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Sunscreen fundamentals: how sunscreens/sunblocks work

Using sunscreen should be an important part of a comprehensive sun protection. To take full advantage of sunscreens, you have to be able to select a sunscreen that is both effective and a good fit for your particular situation (skin type, lifestyle, esthetic needs and so forth). To that end, it is useful to understand how sunscreens work, what impacts their effectiveness, what potential risks are, and so forth. These issues are discussed below.

UV blocking mechanism. Chemical and physical sunblocks.

UV radiation may be blocked either by absorption or reflection/scattering of UV light. Based on their mechanism of protective action, sun-blocking agents are broadly divided into 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 insoluble under typical conditions and only minimally absorbed into the skin. As a result, they are easy to rub off and may need to be reapplied especially frequently. Also, when applied to the skin, both zinc oxide and titanium dioxide create white-tinted matte look. To reduce this unsightly effect, some manufacturers employ microfine zinc oxide and/or titanium dioxide with particles smaller than 200 nm (a.k.a. nanoparticles). Such formulations are more transparent but raise concerns about possible penetration of nanoparticles into the skin and accumulation in vital body tissues.

Chemical sunblocks work mainly by absorbing UV light. Most are synthetic chemicals that are soluble in oil and/or water. Many are absorbed into the skin and, possibly, into systemic circulation (and, therefore, need a thorough safety testing). Some degrade when exposed to UV radiation and/or interact with other sun-blocking agents or other cosmetic ingredients.

There are a few agents that act as both physical and chemical sunblocks. One such hybrid sunblock is bisoctrizole (Tinosorb M). Even though titanium dioxide is typically considered a physical sunblock, some experts consider it a hybrid because considerable part of its activity is through UV absorption.

Overall, physical sunblocks tend to have broader spectrum of UV protection and appear somewhat safer due to the lack of systemic absorption. On the other hand, they are more unsightly (especially the safer, not-microfine versions) and require more frequent application.

In any case, each sunblock, physical or chemical, has its own unique profile and should be evaluated individually. (See our information on individual sun-blocking agents.)

Range of UV protection

There are three subtypes of UV light: UVA (320-400 nm wavelength), UVB (280-320 nm), UVC (100-280 nm). UVA is sometimes subdivided into UVA-1 (350-400 nm) and UVA-2 (320-350 nm). UVC is almost completely absorbed by the ozone layer and does not reach the Earth's surface. As long as we haven't destroyed the ozone layer, we don't have to worry about the UVC. But we do have to worry about the other two. UVB causes sunburn, but has a relatively modest effect on skin wrinkles because most of it is absorbed in the epidermis (the outer skin layer) and does not reach the dermis where wrinkles form. UVA penetrates deeper into the skin and is the major contributor to skin damage and wrinkles. Both UVA and UVB can contribute to the development of skin cancer. (See our article on ultraviolet radiation for more details.)

Sunscreens vary in regard to which part(s) of UV spectrum they block. For example, zinc oxide blocks radiation across entire UVB and UVA, titanium dioxide blocks UVB and short UVA (UVA-1), ecamsule (Mexoryl) blocks UVA only and homosalate blocks UVB only. Whatever sun-blocking ingredients you are using, both UVB and UVA need to be fully covered.

Degree and uniformity of UV protection

Even if a sun blocking agent works across a particular range, it typically has a peak wavelength where it is more effective than elsewhere. Also, even in its active range, an agent generally blocks only a certain percentage of UV radiation. The rule of thumb indicator of the degree of protection against UVB is SPF. There is no widely established protection gauge for UVA; the most commonly used one is PPD. (See our article on gauging UV protection via SPF and PPD.) Increasing the concentration of the a sun-blocking agent, typically increases the degree of protection but may also increase the risk of skin irritation, toxicity, unsightliness, formulation/application difficulty and so forth.

Stability and interactions

Most of today's sun-blocking agents are stable in inert vehicles at room temperature when not exposed to UV light. However, exposed to sunlight, some may degrade, react with other ingredients in a sunscreen formula and/or become catalysts of chemical reactions between other ingredients. These changes can lead to loss of UV protection as well as the formation of possibly harmful byproducts. For example, when exposed to sunlight, avobenzone and octyl methoxycinnamate (octinoxate) degrade and lose sun-blocking effectiveness whereas some forms of titanium dioxide may exhibit photocatalytic activity resulting in the formation of potentially harmful free radicals.

The stability of sun-blocking ingredients can sometimes be improved by using photostabilizers, antioxidants, coatings and so forth. For example, a sunblock octocrylene can partly protect from degradation other sun blocking agents (e.g. avobenzone). Potentially harmful photocatalytic activity of titanium dioxide can be reduced by special coating.

Check the sun blocking ingredients in your sunscreen for stability (especially in sunlight) as well as potentially harmful interactions. (See our information on individual sun-blocking agents.)


Solubility is a factor that affects the compatibility of sun blocking agents with certain vehicles as well as their interaction with the skin. For example, oil soluble sunblocks tend to penetrate skin easier, which, on one hand, may increase the risk of side effects, and, on the other hand, tends to extend the time of skin protection after an application. Water soluble sunblocks may be particularly useful for low-oil, non-comedogenic sunscreens preferred by many acne sufferers. Sun blocks soluble in neither oil nor water tend to work mainly as physical sunscreens (i.e. as particles that reflect/scatter UV light); they are only minimally absorbed by the skin. (For specific solubility data, see our information on individual sun-blocking agents.)

Toxicity and adverse-effects

None of the sun blocking agents in widespread use today appear to possess acute toxicity. None seem to frequently cause skin irritation, except para-amino benzoic acid (PABA), whose use in skin care has been declining for years. Yet, safety issues cannot be discounted altogether. First, similarly to most other skin care ingredients, sun blocking agents lack data on the possibility of low-level skin damage or systemic effect with chronic use. In fact, there are legitimate concerns regarding some of the widely used sun blocks. For instance, octyl methoxycinnamate, a UVB sun block, has been shown to produce hormonal (estrogenic) effects in animal studies. There is no evidence that such effects occur with typical human use but more safety research of this and other sunblocks is clearly needed. (For specific safety data, see our information on individual sun-blocking agents.)

Combinations of sun blocking agents

As of the time of this writing, no single UV blocking agent seems sufficient for a high degree of reliable, comprehensive and safe sun protection. Some protect against UVA but not UVB and vice versa. Others protect from both UVA and UVB but provide insufficiently high degree of protection at safe concentrations. Yet others degrade, rub off easily, have unresolved safety concerns and so forth.

Understandably, most commercial sunscreens (especially those aiming at maximum protection) combine two or more sun blocking agents. Some sunscreens also include special active ingredients that enhance performance of the primary sun blocks by enhancing their UV-blocking capacity, inhibiting their degradation, and/or neutralizing harmful degradation products like free radicals. For example, octocrylene, a rather weak UV blocker itself, protects and augments other UV absorbers, while improving their uniform skin coating.

Bottom line

Formulating a good sunscreen is tricky. Even choosing the right sunscreen and applying it properly is not as simple as it may seem. For optimal sun protection, make sure you are doing all the other (simpler) sun protection steps. And, certainly, inform yourself about sunscreens as much as you reasonably can.


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