Please please please don't make your own sunscreen. I know it seems as simple as adding some titanium dioxide or zinc oxide to a product to reflect the sun's rays, but it really isn't that easy.
I know some of you are dying to write to me and tell me I'm wrong, that your sunscreen works for you, and so on. But there is no argument you can make that will support you making what is considered a drug at home. This isn't as simple as making a lotion or conditioner. The worst thing that happens with those products is a little more dry skin or a bad hair day. We're talking sunburn, premature aging, and skin cancer as the worst case scenarios for a poorly made homemade sunscreen.
I have to say this again: There is no argument you can make that justifies making your own sunscreen. I know this because over the last 8 years I think I've heard them all, and none of them are reasons to make a drug in your kitchen. (If you have, however, had the SPF of the product confirmed by a proper lab with the credentials to perform such tests, I'd entertain the argument.)
Please check out this post for more information on all kinds of things relating to sunscreen!
If you'd like to learn more about SPF, check out my column in Handmade Magazine!
And check out this post by Realize Beauty about her experiences with testing her sunscreens!
And check out this infographic from Compound Interest about the chemistry of sunscreens!