SILVER SPRING, Md. – Reusable grocery bags may be good for the environment, but they could be making you sick.

With Montgomery County, Md., set to impose a tax on disposable grocery bags that’s designed to discourage their use, a new study has found that intestinal bacteria like E. coli flourish in unwashed reusable bags.

The study, published in the latest issue of the International Association for Food Protection’s Food Protection Trends magazine, tested 87 reusable bags obtained at random from shoppers in California and Arizona.

Charles Gerba, a professor at the University of Arizona who conducted the study, found that 8 percent contained E. coli, among other harmful food-borne pathogens.

Putting a reusable bag through the wash can eliminate almost 100 percent of the bacteria. But the study found that only 3 percent of shoppers surveyed had washed their reusable bags between uses.

Starting Jan. 1, Montgomery County will impose a 5-cent tax on all disposable bags, following a similar move by Washington, D.C., last year. One cent of each nickel will go to the retailer. The rest will go to litter cleanup.

The new tax aims to reduce pollution in streams that flow into the Potomac River, degrading the region’s principal source of drinking water, county officials said.

“Right now the county is spending about $3 million a year to clean up after these things. The fewer bags being used and circulated, the fewer will be thrown away,” Montgomery County representative Bonnie Ayers said.

“I don’t think that people always think of cleaning their bags, but it’s like any other item — it’s going to get soiled or dirty,” Ayers said. “If you don’t want to wash them, just wipe them out.”

Other precautions that can eliminate health risks include packing raw meats separately from fruits and vegetables to avoid cross-contamination and not storing reusable bags in the trunk of the car where high temperatures increase bacteria growth.

The bag study found that when reusable bags contaminated with meat juice were left in a trunk for two hours, the number of bacteria grew tenfold.