The solar spectrum is divided into color ranges defined by the frequency of the light. Infrared light is the frequency just below visible red light. We experience this light as heat. Ultraviolet (UV) light is the frequency just above visible violet light. This light causes sunburns and tanning, as well as photo aging (wrinkles and spots) and most skin cancers. This occurs because UV light is ionizing, meaning that it can kick an electron out of an atom. This leads to unwanted, possibly permanent chemical changes in the tissue.
Ultraviolet light is further divided into ultraviolet A (UVA), B (UVB) and C (UVC). Much has been said about the ozone layer blocking UV light. The fact is that the ozone layer does not block UVA (the tanning rays). It does block most of the UVB (the burning rays). The ozone layer filters out all of the UVC, which is fortunate since UVC is lethal. Because of the filtering effect of the ozone layer, 99% of the ultraviolet light that reaches the earth’s surface is UVA.
UV intensity is greatest at midday and greater in the summer than the winter since the sun is more directly overhead. Elevation is also important: every 1000 foot increase brings a 4% increase in these damaging rays. Snow reflects 85% of the UV light, increasing the risk to skiers. Since sand and water are also excellent reflectors, beach umbrellas block only 60% of the UV light. Clouds and fog block infrared and visible light more effectively than UV light. This is why it is possible to damage your skin on a dark, cool, overcast day.
It was once thought that the tanning rays (UVA) where not biologically active and that protection from the burning rays (UVB) was all that was necessary to prevent photo aging and skin cancer. Now, this has been shown not to be the case. Any amount of tanning is harmful to skin including tanning from the popular, but dangerous, tanning lamps.
In order to help consumers choose between the various UV screens on the market, the SPF (Sun Protection Factor) system was adopted in the US in 1978. A sunscreen with an SPF of 15 allows 15 times as much exposure before burning. Therefore a person who might otherwise burn in 20 minutes would burn in 300 minutes (SPF 15 x 20 minutes ). A “water resistant” product is one that maintains its SPF rating for up to 40 minutes of immersion, a “very water resistant” product maintains its rating up to 80 minutes. (Systems used world wide are comparable and one should feel comfortable purchasing sunscreens while abroad.)
Sunscreens screen only the burning rays (UVA) and use the SPF system. Sunblocks also block the tanning rays (UVA and UVB). There is no system to quantify the amount of protection from tanning rays offered by a product.
Sunblocks must be applied properly to obtain maximum protection. Effectiveness is dependent on when, how much and how often it is applied. If a person who burns in 20 minutes applies the product 10 minutes after exposure then the person has only 150 minutes (SPF 15 x 10 minutes) of time left before burning. Creams tend to come off over time and reapplication every two hours is recommended, more frequently if swimming or perspiring. When used properly one ounce of product should cover an adult for one application. A three to five ounce tube should last an adult one day at the beach.
It is important to look for products that protect your skin from both UVA and UVB. All products on the market with an SPF label block UVB. Products that block UVA have ingredients such as benzophenone, oxybenzone, sulisobenzone, avobenzone, titanium dioxide, or zinc oxide. The most effective UVA and UVB blocker is zinc oxide. The following products contain a transparent zinc oxide and are recommended as excellent all around sunblock products:
- Iguana Kids Block SPF 30 & 45
- Iguana Sport SPF 15 & 30
- Oil of Olay Complete
- Forte Daily Protector SPF 15
- No Add Baby SPF 45
- Iguana SPF 15, 30, & 45
- Sun Sense products
- MD Formulations Total Daily Protector SPF 15 MD
- Skinceuticals Z-20 SPF 20 & 30 Sunblock
We can help with a personal skin evaluation. Please ask the doctor or a nurse for answers to your skin care questions.
An excellent, reasonably priced, sunblock from Dermesse is available here.