Dermatoses due to external factors — expanded notes
Irritant contact dermatitis
Irritant contact dermatitis is direct barrier injury from physical or chemical exposures. It does not require immunological sensitisation. Repeated low-grade exposure (wet work, detergents, solvents, handwashing) is common; a single strong acid or alkali exposure may cause acute injury. Burning and stinging may be prominent, with erythema, dryness, scaling, fissuring and sometimes vesicles.
Allergic contact dermatitis
Allergic contact dermatitis is a delayed type IV hypersensitivity reaction. During sensitisation, a hapten binds skin proteins and is presented to T cells. Re-exposure activates memory T cells, producing eczema usually after 24–72 hours. Common allergens include nickel, fragrances, preservatives, rubber accelerators, hair dye PPD, topical antibiotics and adhesives. Patch testing identifies relevant delayed hypersensitivity.
Physical causes
- Friction and pressure produce blisters, callus, corns, abrasions and pressure ulcers.
- Heat causes burns; chronic sub-burn heat exposure may produce erythema ab igne.
- Cold causes frostbite and chilblains/pernio; cold urticaria causes wheals after cold exposure.
- Occlusion and moisture cause maceration, intertrigo and secondary microbial overgrowth.
- Radiation causes acute sunburn and chronic photodamage.
Biological causes
Arthropod bites can produce pruritic papules, wheals or bullae. Plant exposures may cause irritant dermatitis, allergic dermatitis or phytophotodermatitis. In phytophotodermatitis, plant furocoumarins plus UVA produce sharply patterned erythema and later hyperpigmentation. Animal bites require assessment of infection, tetanus and rabies risk according to local epidemiology.
Assessment and prevention
Ask about work, hobbies, products, gloves, detergents, cosmetics, topical medication, plants, animals, travel and timing. Identify whether the eruption is limited to contact sites, whether it improves away from exposure and whether others are affected. Management centres on exposure elimination, barrier repair, appropriate protective equipment and anti-inflammatory treatment.