Sunscreen, Part II: Physical versus Chemical, Alternatives, and Possible Detriments

Products with avobenzone or oxybenzone should not be used on children. Photo source: http://maine.gov

As described by Dr. Leslie Baumann in Cosmetic Dermatology, there are two different categories of sunscreens: physical sunscreens, which scatter or reflect UV radiation, and chemical sunscreens, which absorb UV radiation and then dissipate the radiation as either heat or light, usually of longer wavelength that does not affect the skin.

Physical sunscreens are advantageous over chemical sunscreens because they are rarely associated with allergic reactions. Two of the most desirable physical sunscreens are micronized Zinc oxide (ZnO) or Titanium dioxide (TiO₂), which are advantageous because they have not been shown to demonstrate the allergic reactions nor the unappealing thickness of their non-micronized counterparts, but they still remain on the skin’s surface without being systemically absorbed. However, it should be noted that micronized Zinc oxide and Titanium dioxide are not equal. According to a 1999 study by Mitchnik et al. cited by Baumann, microfine Zinc oxide absorbs more UV light than Titanium dioxide in the long-wave UVA spectrum, from 340 to 380 nm, and hence provides more effective UVA protection. Micronized zinc oxide is available in a multitude of products, including Blue Lizard Australian Sunscreen, SPF 30 ($12.99, Drugstore.com).

Some scientists warn consumers against using Zinc oxide or Titanium dioxide products because it was believed that they may produce oxygen free radicals at the skin’s surface. However, according to Dr. Baumann, particles of microfine Zinc oxide and Titanium dioxide are too large to traverse the stratum corneum, and thus should not be able to induce free radical damage within the skin. According to a 1997 study by Gillies et. al cited by Baumann, most companies further minimize the possibility of photoreactivity of Zinc oxide or Titanium dioxide by coating the surface with dimethicone or silicone. A second caveat of physical blocks mentioned by Paula Begoun in Don’t Go to the Cosmetics Counter Without Me is that zinc oxide and titanium dioxide may clog pores.

Chemical sunscreens are sometimes preferred over physical sunscreens because they are often completely colorless and odorless. Unfortunately, the components in chemical sunscreens, which include oxybenzone, avobenzone, and Parsol 1789, can cause photoallergic contact dermatitis in susceptible patients. Avobenzone and oxybenzone (the latter present in 20-30% of sunscreens) have also been demonstrated by Hayden et. al to be absorbed into the body and secreted into the urine of users. While avobenzone and oxybenzone are not considered to be toxic agents based on animal studies, it is still not recommended that sunscreens with oxybenzone are used on children or by pregnant or nursing women, because they are absorbed into the body. It is thereby best to use a formula with a physical block on children, or if you are pregnant or nursing.

Despite the advances in sunscreen, much work needs to be done in informing the public of its proper use and actual effectiveness. For instance, many consumers are unaware that certain medications may cause photosensitivity. According to Dr. David Bank, these medications include ibuprofen, doxycycline (prescribed for Lyme disease), and Ortho-Tri Cyclen. To be on the safe side, take aspirin or acetaminophen for pain when you know you will be venturing out into the sun.

In addition to using a UVA/UVB sunscreen on a daily basis, the American Academy of Dermatology (AAD) recommends additional forms of sun protection, such as avoiding the sun between 10 A.M. and 4 P.M., when the sun’s rays are the strongest. Sun-protective clothing is also advised by many experts, the effectiveness of which is measured by its Ultraviolet Protection Factor, or UPF. In general, according to Baumann, a UPF of 10 equals protection of about SPF 30. In a 2000 study cited by Edlich et. al of 236 common fabrics in clothing, 33% of fabrics issue a UPF of less than 15, 19% issue a UPF between 15 and 30, and 48% (including wool, polyester, and fabric blends) issue a UPF of 30 or greater. Cotton, linen, and viscose blends were demonstrated to have UPF values less than 30 in 70% of cases. Interestingly, the same study found that, despite fabric type, clothing in black, navy blue, beige, white, and green most commonly displayed UPF values of 30 or greater. However, UPF actually increases with the washing of the UPF protective clothing, perhaps due to the fact that the clothing shrinks and becomes more tight-knit. An additional way to increase the UPF of cotton clothing is by washing with Tinosorb, which has a disulfonic acid triazine backbone that enables it to bind to cotton fabrics. Finally, clothing designed with a high UPF, such as that from Coolibar, are available.

Many dermatologists further recommend the use of a sun hat, such as the Packable Wide Brim Hat shown above from Coolibar ($29.95) which blocks 98% of ultraviolet rays. Other additional options to avoid the sun include the Llumar UV Shield for the windows of the car or the home, which blocks 99.9 percent of ultraviolet rays. Car protection is particularly valuable; many dermatologists note that they can tell which side of the car the patient sits on, simply from visual inspection of signs of sun damage.

Finally, avoidance of the sun has been critiqued, perhaps most famously in The UV Advantage, by Dr. Michael Holick. Holick argues that the UV radiation activates vitamin D synthesis in the skin, and so avoidance should not be encouraged for anti-aging purposes. Vitamin D is important for promoting calcium absorption, and works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization. Insufficient levels of vitamin D have been associated with rickets in children and osteomalacia in adults; sufficient levels for infants, children, and adults are listed below. Most dermatologists recommend stimulating synthesis of adequate levels of vitamin D with ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen, and taking vitamin D supplements, particularly in the winter months in colder areas.

Table: Adequate Intake for vitamin D for infants, children, and adults (Source: US Office of Dietary Supplements)

Age	Children (μg/day)	Men (μg/day)	Women (μg/day)	Pregnancy (μg/day)	Lactation (μg/day)
Birth to 13 years	5 (=200 IU)
14 to 18 years		5 (=200 IU)	5 (=200 IU)	5 (=200 IU)	5 (=200 IU)
19 to 50 years		5 (=200 IU)	5 (=200 IU)	5 (=200 IU)	5 (=200 IU)
51 to 70 years		10 (=400 IU)	10 (=400 IU)
71+ years		15 (=600 IU)	15 (=600 IU)

Therefore, based on current research, it seems that effective UVA/UVB protection is available in a multitude of products, including Neutrogena Ultra Sheer SPF 70 ($17.15 for two, Amazon.com), which has been shown to have a high level of UVA protection. However, because oxybenzone and avobenzone, active ingredients in this product, have been shown to be absorbed into the body, children should use a product with Zinc oxide or Titanium dioxide, such as Blue Lizard Australian Sunscreen, SPF 30 ($12.99, Drugstore.com). Sunscreen should be applied daily, at least 30 minutes prior to sun exposure, and because users typically apply the product so that it is at about 33% of its labeled effectiveness, additional measures against the sun, such as protective clothing, shields for the windows of the home and the car, and avoidance of the sun as much as possible between 10 A.M. and 4 P.M., should be taken. Finally, so that the patient does not jeopardize the health of his or her bones, a vitamin D supplement of the appropriate strength should be taken, in addition to the patient acquiring 10-15 minutes of sunblock-free UV exposure to his/her face, arms, hands or back.

Updates on sunscreen will be issued when they become available!

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