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This article is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND). Usage and distribution for commercial purposes as well as any distribution of modified material requires written permission. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

The number of pigments that could potentially be used in tattoo inks is vast. However, pigments are generally not manufactured for the purpose of being injected into subepidermal layers of the skin. Assuming 100% bioavailability after injection means that pigments can be imminently hazardous to human health. Given the ever-increasing number of pigments being circulated on the market or through the internet, a ‘negative list' (‘black' list) containing pigments with known adverse effects will never be finalised. If incriminated, substances could easily be replaced by structurally similar pigments that might be even more deleterious to human health. Therefore, we and others suggest the establishment of a whitelist (‘positive list') that would only contain pigments that had undergone a risk assessment specifically for their application into the dermis. Some of the problems associated with such a ‘positive list' are discussed. Another important issue with regard to tattoo safety is related to the preservatives used in ink preparations. Notwithstanding the demand for sterile tattoo inks, a whitelist for these compounds would be beneficial. At present, many technical preservatives are being used, despite their known detrimental effects to human health. Criteria for the inclusion of preservatives in a ‘positive list' are also discussed.

According to Colour Index International, which is jointly published by the British Society of Dyers and Colourists (UK) and the American Association of Textile Chemists and Colorists (US) and which serves as standard library for the identification of pigments, about 27,000 products residing under more than 13,000 generic colour index names are used for the manufacturing of colours [1]. To be suitable for tattoo inks, a pigment must display light-fastness, a certain degree of resistance to ultraviolet light, brilliance and equal distribution in suspension. These requirements are met only by a subset of all pigments, but the number of pigments that might be applicable for tattoo inks is still large. Pigments are not specifically manufactured for use in tattoo inks. Instead, pigments initially manufactured for use in paints, plastics, glazes or other applications are being included in viscous ink formulations for insertion into the skin, more specifically, into the upper layer of the dermis right beneath the epidermis. To do so, pricking needles that penetrate the natural (epidermal) skin barrier are used.

The pigments being discussed are not specifically manufactured for any kind of application in the human body. Hence, the raw materials used for their production are not controlled with regard to contamination or other chemical hazards. Consequently, carcinogenic polycyclic aromatic hydrocarbons or nitrosamines and allergenic substances like nickel are frequently reported to be present in tattoo inks [2]. In particular, the ‘modern practice' of applying organic pigments in tattoo inks is not usually backed-up by minimal toxicity testing. Yet, toxicity data for certain pigments are available and thus can be utilised for health risk assessment purposes. Some of these data have previously been compiled into a list of colours that should not be used in tattoo inks (‘negative list') [3].

Currently, there is no authoritative European regulation on tattoo inks; however, some member states of the European Union have set up their own national legislations that are mostly based on resolution ResAP(2008)1 of the Council of Europe in Strasbourg [3]. ResAP(2008)1 includes a list of pigments that should not be included in tattoo inks (‘negative list') as well as provisions stating that pigments that are prohibited or restricted from cosmetic products and that are listed in Annexes II and IV of the European Regulation (EC) No. 1223/2009 on Cosmetic Products should not be contained in tattoo inks [4]. Usually, these annexes are compiled based on existing toxicological data that have been fed into the corresponding risk assessments by expert groups. As a prerequisite for the risk assessment of cosmetic ingredients, a dossier summarising the related toxicological data has to be provided by the industry. However, it should be noted that the majority of substances on the list of cosmetic colourants (Annex IV of Regulation 1223/2009) have not undergone a recent safety assessment.

Given the great number of pigments on the market, ‘negative lists' and restrictions cannot and do not cover all potentially problematic pigments; furthermore, effectively regulating hazardous colourants based on ‘negative lists' is not feasible. First, the inclusion of a pigment in a ‘negative list' would require further substantiation, usually via assessing toxicological data. However, it currently remains unclear who would be in charge of delivering such data. Industry certainly would have no interest in delivering the data due to the lack of any additional benefit from the prohibited compounds and pigments that would fill such black lists. On the other hand, it seems equally questionable to address the taxpayer when it comes to the funding of research that, after all, would ultimately back-up the commercialisation of non-listed alternatives by private companies. Second, the prohibition of one pigment might easily pave the way for structurally similar pigments of equally questionable toxicology. An example of this is provided by phthalocyanine ‘Pigment Green 7' (CI 74260; see fig. 1). According to ResAP(2008)1, this pigment should not be a part of tattoo inks because its use is restricted in EU Regulation (EC) No. 1223/2009 (Annex IV, no. 107, ‘not to be used in eye products'). The very similar substance ‘Pigment Green 36' (CI 74265; see fig. 1), however, is not present in any of the above-mentioned lists and therefore remains currently unrestricted with regard to tattoo inks. Accordingly, ‘Pigment Green 7' is currently being substituted by ‘Pigment Green 36' on the market. When analysing the structures of both compounds, it becomes evident that the chlorine substituents of Pigment Green 7 have just been replaced by bromine in Pigment Green 36 (6 of 16). Since there was only an exchange from one halogen to another, it seems reasonable to expect that both compounds would not differ much in terms of their toxicological properties. It might even be possible that the latter compound would actually show increased potential as a health hazard. Another example is provided by the quinacridones ‘Pigment Violet 19' (CI 73900) and ‘Pigment Red 122' (CI 73915), both of which are restricted from use according to EU Regulation (EC) No. 1223/2009 (Annex IV, no. 102, 103 ‘rinse-off products'). These compounds have been replaced by ‘Pigment Red 202' (CI 73907), which carries two chlorine substituents instead of hydrogen or methyl groups (fig. 1).

Fig. 1

Comparison between structurally similar phthalocyanine and quinacridone pigments.

Fig. 1

Comparison between structurally similar phthalocyanine and quinacridone pigments.

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Although ‘negative lists' of pigments and other compounds prohibiting their use in tattoo inks might be helpful to some extent, ‘positive lists' are now clearly favoured by experts in light of the issues described above. The establishment of whitelists would require submission of dossiers compiling toxicological data, which could then be assessed regarding the potential health risks of the pigments by expert committees. Pigments that pass the risk assessment procedure with no or only little concern could then be included in such a list. This course of action has been generally followed in the cosmetics sector in the EU. Interested companies or, in some cases, industry consortia supply the required toxicological data, and the Scientific Committee on Consumer Safety (SCCS) of the EU commission takes the task of evaluating the dossiers for health issues. It is expected that ‘positive lists' would provide higher levels of safety when compared to ‘negative lists'. However, different from the situation in the cosmetics sector, manufacturers of tattoo inks are mostly small companies that presumably would have difficulties paying for all of the requested toxicological and analytical data, especially if new experiments have to be conducted to fill knowledge gaps. On the other hand, the manufacturers of pigments are mostly large, often international companies that are likely to be in a (financial) position to afford the research. For these companies, however, the market dealing with tattoo pigments simply represents a ‘much-too-small segment' of their portfolio; therefore, they usually refuse to get involved in the safety discussion and evaluation process. Even existing data might not be disclosed because pigment manufacturers, in principal, do not endorse the use of their pigments for the purpose of tattooing. The questions of how to overcome these difficulties and of how to ensure that the pigments used in tattoo inks do not pose a risk to consumers still exist.

According to tattoo ink manufacturers, about 15-20 pigments would be sufficient to satisfy the demands on the market for preparing all shades of tattoo inks imaginable (personal communication, Ralf Michel, Tattoo Ink Manufacturers of Europe). A pragmatic approach would be for tattoo ink manufacturers to form a consortium that would focus on drafting a comprehensive list of indispensable pigments to be assessed for possible health issues. The costs of data compilation - which certainly should be manageable for 15-20 substances - could be shared among all members of the consortium. Subsequently, all prepared dossiers could then be evaluated by a committee of experts similar to the SCCS. Data specific for application of the pigments into skin would have to be included, and in most cases, newly generated data, such as studies on biokinetics (e.g. distribution of pigments or tattoo ink ingredients throughout the body), will probably have to be included. Therefore, another important aspect has to be considered: although tattoo inks are not by any means comparable to cosmetic products, the prohibition of animal experiments for the testing of cosmetic ingredients has begun to cast a shadow over the necessary research on the biokinetics of tattoo ink ingredients in the living body. At present, it is difficult to predict how this will affect and interfere with research on tattoo inks. However, tattooed individuals are not a minority anymore, and these data have to be provided somehow to ensure the safety of consumers.

According to ResAP(2008)1, inks used for tattoos and permanent make-up should be ‘sterile and supplied in a container which maintains the sterility of the product until application, preferably in a packaging size appropriate for single use. In case multi-use containers are used, their design should ensure that the contents will not be contaminated during the period of use'. The resolution also specifies that ‘preservatives should only be used to ensure the preservation of the product after opening and by no means as a correction of insufficient microbiologic purity in the course of manufacture and of inadequate hygiene in tattooing and PMU (permanent make-up) practice' and that they ‘should only be used after a safety assessment and in the lowest effective concentration' [3]. Based on market surveillance, e.g. in Switzerland and Germany, it is known that tattoo inks usually contain preservatives [5,6]. These compounds belong to two categories: (1) preservatives that are included in a ‘positive list' for use in cosmetics (Annex V of EU-Regulation 1223/2009) and that have been approved after a risk assessment by the SCCS or its predecessor committees for use on the skin; and (2) technical preservatives that have not been approved for safe use on skin and/or have never been intended for use in tattoo inks. Examples of this second category include substances like benzisothiazolinone or octylisothiazolinone, both of which are strong sensitisers, or the corrosive phenol. Again, candidate preservatives are numerous, and a ‘negative list' would hardly be adequate to ensure that the health of the consumer would not be adversely affected by tattoo inks.

However, even preservatives in category 1, which, as ingredients of cosmetic products, have been approved for use on the skin, are not necessarily safe for injection into the skin. The latter would occur once they become ingredients of tattoo inks. Yet, at least some toxicological data for these substances that can provide some kind of grounding for the compilation of a whitelist of preservatives to be used in tattoo inks exist. Still, some substances listed in Annex V of the EU cosmetics regulation (i.e. preservatives allowed in cosmetic products) are certainly not suitable for the purpose of tattooing, including:

- mercury-containing preservatives (e.g. thiomersal, phenylmercury and phenylmercury salts)

- substances with a high sensitization potency (e.g. Kathon CG, formaldehyde, glutaraldehyde)

- formaldehyde-releasing substances

- substances for which specific limits are effective or for which a warning must be included on the label according to cosmetics regulations

- corrosive or irritating substances

- triclosan

- iodopropinylbutyl carbamate.

The remaining substances listed in Annex V would still need to undergo a systematic risk assessment for the application route ‘injection into the skin' in order to be eligible to become part of a ‘positive list' of preservatives for tattoo inks. With Annex V as the basis, the number of potential candidates is still limited, thus enabling risk assessment based on the data that are already available in conjunction with the supplementary toxicokinetics data.

In conclusion, although it is useful in the short term to ban some of the most harmful substances, ‘negative lists' will always be incomplete. Although the compilation of ‘positive lists' requires a huge effort by both industry and authorities and might not be possible within a short time frame, these lists seem indispensable for providing the highest level of safety possible. This also holds true for the safety of tattoo inks and for the protection of consumers against any adverse health effects that might occur immediately after the process of tattooing or over the long term.

1.
Society of Dyers and Colorists (SDC) and American Association of Textile Chemists and Colorists (AATCC), Color Index, 1924, 4th edition online, last accessed Jan 9, 2015. http://www.sdc.org.uk/technical-services/colour-index/.
2.
Gesundheitsdepartement des Kantons Basel-Stadt, Bereich Gesundheitsschutz, Kantonales Laboratorium, Jahresbericht (annual report) 2009. 2009, p. 75, last accessed Jan 9, 2015. http://www.gesundheitsschutz.bs.ch/konsum-umwelt/berichte/jahresberichte.html.
3.
Council of Europe, Committee of Ministers, Resolution ResAP(2008)1 on requirements and criteria for the safety of tattoos and permanent make-up, adopted on 20 Feb 2008, last accessed Jan 9, 2015. http://www.coe.int/t/e/social_cohesion/soc-sp/ResAP_2008_1%20E.pdf.
4.
The European Parliament and the Council of the European Union, Regulation (EC) No 1223/2009 of the European parliament and of the Council of November 30 2009 on cosmetic products, 2009, last accessed Jan 9, 2015. http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:342:0059:0209:EN:PDF.
5.
Bundesamt für Verbraucherschutz und Lebensmittel (BVL), Berichte zur Lebensmittelsicherheit 2007, 2007, p.75, last accessed Jan 9, 2015. http://www.bvl.bund.de/SharedDocs/Downloads/01_Lebensmittel/02_BUEp_dokumente/buep_berichte_archiv/BUEp_Bericht_2007.pdf?__blob=publication File&v=5.
6.
Gesundheitsdepartement des Kantons Basel-Stadt, Bereich Gesundheitsschutz, Kantonales Labor, Urs Hauri, Tinten für Permanent Make Up (PMU) und zur Tätowierung / Organische Pigmente, Konservierungsmittel, Verunreinigungen (Nitrosamine, Polyaromatische Kohlenwasserstoffe (PAK), Aromatische Amine), 2012, last accessed Jan 9, 2015. http://www.gesundheitsschutz.bs.ch/konsum-umwelt/berichte/2012.html#page_section3_section2.

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