我们涂防晒霜, 是为了减少皱纹，老年斑，和防皮肤癌。

sunscreen 本身会致癌吗？可能会，也可能不会。但有毒性。

防晒即防UV晒。UV辐射线含有UVA，UVB和UVC。经过地球的臭氧层ozone layer后，UVC和大部分UVB被滤过。达到地面时，是95%UVA， 5%UVB，0%UVC。所以，UVB 会被厚云和窗玻璃过滤掉，但UVA可以穿透, 大阴天和开车时车内都是有UVA的，但基本上没有了UVB。

UVA is a silent killer, it causes all types of cancer and aging spots and wrinkles. UVA 的穿透力强，破坏深层细胞（the Dermis layer), 而UVB只到表层 epidermis layer（见图，来自网络） UVB causes sunburn, discolor on skin and some types of cancer. But，remember, UVB is the one that makes Vit D。

如果每天上班是在室内，我从来不用含SPF的 lotion. 因2小时后无效，反而吸入可能的毒性化学成份。若是周末去户外玩，应该用Sunscreen。选择最安全可靠的zinc oxide防晒霜。这是一层physical barrier，像是给皮肤穿了件薄衣，挡住了UVA和UVB，它不会被吸收到血液造成伤害。戴帽子和围头巾是绝对好的，可以达到SPF50的效果。coconut oil and shea butter 也是安全的，但是防晒效果差很多，比没有好。如果是去雪地，海滩，海水，UV比正常高出至少2倍（reflector)，一定每2小时要涂一次UVA/UVB sunscreen。

在保证有10多分钟晒太阳得到VitaminD后，戴帽子和围头巾，使用zinc oxide防晒霜是我的建议。对了，车窗上我也帖上阻断UVA的FILM，amazon有的。

以下来自网络。

new U.S. FDA data reveals chemicals in sunscreens are absorbed into the human body at levels high enough to raise concerns about potentially toxic effects.

Most sunscreens on the shelf use chemicals such as oxybenzone, avobenzone and octocrylene to block harmful rays.  However,  the chemicals, oxybenzone in particular, might disrupt normal hormone patterns in people。These molecules are chemical rings, essentially, and they absorb light。 Chemical rings are also the fundamental basis for a lot of hormones, and chemical rings tend to enter cells。Oxybenzone has been found in human breast milk, amniotic fluid, urine and blood, the FDA researchers said.

A few studies have suggested that parabens act like estrogen in the body, which can speed up the growth of breast cancer tumours。

A small number of studies have shown that retinyl palmitate can cause minor changes to cells when they are treated with UVA radiation。