TORONTO - The growing popularity of reusable grocery bags could pose a health risk to Canadians by increasing their exposure to dangerous bacteria, says a study commissioned by the plastics industry released Wednesday.

The Canadian Plastics Industry Association hired two independent labs to conduct what it said was the first study of so-called eco-friendly grocery bags in North America, and found 64 per cent of them were contaminated with some level of bacteria.

Forty per cent of the reusable bags tested had yeast or mould, and some had detectable levels of coliforms and fecal intestinal bacteria when there should have been none, said Dr. Richard Summerbell, who was commissioned to evaluate the lab findings.

The problem is similar to a situation where bacteria can be transferred from kitchen countertops and cutting boards to foods, and the more waterproof the reusable bag is, the more likely it is to become a breeding ground for bacteria, Summerbell said.

"Something that's a plastic weave, it takes longer for the moisture level to drop down, and so you can get what microbiologists call a bio-film building up ... a population of bacteria and possibly some yeasts, or even in severe cases, mould growth as well," he said.

"The main actual hazard involved is if there's a little bit of spillage in there from some meat or some eggs, then food-poisoning organisms could be transferred over to other food."

The study also warned of other potential health problems if the reusable bags are used to carry gym clothes or diapers in addition to groceries, which could lead to exposure to the superbug called community-acquired MRSA (methycillin-resistant Staphylococcus aureus).

"The bacterium may enter grocery bags if they are reused to carry athletic equipment," Summerbell said.

"People are so acclimatized to using single-use bags that they throw away, they don't connect bags with hygiene."

The World Wildlife Fund, which worked with grocery chains such as Loblaws to convince retailers to charge five cents for each plastic bag to discourage their use, said the concerns raised in the study could be addressed by washing the reusable bags.

"It's a valid point, but I don't think we need to go back to disposing billions of plastic bags across Canada," said WWF spokesman Steven Price.

"We need to take care of the objectives, which is to be safe and healthy, so let's wash the bags."

Loblaws started charging Toronto customers for plastic bags in January, and in April stopped giving them away free across the country to meet its goal of diverting one billion plastic bags from landfill sites by the end of this year.

The company issued a statement Wednesday saying it stands by its reusable bags.

"Food items placed into reusable bags, for the most part, do not come in direct contact with the bag given their product packaging," Loblaw Companies vice-president Inge van den Berg said in an email.

"Perishable items are put in plastic bags or other types of wrap packaging prior to being placed into the reusable bag, providing minimal opportunity for contamination of the bag."

Summerbell said people need to be educated on the need to properly clean their reusable grocery bags.

"Governments really should think about some standards for how these bags are to be hygienically handled, actually doing studies that validate cleaning procedures," he said.

Ontario's Environment Ministry said Wednesday it is reviewing the findings of the study.