Which type of bag should you use to take your groceries home if you're trying to have as little an impact as possible on the environment?

The answer might seem obvious: a robust and durable bag that can be re-used over and over again.

"If you're able to do that over a long time frame, then you're going to have an environmental advantage," said Associate Professor Karli Verghese from the School of Architecture and Design at RMIT.

But the key words there are "long time frame". Because you might be surprised by the number of times you have to use these bags before they become more environmentally friendly.

How many times should reusable bags be reused?

It's a bit complicated, because there are different types of reusable bags out there and different ways of measuring environmental impact.

But overall, reusable bags need to be used at least 50 times in order for their environmental benefits to be realised.

That's according to a 2009 study led by Dr Verghese, which considered what goes into creating different types of bags (the materials and their weight, as well as energy and water) along with how each of them can be disposed (whether they can be recycled or whether they have to go into landfill).

Green bags must be used more than 100 times to be truly environmentally friendly. ( NMarkRoberts )

Not surprisingly, it takes much more material and energy to create thick reusable bags than the thin single-use high-density polyethylene plastic bags that are given out for free at supermarket checkouts — except in South Australia, Tasmania, the ACT and NT, where they're banned.

That's why Dr Verghese's study found that encouraging people to buy reusable bags without making sure these bags actually get reused an adequate number of times results in a worse result for the environment overall.

As an example, if polypropylene bags (those green bags that you can buy at the supermarket) are only used 52 times then their impact on global warming is actually greater than that of single-use plastic bags.

How many uses should you be getting out of reusable bags? The study from Dr Verghese puts the figure at 104 — that's weekly for two years.

So it might be worth keeping that in mind if you're tempted to buy new green bags when you forget to bring the ones you already own.

Which reusable bag is best?

Two of the most common reusable shopping bags are polypropylene bags and polyethylene terephthalate.

Which of them is better depends on which environmental impact measure you're looking at.

The polypropylene green bag has the lowest impact on most measures including global warming, water use, land use, the release of nutrients into waterways and consumption of minerals.

However, the polyethylene terephthalate has the lowest impact when it comes to solid waste generated and fossil fuels consumed.

Whatever you do, re-use and recycle

The environmental impact of plastic bags is reduced if they're used in other ways — as bin liners, for instance.

And once you're really done with them, Dr Verghese says you should "put them into a recycling collection system so they can be redirected away from landfill and be put back into the economy".

You can find recycling collection bins for plastic bags at the front of many supermarkets.

Supermarket shopping bags not the only culprits

The good work of using reusable bags at the supermarket can quickly be outweighed by the heavy plastic bags you're getting at other retailers.

"If you have a look at some of the plastic bags that are given out by fashion companies, they're pretty thick plastics," Dr Verghese said.

"There's a lot of focus on the supermarket shopping bag but there's lots of plastic bags that are given out at lots of other shops."

Natural doesn't necessarily equal better, either

You might think that paper bags are a more environmentally friendly option than plastic bags because they're not made from synthetic materials.

But it's a mistake to think of products that seem "natural" as better, because paper bags actually have a greater environmental impact than high-density polyethylene plastic bags. It's because of the production process as well as the amount of material needed per bag.

However, paper bags are the most recyclable and have less of an impact as litter.