About UV Rays
UVA:
Can pass through window glass.
Is not affected by a change in altitude or weather.
Is present all day and every day of the year.
Penetrates deep into skin layers.
Is 5% of the sun's rays.
Is 20 times more abundant than UVB rays.
Affects long-term skin damage.
UVB:
Cannot pass through window glass.
Causes sunburn.
Causes tanning.
Helps the body with normal vitamin D production.
Varies with the season. It is more intense in the summer than in the winter.
Varies with weather conditions.
Is more intense at midday than in the morning or late afternoon.
Is more intense at high altitudes and near the equator.
Is 0.5% of the sun's rays.
Is protected against by the sun protection factor (SPF) in sunscreens.
Is related to more than 90% of nonmelanoma skin cancer.
Is related to cataracts.
More:
How Sunlight Works
Sunlight arrives on earth in three forms: infrared (heat), visible light and ultraviolet. Ultraviolet light is classified into three categories:
UVA (315 to 400 nm), also known as black light, which causes tanning
UVB (280 to 315 nm), which causes damage in the form of sunburn
UVC (100 to 280 nm), which is filtered out by the atmosphere and never reaches us.
99% of the sun's UV radiation at sea level is UVA. It is the UVB that causes most of the problems related to sun exposure: things like aging, wrinkles, cancer and so on, although research is increasingly implicating UVA as well.
One of the interesting things about UV radiation is that it is reflected by different surfaces. These reflections can amplify the effects of UV exposure. For example, snow reflects 90% of UV light. That is why you can get snow blindness and severe sunburns from skiing on a sunny day. Sand can reflect up to 20% of UVB that hits it, meaning that you can get extra UV exposure at the beach.
On the other hand, certain things absorb almost all UV radiation partially or completely. Glass is one of these substances - many glasses are very good absorbers of UVB (which is why you may have heard that you cannot get sunburn in a greenhouse - just make sure it is glass and not plastic covering the greenhouse!). Most sunscreens use chemicals that have the same UV-absorbing properties.
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