Sunscreen in the real world

Here’s an oldie, but a goodie: sunscreen use. While not an emergency situation, sunscreen is a topic often overlooked.

A quick review on the meaning of SPF

SPF: the ratio of the minimum erythema dose (MED) with sunscreen to the MED without sunscreen. So what does that mean? However long it would take for your skin to burn without sunscreen (the MED), a sunscreen with SPF 30 would mean your skin would last 30 times longer until it burned. Theoretically. The problem is that sunscreen is very rarely applied properly. In this post I will share some tips to get the most bang out of your SPF buck.

The SPF number holds true under lab conditions, but we all know that lab life rarely reflects the real world. A thickness of 2mg/cm2 is what is utilized for SPF testing. In an average person, that equates to about 1 shot glass full of sunscreen for the entire body. In several studies, the average thickness applied by volunteer beachers at best was 1mg/cm2, only half the recommended thickness.
To get the true extended protection, sunscreen should be applied approximately 30 minutes prior to activity and reapplied every 2 hours. However, it was found that a single reapplication after 20 minutes of exposure provided better protection that a reapplication 2 hours later. Another real world problem is uneven application with common missed places being eyelids, ears, the V of the chest, back of the hands, the calves, and dorsum of the feet.

So tips to get enough sunscreen?

Buy a large-mouthed jar and users will tend to grab bigger globs of it. Too goopy and sticky? The spray sunscreens provide a more even application and are much easier to use at the cost of thinner coverage. The convenience of the spray may draw in users who would otherwise skip out on sun protection because of sticky and time-consuming creams and lotions.

What about those “water resistant” or “sweat proof” formulations?

To claim being water resistant, the SPF measured after water immersion must be equal or more than 50% of the SPF before immersion, with no toweling. In other words, if a bottle boasts SPF 50, then after you go for a swim the SPF should be at least 25 or higher. It is recommended to reapply after swimming or towel-drying. This is where a spray sunscreen comes in handy.

If SPF 30 is good, then isn’t SPF 100 even better?

Technically yes, but practically speaking not really. Your bare skin takes 100% of the UV rays. SPF 15 would allow 7% (or 100 divided by 15) UV rays to hit your skin. SPF 30 allows 3%, SPF 45 allows 2%, and SPF 75 allows 1.3% of the rays. The incremental protection with increasing SPF past 30 is negligible and may not be worth the money for a pricier, “more protective” bottle.

TL;DR
You’ve probably been applying sunscreen wrong, if you have been at all.
SPF 30 is probably all you need.
You should technically use a shot glass amount for your whole body, don’t forget the back of your hands and top of your feet.
Apply 30 minutes before exposure and then again every 2 hours to get the protection on the label.
Sprays are okay if you’re lazy and don’t like greasy hands.

Happy adventuring,
Stephanie

Reference: Jungman, E., and H. I. Maibach. “Enhancing Sunscreen Efficacy in the ‘real’ World?” Journal of Dermatological Treatment J Dermatolog Treat 21.5 (2010): 261-66. Web.

About Stephanie Nguyen

Stephanie Nguyen is a third year medical student at Baylor College of Medicine in Houston, TX. She was born in Pennsylvania and lived in Mexico City for 7 years before moving to Texas. A proud Texas Aggie, she graduated with a degree in Accounting and no intentions on being an accountant. She is an avid rock climber and loves adventure. She is interested in EM because she likes the excitement and wants to be useful outside of the hospital when someone asks if there’s a doctor in the house.