Joining lead on the list of unexpected and potentially health-harming substances found in popular over-the-counter vitamins sold in America is titanium dioxide, a naturally occurring oxide of titanium used in a wide range of consumer products, including cosmetics, paints, food coloring and sunscreens.

Although no long-term human testing has been done on titanium dioxide to date, the FDA filed a notice in late July soliciting public comments in the Federal Register on case studies involving nanoscale titanium dioxide use in sunscreens and water treatment applications. Unfortunately, the document's main intent is to create a foundation for "a long-term research strategy to provide the information needed for comprehensive environmental assessments," but not to draw any conclusions about potential environmental hazards of titanium dioxide.

That said, there's enough public information about titanium dioxide to prompt healthy concern. For one, consider this 2007 study funded by Arizona State University that found high concentrations of titanium dioxide were responsible for slowing the growth and development of one species of green algae. Additionally, the over-expression of stress genes in the presence of nanoparticles showed how titanium dioxide can be toxic to algae.

And, there was a 2006 ruling by the International Agency of Research on Cancer that classified titanium dioxide as a Group 2B carcinogen that could possibly be toxic to humans too.

Recent independent quality testing revealed that one in four supplements were either subpotent, tainted with harmful substances or both. Read which of today's most popular supplements failed quality testing here.

Federal Register July 31, 2009