Sunlight consists of two types of harmful rays: ultraviolet A rays, or UVA, and ultraviolet B rays, or UVB. UVA rays, which can pass through window glass, penetrate deeper into the dermis, the thickest layer of the skin. UVA rays can cause suppression of the immune system, which interferes with its ability to protect against the development and spread of skin cancer. UVA exposure can lead to signs of premature aging of the skin, such as wrinkles or age spots.
UVB rays are the sun's burning rays, which are blocked by window glass, and are the primary cause of sunburn. A good way to remember the difference between these two types of ultraviolet rays is that UVA rays are the aging rays and UVB rays are the burning rays. Excessive exposure to both forms of UV rays can lead to the development of skin cancer. Therefore, sun-safety strategies, such as limiting sun exposure and applying sunscreens that absorb or block UV rays, are important.