Benzophenones are a family of compounds that include dixoxybenzone, sulisobenzone, and benzophenone-3, or oxybenzone. These . Benzophenones (BPs) act as penetration enhancers, as they modify the structure of the skin and facilitate the absorption of other chemical ingredients into the body. The best known uses of these compounds are as perfume fixatives and sunscreen agents.
Sunscreens and benzophenones
BP-2, -3 and -4 are used as sunscreens but have many downsides. They are well known photoallergens, are toxic to aquatic animals (especially BP-3), and are found in urine. BP-2 has weak estrogenic effects, and some studies suggest that it decreases fertility in men. BP-4 can increase absorption of pesticides. BP-3 is banned in Hawaii because of the risk to coral and is the most worrisome.
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In particular, BP-3 is known to protect skin and hair from UV radiation-induced harm.1 Unfortunately, BPs are also associated with photocontact allergies, hypersensitivity, hives, contact urticaria, anaphylaxis, hormone disruption, and DNA damage.2,3 BP-3 has also been implicated as an environmental contaminant. This column will focus on recent studies pertaining to effects on humans, primarily, and on the role of BPs in sunscreen agents.
Effects of BPs in animals
A recent study on the cytotoxicity of BP-3 against thymocytes in rats revealed that cell mortality increased significantly after 3 hours of exposure to 300 μM BP-3, but the membrane potential of thymocytes was unchanged by BP-3 exposure. In a concentration-dependent fashion, intracellular Zn2+ levels increased significantly after administration of at least 30 μM BP-3. The investigators concluded that the cytotoxicity engendered by BP-3 could be the result of oxidative stress linked to elevated intracellular Zn2+ levels.1
Effects of BPs in humans and systemic absorption
In multiple studies, exposure to BP-3, as well as to octinoxate, has been linked to endocrine and hormonal disruptions in humans and animals.4,5 Motivated by several notable observations (global increase in the use of sunscreens with UV filters; rapid rise in malignant melanoma, against which sunscreens should protect; increase in reported experimental findings of UV filters acting as endocrine disruptors), Krause et al. in 2012 reviewed animal and human data on the UV filters BP-3, 3-benzylidene camphor (3-BC), 3-(4-methyl-benzylidene) camphor (4-MBC), 2-ethylhexyl 4-methoxy cinnamate (OMC), homosalate (HMS), 2-ethylhexyl 4-dimethylaminobenzoate, and 4-aminobenzoic acid (PABA). Importantly, BP-3 was present in 96% of human urine samples in the United States, and various filters were found in 85% of the human breast milk samples in Switzerland.6
A 2019 analysis by Wang and Ganley reported that systemic absorption of the active sunscreen ingredient BP-3 can be substantial, justifying the assessment and understanding of systemic exposure to characterize the risks of long-term usage.7
Between January and February 2019, Matta et al. conducted a randomized clinical trial with 48 healthy participants to evaluate the systemic absorption and pharmacokinetics of six active ingredients in four sunscreen formulations, including avobenzone and BP-3. The researchers found that all ingredients were systemically absorbed, with plasma concentrations exceeding the Food and Drug Administration threshold for considering the waiving of further safety studies. They concluded that these results did not warrant discontinuing the use of the tested sunscreen ingredients.8 Yeager and Lim add that, while BP-3 has been incorporated into sunscreen formulations for sale in the United States since 1978, there have been no reports of adverse systemic reactions in human beings.3
However, topical reactions have elicited a different assessment. That is, in 2014, the American Contact Dermatitis Society labeled BPs the Contact Allergen of the Year, as they were identified as the most common source of photoallergic and contact allergic reactions of all UV filters.3,9