Many of the chemicals in sunscreens have been found to cause phototoxic, photoallergic or photogenotoxic (DNA altering) effects. PABA (Paramino benzoic acid) has been found to increase the development of a particular DNA defect in human cells. When this occurs in people who lack the mechanism to repair the defect, they are more susceptible to skin cancer. When exposed to sunlight, PABA also readily generates oxygen radicals which harm DNA strands . An ester of PABA, amyl paradimethylarninobenzoate (Padimate A) was found to cause phototoxic reactions. Padimate A reacts with UVA to produce symptoms widely resembling sunburn. The similarity between sunburn and a phototoxic response has led people to mistakenly believe that the sunscreen causes sunburn. An ingredient commonly used in sunscreens, 2-phenylbenzimidazole-5-sulfonic acid (PBSA), strongly absorbs UVB radiation, thus becoming energised and capable of affecting adjacent skin tissue by damaging the guanine base sites of the cell’s genetic material. This may increase the risk of developing skin cancer.
Physical sunscreens contain inert materials such as titanium dioxide, zinc oxide and/or talc. They work on the basis of reflecting, scattering or blocking UVA and UVB radiation. The effectiveness of this type of sunscreen depends on the diameter or size of the particles or the thickness of the film (application) to reflect or scatter the visible light or UV radiation. No phototoxicity, contact sensitisation or photoallergy have been produced by these agents, although they may be so occlusive (that is, block so effectively) that they may cause blockage of the sweat glands, a condition known as miliaria.
Octyl Methoxycinnamate appears to be the most common chemical used in Australian sunscreens. This forms part of the group of photon absorbing chemicals, the cinnimates. Photoallergy has been reported as well as contact allergy.
Benzophenones, a constituent in sunscreens, have been found to cause contact allergy and photoallergy. Benzophenones, oxybenzone and mexenone have the potential to mimic and exacerbate a disease. One example is chronic actinic dermatitis, found predominantly in elderly men.
Dibenzoylmethanes have been incorporated into sunscreens since 1980 as UVA absorbers. Cases of photoallergy and contact allergy have been reported as well as cross reactions from prior exposure to dibenzoylemethanes. Instances of contact and photocontact allergy with dibenzoylmethane derivatives such as 5-methoxypsolaren (5-MOP) and isopropyl dibenzoyl methane (Eusolex 8020) have been found with sunscreen usage and it is often used in conjunction with Eusolex 6300.
There are instances of people using sunscreens who have reacted to the excipients (base materials/carriers) included in the formulation, such as the preservatives, fragrances and emulsifiers, which have caused contact allergies. So there is cause for concern not only about the active ingredients in sunscreens, but about the inert ingredients as well.
The question of whether sunscreens prevent skin cancer is an ambiguous one. A review of studies on skin cancer and sunscreens by Science News, found that people who use sunscreen are more likely to develop basal cell cancer than people who do not. Science News also examined ten studies of melanoma. Five of them concluded that people who used sunscreen were more likely than non-users to develop melanoma. Three of the studies found no association between melanoma and sunscreen use, and two studies found that people who used sunscreen were the most protected. Science News 65, found that epidemiological data was not conclusive. This may be because people involved in the studies were not wearing effective sunscreen, or were staying in the sun longer because they thought they were protected.
Other PABA derivatives that have caused sensitisation or photocontact sensitisation include octyl dimethyl PABA (padimate 0) and amyl dimethyl PABA (padimate A). Padimate A causes redness of the skin. The frequency of padimate 0 causing adverse reaction is less in comparison to PABA and monoglyceryl PABA because padimate 0 is not a true PABA ester.
In general, elderly people are more susceptible to chemically induced photosensitivity, be it photoallergic or phototoxic. This is due to several factors: their skin is thinner and more easily penetrated, they may be on medication that can enhance photosensitivity and their body’s capacity to excrete such substances once penetration has occurred is much slower.
In 1994, researchers at the Harvard Medical School found that the ingredient psolaren in sunscreens is an extremely efficient carcinogen. Experiments on rodents exposed to UVA also produced the same result - psolaren was found to be carcinogenic. It is worrying that psolaren is known to be a free radical generator when activated by UV radiation, and yet it is used in sunscreen ingredients. People with psoriasis are at greater risk, as their squamous cell carcinoma rate was found to be 83 times higher than that of the general population.
3-(4-Methyl-benzylldene) camphor (known as Eusolex 6300) is a sunscreening agent used widely in Europe. However, the United States Food and Drug Administration (FDA) disapproves of this ingredient due to the high incidence of allergic and photoallergic reactions reported.
Titanium dioxide has also been found to absorb 70% of UV, and in aqueous environments this leads to the formation of hydroxyl radicals which can initiate oxidation. This is known to cause breaks in DNA strands, leading to an increased risk of cancer. In response to titanium dioxide’s potency, manufacturers coat the particles to make them less active. Although this treatment has been found to reduce the chemical’s activity, it does not eliminate DNA damage altogether. Particles of titanium dioxide have been made smaller to stop the white paste effect on the skin, but this enables them to enter cells more easily and block sweat glands. Titanium dioxide does not itself cause photoallergic reactions on the skin, but it can create the skin condition miliaria, mentioned earlier, causing inflamed sweat glands and heat rash. Titanium dioxide is used in PABA sunscreens because it is not a sensitiser and blocks the appearance of photoallergic reactions.
Salicylates are known to cause photocontact allergy. Toxic effects from Octyl salicylate, a major derivative used in many sunscreens, has yet to be reported.
Cinnamates are chemically related to or can be found in coca leaves, cinnamic acid, cinnamic aldehyde and cinnamon oil. They are included in perfumes, topical medication, flavourings and sunscreens. The cinnamate derivative, p-methoxymethylcinnamate has been found to be cytotoxic, while 2 Ethoxyethyl-p-cinnamate (cinoxate), a major derivative of cinnamate, and used in sunscreen, causes the majority of the allergic and photoallergic reactions in humans. Cross reactions with other cinnamate derivatives have also been reported.
Excipients act as the carriers or base materials of a product and ‘receive’ and carry the other ingredients. These chemicals include mineral oil, petrolatum, isopropyl esters, lanolin derivatives, aliphatic alcohols, emulsifiers, fragrances, thickeners and preservatives. Sunscreen toxicity can be caused by excipients in the formulations.
In studies, sensitive subjects tended to be older and often had a history of long-standing dermatitis. In a study of 603 people aged 40 and over, 114 people were tested and found to be allergic to ingredients in sunscreens. Fragrances, used extensively in sunscreens, produced the most common reaction: allergic contact dermatitis.
The FDA expressed their concerns about cancer when it was discovered that a combination of the 21 allowable sunscreen ingredients caused lesions that were not previously seen when the individual ingredients were tested separately.