A group of scientists at the University of California, San Francisco (UCSF) raised concerns about the "potential serious health risks" from the scanners in a letter sent to the White House Office of Science and Technology in April.
Biochemist John Sedat and his colleagues said in the letter that most of the energy from the scanners is delivered to the skin and underlying tissue.
"While the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high," they wrote.
More here from Agence France-Presse.
