Fluoride in some toothpaste and mouthwash, believed to prevent tooth decay, may actually be ineffective as the so-called protective layer it forms is too thin, scientists say.

Experimental physicist Frank Müller and colleagues from Saarland University in Germany point out that tooth decay was a major public health problem worldwide, the journal Langmuir reports.Scientists have known that fluoride makes enamel - the hard white substance covering the surface of teeth - more resistant to decay, said a Saarland university statement.

New research found that the fluorapatite layer formed in this way was only six nanometers thick.It would take almost 10,000 such layers to span the width of a human hair. That s between 10 to 100 times thinner than what previous studies indicated.