Allergology, Contact Dermatitis and Occupational Skin Disease
Two mechanisms
Allergic contact dermatitis is a type IV, delayed, T-cell mediated reaction appearing 24–72 hours after re-exposure and producing eczema. Type I immediate hypersensitivity (urticaria, angioedema, anaphylaxis) is IgE mediated. Irritant contact dermatitis is commoner than allergic contact dermatitis, depends on dose and barrier damage, and is not immune mediated.
Common allergens
Nickel (jewellery, buckles), cobalt and chromate (cement, leather), fragrance mix, methylisothiazolinone (cosmetics, wipes), paraphenylenediamine (hair dye, temporary tattoos), rubber accelerators, epoxy resins, colophony and preservatives. Occupational groups at risk: hairdressers, healthcare workers, construction workers, florists, cleaners and food handlers.
Investigation
Patch testing is the gold standard, read at day 2 and day 4/5; repeat open application testing for products; specific IgE for type I allergy. Interpret each positive reaction for relevance to the exposure, and distinguish true allergy from irritant or doubtful reactions.
Occupational skin disease
Take an exposure history by task, not job title. Confirm improvement away from work, report as a prescribed occupational disease where relevant, and advise correct glove material, after-work emollients and workplace modification.
Traps
- Calling everything "allergy" - most hand dermatitis in cleaners and hairdressers is irritant, sometimes with a superimposed allergic component.
- Patch testing on actively inflamed skin.
- Advising "avoid all chemicals" instead of identifying a specific allergen.
- Missing latex type I allergy in a healthcare worker with anaphylaxis risk.
Rule of thumb
Hand dermatitis that persists despite emollients and appropriate gloves deserves patch testing.