Chemical sunscreen agents protect the skin by absorbing UV energy and transforming it into heat energy. These compounds absorb UV radiation and convert the energy into longer wave radiation. The sunscreen chemical is excited to a higher energy state from its ground state. As the excited molecule returns to the ground state, energy is emitted that is lower in magnitude than the energy initially absorbed. This energy is emitted in the form of longer wavelengths, typically mild heat radiation.
These synthetically derived compounds can be broadly categorized into two groups’ UVB (290-320 nm) and UVA (320-400 nm) absorbing chemicals. Sunscreen chemicals are generally aromatic compounds conjugated with a carbonyl group (37). Chemical sunscreens can be classified based on their chemical properties, and each class has its own characteristic absorption spectra (Table 2).
PABA was a widely used sunscreen in the 1950s and 1960s. Several of the properties pertaining to the limitations of PABA can be attributed to its chemical structure: amino and carboxylic acid groups in a para-orientation on a benzene nucleus. The highly polar nature of PABA made this agent extremely water soluble, but the increased hydrogen bonding between molecules also promoted a crystalline physical state (39). This led to some difficulty in manufacturing a solvent that ensured continuous dissolution of PABA. The amine and carboxyl groups also made the PABA molecule sensitive to pH changes, and therefore somewhat labile in its effectiveness as a UV chemical absorbing agent. The molecule’s lack of stability also led to changes in the color of the product when exposed to air.
Glycerol PABA was subsequently developed to protect the carboxylic acid group from pH changes and therefore was slightly more stable than the original PABA formulation. Other preparations attempted to protect both the carboxyl and the amine
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Table 2 Sunscreen Agents and Their UV Protective Wavelengths
Source’. Adapted from Ref. 38. |
group. Padimate O (N, N-dimethyl PABA octyl ester), addressed many of the original structure’s limitations and became a widely used sunscreen agent. Both the amino and the carboxyl groups are protected, making Padimate O less sensitive to pH changes. This new chemical structure also resulted in decreased intermolecular hydrogen bonding, resulting in a sunscreen agent that is a liquid instead of a crystalline solid.
The original PABA fell out of favor largely because of staining and allergic contact reactions. There is a much higher presence of contact and photocontact allergy to PABA than to other sunscreening agents (40). The PABA derivates also were reported to induce contact sensitization. Sensitization to PABA showed strong reactions to benzocaine, suggesting that reports of glycerol PABA allergy may in fact have been due to impurities in glyceryl PABA preparations (41). Other PABA derivates such as Padimate A, and to a lesser extent, Padimate O, have also been reported to cause sensitization or photocontact sensitization. Padimate A was also found to cause phototoxicity and is no longer used in the United States (42).
Salicylates were the first UV chemical absorbers used in commercially available sunscreen preparations. In contrast to the para-distribution of the carboxyl and amine groups, the salicylates are ortho-distributed (the carboxyl and amine groups are on neighboring carbon atoms on the benzene ring). This spatial arrangement allows hydrogen bonding within the molecule itself, leading to a UV absorbance of about 300 nm (43). This intramolecular hydrogen bonding results in increased molecule stability, less interaction
with other compounds, and good overall safety record. The salicylate group of sunscreen agents include octyl salicylate and homomenthyl salicylate.
Cinnamates are effective sunscreen agents with a peak absorption wavelength of about 305 nm. They are chemically related to balsam of Peru, coca leaves, cinnamic aldehyde, and cinnamic oil. The chemical structure of the cinnamates, as a group, makes the molecule insoluble to water, requiring more frequent reapplication of the preparation. Contact dermatitis to the cinnamates and cross-sensitization to structurally related products have been reported.
Benzophenone derivates and anthranilates are effective at absorbing UVA radiation. Although the primary protective range for benzophenone is in the UVA range, a secondary protective band is also noted in the UVB range. The most commonly used benzophenone agents are oxybenzone and dioxybenzone. Although these ingredients are much less allergenic than PABA, they do nonetheless still carry a risk of photocontact and contact allergy. Anthranilates, such as menthylanthranilate, provide low-level, yet broad-spectrum coverage. They are commonly added to sunscreens to augment protection. Camphor is an agent widely used in Europe, but not approved for use in the United States. They are effective UVB-absorbing agents.
Dibenzoylmenthanes are a relatively new group of sunscreen agents and are especially effective at offering protection against UVA radiation. Tert-butylmethoxy – dibenzoylmethane (Avobenzone, Parsol 1789) is approved for use in the United States, while isopropyldebenzoylmethane (Eusolex 8020) has been widely used in Europe. The latter has been associated with a high incidence of contact dermatitis, and has not been approved in the United States. In a study of 19 patients with positive photopatch tests to sunscreens, eight showed positive reactions to butyl methoxy dibenzoylmethane (44).