Suncare Science - Part 2: Physical Blockers
Direct Physical Photoblockers
Most of the physical photoblockers are metal oxides (iron, chromium, zinc, titanium, etc.) that occur naturally, while some, such as bismuth are man-made. In addition to their photoprotective attributes, these substances also assist in preventing windburns and skin damage from wind driven micro particles of dirt and grime.
An additional significant property of physical blockers is their ability to offer a defense against infrared (“heat”) rays by two distinct means. First, particles large enough to be visible (i.e. reflect visible light) will also reflect and refract the infrared waves most harmful to skin (760nm – 1,800nm). Second, regardless of their particle size, these metal-based materials act as a “heat sink” and thereby reduce the heat effect on the skin.
Three important photoprotective blockers are titanium dioxide, zinc oxide, iron oxides.
This white pigment powder is widely used in cosmetics. The purpose of large particle titanium is to give opacity and lighten (or whiten) the color of the products containing it. Opaque titanium dioxide highly reflects and strongly scatters all UV and visible rays. It also reflects much of the skin-damaging infrared waves, which keeps the skin cooler, reducing “heat” damage and its subsequent photoaging.
To photo-stabilize titanium dioxide, it must be micro-coated with its own protectant such as silicone or aluminum oxide. An alternate procedure to inhibit breakdown is to incorporate other appropriate blockers together with titanium dioxide since titanium dioxide spreads poorly on the skin. Micro-coating the titanium dioxide is also a common way to achieve cosmetic elegance and usefulness; designing a vehicle to assure good, even application to the skin is essential. Large particle titanium dioxide products produce a very white, opaque appearance when applied on the skin, but submicronizing titanium dioxide powder creates small particles that absorb visible light, enabling products to protect the skin from most UVB and some UVA, but are invisible on the skin.
Transparent (sub-micronized) titanium dioxide works by absorbing, reflecting and scattering UVB and some UVA rays. However, protection against UV, visible and infrared is significantly limited when sub-micronized titanium dioxide is the primary protectant.
Zinc Oxide has been known and used topically for centuries as a skin protectant and wound healing adjuvant and is a recognized mild antimicrobial agent. More than 50 years ago, zinc oxide was indicated as a block for ultraviolet light (UVB/UVA). Like titanium dioxide, it also reflects infrared from the skin. Unlike titanium dioxide, it has a much higher ability to protect in the long UVA range (300 – 400 nm), and it absorbs, rather than scatters most UVA. Thus, used in combination with titanium dioxide, ultrafine zinc oxide “closes the window” in the UVA range. Zinc oxide both complements titanium dioxide protective abilities and extends photoprotection to the skin where titanium dioxide is insufficient. The optimal particle size range for ultraviolet-blocking zinc oxide (without blocking visible wavelengths) is approximately 80 to 150 nanometers
Indirect Physical Blocker Aids
Examples of these particles can be natural talc or mica. They are usually flat and oval in shape. These particles are very small, although they are much larger than direct physical blockers. When combined with physical blockers, a portion of very small physical blocker particles will coat the larger flat blocker aids. Because they are flat and smooth, the coated blocker aids will easily slide over each other and overlap to effectively increase protective coverage on the skin.
Polymers can be natural substances from plants; modified semi-natural, animal derived (modified chitin, from the “shells” of shrimp etc. is commonly employed); or synthetic substances such as micronized nylon. Certain polymers, when carefully formulated into a photoprotective preparation, create a “cage” structure that forces ultraviolet and visible rays to follow a maze rather than directly reaching the skin. This protracted route helps increase protection by either preventing some rays from reaching the skin (or reaching the skin with greatly reduced energy) or by increasing the contact time between the rays and the organic filters/physical blockers.
By themselves, such polymers (which incidentally also improve the feel of the cosmetic finished product on the skin) provide little to no useful skin photoprotection, but they do help to defend the skin from wind and wind-blown dirt and grime pollution particles. In the presence of active photoprotective agents, these polymers can increase the Sun Protection Factor (SPF) by 3 to 5 points.