INTRODUCTION
The rising incidence of skin cancers in the United States and abroad has led to an acute awareness of the need for increased sun protection. While the prevailing consensus holds that topical sunscreens decrease the occurrence of both melanoma and non-melanoma malignancies, their safety and efficacy claims have been under increasing scrutiny in this era marked by a growing skepticism towards medical pronouncements. Remarkably, for quite some time this was not a completely accepted dogma. There is extant literature on sunscreens being associated with an increased risk of melanoma as well as other cancers, and many concerns about sunscreen safety to both humans and the environment remain.1 This article will provide an in depth review of the mechanism of action and efficacy of sunscreens as well as some of the safety concerns and adverse effects associated with their use, with an eye towards dispelling myths with the most comprehensive current understanding.
Sunscreens: Mechanism of Action
Ultraviolet (UV) radiation spans a spectrum from 40 nm to 400 nm. It can be divided into multiple subgroups but the most medically relevant are UVB (290–320 nm), UVA2 (320–340 nm), and UVA1 (340–400 nm) since they are capable of damaging the skin. Both UVA and UVB rays are associated with sunburn and photoaging. However, while UVB radiation is mostly absorbed by the epidermis, UVA radiation is able to penetrate more deeply and reach the dermis, resulting in more severe and longer-term damage. Importantly, UVA radiation stays constant throughout the seasons of the year, while UVB radiation is much stronger in the summer, leading to a higher incidence of sunburn in those months.2 Photoprotection is critical when preventing UV-induced skin damage, and topical sunscreens have been an important component of protection for decades.
Sunscreens are often categorized based on their mechanism of action: there are chemical (organic) absorbers and physical (inorganic) "deflectors" - but things are far from that simple. While chemical sunscreens do truly act to absorb UV radiation, physical sunscreens actually have a much more complex mechanism of action than simple reflection, and there are some important myths that should be dispelled.
Chemical Sunscreens
Chemical sunscreens are typically composed of an aromatic compound combined with a carbonyl group forming a unique structure that absorbs UV radiation and then emits the energy as less harmful, lower energy rays. These sunscreens are usually composed of multiple different organic compounds in order to maximize protection against a wider range of the UV spectrum. Chemical sunscreens can be subdivided into UVA, UVB, or broad-spectrum agents depending on the organic compounds they contain.
Cinnamates such as octinoxate and cinoxate are the most commonly used UVB-absorbing compounds in current sunscreen formulations.3 The increase in formulations containing cinnamates reflects a move away from aminobenzoate-based sunscreens, which were classified as not "generally recognized as safe and effective" (Non-GRASE-Category II) by the US Food and Drug Administration (FDA) and can no longer be used in sunscreen formulations in the US.4 PABA was associated with a multitude of adverse effects including endocrine disruption, specifically altered thyroid activity, as well as allergic contact dermatitis in some users.5
The most common UVA-targeting agents in the US are benzophenones such as sulisobenzone and oxybenzone. It
Sunscreens: Mechanism of Action
Ultraviolet (UV) radiation spans a spectrum from 40 nm to 400 nm. It can be divided into multiple subgroups but the most medically relevant are UVB (290–320 nm), UVA2 (320–340 nm), and UVA1 (340–400 nm) since they are capable of damaging the skin. Both UVA and UVB rays are associated with sunburn and photoaging. However, while UVB radiation is mostly absorbed by the epidermis, UVA radiation is able to penetrate more deeply and reach the dermis, resulting in more severe and longer-term damage. Importantly, UVA radiation stays constant throughout the seasons of the year, while UVB radiation is much stronger in the summer, leading to a higher incidence of sunburn in those months.2 Photoprotection is critical when preventing UV-induced skin damage, and topical sunscreens have been an important component of protection for decades.
Sunscreens are often categorized based on their mechanism of action: there are chemical (organic) absorbers and physical (inorganic) "deflectors" - but things are far from that simple. While chemical sunscreens do truly act to absorb UV radiation, physical sunscreens actually have a much more complex mechanism of action than simple reflection, and there are some important myths that should be dispelled.
Chemical Sunscreens
Chemical sunscreens are typically composed of an aromatic compound combined with a carbonyl group forming a unique structure that absorbs UV radiation and then emits the energy as less harmful, lower energy rays. These sunscreens are usually composed of multiple different organic compounds in order to maximize protection against a wider range of the UV spectrum. Chemical sunscreens can be subdivided into UVA, UVB, or broad-spectrum agents depending on the organic compounds they contain.
Cinnamates such as octinoxate and cinoxate are the most commonly used UVB-absorbing compounds in current sunscreen formulations.3 The increase in formulations containing cinnamates reflects a move away from aminobenzoate-based sunscreens, which were classified as not "generally recognized as safe and effective" (Non-GRASE-Category II) by the US Food and Drug Administration (FDA) and can no longer be used in sunscreen formulations in the US.4 PABA was associated with a multitude of adverse effects including endocrine disruption, specifically altered thyroid activity, as well as allergic contact dermatitis in some users.5
The most common UVA-targeting agents in the US are benzophenones such as sulisobenzone and oxybenzone. It
