The active ingredients in most sunscreens are chemical based, the most common ones being:  PABA, octinoxate, benzophenone-3 also known as oxybenzone, and homosalate.   When you're shopping for sunscreens look for words like, "plant based", "natural" etc which means the ingredients are likely to be the natural minerals zinc oxide or titanium dioxide.

One word of warning: these mineral sunscreens can often leave a very white film on your skin which does not rub in easily, even after a few hours of wear!  Although that's great for sun protection it's not so good for vanity protection.  The one pictured here however (UV Naturals), is fantastic!  It is 100% natural and leaves no film at all.  It's a tad greasy but it soaks in quickly.  It's not cheap ($37 for just over 5 oz) but look at it this way - cosmetic surgery to remove  sun damage will cost you a lot more :)