A motorist pumps fuel into his vehicle at JJ's Express Gas Plus station in Phoenix gas station in Phoenix, Arizona August 10, 2011. REUTERS/Joshua Lott

(Reuters) - Just when you thought filling up your car could not hurt any more, researchers may have found another reason to avoid touching the gas pump: germs.

Gas pump handles turned out to be the filthiest surface that Americans encounter on the way to work, according to a study released on Tuesday by Kimberly-Clark Professional, a unit of personal hygiene giant Kimberly-Clark Corp.

A team of hygienists swabbed hundreds of surfaces around six U.S. cities to see what everyday objects are breeding grounds for the worst bacteria and viruses.

The top offenders, following gas pumps, were handles on public mailboxes, escalator rails and ATM buttons.

Closely following on the filthiest list were parking meters and kiosks, crosswalk buttons and buttons on vending machines in shopping malls.

“It comes down to the fact that nobody cleans the things that you’re going to touch on a daily basis,” said Dr. Kelly Arehart, program leader of Kimberly-Clark’s Healthy Workplace Project.

Testers analyzed swabs of the surfaces for levels of adenosine triphosphate, which signals the presence of animal, vegetable, bacteria, yeast or mold cells, and the high levels found suggest they can be transmitting illness, researchers said.

Swabs were taken in Atlanta, Chicago, Dallas, Los Angeles, Miami and Philadelphia.

Arehart’s colleague Brad Reynolds said germs from people’s hands can transfer seven times before leaving the skin. People should wash their hands as soon as they get to work, he said.