just found this website, very useful and reliable.

http://www.aad.org/SkinCancerNews/SafeSunTips/sunscreenfacts.html

the following are some important points:

1.Unless indicated by an expiration date, the FDA requires that all sunscreens be stable and at their original strength for at least three years.

2.Sunlight consists of two types of harmful rays ?UVA rays and UVB rays. The UVB rays are the sun抯 burning rays (which are blocked by window glass) and are the primary cause of sunburn and skin cancer. UVA rays (which pass through window glass) penetrate deeper into the dermis, or base layer of the skin. They also contribute to sunburns and skin cancer. Both UVA and UVB rays can cause suppression of the immune system which helps to protect you against the development and spread of skin cancer.

3.The SPF number on sunscreens only reflects the product抯 screening ability for UVB rays. At present, there is no FDA-approved rating system that identifies UVA protection. Scientists are working to create a standardized testing system to measure UVA protection.

4.It抯 important to find a sunscreen that offers both UVA and UVB protection and includes ingredients such as benzophenones, oxybenzone, sulisobenzone, titanium dioxide, zinc oxide, and avobenzone (Parsol 1789).

[此贴子已经被作者于2004-7-7 22:56:02编辑过]