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What Are The Benefits Of Iron Oxides in Sunscreen?

Sunscreens help filter out ultraviolet (UV) light using two main types of ingredients. Organic sunscreens, also known as chemical sunscreens, work by absorbing UV light. Chemical sunscreens include ingredients such as oxybenzone, avobenzone, octisalate, octocrylene, homosalate, ecamsule, and octinoxate.

Inorganic, or physical sunscreens contain metal oxides such as zinc oxide and titanium dioxide that reflect and scatter UV light. Zinc oxide provides better UVA protection than titanium dioxide, including protection against UVA 1 (340-400 nm), but it is less efficient than titanium against UVB radiation.[1] Titanium dioxide protects against UVA 2 (320-340 nm) and UVB protection, but does not protect against UVA 1 radiation in the way zinc oxide does.[2]

Ultraviolet Radiation and Visible Light

There are many different types of rays in sunlight. Ultraviolet radiation is the most damaging to our skin and has wavelengths shorter than visible light, making it invisible to the naked eye. Within the UV spectrum, there are UVA, UVB, and UVC rays.

UVC rays (100-290 nm) are the shortest wavelength and are mostly absorbed in the ozone layer. Generally, they do not reach the earth&#;s surface and thus are not thought to be major contributors to skin damage by the sun.

UVB rays (290-320 nm) are shorter wavelengths that can penetrate the atmosphere. They cause damage to the superficial layers of the skin, resulting in reddening of the skin (sunburns), and DNA damage, which contributes to the development of skin cancers.[3] SPF ratings on sunscreens refer mostly to protection against UVB rays.

UVA rays (320-400 nm) are longer wavelengths that can penetrate the atmosphere and can be further broken up into UVA 1 (340-400 nm) and UVA 2 (320-340 nm) wavelengths. These rays penetrate deeper into the skin and can therefore damage collagen, resulting in premature aging and wrinkles. UVA rays can also damage DNA and thus play a role in the development of skin cancers.[3] Because the SPF value is based primarily on UVB rays, the &#;PA+&#; rating system was developed in Japan to represent how much UVA protection a product has. However, there are issues in how the value is achieved and applied, so regulatory agencies in Europe and the US use the term &#;broad-spectrum&#; to ensure a product has coverage against UVA and UVB wavelengths.

Visible light (400-700 nm) can penetrate much deeper into the skin than UVA or UVB rays. Though not traditionally thought of as damaging to the skin, visible light can exert negative effects through the production of reactive oxygen species, causing indirect DNA damage.[4] In people with darker skin, visible light can cause more potent and long-lasting pigmentation compared with UVA1 radiation.[5, 6] Most sunscreens do not protect against visible light. 

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Shorter wavelengths of visible light (blue) closest to the UV range are more concerning than longer wavelengths (red) when it comes to sun damage, since they are more energetic. Wavelengths in the blue region (~415 nm) have been found more likely to induce hyperpigmentation compared to wavelengths in the red region (630 nm)[7] and contribute to relapse of pigmentary disorders such as melasma, especially in darker-skinned individuals.[8] 

What Does SPF Mean?

SPF (Sun Protective Factor) is a relative measure of how long a sunscreen will protect you from UVB. For example, if your skin normally burns after 10 minutes in the sun, using an SPF 15 sunscreen will theoretically allow you to stay in the sun for a factor of 15 times longer, or 150 minutes, without burning. However, this can be altered by how much sunscreen is applied, the length of time spent in the sun, time of day, weather conditions, activities, and geographic location.

What Are Micronized Sunscreens?

Due to their opaque nature that often leaves a &#;white cast&#; on skin that is undesirable to consumers, zinc oxide and titanium dioxide are now frequently broken down into smaller nanoparticles (10-200 nm) to minimize their white appearance, and this is termed &#;micronized.&#; However, due to their smaller particle size, protection is shifted towards shorter wavelengths and do not protect against visible or blue light.[9]

Because of the small particle size, there is potentially an increased risk of systemic absorption, leading to concerns about their safety. Though studies have demonstrated micronized zinc oxide and titanium dioxide do not penetrate into deeper layers of the skin,[10, 11] the risk of long-term exposure and absorption has yet to be established. Finally, micronized particles tend to clump together, leaving areas of sun-exposed skin if not properly applied. 

What is the Cosmetic Role of Iron Oxide?

Most physical sunscreens consist of zinc oxide and titanium dioxide that act as physical blockers of UV light and thus can leave a whitish residue on skin that is cosmetically unappealing. Titanium dioxide has a smaller particle size and a higher refractive index than zinc oxide, which causes it to appear whiter on the skin. Iron oxide has gained popularity as an addition to physical sunscreens for its natural reddish hue, which serves a cosmetic role in improving the &#;white cast&#; of zinc and titanium dioxide and easily blends in with skin tones.

How do Iron Oxides Protect Us from the Sun?

Iron oxide can be added to physical sunscreens to increase the absorption and protection against visible light and UVA radiation. While titanium dioxide and zinc oxide have good absorption in the UV range, they do not confer protection in the visible light range if they are micronized.

Iron oxide is effective at absorbing visible light. One study by Kaye et. al showed that the addition of iron oxide to physical sunscreens such as zinc oxide or titanium dioxide transmits less light and thus provides greater photoprotection than either zinc oxide or titanium dioxide alone.[12]

Iron oxide is effective at protecting against UVA light.[13, 14] When combined with zinc oxide, iron oxide works synergistically to reduce the amount of UVA radiation transmitted to the skin to as little as 1.5%.[2]

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