Why do the dorsal sides of the hands turn purple under black light, while the nails and palms remain white?

Wood's Light Examination: Normal Fluorescence Pattern of Hands

The dorsal hands appear purple under black light (Wood's lamp) while the nails and palms remain white because of differences in keratin structure and melanin distribution—this is a normal finding, not a pathologic condition. 1

Understanding Wood's Light Physics

Wood's light is a hand-held ultraviolet (UVA) irradiation device used primarily to identify areas of depigmentation or pigmentary changes that may not be visible to the naked eye. 1

Normal Fluorescence Patterns by Anatomic Location

Dorsal Hand Surface (Purple Fluorescence):

• The dorsal hand skin contains melanin in the epidermis and has thinner, more UV-reactive keratin compared to palmar surfaces 1
• UV light interacts with melanin and other chromophores in the skin, producing the characteristic purple/violet fluorescence seen on sun-exposed skin 1
• This represents normal pigmentation and is more pronounced in individuals with chronic UV exposure to the dorsal hands 1

Nail Plates (White/No Fluorescence):

• Nails are composed of hard, compact keratin that acts as a barrier to UV penetration and does not fluoresce 1
• The nail plate's hydrophilic nature and dense structure prevent absorption of UV light, resulting in white appearance under Wood's lamp 1
• This is the expected normal appearance of healthy nail plates under UV examination 2

Palmar Surfaces (White/No Fluorescence):

• Palmar skin has significantly thicker stratum corneum (up to 0.6mm) compared to dorsal skin 1
• Palms contain minimal melanin and have different keratin composition that does not fluoresce under UV light 1
• The thick keratin layer blocks UV penetration, preventing fluorescence 1

Clinical Context and Differential Considerations

When This Pattern Is Normal:

• Symmetric purple fluorescence on both dorsal hands with white nails and palms is the expected physiologic finding 1
• This pattern does not indicate vitiligo, fungal infection, or other pathology 1

When to Consider Pathology:

• Asymmetric patterns or focal areas of increased/decreased fluorescence may indicate pigmentary disorders 1
• Green-black fluorescence suggests bacterial infection (Pseudomonas) 3
• Areas appearing larger under UV than visible light may indicate active depigmentation (vitiligo) 1
• Yellow-green fluorescence of nails suggests fungal infection (onychomycosis), though this would be accompanied by nail thickening and dystrophy 1, 4

Important Clinical Pitfalls

Do not misinterpret normal UV fluorescence patterns as disease. The purple dorsal hands with white nails and palms is the standard appearance in healthy individuals under Wood's lamp examination. 1

Excessive UV nail lamp exposure (used for gel manicures) can cause actinic damage to dorsal hands, potentially leading to actinic keratoses or squamous cell carcinoma with chronic use, but this would present with visible skin changes, not just fluorescence patterns. 5, 6