Potential health risks of sunscreen

Sunscreen is widely acknowledged to protect against sunburn and two common forms of skin cancer associated with direct DNA damage, squamous cell carcinoma (SCC) and basal cell carcinoma (BCC).^[1][2] However, there is some evidence that sunscreen use can contribute to negative health outcomes. The major claims include:

• Some sunscreens only protect against UVB radiation, and not against the UVA component of the ultraviolet spectrum. Incomplete protection against the full spectrum, combined with increased time spent in the sun, can lead to an increase in the risk of developing malignant melanoma, a rarer but more deadly form of skin cancer associated with indirect DNA damage.
• Some sunscreen chemicals can penetrate into the skin and potentially cause more cell damage than they prevent.
• Reduced exposure to ultraviolet light in sunlight can contribute to Vitamin D deficiency.

These issues have precipitated various levels of disagreement over the benefits and risks of sunscreen use. Most health authorities and medical associations have concluded that, on the whole, sunscreen use is beneficial,^[3] and the industry continues to develop improved formulations that are both longer-lasting and broader-spectrum (UVA/UVB).^[4]

Studies of melanoma rates

Even though it is rare, malignant melanoma is responsible for 75% of all skin cancer-related deaths, making it the most lethal form of skin cancer.^[5][6] There is a correlation between high UV exposure, especially during childhood, and the risk to develop melanoma,^{[7][8][9][10]} resulting in a WHO recommendation for persons under 18 to avoid sunbeds.^[11]

Some scientists have argued that the sun-avoiding health message increases some forms of skin cancer.^[12] Malignant melanoma was found more frequently in sunscreen users compared to non-users in some studies.^[13] Other studies found that fair-skinned people used more suncreen and had more skin cancer, but did not establish a causal relationship.^{[14][15][16][17][18][19]} It was suggested that those sunscreens blocked the natural warnings and adaptations mediated by UVB, but allowed damage from UVA to go unchecked.^[20] However, these claims could not be supported in two meta-analyses, including one of 9,067 patients from 11 case–control studies, finding no association between sunscreen use and development of malignant melanoma.^[21][22] The only evidence suggesting a relationship between sunscreen and increased melanoma was correlational, and thus cannot be used to establish cause and effect.

Several studies with mice suggested that sunscreen failed to protect against melanoma,^[23][24][25] or even increased its incidence;^[26] but these were pre-2000 sunscreens that offered only limited protection against UVA radiation.

Alternative view

Authors who claim that sunscreen use causes melanoma have speculated that this occurs by one or more of the following mechanisms:

• the absence of UVA filters combined with a longer exposure time of the sunscreen user^{[14][27][28][29]}
• suppression of the skin's production of melanin, a natural broad-spectrum photoprotectant^[14][30]
• free radical generation by sunscreen chemicals that have penetrated into the skin^{[31][32][33][34][35]}
• cytotoxic and carcinogenic effects of nanoparticles^[36]
• carcinogenic effects of sunscreen ingredients related to vitamin A^[37][38]

Skin penetration

Between 1% and 10% of some sunscreen ingredients are absorbed into the body through the skin.^{[31][33][39][40][41][42][43][44][45]} The absorption does not occur instantaneously, but the sunscreen concentration in the deeper levels of the skin increases over time.^[34] For this reason, the amount of time between the topical application of sunscreen and the end of the illumination period is an important parameter in experimental studies.

DNA damage

Although sunscreen ingredients are designed to block UV radiation that damages skin cells, some cell damage can be caused by the sunscreen ingredients themselves when absorbed into the skin and illuminated by sunlight.^{[46][47][48][49]} DNA, in particular, is susceptible to damage caused by photo-excited compounds.^[50] It is also well known that DNA alterations are necessary for cancer to occur. PABA causes DNA damage in human cells,^[51] and was banned as a sunscreen ingredient several years after these findings were published. Phenylbenzimidazole (PBI) causes DNA photodamage when illuminated while in contact with bacteria or human keratinocytes.^[35][50][52]

When exposed to UV radiation, some sunscreen ingredients generate reactive oxygen species,^{[34][53][54][55]} a class of free radicals that can damage DNA throughout the body.

Studies have found that titanium dioxide nanoparticles cause genetic damage in mice when given orally in high doses.^[56] Nanoparticles are commonly used in American sunscreens to make them transparent on the skin; the EU has treated these with more caution, demanding they be treated like new chemicals, subject to safety assessments before being deployed in consumer products. In 2009, the EU initiated labeling requirements for nanoparticle ingredients in certain sunscreens and cosmetics.^[57]

Vitamin D deficiency

The use of even a weak sunscreen with SPF 8 inhibits more than 95% of vitamin D production in the skin, more effectively than it inhibits sunburn itself.^[58][59] Recent studies suggest that more Americans, Australians and New Zealanders are vitamin D deficient than previously thought.^[60][61] Ironically, there are indications that vitamin D deficiency may lead to higher cancer mortality.^[62] To avoid deficiency, dietary sources or vitamin supplements can be used, or adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs, averaging 5–30 minutes twice per week without sunscreen. (The darker the complexion, or the weaker the sunlight, the more minutes of exposure are needed, approximating 25% of the time for minimal sunburn. Vitamin D overdose is impossible from UV exposure; the skin reaches an equilibrium where the vitamin degrades as fast as it is created.)^[63][64][65] If sunscreen slows vitamin D synthesis to just 5% of the normal rate, then normal production requires 20 times more exposure, from longer sessions, greater frequency and/or uncovered skin area.

Legal and regulatory issues

In 2006, a number of class-action lawsuits alleged that sunscreen manufacturers misled consumers into believing that these products provided full sun protection, while the products actually lacked effective UVA filters.^[66] The lawsuits were settled in 2009.^[67]

In 2007, the United States Food and Drug Administration "tentatively concluded that the available evidence fails to show that sunscreen use alone prevents skin cancer".^[68]

Sunscreen ingredients are not tested for photocarcinogenic effects in Europe, Japan or Australia before being introduced to the market. In the US, most sunscreens sold in 2008 have not passed regulatory testing either, due to a grandfather clause. Three new sunscreen active ingredients introduced in the US since 1978 have fulfilled new testing requirements.^[69]

See also

• Health effects of sun exposure
• Sun protective clothing
• Tanning activator

References