First and foremost, forget about SPF when it comes to UV-induced skin aging. SPF, or sun protection factor, is simply a measure of how well a sunscreen can protect against UVB-induced sunburn. (1) While UVB does play a role in skin damage, it is not what is primarily responsible for the aging of skin. In other words, high SPF is not synonymous with high protection from photodamage. If a sunscreen prevents a sunburn yet allows for a “nice tan” (like many high SPF sunscreens), then said sunscreen is still allowing skin aging to occur.

Secondly, the label “broad spectrum” is a step in the right direction, but it is no guarantee for sufficient protection. Although “broad spectrum” signifies UVB and UVA protection, that does not mean that the UVA protection is necessarily high and reliable. (Ibid)

There are two important ways to measure the UVA-blocking potential of sunscreen. One of these ways is called persisment pigment darkening (PDD), and another is assessing its absorbance of specific wavelengths. Although UVA wavelengths span from about 320–400 nm, it is the rays at or beyond 360 nm (also known as “UVA1” or “longwave UVA rays” — 340–400 nm) that appear to confer the most damage to skin, such as induction of matrix metalloproteinases (i.e., MMPs/collagen degradation enzymes) and secretion of inflammatory cytokines. (2) Using PPD as a measure of UVA-blocking efficacy for sunscreens is a rather incomplete assessment and does not provide a satisfactory explanation of its ability to inhibit skin aging as PPD is primarily a measure of UVA2(short) rays. Measuring the ratio of UVA1 absorbance to total UV absorbance (as proposed by the FDA) (3), or even — as primarily stated — determining the absorbance in the UVA1 range would be a much more informative criterion for anti-aging ability.

The filters (chemical and physical) that are useful against the penetration of UVA1 are avobenzone, Tinosorb (S and M), Mexoryl (SX), Uvinul A Plus, and zinc oxide. Of course, within this selection, stability, concentration, and the full range of wavelength protection further impacts the quality of the overall sunscreen product. For example, avobenzone is an excellent UVA1 blocker with its maximum peak absorption being 357–360 nm (3–4), but its major downfall is that it is highly unstable, degrading under sunlight. With 15 minutes of solar simulated irradiation, roughly 36% of avobenzone is destroyed. (4) Within 1 hr of UV exposure, avobenzone’s ability to filter long wave UVA rays drops strikingly. (5) Compared to a stabilized avobenzone-containing formula, unstabilized formulations allow about 35% higher MMP-1 production under UV conditions. (Ibid) If avobenzone is paired with another useful UVA1-blocker — Tinosorb, for example — its photostability increases dramatically and as a whole, the overall formula becomes a highly efficient broad spectrum sunscreen. Such a formation has actually been shown to strongly restrict MMP-1, MMP-3, and MMP-9 activation during UV simulation. (6) * (see last paragraph) Tinosorb M,with its peak absorbance being around 360 nm, has in fact been shown to be quite beneficial on its own as a stable UVA1 inhibitor in natural sunlight conditions. (3,7) Mexoryl XS is yet another filter that has been proven in experimental (yet natural UV) conditions to offer high UVA1 protection, especially when combined with avobenzone as avobenzone stability is improved and UVA protection is enhanced. (7) With a peak absorption at 345 nm (8), Mexoryl SX is slightly inferior to Tinosorb M, but it is nonetheless one of the top UVA1 filters. Uvinul A Plus is also moderately better than Mexoryl XS as it absorbs UV at a peak of 354 nm. Uvinul A Plus is clearly similar to avobenzone in terms of absorptive capabilities, but due to superior photostability, it is known to be its successor. (3) With the exception of Mexoryl SX, these chemical filters are currently not available in the US, but they may be easily purchased online.

Zinc oxide differs from the rest in that its a physical sunblock. Physical sunblocks are advantageous in some ways in that they do not cause photoallergic reactions; they provide stronger coverage that may be harder to wash off, yet cuts down the need for constant re-application every few hours; and they block UV absorbance immediately, unlike chemical sunscreens that are not effective until 15–20 minutes after contact with the skin. Zinc oxide does not really have a peak absorbance value as it is a uniform UV protectant, protecting against wavelengths from the UVB spectrum to the UVA spectrum. At crucial high wavelengths (385–400 nm) — those that induce significant collaganese (i.e., collagen breakdown) induction — zinc oxide offers greater protection than organic filter Mexoryl SX. (7)

There are two caveats with zinc oxide: 1) It is only useful at high concentrations (~20%) and 2) Regrettably, most sunscreens that are easily accessible are not in high enough concentrations of zinc oxide to be useful in protecting against photoaging due to the cosmetically unappealing “white cast” on the skin left by high amounts of zinc oxide. A lot of sunscreens and SPF makeup now are mixtures of zinc oxide (3–6%, usually) and octinoxate (a UVB blocker). These meager formulations are proven to perform inferiorly as far as inhibiting UV-induced collagen breakdown compared to Mexoryl SX/avobenzone/octocrylene products that are also available on the market. (9) Luckily, there are now high zinc oxide sunscreens (+20%) that are available for purchase online. Fortunately, the latter caveat is easily remedied by tinted pigments added to the formula. Nowadays, there are many tints available that are likely to suit most skin tones.

Since study after study shows sunscreen to vary considerably in UVA1-inhibiting ability (and indirectly, MMP release), it is imperative to choose wisely. Research shows that Tinosorb M is the best anti-aging (i.e., UVA1-blocking) filter available, with high (~20%) zinc oxide being a close second. Using either filter (or even Uvinul A Plus or Mexoryl SX, if one so chooses) in one’s daily skincare regimen will go a long way prevent sun-associated skin aging

* Despite improvements in stability based on the pairing of certain filters, those who wear makeup on top of avobenzone-containing sunscreens should consider using an avobenzone-free sunscreen. Based on a stability study of avobenzone, titanium dioxide, zinc oxide, and iron oxides (specifically: yellow and black pigments) that can be found in makeup can significantly enhance the degradation of the compound during UV exposure.

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