and skin charge [137]. In a recent study [36], trivalent and hexavalent chromium salts were chosen that would form similarly charged (negatively charged) and relatively stable species over the entire concentration range of the patch test study, Fig. 2b-c. The trivalent chromium solution was buffered to a pH of 4.1 and the hexavalent chromium solution to a pH of 8.5. The majority of chromium-allergic individuals in that study reacted to both trivalent and hexavalent chromium, although the required dose for a positive and stronger reaction was significantly lower for hexavalent chromium as compared to trivalent chromium, Fig. 2d-f, which is in agreement with a number of previous studies [37-39, 97, 137-140]. Figure 3 shows the lowest skin doses of the trivalent and hexavalent chromium during patch testing that the ten chromium-allergic individuals in the clinical study [36] reacted to. The asterisks in Fig. 3a-b mark those four chromium-allergic individuals that also reacted to a chromium-tanned leather bracelet within 3 weeks of use (12 h per day like a normal bracelet). Only one of these individuals reacted to the same leather when tested on the back in an occluded patch test for 48 h. As also confirmed in other studies [97,141], a lower elicitation threshold and positive reactions to trivalent chromium increased the risk of reacting to the chromium-tanned leather bracelet within the study period. In a Danish study on 2211 dermatitis patients, 71 (3.1%) had a positive reaction to hexavalent chromium (0.5% potassium dichromate), of which 31 also had a positive reaction to trivalent chromium (13% chromium trichloride) [97]. An increased risk of foot dermatitis was found for those patients reacting to both trivalent and hexavalent chromium. This increased risk of foot dermatitis was not caused by a higher degree of sensitivity to hexavalent chromium, suggesting that trivalent chromium played a role for shoe dermatitis for these patients, along with other shoe allergens [97]. The deposited amount of chromium from the chromium-leather bracelets on the skin of the participants in the bracelet study [36] was very low and no hexavalent chromium was detectable on the chromium-tanned leather bracelets after the threeweeks use test, Fig. 3d. These findings suggested that repeated skin exposure to very low amounts of chromium from chromium-tanned leather items and released trivalent chromium are sufficient to elicit ACD to chromium from leather items in chromium-allergic individuals. This observation is in agreement with other use test and repeated open application test studies and allergens, suggesting that lower doses are required if the exposure occurs repeatedly over longer time as compared to an occluded patch test during 48 h [141][142][143][144]. In that bracelet study [36], the two leathers were also tested for traces of other metals present (there were none). Both leathers were non-finished to avoid the presence of dyes or other allergens originating from the finishing process. The chromium-tanned leather was not tanned with other tannins. It is hence unlikely, but not completely impossible, that another organic allergen that might have been present in the chromium-tanned leather and absent   [36], number of positive skin reactions in ten chromium-allergic individuals as a function of patch test dose (d), from [36], and severity of skin reactions, expressed as percentage of the possible maximum score, of ten chromium-allergic individuals and 22 non-allergic controls, when tested with hexavalent chromium (e) or trivalent chromium (f) in the same study. Crchromium; Cr(VI)hexavalent chromium; Cr(III)trivalent chromium; skin or patch test dose is expressed as μg chromium species per cm 2 skin area in the vegetable-tanned leather would have elicited the reactions in the four chromium-allergic individuals, but not in the 20 controls. None of the ten chromiumallergic individuals in that study has been allergic to formaldehyde.

The formation of hexavalent chromium during use of chromium-tanned leather
While there is today enough evidence that released trivalent chromium from chromium-tanned leather is a problem for chromium-allergic individuals, the avoidance of the formation of hexavalent chromium in, or more exactly on, chromium-tanned leather remains the most important measure regarding chromium-tanned leathers for both chromium-allergic individuals and those that should be protected from sensitization.

Age of chromium-tanned leather article in use
There are reports that show that the extent of released hexavalent chromium from chromium-tanned leather increases with age/use [43,133,134], most probably since antioxidants, acids, and colors are washed out. The chromium-allergic participant number 2 in Fig. 3a-b reported that new leather shoes did not cause any problems, but old shoes did. A similar observation was reported in [145]. Instead, a study investigating the effect of different stable inhibitors for chromium oxidation found that leathers that were prepared with this inhibitor mixture did withstand the formation of hexavalent chromium (under dry conditions) even after 12 months of storage, while chromium-tanned leathers without that inhibitors could re-form hexavalent chromium after a heating step [43].

Environmental factors during use of leather articles
For those chromium-tanned leathers that lack antioxidants due to the tanning process or their age, hexavalent chromium can be formed as a function of environmental factors, such as relative humidity and UV irradiation during storage, solution pH, and contact with alkaline solutions (such as contact with cement), illustrated in Fig. 4. There is generally a high risk of the formation of hexavalent chromium for work gloves in contact with alkaline solutions or cement, especially if they are used repetitively and in regions with low relative humidity, such as Northern countries in the wintertime. It should be emphasized that the total chromium release from chromium-tanned leather items is even more important and that the total chromium release is primarily influenced by the amount of sweat and therefore highest in warm countries or during summertime [40,133,[146][147][148].
4 Obstacles related to the standard test for hexavalent chromium in leather (ISO 17075) Since 2015, all leather items in the European Union are required to comply to the regulation Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) [149,150] and to have a lower test result than 3 mg/kg hexavalent chromium according to the standard test ISO 17075 [151]. However, an initially compliant test result with < 3 mg/kg hexavalent chromium does not mean that the leather item is not able to release hexavalent chromium later under different environmental conditions. The reasons are several.
First, it has been reported that manufacturers by intention spray the leather items prior to testing or retesting occasions with antioxidants, such as ascorbic acid [152]. These sprayed-on antioxidants prevent a test result that would prohibit the leather item to be placed on the market, but may not necessarily hinder the formation of hexavalent chromium on the leather item in the long term (if washed out). In contrast, stable inhibitors of hexavalent chromium formation may be useful, as they have been shown to be intact even after long-term storage [43].
Second, as discussed in the previous sections, the relative humidity prior to testing with ISO 17075 is crucial. If that relative humidity is greater than 35%, the test results will most probably be below the restriction limit of 3 mg/kg. In the current version of ISO 17075, it is Fig. 4 Different pre-exposure (storage) and exposure scenarios for a chromium-tanned leather without antioxidants, such as common purely chromium-tanned work gloves: The influence of relative humidity prior to exposure / skin contact (a), the influence of solution pH during exposure / skin contact (b), and an occupational exposure scenario involving exposure to rain, cement or alkaline solutions, and UV irradiation during drying / storage. Schematically redrawn from the results shown in [41]. Cr(VI)hexavalent chromium; Cr(III)trivalent chromium; UVultraviolet irradiation Hedberg Journal of Leather Science and Engineering (2020) 2:20 Page 9 of 15 referred to ISO 4044 [153], which in turn refers to ISO 2419 [154]. In ISO 2419, the conditioning of dry leather samples is specified at 23°C and 50% relative humidity (reference standard atmosphere) for 24 h for dry leather samples (dried at a temperature not exceeding 40°C according to ISO 4044), after which the sample should be ground, cut, and stored (in a clean, dry, and airtight container, kept away from sources of heat) according to ISO 4044. Alternative atmospheric conditions are also specified and allowed, when reported, according to ISO 2419. The storage time and relative humidity during storage is not further specified. Several studies on hexavalent chromium in leathers pre-conditioned the samples by heating at 80°C for at least 24 h [43,123], which resulted in higher hexavalent chromium release as compared to other pre-conditioning. In one study, it was also confirmed that hexavalent chromium could be re-formed after up to 12 months of storage when heated again at 80°C [43]. Third, ISO 17075 requires the leather to be tested to be cut/milled into small pieces. This increases largely the surface area to mass ratio, which increases the overall chromium release [132]. However, for hexavalent chromium, the increase of the specific surface area does not necessarily result in an increased release and may under some conditions even result in a slightly reduced release due to two reasons: i) co-released acids and antioxidants may decrease the amount of hexavalent chromium in solution [42,132,134], and ii) without new conditioning at low relative humidity, the increased surface area does not result in new formed hexavalent chromium all over the surface (hexavalent chromium is only found and formed on the surface in contact with oxygen at dry conditions) [41,104,116,125,132].
Several conditions of the ISO 17075 test are optimal for testing leathers for hexavalent chromium. The deaerated phosphate buffer, extraction time, and mass/solution ratio of the standard test of ISO 17075 can be considered ideal for the extraction of all surfaceavailable hexavalent chromium of the test item [132]. There is no risk of oxidizing trivalent chromium to hexavalent chromium during the test procedure of ISO 17075 [115], hence avoiding false positive results.
It can be questioned how meaningful it is to test leather items for hexavalent chromium, without testing the release of trivalent chromium. From an allergic and exposure perspective, trivalent chromium is also important. The one-time testing of hexavalent chromium of a new leather product can rather be seen as a, relatively irrelevant, snapshot in a dynamic history of chromium speciation throughout the lifecycle of leather. It could be argued that the release of hexavalent chromium to some extent correlates with the total allergic potential of that leather item. But there is no relationship between released hexavalent and trivalent chromium, Fig. 5.  Table S2), excluding non-measurable levels of hexavalent chromium due to color interferences. The red dotted line marks the restriction limit of 3 mg/kg for hexavalent chromium. Only leather items below that restriction limit are allowed to be placed on the market in the EU. Crchromium Hedberg Journal of Leather Science and Engineering (2020) 2:20 Page 10 of 15 5 Practical implications for chromium-allergic individuals as consumers of leather products ACD to chromium can be severe and chronic. Its severity and persistency depend on the extent of exposure to chromium. If a chromium-allergic individual learns about his/ her ACD to chromium and which sources of chromium exist, the person can avoid sources of chromium and the eczema will improve over time. Also, the elicitation threshold can increase in the long-term if chromium exposure is avoided for long time. Not all chromium-allergic individuals report problems with leather items. Some chromiumallergic persons report issues only for some types of leather items or occasions, such as for direct skin contact or certain types of shoes (e.g. old shoes). Other chromium-allergic individuals need to strictly avoid all skin contact with all chromium-tanned leather items. For all chromium-allergic persons, clearly labelled leather products would be helpful. Information on whether the leather has been chromium-tanned or not is not easily available and sometimes wrong. Chromiumallergic persons in Sweden in the clinical study [36] reported that it was difficult to find some chromiumfree leather products, such as special shoes. So far, most standards/labels only consider hexavalent chromium tested by ISO 17075, but not the release of trivalent chromium. The OEKO-TEX label, which has been initiated in 1992 in Germany, Austria, and Switzerland, requires testing for both hexavalent chromium (by ISO 17075) and for trivalent chromium extracted in artificial sweat for 4 h at 37°C (by ISO 17072) with restriction limits of < 2 mg/kg for leather products intended for babies and of < 200 mg/kg for other leather products intended for direct skin contact [155]. For comparison, the leather that elicited allergic reactions in the bracelet study [36] released 313 mg/kg trivalent chromium into artificial sweat after 3 h at room temperature [132].
ACD to chromium is not specific to chromium-tanned leather but elicited by any sources of soluble chromium. These include cement, chemicals, chromated metals, metal fumes, detergents, and bleaching agents. A comprehensive overview is given in [7,156]. Many occupations involve exposure to chromium, e.g. metal work and construction work, which makes it difficult for chromium-allergic persons to avoid long-lasting eczema and consequences.
6 Alternatives to chromium-tanning from an allergy perspective Alternative methods to chromium-tanning exist, such as aldehyde-tanning, mineral tanning (other metal ions), and vegetable-tanning [5]. The allergenic potential of vegetable-tanning is so far relatively unknown. Vegetable-tanning has been claimed to have considerable disadvantages as compared with chromium-tanning [5]. Many attempts have been made to overcome these disadvantages and find alternative tanning methods [157][158][159][160][161]. The allergic potential for aldehyde-tanned leather is known: ACD to aldehydetanned leather has been reported [162,163]. Of other practically possible mineral tanning methods, aluminium(III), titanium, zirconium, and iron have been named [5] and have a relatively low, but not totally absent, allergic potential [15].
From an allergic perspective, it is important to clearly label tanned leathers for any allergenic substances and to provide alternatives to chromium-tanned leathers, which today are dominating the market.

Conclusions
1. Chromium-tanning is the dominant tanning method for leathers today. Chromium-tanned leather increases in importance as cause for allergic contact dermatitis to chromium in some European and North American countries with a declining number of other chromium sources. 2. Around 1% of the general population, and more in some occupations and regions, are allergic to chromium. This means, that these chromiumallergic individuals can develop ACD upon sufficient skin exposure to chromium-releasing items. 3. ACD to chromium-tanned leathers can be elicited in chromium-allergic persons by both hexavalent and trivalent chromium released from chromiumtanned leather. Positive allergic reactions to trivalent chromium are more common for chromium-allergic persons that react to chromiumtanned leather as compared to other chromiumallergic individuals, which should further be investigated in future studies. 4. Hexavalent chromium is the more potent chromium form and requires a lower amount to elicit allergic reactions. It is formed on the surface of some, antioxidant-free, leathers at dry conditions (< 35% relative humidity) and is influenced by the tanning process and other environmental conditions, such as UV irradiation or contact with alkaline solutions. Hexavalent chromium can be released to a higher extent for older / aged leather as compared to new leather. 5. Trivalent chromium is the dominant form released from chromium-tanned leather and its amount released is sufficient to elicit allergic reactions in some chromium-allergic individuals when they are exposed repetitively and over longer time (daysmonths). Trivalent chromium is mostly released initially, and its release is significantly lower for older or used leather. 6. A low test result (< 3 mg/kg) for hexavalent chromium with the current standard test ISO 17075 does not guarantee a low release of total chromium from the leather or low release of hexavalent chromium at another timepoint under certain exposure conditions. This means that certain leathers that have a negative test result in ISO 17075 at one time point can form hexavalent chromium later under certain environmental conditions, such as low relative humidity during storage and exposure to alkaline solutions. 7. Current information, labels and certificates of leather products are insufficient to protect chromium-allergic individuals. 8. Alternative tanning methods might not necessarily be better from an allergic, environmental, toxic, or economic point of view. Correct labelling and information regarding the content of potential allergens, as well as different tanning alternatives, are crucial.

Additional file 2.
Abbreviation ACD: Allergic contact dermatitis