Coal tar phototoxicity: kinetics and exposure parameters

Area of Science:

• Dermatology
• Photobiology
• Toxicology

Background:

• Coal tar is used to treat skin conditions like psoriasis and eczema.
• Coal tar can cause phototoxicity, leading to delayed erythema and skin pain (tar smarts).

Purpose of the Study:

• To quantify the UVA dose and exposure conditions required to induce delayed erythema and skin pain from coal tar in human skin.
• To investigate the relationship between the duration of coal tar application and phototoxicity.
• To evaluate the effectiveness of different coal tar removal methods on subsequent phototoxicity.

Main Methods:

• Quantified minimal UVA dose for delayed erythema (minimal phototoxic dose, MPD) and immediate skin pain (minimal smarting dose, MSD) in 32 subjects.
• Utilized a log-log dose-response model to analyze the effect of tar contact time on MPD.
• Assessed four different coal tar removal methods and the impact of the time interval between tar removal and UVA exposure.

Main Results:

• A log-log dose-response model demonstrated that longer coal tar contact time decreased the MPD.
• Several tested coal tar removal methods were as effective as water alone in reducing phototoxicity.
• Increasing the interval between tar removal and UVA exposure significantly increased the MPD, with 30 minutes showing a notable increase.
• The minimal smarting dose (MSD) was consistently lower than the minimal phototoxic dose (MPD).
• Both delayed erythema and skin pain susceptibility persisted for at least 30 hours post-tar removal.

Conclusions:

• The time interval between coal tar removal and UVA exposure is critical in modulating phototoxicity.
• Coal tar phototoxicity, encompassing both erythema and smarting, has a prolonged duration, persisting for over 30 hours.
• Understanding these parameters is essential for managing coal tar therapy and preventing adverse phototoxic reactions.