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Most of us think of the Sun as our friend.

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It helps plants grow,

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keeps us warm,

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and who doesn't love to lie on the beach on a sunny day?

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But for all of it's good qualities,

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the Sun can also be harmful in large amounts.

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That's why we invented sunscreen.

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The purpose of sunscreen is to shield the body

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from the Sun's ultraviolet rays,

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which have several harmful effects,

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including sunburn,

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aging,

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and skin cancer promotion.

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These rays are separated

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by their different wave lengths,

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into types such as UVA

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and UVB,

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which exert a variety of effects in the skin

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due to the absorption patterns of chromophores,

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the parts of the molecules

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responsible for their color.

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The primary two chromophores are hemoglobin,

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found in our red blood cells,

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and melanin, which gives our skin its pigment.

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We know that UVB rays cause the skin to burn.

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The role of UVA rays is less well understood

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and appears to have an effect

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on our tanning response,

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carcinogenesis,

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and aging.

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So, how does the sunscreen protect us from these rays?

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There are two basic types of sunscreen,

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physical and chemical blockers.

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Physical blockers, like zinc oxide

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or titanium dioxide,

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reflect the Sun's rays by acting as a physical barrier.

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If you've seen lifeguards with noses covered in white,

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then you know what this looks like.

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The same ingredients are primary components

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of diaper creams,

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where the goal is also to create a physical barrier.

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Historically, they haven't always been easy to apply

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and were conspicuously visible on the skin,

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but new formulations have made this less of an issue.

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Chemical blockers, on the other hand,

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absorb the Sun's rays.

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They deteriorate more quickly than physical sunscreens

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because their ability to absorb the Sun diminishes.

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Generally, these are more transparent

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when rubbed on the skin,

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but some people develop allergric reactions

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to some of the chemicals.

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Regardless of the type of sunscreen,

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all are subjected to testing

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to determine their sunburn protection factor,

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or SPF.

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This is essentially a measure of the protection

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that the sunscreen will provide from UVB rays

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before one begins to burn.

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But even if you don't burn,

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you still need to use sunscreen

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because unless you live in a cave,

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you're not immune to the effects of the Sun.

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It is true that darker skinned people

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and those who tan easily

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have more built-in protection from sunburns,

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but they are still vulnerable

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to the effects of UVA.

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Children under the age of six months,

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on the other hand,

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should have almost no sun exposure

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as their protective mechanisms

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are not fully functioning,

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and their skin is more likely to absorb

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any sunscreen that is applied.

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Wearing sunscreen helps protect

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against the development of all three types of skin cancer:

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basal cell carcinoma,

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squamous cell carcinoma,

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and melanoma.

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On a daily basis, the DNA in your cells

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is developing mutations and errors

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that are generally handled

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by machinery within your cells,

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but ultraviolet rays from the Sun lead to mutations

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that the cell may not be able to overcome,

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leading to uncontrolled growth

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and eventual skin cancer.

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The scariest thing about this

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is that usually you can't even see it happening

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until its too late.

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But if these concrete risks to your health

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are not enough to convince you to use sunscreen,

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there are aesthetic reasons as well.

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Along with cigarette smoking,

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sun damage is the leading cause of premature aging.

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Photoaging from chronic sun exposure

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leads to a loss of elasticity in the skin,

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in other words, making it look saggy.

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Take a look at this truck driver

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who's left side was chronically exposed to the sun

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and notice the difference.

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This is an important point.

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Car windows block UVB, the burn rays,

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but not UVA, the aging rays.

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It is recommended to use sunscreen daily,

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but you should pay special attention

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before prolonged sun exposure

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or when at the beach

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or among snow

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since the reflectivity of water and ice

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amplifies the Sun's rays.

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For these cases, apply about an ounce

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fifteen to thirty minutes before you go out

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and once again soon after you get outside.

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After that, you should reapply it every two to three hours,

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especially after swimming or sweating.

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Otherwise you should wear protective clothing

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with ultraviolet protection factor, or UPF.

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Stay in shaded areas,

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such as under trees or an umbrella,

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and avoid the sun at the peak hours

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of 10 a.m. to 4 p.m.

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And what's the best kind of sunscreen?

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Everyone will have their preference,

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but look for the following things:

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broad spectra,

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SPF of at least 30,

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and water-resistant.

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A light moisturizer with SPF 30

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should be good for daily use.

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Take note if you decide to use a spray.

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They take several coats to effectively cover your skin,

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like painting a wall with a spray can

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versus a paint brush.

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So, enjoy the sun,

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but enjoy it with sunscreen.