London: Scientists looking at different types of bags and their impact on the environment have found that cotton bags offered by many supermarkets may be less "green" than plastic carriers.
The government sponsored research, ``Life Cycle Assessment of Supermarket Carrier Bags`` by Dr Chris Edwards and Jonna Meyhoff Fry looked at the environmental impact of six different types of bags.
They found that cotton bags may cause more global warming, as a greater amount of energy goes into making a cloth carrier than a polythene one.
And that a cotton bag has to be used 131 times before it has the same environmental impact like its plastic counterpart.
And if a plastic bag is re-used as a bin liner, a cotton bag has to be used 173 times - nearly every day of the year - before its ecological impact is as low as a plastic bag on a host of factors including greenhouse gas emissions over its lifetime.
But researchers found that most of us only use the bags around 51 times before they are thrown away.
Paper bags, used by some clothes chains such as Primark, need to be used three times to fall below the environmental impact of the thin plastic carrier.
While bags for life, made of stronger plastic, have to be used four times to start having less ecological impact.
The report said using a thin plastic bag, made from a plastic called high-density polyethylene (HDPE), equates to generating 1.57kg of carbon dioxide, the greenhouse gas that scientist believe leads to global warming.
A cotton bag would have to be re-used 171 times to emit the same level of CO2.
Cotton bags typically made in China have a greater environmental impact because of the water and fertiliser required in their production, as well as their transportation and greater weight.
"The HDPE bag had the lowest environmental impacts of the single use options in nine of the 10 impact categories," the Daily Mail quoted the researchers as saying.
"The bag performed well because it was the lightest single use bag considered," they said.
Plastic bags have also come under fire for using up oil and for littering the countryside and fouling the marine environment for wildlife.
However, the research found that biodegradable bags made of starch were not a greener option than HDPE bags as they are less environmentally friendly to make and heavier.
"In practical terms of global warming potential, eutrophication [a form of river pollution] ozone layer depletion, toxicity and ecotoxicity the current starch polyester blend bag is significantly worse than conventional single-use options due to the high impact of raw material production on those categories," the authors wrote.
An Environment Agency spokesperson said the report focuses on the greenhouse gas emissions of manufacturing different types of carrier bags.
"Much of the environmental impact of these bags is associated with the primary resource use and production," the spokesperson said.
"The final report due to be published in the next two weeks, will show that all multi-use bags - plastic, cotton or paper - need to be reused on multiple occasions to justify the additional carbon footprint of their production.
"If they are, then their overall carbon footprint can be less than single use plastic bags," the spokesperson added.