Why You Need Sunscreen: The Power of UVRs

In this first installment of 'Why You Need Sunscreen,' we'll be discussing the power of UVRs.

Ultra Violet Radiation (UVRs) is made up of the electromagnetic light waves that reach earth from the sun. UVRs are divided into three wavelengths: UVA, UVB, and UVC.

(psst, let's pretend for a second that that actually lists radio waves after microwaves)

UVC is the shortest and most damaging of the UVRs. Thankfully UVC is entirely absorbed by the ozone layer. And if you're worried about the hole in the ozone- don't be. Not only is it repairing, but it's currently
hanging out over Antarctica. Fluorescent lamps made for the purposes of disinfecting surfaces in labs and disinfecting water emit UVCs.

These lamps use ultraviolet germicidal irradiation (UVGI) to effectively kill infectious bacteria like Tuberculosis bacteria. Otherwise, UVCs usually aren't present on earth and not something you'll typically need to worry about.

UVB is a medium length wave and mostly absorbed by the ozone layer, too. UVBs hit the U.S. with highest intensity during April through October, and from 10am to 4pm everyday. Seasonal UVB intensity varies upon location- it's during the summer months and closer to the earth's equator that UVBs are most intense. UVBs are mostly responsible for skin tanning and burning; moreover, UVBs play a key role in the development of skin cancers. UVBs typically do not penetrate glass. The tan produced by UVB rays is slower developing and longer lasting, fully developing 2 days after sun exposure. UVB tans produce more melanin (melanogenisis) in the skin, which is why the tan produced by UVBs lasts longer.

UVA is the longest wave and not absorbed by the ozone layer. In fact, 95% of UVRs that reach the earth's surface -and our skin- is made up of UVAs. They are present with roughly equal intensity during all daylight hours throughout the year, penetrate clouds, and penetrate glass. So no matter how much visible light there is, UVAs will still be present during the day (places with permaclouds and places with clear blue skies have just as much UVA present during the day).

As you can see from this table, concentration levels do vary, however, the concentration of UVAs is still incredibly high early in the morning. It is important to wear sunscreen all hours of the day and not just between 10am and 2pm.

Further, UVAs penetrate the skin much more deeply than UVBs, causing photodamage and photoaging (wrinkles), and damages skin cells in the basal layer of the epidermis. So while you won't see a burn, you'll still get the damage. Because of the depth of penetration, evidence suggests that UVAs may therefore contribute to the initiation events of cancer in which a cell/small groups of cells acquire mutations causing them to basically 'forget how to die' and proliferate uncontrollably. Which is precisely what cancer is. Moreover, the basal layer of the epidermis is exactly where most skin cancers (Basal Cell Carcinoma) occur.

Now you might be wondering why UVAs penetrate deeper than UVBs. Drs Zimmerman and Swenson have got your back.

With light (and remember, UVAs and UVBs are wavelengths of light), penetration depends upon the object it is hitting. As UVs move through the atmosphere, their photons (the matter light is made up of) are absorbed and dispersed. When UVs finally reach the skin, the photon's energy is absorbed by melanin (as well as other bio-molecules in your skin; melanin's absorption of light energy is why those with darker skin have less photodamage than those with lighter skin). Depth of penetration will depend on the energy of the wavelength.

In light, "the ray that has the shorter wavelength will have more energy, thus the ray with more energy will penetrate farther." For UV light, the opposite is usually true. Therefore, the shorter the wavelength, the faster the light is absorbed. In this case, UVBs are shorter than UVAs. Thus, UVAs are able to penetrate much deeper than UVBs.

Well what about fake sun? Surely tanning beds can't be as dangerous as tanning outside? Unfortunately, you are wrong. Tanning beds typically emit only UVAs. So as you can imagine those that use tanning beds have a largely increased risk of developing skin cancer. Unfortunately for tanners, UVAs do not produce new melanin, they simply darken current melanin in the skin. Therefore, tans produced by UVAs fade quickly. And more bad news for those that use tanning beds- since tanning beds use UVAs and UVAs don't create new melanin, tanning bed 'base tans' do not provide any extra protection. So you will still burn just as badly on vacation.

No matter the physical presentation- sun tanning or sun burning results in DNA damage. The most deadly skin cancer, melanoma, results from indirect DNA damage (or free radicals and oxidative stress). 92% of all melanoma lacks a UV-signature, thereby providing evidence that indirect damage causes this skin cancer. Further it's proposed that upwards of 80% of all sun exposure over the lifetime is incidental (the exposure one gets from walking from inside a building to their car or the time it takes to get the mail). Regardless of this, 1 in 5 Americans will still be diagnosed with skin cancer in their lifetime.

Free radicals are highly unstable atoms that cause cell damage. UVRs are one of the biggest contributors to the production of free radicals in the body. Without getting too technical, "DNA mutation is a critical step in carcinogenesis and elevated levels of oxidative DNA lesions [caused by free radicals] have been noted in various tumors, strongly implicating such damage in the etiology of cancer." It all boils down to, in order to prevent premature aging and skin cancer, broad-spectrum sunscreen must be worn everyday in every weather in every locale. To explain in the plainest of terms:

sun exposure = free radicals = DNA damage = cancer

Be sure to tune in next time to the next installment of Why You Need Sunscreen: What the Hell is Sunscreen Anyway?

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