Allergic contact photodermatitis to para-aminobenzoic acid

Area of Science:

• Dermatology
• Toxicology
• Photobiology

Background:

• Allergic photocontact dermatitis is a hypersensitivity reaction to a substance that becomes allergenic when exposed to ultraviolet radiation.
• Para-aminobenzoic acid (PABA) has been a common ingredient in sunscreens, but its potential for causing photosensitivity is recognized.

Observation:

• A patient developed allergic photocontact dermatitis after using a commercial sunscreen.
• The sunscreen contained para-aminobenzoic acid (PABA) dissolved in an alcohol base.

Findings:

• Photopatch testing revealed that the patient was photosensitive to PABA.
• Crucially, PABA sensitization was only detected when tested in an alcohol vehicle.
• A photopatch test using PABA in a petrolatum vehicle yielded negative results.

Implications:

• The vehicle used for topical application can significantly influence the detection of PABA sensitization.
• This highlights the importance of considering vehicle composition in diagnosing photocontact dermatitis.
• Clinicians should be aware that negative photopatch tests with certain vehicles may not rule out PABA allergy.