TEMPE, Ari.-You might think you're doing the planet good by using reusable grocery bags, but be careful!

Researchers at the University of Arizona conducted a study and found that 97% of shoppers who use those reusable bags, have never washed them.  75% say they don't use separate bags for meats and vegetables.  One-third of those surveyed say they use those bags for everything from storing snacks to toting books around.

The scary part? When researchers tested 84 of the bags for bacteria.  Half of the bags had potentially dangerous bacteria.  One in ten tested positive for E-Coli.

"I never even thought about that stuff.  I come home, wash my hands, always afraid to touch those baskets.  But I never thought about the bags," said shopper Alice Reeves.

The best advice for those who want to go green: keep using the bags, but throw them in the wash.