Math 101 - SPF by the Numbers

In Weather 101 - Understanding Sun Safety, I briefly discussed Ultraviolet (UV) radiation, while keying heavily on sun safety. The intent of that article was to help readers protect themselves from UV radiation, associated skin damage and potential skin cancer.

Here, we'll delve a bit more into what UV radiation is and then what the Sun Protection Factor (SPF) really means.

Ozone

Sunlight, light from the Sun, contains many different types of electromagnetic radiation. The Earth's electromagnetic field and its atmosphere block some of these, like x-rays. Others like visible light pass through the atmosphere (unless they are reflected back into space by clouds). UV rays, on the other hand can be partially blocked by stratospheric ozone, a colorless gas containing 3 oxygen molecules. By contrast, oxygen as we know and breathe it contains two oxygen molecules and has no effect on UV radiation.

Stratospheric ozone does not completely block incoming UV radiation. However, it significantly lowers the amount that would reach the Earth's surface without an atmosphere. And until gases (known as CFC's - chloroflurocarbons) were introduced into the atmosphere by refrigerants and related sources, stratospheric ozone did a pretty good job of protecting us.

But, the CFC's, in the presence of sunlight, act to break up ozone. This decreases the stratospheric ozone population and leads to the so-called "Ozone Hole." There's really not a hole up there, but rather a significant thinning of the ozone content. Scientists measure this in terms of optical thickness, or the thickness that the particular gas (in this case, ozone) would have if all the gas above a particular spot were made into a pile.

When the sunlight goes away at night or during winter at high latitudes, ozone reforms.

It so happens that the most dramatic thinning of the ozone layer occurs around the South Pole. Hence, scientists working in Antarctica and people living in places like Australia and extreme southern South America are most at risk from the renegade UV rays that get through the stratosphere.

Understanding UV radiation

Regardless of the amount of ozone above, it's always a good idea to add a little extra skin protection. Hats, shirts, sunglasses and other protective coverings are a great first line of defense. But, when you remove the excess clothing to "catch those rays" or go into the water, you need some other protection.

Sunscreens and sunblocks come to the rescue (Figure 1). These liquids, ointments and creams, when applied properly according to the American Academy of Dermatology (www.aad.org), absorb, reflect and/or scatter the incoming UV rays.

But, UV rays come in three varieties - A, B and C and each must be understood in order to effectively protect against it. Dermatologist Henry W. Lim, M.D., F.A.A.D., of the Department of Dermatology at Henry Ford Hospital in Detroit, MI, advised that, "UVA has the longest wavelength, is not absorbed by the ozone layer and it penetrates the skin deeper than UVB." In fact, it is UVA that causes premature aging of the skin, as well as skin cancer. Lim also noted that UVB, although partially blocked by the ozone layer, still causes sunburns. UVC is totally absorbed by the earth's atmosphere; hence, we receive it only from artificial radiation sources.



SPF - A Math Problem

Many of the new sunscreen formulations now contain ingredients that block both UVA and UVB. While many do a good job of battling UVB, protection from UVA is compromised after a few hours of sun exposure.

Assuming that you apply and reapply sunscreen / sunblock appropriately, the next thing you need know is what the SPF really means.

Many people incorrectly think that a sunscreen / sunblock with an SPF of 30 offers twice as much protection from UV rays as one with an SPF of 15. Nothing could be more incorrect.

The SPF tells how what percent of UV rays "get through" the sunscreen / sunblock, not about how many are blocked. And this is done through a not so easy to understand mathematical relationship.

Since we are dealing with percent, the amount of UV rays getting through is determined by dividing the SPF value into 100%. Hence, an SPF of 15 lets in about 6.7% of the incident UV radiation (100%/15). An SPF of 30 lets in only 3.3% (100%/30).

This can also be expressed as an inverse relationship (SPF * amount of UVB rays coming through = 100%). In math textbooks, you'll see this shown as X*Y=K where X and Y are variables and K is a constant.

While doubling the SPF reduces the UV penetration by 50%, the amount of UV blocked only changes from 93.3% to 96.7%, a percentage increase of only 3.6% [(96.7%-93.3%)/93.3%]

Figure 2 shows percentage of UV blocked by different SPF values.

Interpretations and Actions

Unfortunately, people often misinterpret these higher SPF numbers thinking that a higher SPF means a much higher level of protection and/or even a guarantee of all-day protection.

In fact, some folks think that an SPF of 50 means that they can stay outdoors 50 times longer than usual without succumbing to a burn or lots of internal skin damage. But who is going to try and compute that? Rather folks then tend to stay outdoors far longer than they should. Ouch!!!

Another factor going against thinking there's a lot of sun protection is that most of us don't apply enough sunscreen or sunblock. In fact, scientists estimate that we use only about half the amount we should to receive the advertised protection factor.

And, If you don't reapply the sunscreen / sunblock after exposure to water and/or perspiration, even a short bake in the sun could give you a very serious sunburn.

This doesn't mean that you shouldn't use a higher SPF (especially if you are fair-skinned and/or have reddish or naturally blonde hair). It just means that the percent of protection increase and even the basic protection you thought you were getting is much less.

Further, the SPF only applies to UVB rays, not the potentially more dangerous UVA rays.

So, enjoy your time out in the sun, but do so intelligently. That way you'll be able to enjoy the sun tomorrow rather than battling a nasty sunburn or something worse.