Skincare Science:
Sunscreen

If I had to pick just one skin treatment to recommend, it would be sunscreen, hands-down. It may feel nice to have the warm sun on your face and the breeze in your hair while you sip a mimosa, but you should do it while wearing sunscreen. As detailed in the skin aging post, UV rays in sunlight damage collagen and elastin, giving skin a “leathery” look. UV rays also create Reactive Oxygen Species (ROS) which can damage cell DNA, leading to benign and cancerous skin growths.

Electromagnetic radiation

Electromagnetic spectrumThe full electromagnetic spectrum

First, some terminology. Radio waves, infrared, visible light, ultraviolet, X rays, and gamma rays are all types of electromagnetic radiation. The only difference between them is how much energy is contained in them. Types that contain a very large amount of energy, like X-rays or gamma rays, are particularly dangerous, because they can cause severe cellular damage. Visible light on the other hand, is quite safe. UV light, which has more energy than visible light but less than X rays, can also cause DNA damage. Fortunately, the sun doesn’t produce too much high-energy radiation.

Solar radiation spectrumSpectrum of radiation from our sun

Even better, X-rays, gamma rays, and cosmic rays from the sun (or from outside the solar system) are blocked by the Earth’s atmosphere. The ozone layer is responsible for stopping most of the UV radiation from the sun. However, some UV rays still get through, and these are the ones we have to worry about.

UV light

About 95% of the UV radiation that gets through the atmosphere is UVA, and the rest is UVB. UVB has more energy than UVA, and it has a greater propensity to cause sunburn and also causes more DNA damage. That being said, UVA can also indirectly cause DNA damage. Finally, both types of UV radiation damage the collagen and elastin fibers in the skin, causing skin aging and damage. Sunscreens can work either by reflecting and scattering the UV radiation, or by absorbing it.

Physical sunscreens

The sunscreens that work by reflecting and scattering UV radiation are called physical sunscreens. They just form a barrier on top of the skin, and are not absorbed into the skin. As long as the barrier is present, the sunscreen is working. Zinc oxide and titanium oxide are the two physical sunscreens. They are both a whitish cream, so they are quite visible when you apply them.

Chemical sunscreens

The original chemical sunscreen is, of course, melanin. This is the pigment produced by specialized skin cells, giving your skin and hair their color. Melanin absorbs UV radiation and releases it as lower-energy radiation. There are other organic chemicals that can do the same thing, and they are used in sunscreen. Unlike physical sunscreens, the chemical sunscreens are absorbed into the skin. As they are absorbed, they stop working, so chemical sunscreens have to be reapplied every 2 hours or so to remain effective.

SPF

Sunscreen protection factor (SPF) is a measure of how effectively a sunscreen prevents sunburn. It represents the fraction of sunburn causing radiation that is allowed to pass through by the sunscreen. So SPF30 means that 1/30th of the radiation is allowed through to the skin. Another way of putting it: if you are wearing SPF30, you will require 30 times longer to sustain a sunburn than you normally would. SPF is measured by actually applying it to a person’s skin, exposing them to UV light, and measuring how long it takes to cause skin redness and irritation! As mentioned above, UVA does not have much propensity for sunburn, so SPF does not measure UVA protection. But UVA can cause DNA damage, so even if your sunscreen is a high SPF, it may not be preventing damage from UVA radiation.

UVA protection

Measuring UVA protection requires more complicated techniques. In the US, if a sunscreen is labeled ‘Broad spectrum’, it has been shown to provide UVA protection proportional to its UVB protection. In Europe, sunscreens that provide UVA protection (equal to at least 1/3 of the SPF), have a ‘UVA’ seal.

Conclusion

There are a few things to remember about sunscreen:

  1. Wear a good sunscreen! This is incredibly important to reduce the risk of skin cancers, and to reduce the signs of skin aging.
  2. Reapply every 2 hours. Chemical sunscreens (which is what most people use) stop being effective over time.
  3. Make sure your sunscreen is labeled ‘broad spectrum’ or ‘UVA’ (if its from Europe).

Footnotes

1. EM spectrum image copyright 2011 Entirely Subjective and made available under an Attribution 2.0 Generic license
2. Solar spectrum image courtesy of Wikimedia