BOSTON  The next time a man hands you a hot dog after making a run to the restroom and the concession stand at an Atlanta Braves baseball game, be careful: there is a good chance he did not wash his hands, according to a report released Monday by a group that sends spies into public restrooms in the name of science.

Only about two-thirds of the men observed washed their hands after using the restroom at Turner Field  the lowest rate for any of the locations cited in the observational study and survey on the hand-washing habits of Americans. The study, conducted every few years, was released by the American Society for Microbiology and the American Cleaning Institute at a microbiology conference in Boston.

Some subjects were asked about their washing habits in telephone interviews; others were watched by undercover observers in public restrooms. Some of what the sink spectators witnessed was, well, filthy. Consider: 20 percent of people using the restrooms at Pennsylvania Station and Grand Central Terminal in New York did not wash their hands.

The researchers, from Harris Interactive, stood in restrooms while pretending to fix their hair or put on makeup, said Brian Sansoni, a spokesman for the American Cleaning Institute, a trade group for producers of cleaning products. “After they took care of business, the observer checked whether or not they actually washed their hands,” Mr. Sansoni said.