PPD is a chemical most frequently encountered in permanent hair dye – usually, the darker the hair dye the higher the PPD content. Hair dyes containing PPD or its derivatives usually carry a warning on the packaging recommending a self patch test prior to use. Black temporary or henna tattoos are another important source of PPD exposure.

Allergy to PPD is common and patients can have recognised cross-sensitization with the following due to their chemical structural similarities:

• Azo dyes – certain semi-permanent/temporary hair dyes, pen inks, petrol/diesel, certain textiles, and as a colouring agent in certain foods and medications
• Para-amino benzoic acid (PABA) – over the counter sunscreens and certain cosmetics
• Para-amino salicylic acid -
• Sulfonamides
• Local anesthetics such as benzocaine and procaine

References:

• Hair dye dermatitis and para-phenylenediamine contact sensitivity https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4513402/

Nickel sensitivity is present in around 15-20% of patch tested individuals, affecting females more so than males. It is present in many metal containing products such as jewellery, kitchen tools, silverware, medical devices and in particular metal fasteners such as in belts (as in this case), as well as costume jewellery. Nickel is banned from use in jewellery in the Europse.

Testing a metal with DMG (dimethyl-glyoxime) turns it pink if nickel is present.

Cobalt is a frequent co-reactor with nickel, but this does not mean that the patient has a cobalt sensitivity. It is cobalt that provides a blue colour to cosmetics, eye makeup, and tattoos.

Nickel can be found in certain foods such as chocolate, nuts, oats, green beans, peas, and canned foods, and there have been reports of systemic allergic contact dermatitis from their ingestion in those significantly sensitised.

Formaldehyde-releasing preservatives include quaternium-15, DMDM hydantoin, and 2-bromo-2-nitropropane-1,3-diol (also referred to as bronopol). Non-formaldehyde-releasing preservatives include methylchloroisothiazolinone (MCI)/methylisothiazolinone (MI).

These are frequently found in hair care, cosmetics, topical preparations, and cleaning agents due to their antimicrobial properties. They can also be found in baby wipes.

References:

• Contact dermatitis caused by baby wipes https://pubmed.ncbi.nlm.nih.gov/16631948/

Methylisothiazolinone (MI) and methylchloroisothiazolinone (MCI) are preservatives often found in:

• Leave on products: moisturizing creams/lotions, sunscreens, makeup. Although in Europe MI has been banned in leave on products.
• Wash-off products: hair care products, body washes, premoistened toilet wipes
• Water-based paints: can cause an allergic contact dermatitis from airborne exposure in recently painted indoor environments
• Cross react with others: benzisothiazolinone and octylisothiazolinone

p-phenylenediamine (PPD) is a chemical most frequently encountered in permanent hair dye – usually, the darker the hair dye the higher the PPD content. Black temporary or henna tattoos are another important source of PPD exposure.

Whilst those sensitised to PPP who use PPD-containing hair dyes are likely to experience a head and neck dermatitis, those applying the hair dye, such as hairdressers, may develop hand dermatitis as a result.

Nickel allergic contact dermatitis is another cause of hand dermatitis in hairdressers secondary to instrument use.

References:

• Hand contact dermatitis in hairdressers: clinical and causative allergens, experience in Bangkok https://pubmed.ncbi.nlm.nih.gov/23393911/

Knowledge of local anaesthetic allergy is important when performing skin surgery.

Ester local anaesthetics include benzocaine, dibucaine, and tetracaine. As their names suggest, these are tested for using Caine mix which is part of the European baseline patch test series. Testing positive also risks cross-reacting with p-phenylenediamine. Esters have a greater chance of producing type 1 anaphylactoid reactions compared to amides. There are also reports of peri-anal dermatitis in those using local anaesthetic preparations peri-anally eg. For the treatment of haemorrhoids.

Amide local anaesthetics include lignocaine and prilocaine. These are the two anaesthetic components of Emla 5% cream, which is often applied topically 45-60 minutes before a procedure in children. Rarely do amides cause allergic reactions.

Colophonium is derived from the sap of pine trees. It’s unique stickiness means it is used in a broad range of products.

It can be found in the following:

• Cosmetics - mascaras, lipsticks, eyeshadows
• Adhesives - plasters and tapes, glues
• Toiletries – hair removal wax, blister creams and first-aid ointments
• Recreational - sport racket handles, bows for stringed instruments
• Chewing gum

Industrial – paper manufacturing, paints/varnishes/lacquers

Acrylates are monomer chemicals that usually come as a powder or liquid, which can then be moulder during polymerisation to form strong plastics as they harden. It is in their monomer states that they are most allergenic, and less so once polymerised. They were dubbed contact allergen of the year in 2012.

Acrylates can be found in:

• Artificial nails – ultraviolet light is often used to cure gel nails ie. Polymerise the acrylate
• Dental fillings and sealants
• Joint replacement cements & glues
• Paints, polishes and sealants

Typically, allergic contact dermatitis to acrylates presents with fingers or hand dermatitis at the site of exposure, but this can be transported to other sites touched eg. Periorbital dermatitis when scratching eyes

Acrylates can diffuse through surgical gloves so this does not provide complete protection.

A circumferential rash is not indicative of nickel allergy, unless the strap was also metal, and purchased from outside Europe. The remainder of the answers are also adhesives, however, PTBP resin is a common adhesive used in leather products, and can cause accentuation at the borders of a watch strap rash where the leather has been glued down. PTBP can also be used as a rubber adhesive, such as in trainers.

Phytophotodermatitis is a phototoxic reaction that generally occurs in the spring or summer, when plant content exposed to the skin alongside UV light. It typically presents with bizarrely distributes dermatitis, bullae and oedema, often in a linear configuration.

A number of plants have been reported to cause phytophotodermatitis. Below are the most commonly encountered:

• Parsley
• Celery
• Lemon/lime and other citrus fruits
• Figs
• Parsnips

Octyl gallate is an additive, preservative, and antioxidant usually found in foods or cosmetics, most notably in lipsticks. Chlorocresol is found in multiple topical preparations, such as topical steroids. Cyanoacrylate is an adhesive found in crazy glue and Dermabond. Balsam of Peru is a fragrance. Sisquiterpene lactone is an allergen found in various plants.

Elastic dermatitis is a recognised presentation of allergic contact dermatitis to carba chemicals, which are tested for using carba mix patch testing reagent. The more elastic is bleached, the greater the carbamate content. They are also found in leather shoes.

Carbas are part of a group of chemicals termed rubber accelerators, as they play a role in speeding up the production of rubber (also termed vulcanisation). Rubber compounds have numerous uses, and some of the key reagents and some of their main uses are summarised below:

• Thiurams – rubber shoes, rubber gloves, barrier contraception. Note: cross-reacts with disulfiram (Antabuse) used to treat alcohol abuse/dependence
• Mercaptobenzothiazole – most common cause of allergic shoe dermatitis, also in tires, antifreeze, and cutting oils
• Mercapto mix – chemicals in tires, elastic and other rubber products
• Carba mix – elastic dermatitis, leather shoes, can cross-react with thiuram derivatives

The clinical picture is that of an airborne contact dermatitis. Sisquiterpene lactone is an allergen found in the compositae family of plants. This can cause a hand dermatitis if handled, but can also cause an airborne contact dermatitis at body sites exposed to the plant pollen in the wind. This can result in a dramatic eczematous, oedematous and/or bullous rash. This tends to occur in middle-aged/older adults, during the summer months.

Plants in the compositae family include:

• Chrysanthemum
• Dandelion
• Echinacea
• Hogweed
• Sunflower

Questions relating to contact allergy to tattoo constituents are easy exam questions.  Below are the key tattoo ink colours, and their relevant components that can lead to allergic contact dermatitis:

• Black = carbon or iron oxide
• Red = cinnabar (mercury sulfide)
• Blue = cobalt
• Brown = ferric oxide
• White = titanium
• Yellow = cadmium sulfide
• Purple = manganese
• Black henna = p-phenylenediamine (PPD)

Tattoos can also lead to granulomatous, lichenoid, photo-aggravated, and pseudolymphomatous reactions. Pre-existing conditions can koebnerise into tattoos such as psoriasis, vitiligo or lichen planus.

Questions relating to contact allergy to tattoo constituents are easy exam questions.  Below are the key tattoo ink colours, and their relevant components that can lead to allergic contact dermatitis:

• Black = carbon or iron oxide
• Red = cinnabar (mercury sulfide)
• Blue = cobalt
• Brown = ferric oxide
• White = titanium
• Yellow = cadmium sulfide
• Purple = manganese
• Black henna = p-phenylenediamine (PPD)

Tattoos can also lead to granulomatous, lichenoid, photo-aggravated, and pseudolymphomatous reactions. Pre-existing conditions can koebnerise into tattoos such as psoriasis, vitiligo or lichen planus.

Photoallergic contact dermatitis is caused by the combination of a chemical on the skin, as well as exposure to ultraviolet radiation.

Benzophenones are ketones that have the ability to absorb UV, mostly UVB, so function well as ingredients in sunscreen. Oxybenzone (Benzophenone 3) is the most common compound that causes photoallergic contact dermatitis.

Topical corticosteroid allergy can present as failure of the eczema to improve, or with a deterioration in the patients’ dermatitis. Patients may develop an allergic contact dermatitis to other components within topical corticosteroid preparations, such as quaternium-15 and antibiotics combination preparations.

It is important to know the class and patch testing substance (in brackets below) used to diagnosed allergic contact dermatitis for each group of corticosteroids:

• Class A (tixocortol pivalate) – hydrocortisone Acetate, prednisolone, methylprednisolone
• Class B (budesonide & triamcinolone) – budesonide, triamcinolone
• Class C (no specific test substance) – dexamethasone
• Class D1 (clobetasol-17-propionate) - Betamethasone dipropionate, Betamethasone valerate (Betnovate), Clobetasol propionate (Dermovate), Clobetasone butyrate (Eumovate), Mometasone furoate (Elocon)
• Class D2 (hydrocortisone-17-butyrate) – hydrocortisone preparations (including butyrate and valerate) except hydrocortisone acetate (Class A)