A few facts I found:
Some of the ingredients are organic compounds and therefore breakdown, hence the need for constant reapplication. There is still debate surrounding how often to reapply and whether you should apply before going into the sun versus while in the sun.
What I found on Wikipedia alarmed me a bit:
The ingredients that hurt:
- Benzophenone is easily absorbed by the skin.
- This chemical is present in most sunscreens. It tends to cause skin irritations and allergies.
- Benzophenone- 3 behaves like the hormone oestrogen and increases the numbers of oestrogen sensitive breast cancer cells. It also has the potential to disrupt the hormonal balance of users.
- Other products that harm include
- Homosalate and octyl-methoxycinnamate (also called octinoxate): this also acts like oestrogen in test tubes
- Padimate-O is a derivative of PABA. PABA was earlier widespread in sunsreens but caused irritations and was discontinued. Padimate-O is said to cause damage to the DNA which could cause cancer.
- Titanium dioxide application also indicates DNA damage. These last two are as yet only lab and not human and living animal experiments
- Diethanolamine (DEA) and associated compounds like triethanolamine or TEA may lead to cancer causing compounds if the sunscreen contains nitrites. The FDA agrees that this is possible and is attempting to examine this. They do not as yet acknowledge risk to users.
- Parabens including butyl-, ethyl-, methyl-, and propyl-paraben are used as preservatives in almost all sunscreens. They, like benzophenone act like oestrogen and therefore carry similar risk. Not using them means cutting out all sunscreen use.
- Synthetic fragrances could cause allergies or asthma.
And how about that SPF rating? Is it really possible to be protected from the sun x70? According to the Environmental Working Group:
"There are more high SPF products than ever before, but no proof that they’re better. In 2007 the FDA published draft regulations that would prohibit companies from labeling sunscreens with an SPF (sun protection factor) higher than “SPF 50+.” The agency wrote that higher values were “inherently misleading,” given that “there is no assurance that the specific values themselves are in fact truthful…” (FDA 2007). Scientists are also worried that high-SPF products may tempt people to stay in the sun too long, suppressing sunburns (a late, key warning of overexposure) while upping the risks of other kinds of skin damage."
As with any product you plan to put on skin, read the label. What goes on your skin typically goes into your body eventually.
Some of the best rated sunscreens? Badger and California Baby just to name a few.
Find your sunscreen on this site to find out what the ingredients are and how the EWP rates the safety of the product: http://www.ewg.org/2010sunscreen/finding-the-best-sunscreens/