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Sunburn, Skin Care and Cancer Prevention - How to Read Sun Block SPF Numbers

Updated on June 12, 2017
Patty Inglish, MS profile image

Patty has advanced degrees in preventive medicine and health psychology, with 35 years of work in allergy and other autoimmune treatment.

The Sun Sends Harmful Radiation Our Way


Sun Burn is Serious

What are SPF numbers and what do they mean?

This is important to me, because I recently incurred a painful sunburn, because I forgot to wear sun block at an all-day outdoor activity on a clear, sunny Saturday.

The temperature measured in the upper 70s F that day and a consistent breeze wafted through the crowd. I wore a hat and avoided a sunburned scalp, but my face and forearms were a bit sore by 4:00 PM and very red later in the evening. This problem could have been avoided with sun screen.

Have you ever been badly sun burned as a child?

As a nine-year-old, I did not know what sun block was, and my parents did not use it. After a day outside without a hat and in a kid's bathing suit, I was so sore, that the sheets on the bed hurt my skin. Interestingly, Astronaut Scott Kelley has reported that same kind of skin pain from being in outer space for 340 days ending in early March 2016 - skin so sensitive that everything hurt.

I received no treatment for my skin burn and it was torture. The first two days were the worst, after which, I could ignore the pain. However, I later learned how serious a sun burn can be and am thankful that I did not suffer infections or other complications.

Do Not Ignore Sun Burn!

Sun burned skin can become infected and lead to the cancer melanoma later, whether an infection occurs or not. Sun burn coupled with heat exhaustion or heat stroke can be deadly.

The Sun Can Burn Even On A Cloudy Day

Unexpected Skin Burn

One year, I was at a picnic in overcast weather on an August afternoon. Even though I was wearing a V-neck blouse and a hat, plus sun screen, the V-neck area burned anyway. The SPF rating of sun screen is important in preventing sun burn.

Dermatologists around the nation have declared the month of May "Skin Cancer Detection Month."

Wear a hat and sunglasses in the sun.
Wear a hat and sunglasses in the sun. | Source

As the SPF value increases, sunburn protection increases.

What is SPF?

SPF means sun-protection factor.

Most broadly speaking, the SPF number ratings, multiplied by 10, indicate the number of minutes that the protective effects of the sunscreen or sun block will last on average.

For your individual protection requirements, you need to consider your skin tone (light to dark), geographic location, length of exposure to the sun, and season of the year. Winter or a cloudy day can still be a season of sun block and sun glasses, depending on these factors. As with most products, sun block does not work for everyone.

On average, for times other than during the midday hours of most extreme and intense heat:

Most broadly speaking, SPF 5 lasts 50 minutes of protection and you must reapply every 50 minutes. During a summer in August in Florida or Arizona, a higher SPF would be required.

  • That's less than one hour and time consuming. (I believe that SPF 5 sunscreen is actually no longer made, but have seen SPF 8 = 80 minutes.)

SPF 15 lasts 150 minutes, or 2 hours and a half.

  • SPF 15 filters out or blocks 92% of UVB rays from the sun. It allows an individual that would normally burn in 10 minutes to avoid a burn for 150 minutes, on average.

SPF 30 lasts 300 minutes, or 5 hours.

SPF 90 lasts for 900 minutes, or 15 hours.

SPF ratings are also available at 40, 45, 50, 60, and 100 in local drug stores and big box retail outlets. Prescription sun block may come in even higher ratings, so ask your health care provider if you think you might need one of those.

Cold Weather Sun Blocking

If you are outside in Alaska above the Arctic Circle during the Midnight Sun days of 24-hour sunshine, you need 90 SPF, even though it is cold outside.

Ambient air temperature has nothing to do with the sunburn situation. UV rays are radiation that causes sunburn as they pass through any clouds, the air, and even through your clothing.

One would not likely be outdoors for 15 hours at subzero temperatures, but should use the highest SPF available if outdoors for an extended period of time. Apply it once and keep it with you. If you are stranded outside, you'll have it for additional protection.

Ambient air temperature has nothing to do with sunburn.

What are UVA, UVB, and UVC?

  • UVA Radiation was wrongly considered safe in the 1980s and used in tanning beds, but UVA actually does the most harm. UV = Ultra Violet.
  • UVB Radiation is prominent in summer months in any Earth hemisphere. Sun screens formerly blocked only UVB radiation, until we found out that UVA is even worse for the body.
  • UVC Radiation is largely absorbed by the earth's Ozone Layer, one reason that the Ozone Layer is so important. Prolonged UVC exposure is fatal.

How Sunburn Happens

An umbrella will help!
An umbrella will help! | Source

Radiation Is Dangerous

Remember the victims of the Hiroshima atom bombing? Some of them turned bright pink that night and died.

UVB radiation burns usually manifest more fully in the evening. If a sunburn is painful and/or widespread, seek professional medical help at once. Sunburned skin can become infected, just like other skin wounds. Sunburn is a skin wound.

A national UV Forecast is presented at the National Weather Service of major cities in the United States. Before you go outside, check the UV forecast as you might consult any other weather event. The seriousness of the UV ratings used are as follows:


0 - 2 = LOW - Sunscreen should still be used.

3 - 5 = MODERATE

6 - 7 = HIGH

8 - 10 = VERY HIGH

11+ = EXTREME: The very young and the very old should likely not go out in the sun at all.



Please do not take radiation lightly. Use Sun Block and wear UV-Blocking sunglasses.

Radiation will age the skin and eyes, diminish eyesight and skin elasticity, and cause skin cancer in many individuals. Skin cancer does not often stay on the skin! It can enter the bloodstream and spread to vital organs, causing death.


© 2008 Patty Inglish MS


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