What is phototoxicity and provide an example of a chemical class that can cause it.

• A. Phototoxicity occurs when some chemicals become toxic after exposure to light, generating reactive species; example: psoralen compounds (furocoumarins) in certain plants.
• B. Phototoxicity is the inherent toxicity of a chemical regardless of light.
• C. Phototoxicity only occurs in photodynamic therapy context.
• D. Phototoxicity refers to protective effects of light on chemicals.

Answer: (A)

Phototoxicity happens when a chemical absorbs light and becomes activated, then transfers energy or forms reactive species that damage cells. It’s a nonimmune reaction—no prior sensitization is needed—so light exposure plus the chemical leads to tissue injury, usually in sun-exposed skin.

A classic example is psoralen compounds, a group of furocoumarins found in certain plants (such as citrus fruits like limes and celery, parsley, figs). When these substances absorb UVA light, they become activated and can form DNA adducts or generate reactive oxygen species, resulting in skin inflammation and damage known as phytophotodermatitis. This class is also used therapeutically in controlled PUVA treatment, illustrating how light-activated chemistry can be therapeutic or harmful depending on context.