Understanding UVA and UVB
Our purpose here is to explain the differences in UVA and UVB and to show the changes that have been mandated by FDA to call a sunscreen "Broad Spectrum." We will cover the following topics.
To better understand the difference between UVA and UVB, let's look at it this way. The sun has two properties that apply, heat and light. Let's start with heat. The suns rays are hotter at the equator and cold at the poles. Hotter in summer than in winter and hottest at mid day when the sun is overhead. This describes UVB. The intensity of UVB starts out at dawn very low and gradually increases till midday when the sun is overhead and then the intensity gradually decreases as the afternoon progresses. We could roughly say that UVB travels with the heat of the sun. When they advise us to stay out of the peak sun hours between 10am and 2pm they are actually talking about UVB intensity.
UVA is the opposite of UVB in these ways: UVA is weakest at the equator and stronger at the poles. That means that UVA is stronger in Canada than Florida. UVA relates to daylight and not heat. Daylight is fairly constant throughout the day. When you see daylight you are exposed to UVA. It remains fairly constant through the day as compared to UVB.
Fig 1 shows how UVB builds to a peak during the day and UVA remains fairly constant from dawn to dusk.
No one is saying that UVA isn't dangerous, however, the damage potential between UVB and UVA is huge. Human skin is very sensitive to a narrow band in the UVB spectrum. After that the sensitivity drops off dramatically. At the low end of the UVA spectrum the sensitivity is approximately 1000 times less than UVB and as it moves towards visible light it becomes 10,000 times less sensitive. Claims that UVA is the most damaging are simply not true.
Fig 2 shows the overall skin sensitivity of UVB as compared to UVA. Note that UVA diminishes as it gets closer to visible light.
SPF numbers can be misleading. The general public has been led to believe that the higher the SPF the more protection is provided. This is certainly true, however, the additional protection offered above an SPF 30 is minimal. AN SPF 30 will block 97% of UV rays, SPF 50 98% and SPF 100 99%. This difference is so small that the average person can't tell the difference between them. The FDA was wise to put the top limit at SPF 50.
A well designed SPF 15 will provide good protection even in tropical locations.
Fig 3 shows the protective capabilities of different SPF sunscreens.
Observing UVA and UVB outside the laboratory
The new FDA monograph creates a new sunscreen category called Broad Spectrum. Essentially, to classify as a Broad Spectrum sunscreen, protection has to be provided well into the UVA spectrum. They also added a host of new labeling laws, which will be covered at a later time.
Fig 4 shows the additional protection required by the new Broad Spectrum category.
There is a trade off everyone should be aware of with this new category. These sunscreens are more fragile, dissipate rapidly and must be reapplied every 2 hours. Reapplication isn't a problem for the average sunbather, however, active people may not always be able to stop to reapply. The choice is between an original sunscreen that will stay put all day or to use the new Broad spectrum and keep reapplying.
Protection of Sunscreens vs new Broad Spectrum products
Fortunately we have an excellent method to observe UVA outside the laboratory. UVA passes straight through glass while UVB is blocked and doesn't penetrate. See Fig 5.
Trucks and automobiles give us an excellent method to observe any damaging impact of UVA. Since UVB can't penetrate glass, all the driver and passengers receive inside is pure UVA, unless, of course, the window is rolled down. The trucking industry in the US started in the 1920's and expanded in the 1930's when trucks started competing with the railroads for freight. Truck drivers picked up skin cancer on their left arms and the side of the face and ear from riding with the window down. In the 1950's when air conditioning started to become common drivers started to roll up the windows and these injuries went away. As it turns out, there is no record of anyone riding in a car or truck with the windows up being injured by sun exposure.
Several years ago Sawyer spent a long weekend at an interstate truck stop in mid July. They mingled with the drivers young and old and ate the huge portions of food served. The drivers didn't look like they had any sun exposure. For the most part, they were pasty white and without a wrinkled complexion. The FDA settled for an instrument test for the protection offered by the broad spectrum products. They could never produce a change in human skin from UVA exposure that would serve as SPF indication.
While there is some damage from UVA, it is difficult to find outside the laboratory.