REVIEW ARTICLES Sunscreens: Is Looking at Sun Protection Factor Enough? Dr. T. Y. Ho Social Hygiene Service (Dermatology), Department of Health, Hong Kong ABSTRACT As mounting evidence points to the photocarcinogenicity and the photoaging potential of chronic ultraviolet A (UVA) exposure, broad-spectrum sunscreens that are effective against both ultraviolet B (UVB) and UVA radiation are obviously preferred. Older chemical filters are not effective against the longer UVA. Several chemical filters have been developed recently to provide invisible protection for the UVA spectrum. The sun protection factor (SPF) can only indicate the level of protection against UVB, but not UVA. There is not yet any widely accepted method to measure UVA protection. Keywords: Review, sunscreen, ultraviolet A INTRODUCTION unacceptable. However, several organic UVA filters have been developed recently which aim to provide The sun protection factor (SPF) has widely been invisible protection against the whole UVA spectrum. used to evaluate the protective value of sunscreens. However, SPF only reflects the effectiveness of the This review will examine the evidence for the sunscreen to protect against acute sunburn which is harmful potential of UVA, discuss the safety issues mainly due to ultraviolet B (UVB) radiation. The concerning sunscreen use and the problems with UVA deleterious effects of UVB are well known and UVB protection indicators, look at the UVA protective agents was believed in the past to be the spectrum responsible available commercially and advise on the choice and for the damages arising from ultraviolet radiation the use of sunscreen products. (UVR). However, there has been increasing evidence pointing to the photocarcinogenicity and the photoaging potential of ultraviolet A (UVA) radiation even at ULTRAVIOLET RADIATION suberythemogenic doses. Therefore, broad-spectrum solar protection covering both the UVA and the UVB Ultraviolet radiation (UVR) is defined as those range is desirable, and choosing sunscreen products electromagnetic radiation with wavelengths between simply basing on the SPF value will not be adequate. 100 and 400 nm, and is divided into UVA (320-400 nm), UVB (290-320 nm) and UVC (100-290 nm). UVA can Ultraviolet filters developed in the past are mainly be further divided into UVA I (340-400 nm) and UVA effective across the UVB wavelengths, though some II (320-340 nm). Solar UVR at the earth's surface extend into the short UVA spectrum. These offer little comprises approximately 95-98% UVA and 2-5% UVB, protection against the longer UVA radiation. Previously, all the UVC having been absorbed by stratospheric broad-spectrum coverage could only be achieved by ozone. opaque sunscreens that are often cosmetically Exposure to UVR has pronounced acute and chronic effects on the skin. The UVR-induced skin effects manifest as acute responses such as inflammation Correspondence address: (sunburn or erythema), pigmentation, hyperplasia, Dr. T. Y. Ho Yaumatei Dermatological Clinic immunosuppression, vitamin D synthesis, and chronic 12/F Yaumatei Dermatological Clinic effects such as photocarinogenesis and photoaging. 143 Battery Street, Kowloon These acute and chronic effects are dependent of the Hong Kong spectrum, intensity and cumulative dose of UVR. The 100 Hong Kong Dermatology & Venereology Bulletin Review Articles complete action spectrum for the majority of UVR- there is now increasing evidence to suggest the induced effects has not yet been completely defined in pathogenic role of UVA in cutaneous malignancies. human skin. In addition, these responses have different thresholds such that the prevention of UVR-induced It has been shown that UVA can have an changes for an endpoint does not guarantee a similar immunosuppressive effect on the human skin.4 UVR- level of protection for any other. induced immunosuppression is important as it occurs not only with environmental antigens but also with tumour antigens. It can be directly demonstrated in Effects of ultraviolet A radiation animals that UVA, including those with wavelengths >340 nm, induce skin tumours.5 In human skin, UVA Inflammation has been demonstrated to cause DNA breaks and Erythemal effectiveness declines rapidly with accumulation of p53 protein in melanocytes.6 increasing wavelength, such that approximately 1000 times more UVA energy than UVB is required to produce The epidemiological evidence for the photo- the same erythema response.1 However, due to its carcinogenicity of UVA is strong. Among individuals preponderance in terrestrial sunlight, UVA nonetheless with long-term exposure to UVA tanning beds or contributes significantly to sunburn erythema. psoralen plus UVA therapy (PUVA), increased incidence is observed for non-melanoma skin cancer.7,8 However, Pigmentation the association is less consistent for melanoma.8-10 Ultraviolet A radiation causes immediate transient tanning which is believed to be due to the photo- oxidation of a precursor of melanin. The action spectrum Protection by sunscreens is broad, extending from 320-400 nm, with a peak at It has been shown that photodamage due to UVA around 340 nm. The physiologic significance of this can be reduced or abolished by broad-spectrum immediate tanning response in humans is not known, sunscreens or the UVA filters but not by sunscreens with as unlike neomelanogenisis, it does not confer attenuation spectrum mainly in the UVB range.4,11 photoprotective properties.2 Delayed tanning, which is more persistent, is a result of increased formation of Concerning protection against cutaneous epidermal melanin. This is induced by UVB and the malignancies, these endpoints are difficult to evaluate shorter UVA wavelengths.1 in humans for ethical reasons. However, it has been reported in humans that the use of a broad-spectrum Photoageing sunscreen can effectively prevent and even reduce solar Ultraviolet A radiation, due to the longer keratosis, and so by implication, possibly reduces the wavelengths, can penetrate into and cause damage in risk of skin cancer in the long term.12 In vivo studies the dermis. All UVA bands have been shown to be support that broad-spectrum sunscreens, compared with equally effective in inducing inflammatory changes in UVB filters, provide better protection against UV- the dermis which can lead to connective tissue damage, induced cutaneous malignancies. Broad-spectrum whereas UVA II is more effective at inducing epidermal sunscreens are more effective in the reduction of changes.3 Although acute exposure to UVA may not lead immunosuppression and skin tumours in mice,13,14 and to any detectable changes, repeated exposure to UVA, DNA breaks in human melanocytes.6 even at suberythemogenic doses, has been shown to have a cumulative effect in human skin causing significant photodamage and immunosuppression.4 SAFETY OF SUNSCREENS Immunosuppression and carcinogenesis Acute adverse reactions In the past, it was thought that the cancer risk posed Acute adverse reactions to sunscreens are quite by UVA is negligible, as unlike UVB, it does not cause frequent and all sunscreen agents have been reported to sunburn. Although we cannot subject humans for cause contact or photocontact allergic reactions. prospective studies to document the carcinogenicity of However, most inflammatory reactions occurring after UVA using tumour occurrence as the study endpoint, sunscreen application are irritant in nature, which can Vol.9 No.3, September 2001 101 Review Articles be due to the active sunscreen agents or the base cream. place after perspiration or swimming. In addition, it For the small proportion of cases which are allergic in should be non-toxic and does not cause irritation or nature, as demonstrated by patch testing, photopatch contact allergy, and from this point of view, there is testing or scratch testing, the offending agent is more usually a maximum concentration recommended for often an ingredient in the base cream rather than an each filter. The concentration is important because it active sunscreen agent.15 determines the degree of protection of the final sunscreen product, and increasing this protection can be achieved by increasing the concentration of the filter Carcinogenicity and the melanoma-sunscreen (up to a certain point) or by combining different filters controversy in the same product. Some in vitro studies have suggested that certain sunscreen agents could have a carcinogenic effect after UV irradiation. However, these findings have not been Protection against UVB confirmed by in vivo studies using animals and humans. The results of the in vivo studies overwhelmingly The sun protection factor demonstrated that sunscreen agents protect, instead of The sun protection factor (SPF) was first designed promote, skin cancers due to acute and chronic UVR to indicate the level of protection offered by a sunscreen exposure. product against acute sunburn. According to the Food and Drug Administration of the United States, the SPF Despite laboratory evidence that sunscreens can of a sunscreen product is calculated as the ratio of the reduce or prevent UV-induced malignancies, there are minimal erythema dose (MED) of sunscreen-protected epidemiological studies which suggest that sunscreen skin to the MED of unprotected skin. The light source use is associated with an increase in the frequency of used for SPF testing should provide a continuous malignant melanoma.16 Other epidemiological studies emission spectrum from 290 to 400 nm similar to either observed no association between