UV exposure is responsible for 90% of human nonmelanoma skin cancer. Of the UV light reaching the earth, 90% of the waves are UVA lengths (320-400 nm) and only 10% are the shorter UVB wavelengths (280-320 nm). Both UVA and UVB are studied for their skin cancer-causing potential, however focus has been on UVB. This is at least partially because UVB is considered a complete carcinogen—a substance able to initiate, promote and progress the development of skin cancer.
Although many of us react to the oft-repeated caution to “wear sunscreen daily,” a study in the Archives of Dermatology reported a 300% increase of skin cancer incidence in the USA since 1994, when rates were last estimated. Reasons for this jump are complicated and numerous, but a major factor is people not heeding the message and/or the effectiveness of the sunscreens they are using.
Many sunscreens, if not most, contain chemicals that may cause more damage than they prevent, because once broken down (some rather quickly) they begin generating DNA-damaging free radicals. We also can’t discount the effect of stratospheric ozone depletion, since ozone specifically blocks UVB penetration. UVB levels are expected to rise disproportionately in temperate and arctic zones. Researchers using a sensitive new spectroradiometer in Toronto, Canada, discovered that current ozone depletion has already caused local increases in ambient UVB radiation of 35% in winter and 7% in summer, relative to 1989 levels (Kerr and McElroy 1993). This means that applying sunscreen on a daily basis, regardless of the season, has become even more of an imperative.
As a complete carcinogen, UVB attacks not only at the initiation stage but at the promotion stage as well.
During initiation, UVB can cause chromosomal alterations and mutations via direct DNA damage and/or production of reactive oxygen species. Tumor promotion occurs via epigenetic effects, such as altered gene expression cell membrane damage, apoptosis and a compromised immune system. Tumor progression appears to involve additional genetic alterations including chromosomal alterations.
Sunscreen agents prevent the onset of UVB by either reflecting the rays in the case of sunblocks containing zinc oxide, or by absorbing UVB before it interacts with the cell, as is the case with chemical sunscreens.
Of course, your first line of defense against sun damage is sunscreen, especially non-nano zinc oxide, which is photostable and won’t break down to generate free radicals. A 2006 study from the University of California, Riverside, reported that sunscreens containing widely used chemicals like benzophenone-3, octyl methoxycinnamate and octocrylene can degrade in as little as one hour, elevating free radicals by as much as 64%.
And on the mineral sunscreen front, titanium dioxide is also very photoactive—it readily absorbs UV and in the presence of water produces the chemically active hydroxyl radical. Coated versions of TiO2, designed to be more stable, can break down in chlorinated pools to form free radicals. The breaking news is that some manufacturers, finally responding to public demand, are removing TiO2 from their products pending further safety studies.
Little known fact: The reason you're advised to repeat sunscreen application every two hours is not because it rubs off, but because UV exposure degrades the chemicals to the point where they could start generating free radicals at rates higher than if you wore no sunscreen at all. The manufacturers’ solution is to keep reapplying, thus suppressing free-radical generation, either directly from UV rays or from chemical breakdown.
This 'band aid' helps answer the question heard so often from frustrated sunscreen users: "How come I wear an SPF 55 and still get red?" Chances are they have not reapplied sunscreen as often as needed, which could be as frequently as every hour if it contains benzophenone-3. Another solution would be to switch to a brand containing zinc oxide. Non-nano zinc oxide is photostable because it works by reflecting UV rays rather than absorbing them. Zinc oxide is also anti-inflammatory, and it protects against the entire UV spectrum, both UVA and UVB.
An individual's need for sun protection varies, depending on factors like geographical location, skin type and even genetic predisposition. For some people, preventing UV damage means taking measures in addition to wearing a sunscreen containing zinc oxide—though that still should be first on everybody’s list of must-dos.