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Sunscreen Explained: What is SPF and How Much Do You Really Need?

Since ancient times, humans have protected themselves from the scorching sun. Back then, it was purely for comfort. It took thousands of years to realize that beyond discomfort, excessive exposure to sun causes physical damage, including premature wrinkling and skin cancers. It was not until 1918 that sun exposure was first linked to skin cancers. It would still be many decades before society took note.

First, we had to go from preferring pale skin to our love affair of bronzed skin and introduction to bikinis. Our beloved Coco Chanel had much to do with popularizing the tanned look after falling asleep on a yacht in the early 1920s. As a style icon of the time, her preference of being tan influenced generations to come. Until the late 1970s, a tanned complexion was still the hallmark of health and attractiveness, skin cancer from sun was barely spoken about, and sunscreens were still used primarily as a way to spend more time in the sun rather than protecting from skin cancers.

It was not until the 1980s that “suntan lotion” was replaced with “sunscreens” and the Skin Cancer Foundation’s Seal of Recommendation was born. In order to dig deeper into current sunscreens, we have to understand their primary action and what they are protecting us from.

What does SPF mean ?

As an esthetician, I hear clients proclaim they use SPF 100, and consider that a perfect excuse to stay in the sun all day. SPF - Sun Protection Factor - measures the fraction of ultraviolet rays blocked during exposure. An SPF 15 would block one-fifteenth of radiation from reaching skin, assuming sunscreen is applied at two milligrams per square centimeter of skin. This is about six tablespoons of sunscreen for an average body – way more sunscreen than anyone I know applies.

Another way of looking at SPF is the length of time a person would burn without sunscreen and multiplying that by the SPF number. For example, if a light-skinned person generally shows signs of redness after 10 minutes in the sun, the idea is that an SPF of 15 would afford that person 150 minutes in the sun, except that by then most sunscreen ingredients have started to break down. It is generally understood that protection lasts for roughly two hours before it must be re-applied.

The original SPF measured effectiveness against sunburn caused primarily by UVB rays. However, there are two types of wavelengths that affect our skin: UVA and UVB.

UVA is the longest and causes a milder sensation than UVB, but UVA rays travel deeper into the skin. These rays are primarily responsible for tanning and pigment irregularities, such as hyper- and hypopigmentation, and are now believed to cause genetic mutation in the sublayers of skin, which ultimately plays a role in premature aging and cancers.

UVA rays are the predominant rays in tanning beds. UVB is known to be the primary cause of actual sunburn and its damage is immediately visible on the skin’s surface. It plays a key role in skin cancer formation, and contributes to photo-aging and tanning.

In the United States, UVB rays are most intense between 10 a.m. and 4 p.m. from April through October. In high altitudes, they bounce off of snow and ice and reflect back 80 percent of the radiation, causing a double-hit on skin, remaining a concern year-round.

UVA rays can penetrate glass while UVB cannot. This explains hyperpigmentation through car windows when we have not even gotten pink.

There is a massive difference between SPF 15 and broad-spectrum SPF 15. Even in that simplified comparison, the actual formulation of ingredients and quality varies drastically from brand to brand. We are still not sure if sunscreen will actually prevent the deadliest skin cancers. Although some can be prevented through smart sunscreen use, this should not be the only approach.

What exactly is in your sunscreen, how does it work and how else can we protect our skin and health from sun damage?

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