Sunscreen and Sunblock

Sunscreen and Sunblock Facts

  • The sun produces ultraviolet radiation that can result in undesirable changes in human skin.
  • A limited amount of sun exposure can produce beneficial effects to the body as a whole.
  • Skin darkening (tanning) in response to exposure to sunlight is a sign of potential skin damage.
  • Chemicals or clothing placed on the surface of the skin that either absorb or block ultraviolet solar radiation can act as a sunscreen or sunblock.
  • The major long-term undesirable effects of sunlight on skin are the development of skin cancer and photo-aging.
  • Sunburn is an unpleasant and painful immediate short-term effect of excess exposure to sunlight.
  • There are naturally occurring substances in the skin, melanin for example, that offer a degree of protection from ultraviolet radiation.

History of Sunscreen

Originally sunscreens were looked upon as preventatives for sunburn. Over the last 50 years, our understanding of the negative effects of ultraviolet light has evolved. It was believed that the sun was beneficial for skin, producing a darker coloration and an enhanced feeling of vitality. Now it is known that extended exposure to sunlight has very limited beneficial effects and will cause wrinkling and produce mutations that predispose to almost all cases of skin cancer.

How Does Sunscreen Work?

Sunscreens work by absorbing or reflecting ultraviolet light so that it does not reach the living cells of the skin or the deeper, collagen-rich dermis.

The Meaning of SPF

SPF stands for Sun Protection Factor. It is determined in the laboratory by exposing human skin to an ultraviolet light source producing a fixed amount of energy and determining the length of time it takes for the skin to turn red. This is then compared to the length of time it takes for the skin to turn red after sunscreen has been applied to it. For example, if it takes one minute for the skin to turn red without sunscreen and 50 minutes for the skin to turn red when coated with a specific amount of sunscreen, the SPF equals 50. Please note that this test does not measure the sunscreen's ability to prevent photo-aging or skin cancer. Conveniently, it turns out that the higher the SPF, the better the sunscreen is at preventing skin cancer and aging.


The spectrum of ultraviolet light produced by the sun has been separated into three divisions. Generally, the only ultraviolet light we need to worry about is light between the wavelengths of 290 nm to 400 nm. Wavelengths shorter than 290 nm never reach the earth's surface because they are absorbed by the atmosphere. Wavelengths longer than 400 nm are visible light and seem to have very little effect on the skin. Wavelengths between 290 nm-320 nm are designated as ultraviolet B rays (UVB) while wavelengths from 320 nm to 400 nm are ultraviolet A rays, or UVA. Although the quantity of UVB light varies and makes up only 5% or less of all UV radiation (depending on time of day and latitude), UVB is most responsible for the sunburn response and coincidently the most efficient at producing mutations in the DNA of skin cells as well as producing wrinkles by damaging collagen. That is not to say the UVA cannot produce mutations and skin cancers along with wrinkles. It is just not as effective.

Sunscreen vs. Sunblock vs. Suntan Lotion

The use of the terms screen, block, and tan with the prefix sun certainly complicates our understanding of what these products can do. Currently, the only term allowed by the FDA is "sunscreen." Sunscreens must have documentary evidence that they either absorb or physically block ultraviolet light. The only real sunblock is presumably a physical substance like clothing that is entirely opaque to ultraviolet light. The term suntan lotion probably means virtually nothing except as an advertising device.

Types of Sunscreens and Ingredients

Sunscreens categories are rather subjective. One way of dividing them is by what wavelengths they block. Broad-spectrum sunscreens block ultraviolet light throughout the ultraviolet spectrum (UVA and UVB). Physical sunscreens utilize very minute bits of metallic salts like zinc or titanium to block ultraviolet light. Chemically based sunscreens use the ability of chemicals to absorb the ultraviolet light.

Safety of Sunscreens

Although public anxiety is periodically raised by the publication of poorly documented concerns about the safety of the chemicals used in sunscreens, there is no compelling evidence that sunscreens pose any danger. On the other hand, there are massive amounts of evidence that ultraviolet light is potentially dangerous to human skin. There is abundant evidence that the use of sunscreens prevents wrinkling and skin cancers.

The Perfect Sunscreen Should...

  1. Last all day after one application
  2. Be waterproof so it stays intact after sweating or bathing
  3. Be entirely nontoxic
  4. Be entirely non-allergenic
  5. Not produce eye irritation
  6. Block 100% of ultraviolet light -- it should be broad spectrum.
  7. Be cosmetically elegant
  8. Be very inexpensive

Proper Application of Sunscreen

The major problem with sunscreen is forgetting to apply it. There are certain criteria recommended by the U.S. FDA for labeling of sunscreens, including the permission to use the term "water resistant." This information is not immediately applicable to our normal daily activities. Sunscreens should be applied to skin not covered by clothing that is likely to be exposed to sunlight. This includes the forearms, the rims of the ears, and the top of the head (if you are somewhat hair compromised).

When and How Often to Apply Sunscreen

Sunscreens should be applied prior to spending more than 15 or 20 minutes in direct or reflected sunlight. Since sometimes we really do not know how long we will really be exposed, it is not a bad idea to apply it first thing in the morning. If one expects to be getting wet, it seems prudent to apply it frequently since even if it is water resistant it will disappear from the skin surface over time.


Sun Safety Quiz IQ

How to Prevent Sunburns, Skin Cancers, and Photo-Aging

Although sunscreens are prohibited from making claims regarding their ability to prevent skin cancer and skin aging, they are able to do these things reasonably well. One must remember however that avoiding the sun, sitting in the shade, and wearing opaque clothing is better than relying on sunscreen alone. Developing a tan is only a sign that the skin is damaged by ultraviolet light. The protective effects of tanning are poorly documented. If your skin is darkly pigmented naturally, this is a genetic gift that can protect you from some of the dangerous effects of ultraviolet light. The use of sunscreens lower than SPF 50 makes little logical sense aside from the mistaken belief that there is a significant health or social benefit to tanning. In addition, it is important to use a broad-spectrum sunscreen that blocks both UVA and UVB rays. Currently, there is no consensus on the grading of the ability of sunscreens to block UVA radiation. Some people believe that darker skin is more esthetically pleasing than lighter-colored skin. There are coloring agents available that do a creditable job at producing a faux tan.

Sun Protection for Babies and Children

Since babies and children do not understand the damage ultraviolet light can produce, it is our duty as parents and adults to do our best to limit their exposure within reasonable limits. The use of shade, clothing, and sunscreens to prevent tanning and sunburn requires vigilance. It is not recommended to use sunscreen on infants younger than 6 months of age, as keeping young infants out of the sun is preferred to using sunscreen. Use sunscreen only if sun exposure cannot be avoided in young infants.

Sunscreens and Vitamin D Deficiency

Aside from a feeling of well-being and skin darkening, the major benefit of ultraviolet light is to induce the synthesis and storage of vitamin D in the skin. There is no question that if one is darkly pigmented, lives in a northern latitude, and practices sun avoidance it is possible to develop vitamin D deficiency. This can be avoided by taking vitamin supplements containing sufficient amounts of vitamin D. The American Academy of Dermatology recommends taking 1,000 international units daily. In addition, vitamin D-fortified foods like milk are helpful. Although there is accumulating evidence that vitamin D may have important effects on cancer prevention and bone health, the optimum blood level is as yet unknown.

Does Sunscreen Expire?

Many manufactures print an expiration date on the product container. If there is no expiration date, then it is assumed that the product will last three years from the date of purchase.

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Medically reviewed by Joseph Palermo, DO; American Osteopathic Board Certified Internal Medicine


Burnett, Mark E., and Steven Q. Wang. "Current Sunscreen Controversies: A Critical Review." Photodermatology, Photoimmunology & Photomedicine 27. Chichester, U.K.: John Wiley & Sons A/S, 2011: 58-67.