Update on Sunscreens

Dr. Robert Bissonnette, Vancouver

ABSTRACT
Sunscreens have been in use for nearly 70 years. Originally designed to protect against sunburn, enable longer sun exposure and promote tanning, sunscreens are now often promoted as a method for preventing skin cancer and skin aging. Although there is experimental evidence in animals suggesting that used adequately, sunscreens can prevent skin cancer, no prospective study has shown a decrease in incidence of skin cancer in sunscreen users, and sunburn prophylaxis is currently the only FDA-approved use of sunscreens. Preliminary studies suggest that new-generation sunscreens could prevent photodermatoses like polymorphous light eruption.¹

Key Words:
Sunscreen, UVB, UVA, Photodegradation, SPF

The regular and adequate use of sunscreens, avoiding the sun around mid-day, staying in the shade, wearing hats with brims which can shade the head, face and neck, and wearing clothes made from tightly woven fabrics, are all important factors that could prevent the deleterious effects of sunlight.

Table 1 lists some sunscreen agents available with their corresponding spectral photoprotection. Chemical agents protect by absorbing light, whereas physical agents act mainly by reflecting and scattering light. The recent development of micronized preparations of physical agents has improved the cosmetic acceptability of physical sunscreens. We will review some key factors to consider when selecting a sunscreen and will compare some commercially available sunscreens, including new sunscreens made recently available in North America (Table 2).

TABLE 1: Protection wavebands of sunscreens 2,3

TABLE 2: Comparison of Some Sunscreens Available in North America
a Could prevent polymorphous light eruption¹
b Contains no chemical sunscreen

Factors to consider when selecting a sunscreen

UVB protection

UVB (290-320nm) is the most erythemogenic solar radiation reaching the surface of the earth. It is also a potent skin carcinogen in animal studies. Sun Protection Factor (SPF) indicates the degree of protection against UVB induced erythema. It is measured in the laboratory by applying 2 mg/cm² of sunscreen to the skin of volunteers and irradiating with an artificial light source. Studies have shown that people use an average of 0.5 to 1mg/cm² of sunscreen,^4,5 and that the SPF may overestimate the actual protection against sunlight.⁶ The real protection against sun-induced erythema of a self-applied SPF 15 sunscreen is thus lower than 15, probably not much more than half this figure. By applying a sunscreen with a SPF of 30 or higher, most people should get a SPF protection equivalent to at least 15. UVB also induces immunosuppression, which in animal studies is not totally prevented by sunscreens.^7,8 One consequence of high SPF sunscreens is that, by preventing sunburn, some people may stay under the sun for long periods of time and thus receive high total daily UVA exposures.

UVA protection

In animal models, UVA (320-400 nm) has been implicated in skin sagging, skin cancer and immunosuppression. UVA protection is now offered in most sunscreens, but unfortunately the labels usually provide no indication of the level or wavelengths of UVA protection. Most UVA protecting sunscreen agents only offer protection against short wave UVA (Table 1). Relative protection against long wave UVA can be achieved by Avobenzone (Parsol 1789), and physical agents. Mexoryl SX is a new sunscreen agent with maximal absorption in the mid-UVA that also offers some UVB protection. Spectral protection, including UVA protection, from micronized physical agents varies according to the size of the micronized particles, with smaller particles providing more UVB and less UVA protection.⁹

Children

Sun exposure in the first 20 years of life is a strong determinant for the risk of skin cancer.¹⁰ Therefore sun protection throughout childhood and teenage years is probably crucial to preventing such carcinogenesis. Direct sun exposure should probably be minimized in children, and if they must spend periods of time outside during the day, physical blockers such as clothing should be used; failing that, sunscreens. Sprays and gels should be used with caution in young children as they can irritate the skin and exacerbate atopic dermatitis. Sunscreens are not recommended for use in children less than 6-12 months of age in order to discourage unnecessary sun exposure. However, there is no strong reason to suggest that sunscreens are harmful in this age group.

Substantivity

Substantivity is the ability of a sunscreen to resist its removal by physical means such as sweating or contact with water. If the SPF of a sunscreen stays unchanged after 40 minutes of contact with water, it is said to be water resistant, whereas if it stays unchanged for 80 minutes or more, it is said to be waterproof. Thus a person staying outside in a pool for six hours may wish to reapply a waterproof sunscreen at least four times to ensure that the SPF remains unchanged. Some manufacturers state the actual time their product remains waterproof, and products which are waterproof for six hours or more should be used if prolonged exposure to water or prolonged sweating is anticipated.

Allergic potential

The prevalence of allergic reactions to sunscreens is low and most reactions reported by patients are of the irritant type. PABA and its derivatives, benzophenone and fragrances are among the most allergenic ingredients in sunscreens, explaining why many commercial products no longer contain PABA. Physical sunscreen agents do not cause allergic contact dermatitis.

Photodegradation

Certain sunscreen agents like avobenzone (Parsol 1789) have been shown to isomerize and lose part of their sun protection properties when exposed to light, whereas others like the newer agent Mexoryl-SX are especially photostable.¹¹ In vitro studies have shown that certain sunscreen formulations can lose more than half their SPF value after one hour of artificial light exposure, suggesting that photodegradation is an important factor to consider when evaluating sunscreens.¹² In vivo studies are needed comparing the photostability of sunscreens to both UVA and UVB.

References

1. Moyal D, Binet O. Polymorphous light eruption: Its reproduction and prevention by sunscreens. In: Lowe NJ, Shaath NA, Pathak MA, eds. Sunscreens development, evaluation, and regulatory aspects. New York: Marcel Dekker, 1997: 611-617.
2. Drug information for the health care professional. USP DI. Taunton: Rand McNally, 1997: 2713-2729.
3. Shaath NA. Evolution of modern sunscreen chemicals. In: Lowe NJ, Shaath NA, Pathak MA, eds. Sunscreens development, evaluation, and regulatory aspects.New York: Marcel Dekker, 1997: 3 – 33 .
4. Stender IM, Andersen JL, Wulf HC. Sun exposure and sunscreen use among sunbathers in Denmark. Acta Dermato-Venereologica 1996; 76: 31-3.
5. Bech-Tomsen N, Wulf HC. Sunbather’s application of sunscreen is probably inadequate to obtain the sun protection factor assigned to the preparation. Photodermatol Photoimmunol Photomed 1992/1993; 9: 242-244.
6. Sayre RM, Kollias N, Ley RD et al. Changing the risk spectrum of injury and the performance of sunscreen products throughout the day. Photodermatol Photoimmunol Photomed 1994; 10: 148-153.
7. Wolf P, Yarosh DB, Kripke ML. Effects of sunscreens and a DNA excision repair enzyme on ultraviolet radiation-induced inflammation, immune suppression, and cyclobutane pyrimidine dimer formation in mice. J Invest Dermatol 1993; 101: 523-527.
8. Walker SL, Young AR. Sunscreens offer the same UVB protection factors for inflammation and immunosuppression in the mouse. J Invest Dermatol 1997; 108: 133-138.
9. Anderson MW, Hewitt JP, Spruce SR. Broad-spectrum physical sunscreens: Titanium dioxide and zinc oxide. In: Lowe NJ, Shaath NA, Pathak MA, eds. Sunscreens development, evaluation, and regulatory aspects. New York: Marcel Dekker, 1997: 353-397.
10. Gallagher RP, Hill GB, Bajdik CD et al. Sunlight exposure, pigmentary factors, and risk of nonmelanocytic skin cancer. I. Basal cell carcinoma. Arch Dermatol 1995; 131: 157-163.
11. Deflandre A, Lang G. Photostability assessment of sunscreens. Benzylidene camphor and bibenzoylmethane derivatives. Int J Cosmetic Sci 1988; 10: 53-62.
12. Diffey BL, Stokes RP, Forestier S. et al. Suncare product photostability: a key parameter for a more realistic in vitro efficacy evaluation. Eur J Dermatol 1997; 7: 226-228.