Rubber vulcanisation additives and antioxidants
The real haptens in thiurams and dithiocarbamates remain unknown. Hansson et al. tested 24 patients with known contact allergy to rubber accelerators (thiurams, dithiocarbamates, and/or mercaptobenzothiazoles) with a series of 21 compounds identified based on the chemical analyses of vulcanised rubber products (table 2) . Diphenylguanidine, p-phenylenediamine oxidants, and thioureas were not included in the study. The baseline series included allergens usually found in the TRUE Test® or Chemotechnique® series, as well as potentially sensitising molecules (table 2). Zinc dibenzyldithiocarbamate in 1% pet. from the former Trolab® series, currently marketed by SmartPratice®, was not tested. Thiuram monosulfides induced stronger and more frequent patch test reactions than the corresponding thiuram disulfides. In this study, a positive reaction to a dithiocarbamate was always accompanied by a positive reaction to the corresponding thiuram, except in one case. Thiuram disulfides and dithiocarbamates constitute a redox pair, thus during oxidation of a dithiocarbamate, the corresponding thiuram disulphide is formed, while during reduction of thiuram disulphide, a dithiocarbamate is formed. Today, dithiocarbamates are the main accelerators used in rubber gloves. However, positive patch test reactions to thiurams remain more frequent than positive patch test reactions to dithiocarbamates, which confirms the results from previous studies that showed that thiurams are better markers for sensitisation to the dithiocarbamate/thiuram redox pair. The study also confirms that butyl-substituted thiurams and dithiocarbamates have lower reactivity. Dialkylthiocarbamyl benzothiazole sulfides, formed between thiurams and mercaptobenzothiazoles during vulcanization, showed strong test reactions in almost all patients who were sensitive to dithiocarbamates, thiurams, or mercaptobenzothiazoles. Dialkylthiocarbamyl benzothiazole sulfide is the best marker of rubber allergy to substances of any of the three groups (thiurams, dithiocarbamates, and mercaptobenzothiazoles). The authors suggest that it should be evaluated in a multicentre study.
Several one-off cases of ACD induced by thiurams and dithiocarbamates have been published over the past few years. Creytens et al. reported the first case of connubial airborne contact dermatitis caused by a thiuram, i.e. disulfiram (tetraethylthiuram) in an atopic patient . The patient had eczema on his face, neck, upper chest, shoulders, and elbow folds. Patch test results were positive to thiuram mix, carba mix, and methylisothiazolinone 0.05% aq. The patient's history revealed that he had crushed Antabuse® tablets (disulfiram) for his wife. His symptoms completely resolved after he started using a pill crusher. Pföhler et al. reported a case of occupational allergic contact dermatitis of the ears in a female working as a secretary . She had been typing dictated letters for over 30 years and she wore a headset for over five hours a day, five days a week. Patch tests showed positive reactions to thiurams and rubber parts of the headset. After her headset was replaced by a rubber-free headset, her symptoms resolved completely.
Mercaptobenzothiazole sensitisation is mainly associated with ACD of the feet . Recently, Munk et al. reported four cases of patients who developed ACD on their feet after wearing Keds® canvas sneakers. Patch test results showed positive reactions to thiuram mix, as well as to pieces of their shoes. High-performance liquid chromatography (HPLC) identified MBT in the canvas of the shoes, but no thiurams nor dithiocarbamates. Symptoms resolved after the patients stopped wearing the shoes. Information on the chemical composition of the shoes was difficult to obtain. The company's website mentions that the shoe is manufactured from an unvulcanised rubber sole attached to a canvas fabric, which is subsequently vulcanised in order to attach the top and bottom of the shoe.
The prevalence of positive patch results to 1,3-diphenylguanidine has been increasing over the past few years. In 2013, Baeck et al.  reported five cases of ACD caused by 1,3-diphenylguanidine. The authors hypothesised that this was due to the replacement of natural rubber latex gloves by synthetic rubber ones, as part of a “latex-free hospital”. The same authors have reported 32 unpublished cases of ACD induced by 1,3-diphenylguanidine in synthetic rubber gloves since 2011 (Baeck, personal communication). In Sweden, Pontén et al. investigated 16 patients with ACD induced by sterile synthetic polyisoprene rubber gloves among surgical operating theatre personnel. Chemical analysis was carried out on five different brands of gloves using HPLC . Rubber chemicals yielding positive patch test results were 1,3-diphenylguanidine (12 patients), thiurams (eight patients), and ZDEC (two patients). It is worth noting that although no thiurams were detected in any of the gloves, eight patients with a clinical history suggesting glove allergy had positive patch test results to thiurams. For two of the gloves, the concentrations of 1,3-diphenylguanidine on the inside were ten times higher than those on the outside of the gloves.
Dahlin et al. reported two cases of ACD induced by triphenylguanidine (CAS no.101-01-9) in synthetic rubber surgical gloves in a female surgeon and a scrub nurse . Chemical analysis of synthetic rubber surgical gloves confirmed the presence of triphenylguanidine (gas chromatography/mass spectrometry (GC-MS) and liquid chromatography). Over 122 patients were tested using a rubber series including triphenylguanidine (1.35% pet.) between 2011 and 2013. Two other cases were positive to triphenylguanidine. All patients with positive patch test results to triphenylguanidine also had positive patch test results to 1,3 diphenylguanidine. There are at least three different guanidine-type accelerators that are used in the production of rubber, i.e. 1,3-diphenylguanidine, triphenylguanidine, and di-o-tolylguanidine (CAS no. 97-39-2). 1,3-diphenylguanidine is now a well-known sensitiser; it is tested in the rubber series (1% pet.) and elicits positive reactions in up to 3% of patients . It is currently unknown whether there is cross-reactivity between triphenylguanidine and 1,3-diphenylguanidine.
Hamnerius et al. studied the influence of exposure time to gloves and the use of skin disinfectants on the amount of 1,3-diphenylguanidine released by synthetic rubber gloves . They used HPLC to measure the amount of 1,3-diphenylguanidine released from the inside of the gloves after exposure to artificial sweat. After approximately 10, 30, 60, and 180 minutes, 73%, 79%, 82% and 87% of the total amount of 1,3-diphenylguanidine was released from the inside of the gloves (measured by HPLC), respectively. The remaining amount of extractable 1,3-diphenylguanidine was estimated by washing the inside of the gloves with ethanol for 10 minutes. The authors also evaluated the amount of 1,3-diphenylguanidine on the hands exposed to 3 ml of skin disinfectant and then after wearing gloves for 60 minutes, and compared this with a control group which did not use disinfectant. The amount of 1,3-diphenylguanidine released was higher in the group of patients whose hands were exposed to disinfectant.
Diphenylthiourea (DPTU), diethylthiourea (DETU), and dibutylthiourea (DBTU) are used in the series of rubber additives as the three diagnostic markers for polychloroprene rubber allergy . In experimental studies, thiourea compounds are classified as weak sensitisers. Recently, it was found that DPTU was activated to metabolites including phenylisothiocyanate (PITC) and phenylisocyanate (PIC), which are strong sensitisers . The authors carried out a chemical analysis with measures of DPTU, DETU, and DBTU in three main categories of products made of polychloroprene and likely to have prolonged contact with water, i.e. medical devices, and sports and diving gear. Only DETU was detected. DETU is by far the most common thiourea compound used in the manufacturing of polychloroprene goods. At 37̊C, DETU is continuously degraded into ethyl isothiocyanate (ETIC). The authors concluded that DETU can be considered as a prehapten which, at room temperature, degrades into a strong sensitiser, i.e. EITC. EITC could thus account for severe ACD induced by polychloroprene rubber. ACD to thioureas in wetsuits was the subject of two presentations at the 2014 ESCD congress. Poreaux et al. reported a case of generalised eczema in a sea lion trainer . Symptoms appeared after he started wearing a new Spandex wetsuit and persisted even after he returned to his previous wetsuits. Patch test results were positive for thioureas from the plastic-glue series (DETU and DBTU). Patch tests with 2.5 × 2.5-cm pieces of wetsuits were negative but positive with 5 × 5-cm pieces of wetted wetsuits. Chemical analysis of the wetsuits confirmed the presence of higher thiourea quantities than in the Spandex wetsuit.
Ghys and Goossens reported another case in a six-year-old child wearing a jet ski wetsuit . Since she was three, she had been suffering from a recurrent generalised papular and urticarial eruption associated with the wearing of a wetsuit. Patch test results showed positive reactions to DETU and disperse dye mix from the European standard series, as well as to red, blue, and black pieces of the wetsuit. The lesions decreased when she started wearing cotton clothes under her wetsuit.
Liippo et al.  reported two cases of ACD induced by thioureas in the neoprene handles of cleaning trolleys in two cleaners with hand dermatitis. Patch test results were positive for thiourea mix (consisting of 0.5% [wt/wt] DETU, 0.5% DBTU, and 0.5% DPTU in petrolatum), DETU, and the handles of the trolleys.
Benton et al. reported a case of ACD induced by a rubber respirator. The patient developed lesions limited to the face after he had worn the respirator for his military training. Patch test results were positive to pieces of the respirator and methyl hydroxystearate (1% pet.), a compound supplied by the manufacturer. Methyl hydroxystearate is derived from hydrogenated castor oil and is used as a processing aid as it reduces the coefficient of friction and surface tack.
Vanden Broecke et al. reported a case of severe ACD of the right hand in a retired farmer after he had cleaned his garden shed, wearing a rubber glove coated with a moisturiser on the inside (Vileda Comfort and Care, Comfort plus, extra-absorbent®; Vileda, Verviers, Belgium) . He had severe dermatitis on the right hand, wrist, and forearm and a more discrete reaction on his left hand. He had been wearing the glove for about two hours, on his right hand only. The patient had a history of dermatitis induced by moisturisers and positive patch test results to cetrimide, a quaternary ammonium compound (cetyltrimethylammonium bromide), isopropanol, iodine, and povidone iodine. Patch testing was performed with the European baseline series, the rubber series, chamomile, the ingredients of the creams he had used, and with pieces of the inner and outer sides of the gloves. Positive results were yielded with cetyl alcohol and the pieces of the gloves. Chemical analysis of the gloves with GC-MS showed the presence of fatty alcohols, as well as docecytrimethylammonium chloride, a compound closely related to cetrimide. Quarternary ammoniums are surfactants used as stabilisers or wetting agents. They are found in the baths used on the glove production lines (Palu, personal communication). Additional patch tests with the ingredients from the gloves showed strong positive reactions to stearyl alcohol and behenyl alcohol (which includes various unsaturated and polyunsaturated alcohols). Dodecyltrimethylammonium chloride identified in the glove was not tested as it was not available, however, patch testing with benzalkonium (or alkyldimethylbenzylammonium chloride) yielded positive reactions.
In Pontén et al.’s study mentioned above (16 cases of ACD induced by sterile synthetic polyisoprene rubber gloves), cetylpyridinium chloride was positive in seven patients . Since 2010, cetylpyridinium chloride has been tested at 0.1% (wt/wt) in water. The authors recommend patch testing with fully dissolved cetylpyridinium chloride at room temperature as, when stored in a refrigerator, it crystallises in the bottom of the test tube. The content of cetylpyridinium chloride was also analysed using HPLC, and was shown to be higher on the inside of the glove than on the outside of the glove.
There are currently 15 latex proteins internationally considered as allergens. They are referred to as Hev b and are listed in table 3. The following website provides information on latex allergens: http://www.allergome.org or http://www.allergen.org.
Molecular allergy diagnosis can help to differentiate between patients with a severe risk of anaphylaxis and those with asymptomatic polysensitisation (carbohydrate determinants or CDD) [31-33]. EAACI recently published a position paper on food allergy and immunological cross-reactions . Hev b 5, Hev b 6.01, and Hev b 6.02 are considered to be major allergens involved in latex sensitisation, in particular, in healthcare workers . Hev b 1 and Hev b 3 affect patients who undergo surgery frequently (e.g. patients with spina bifida) . Hev b 2, Hev b 6.01, Hev b 6.02, Hev b 6.03, Hev b 7, Hev b 8, and Hev b 11 are cross-reactive allergens that cause latex-fruit syndrome . CDD have little or no clinical relevance . Canonica et al. published a consensus document in 2013 . Sensitisation to Hev b 8 (profilin), a cytoskeletal protein found in many plants (also a panallergen), is not related to clinical latex allergic reactions . Patients with positive IgE against latex with negative prick test results with latex, monosensitisation to Hev b 8, and who show no latex-specific symptoms, are not considered allergic to latex and can thus undergo medical and surgical procedures (using latex gloves) without any risk [33, 35]. Hev b 8 is considered as a marker of asymptomatic latex sensitisation. Hev b 13, another allergen, can also be used to identify other cases of latex allergy, though in a lower percentage of cases . Prick tests with latex standardised extracts are used to confirm latex sensitisation. Gabriel et al. analysed the protein and allergen composition of natural latex extracts from seven different manufacturers: Alk-Abelló, Allergopharma, Bial-Aristegui, Leti, Lofarma, Q-Pharma, and Stallergènes . They analysed the protein content using sodium dodecylsulfate polyacrylamide gel electrophoresis and quantified four major allergens (Hev b 1, Hev b 3, Hev b 5, and Hev b 6.02) involved in latex allergy using an enzyme immunoassay. Allergenic capacity was assessed using microarray inhibition assays and prick tests in 11 patients with known latex allergy. Results showed large differences in protein profiles between the seven standardised latex extracts. A 65-fold variation in the protein content was observed, ranging from 8.0 μg/mL to 526.5 μg/mL. The levels of the four main latex allergens were also highly variable, particularly Hev b 3 and Hev b 5, as these were below the detection limit in some extracts. Similarly, allergenic capacity assessed using microarray inhibition assays and prick tests showed large differences between the extracts. Almost all patients allergic to latex had at least one negative prick test result (<3 mm) to one of the seven extracts. One of the patients had a prick test with a wheal diameter >8 mm with one extract, but no other visible skin reaction with the two others. The authors suggested that if latex allergy is suspected, prick tests should be carried out using at least two extracts from different companies in order to reduce the risk of false negative results.