Sunscreens can be categorized as and Physical or Mineral, and Chemical or Organic. Physical sunscreens block UV rays and are ingredients like Zinc Oxide and Titanium Dioxide. Chemical sunscreens absorb the UV rays and are ingredients like avobenzone, homosalate, octinoxate, octisalate, octocrylene and oxybenzone. The most widely used chemical ingredient being avobenzone.
Chemical sunscreens have had bad press about causing hormone disruption. The ingredients have shown to break down to exposure to sunlight and must be stabilized, usually with a chemical octocrylene. As of now, the chemical ingredients Avobenzone and Homosalate have received FDA approval.
Many of the research you will find on Avobenzone will show that it is an endocrine hormone disrupting ingredient, and therefore it is toxic and to stay away. Although studies have shown this is correct, the reality is, this has been blown WAY out of proportion. The levels of causing the hormone disruption is "several orders of magnitude lower than that of natural estrogens". (Source: Environment International, July 2007, pages 654–669) Another common ingredient that has the same endocrine hormone disruptors is acetaminophen. The effects are so small, it is not a factor, as well as topical use stays on the surface of the skin, not penetrating internally.
As for how chemical sunscreens break down under sunlight, that is correct. However, all ingredients, physical and chemical break down, and this is why reapplication every two hours is needed on sunscreen. Chemical sunscreens can result in allergic reactions and irritations because of chemical filters, however only a small percentage of people are effected by the adverse effects.
How Chemical Sunscreens Work
Chemical sunscreens are colorless, orderless and usually runny that when applied start being effective after they have been absorbed by the skin, usually taking twenty minutes until it is effective. Mineral sunscreen offers protection only for UVB protection, making avobenzone an important ingredient to make a broad spectrum sunscreen. Free radical damage can occur with all sunscreens, and a Vitamin C serum to correct skin damage from free radicals should be used with sunscreen.
Here some information about each chemical ingredient:
Avobenzone - Also known as Parsol 1789 and butyl methoxydibenzoylmethane, it is FDA approved and protects for UVA rays, giving a broad range to the sunscreen.
Homosalate - An FDA approved ingredient, it protects mostly for UVB rays.
Octinoxate - Also known as octyl methozycinnamate protect for UVB rays.
Octisalate - Also known as octyl salicylate protects for UVB rays.
Oxybenzone - Also known as benzophenone-3 protects from UVB and for a small amount of UVA rays. It is used as a stabilizer to help the ingredients not break down. Controversy for this ingredient has been found for being a hormone disrupter. When used at the concentration allowed though it has not shown through studies to have any adverse effects. Because Oxybenzone is in a lot of products as a preservative, it can be found in urine in most people and mothers milk, but has shown to have no hazardous effects.