Bad chemistry.

In the third paragraph of ["The Truth in Beauty," Green Guidance, May/June] is the sentence, "These include zinc oxide and titanium dioxide, which are 'much less likely than [chemical] sunscreens to penetrate human skin,' according to biochemist John Knowland of Oxford University." I'll bet that John Knowland did not make the distinction your writers have made. Zinc oxide and titanium dioxide are inorganic chemicals while the other sunscreens are organic chemicals. Both are chemicals! Organic compounds are often more easily absorbed by the skin than inorganic chemicals, but not always. I would guess the writers didn't take much chemistry in college.

DOUG HAWES

Dallas, Texas, USA

Mindy Pennybacker responds: I should have described zinc oxide and titanium dioxide as inorganic sunscreens and benzophenone, homosalate, and octymethoxycinnamate as organic sunscreens; both types are chemicals. I have checked with Dr. Knowland, who confirms, "The available published evidence does indeed indicate that the inorganic sunscreens zinc oxide and titanium dioxide are much less likely than organic sunscreens to penetrate skin."