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You are here: Skin Protection >

The dark side of chemical sunscreens. Should you be concerned about photosensitization?

In the world of medicine, the situation when the remedy may be almost as harmful as the problem is not uncommon. Skin protection with chemical sun blocking agents might be a case in point, according to some experts.

First, a little background. Protecting the skin from harmful ultraviolet (UV) rays is important for both preventing skin cancer and reducing the rate of skin aging. The exposure of the skin to UV rays can be reduced by UV blocking agents (a.k.a. sunscreens, sunblocks, UV/sun blockers), which fall into two broad categories: physical and chemical.

Physical blockers are usually finely powdered and dispersed minerals, the most common being zinc oxide and titanium dioxide. They block UV radiation mainly by reflecting/scattering the rays. They are generally insoluble and do not penetrate the skin. Conversely, chemical sunblocks work mainly by absorbing UV light. Most are synthetic chemicals that are soluble in oil and/or water. Many can penetrate the skin at least to some degree. (See our article on sunscreen fundamentals for details.)

The useful ability of chemical sunblocks to absorb UV light is also a potential source of harmful effects. When a molecule of a chemical sunblock absorbs a UV photon (a quantum of UV light), it becomes excited (energized). However, the sunblock molecule does not stay excited forever. Eventually, it releases the absorbed energy by emitting lower energy photons and/or interacting with other molecules. These secondary effects oftentimes lead to the formation of harmful chemical byproducts, particularly free radicals. If this occurs inside the skin (i.e. the sunblock molecule has penetrated the skin before absorbing UV photon), the free radicals (and possibly other secondary effects) may cause skin damage and irritation, increase the risk of cancer as well as contribute to skin aging.

The process where a chemical makes the skin (or other tissues) prone to damage when exposed to light is called photosensitization. There is evidence that at least some chemical blockers can trigger the production of free radicals and thus cause photosensitization. In particular, the following blockers were shown to be potential photosensitizers: octocrylene, octyl methoxycinnamate (octinoxate), benzophenone-3 (oxybenzone) and benzophenone-4 (sulisobenzone). More research is needed to determine whether any of these or other absorbable chemical blockers can cause significant skin damage under the conditions of typical use.

What does this all mean in terms of best practices for sun protection? Should one abandon the use of all sunscreens containing chemical blockers until they are comprehensively researched and improved? At present, the answer is not clear-cut so let me provide some context.

First, no sunscreen is perfect. Even if there was one, the chances of it being applied consistently and correctly are low. Hence the best strategy to avoid UV damage to the skin is to reduce exposure while taking steps to minimize the downside of sun avoidance. When reducing sun exposure is not an option, physical sunblocks (e.g. zinc oxide and titanium dioxide) tend to be safer (as long as they are not in the form of nanoparticles). Unfortunately, physical blockers have drawbacks too. They rub off too easily, they may be cosmetically problematic (powdery white) and, more importantly, it is difficult to achieve very high degree of broad (UVA+UVB) protection with physical blockers alone. Therefore you may still want to consider chemical sunscreens as an option if facing particularly intense and/or prolonged sun exposure or if your skin is particularly sensitive to the sun and avoidance is not feasible. In such a situation, what would be the lesser of two evils - direct sun damage or the secondary damage from free radicals triggered by a chemical blockers? At present, there is not enough research data to provide a definitive answer. It appears at least some chemical sunblocks prevent more damage than they cause.

Not all chemical sunblocks are created equal. For example, ecamsule (Mexoryl SX), homosalate and a few others appear to have a relatively good safety profile. On the other hand, such agents as octocrylene, octinoxate, oxybenzone and sulisobenzone appear to be more damaging. If choosing a chemical sunscreen formula, examine the list of ingredients and look up the information (on this site and elsewhere) on the specific blockers used in the formula.

Some manufacturers are attempting to mitigate the side effect of chemical sunscreens with new technologies. One approach is microencapsulation: the sun blocking agent is encased in tiny capsules, which makes it less prone to react with other chemicals and/or be absorbed by the skin. Another approach is to add antioxidants to the sunscreen formula. Since much of the damage from chemical sun blocks occurs due the increased production of free radicals, the addition of an effective combination of antioxidants could be a mitigating factor. Yet another approach is to add photostabilizers, which can modify the release of energy from chemical sun blocks and thus reduce the consequent skin damage. If choosing a sunscreen with chemical blockers, opt for the ones employing at least some of these new technologies (if available).

Bottom line

Evidence indicates that at least some absorbable chemical UV blocking agents (particularly octocrylene, octinoxate, oxybenzone and sulisobenzone) may cause damage to the skin upon exposure to sunlight. Until further research has established optimal usage practices, if any, for these agents, it is prudent to put emphasis on other methods of UV protection, such as reducing sun exposure and using physical sunscreens. However, in some situations, the benefits of chemical UV blockers may outweigh the risks. If so, one should try to use sunscreen formulas containing the least damaging of the chemical blockers and/or the formulas employing the methods to mitigating such damage, such as encapsulation, antioxidants and/or photostabilizers.


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