Americans chucked more than 21 billion pounds of clothing and other textiles into landfills in 2015, according to the latest available estimates from the Environmental Protection Agency, a steep increase from 12.5 billion pounds in 2000 and 4.6 billion pounds in 1980. And that growing trash pile is just one of the apparel industry’s array of environmental problems—which also include water pollution and greenhouse gas emissions—fueled in part by the rise of the cheap, disposable clothing called “fast fashion.”

“The major impact of the industry comes from ever and fast-increasing clothing consumption that has further accelerated with fast fashion since the 2000s,” says Elena Karpova, a textiles scientist at Iowa State University and co-author of the third edition of the book Going Global: The Textile and Apparel Industry.

But as consumers become more environmentally aware, a new countertrend toward “sustainable fashion” has emerged over the past decade. Among its core tenets is secondhand shopping, which follows the “reuse” component of long-standing sustainability philosophy (along with “reduce” and “recycle”). Secondhand shopping has recently expanded from charity thrift stores to the digital age, with popular Web sites and apps such as ThredUp and Poshmark that aim to reach a broader market and to make buying and selling used clothing more convenient. Some of these venues specifically promote themselves as a more environmentally friendly alternative to buying new clothes and have created reports that cite third-party research and suggest secondhand fashion plays a key role in creating a less wasteful industry. While some studies do indeed back this claim, scientists are now trying to fill in some important gaps and get a more complete picture of how much finding a new life for discarded apparel can move the needle on reducing fashion’s environmental impact.

The global fashion industry used some 21 trillion gallons of water in 2015 alone, according to a 2017 report by Global Fashion Agenda, a nonprofit organization that encourages sustainable practices in the trade. And the apparel and footwear sectors combined accounted for 8 percent of global carbon emissions in 2016, says a report from Quantis, a consultancy that advises organizations on implementing environmental sustainability. These figures are on par with one 2018 study’s estimates of the emissions from the tourism industry worldwide.

In theory, reusing clothes avoids some of the water and fertilizer use, greenhouse gas emissions and other impacts that result from producing a new garment. A 2017 study in the Journal of Cleaner Production conducted life-cycle assessments on a cotton T-shirt, a pair of jeans and a polyester dress. It found that quadrupling the average life span of these items resulted in a 75 percent savings in freshwater used for dyeing and other processes. A 2018 review published in the same journal looked at 41 studies and found all but one concluded that lengthening a garment’s life by reusing it reduced its environmental impact. Research by Waste and Resources Action Program (WRAP), a sustainability advisory group in England, “shows extending the average life of clothes by just three months of active use per item would lead to a 5 to 10 percent reduction in each [item’s] carbon, water and waste footprints,” says Sonali Diddi, a design and textile researcher at Colorado State University. Those reductions can add up: A 2016 report commissioned by the Nordic Council of Ministers found that the reuse and recycling of textiles that are made in and exported from Nordic countries saves the equivalent of 425 million pounds of CO2 annually, along with 19 billion gallons of water. That is equivalent to the CO2 expelled by around 42,000 cars each year and the annual water usage of about 174,000 American households, according to U.S. government estimates.

But in order to successfully track all carbon emission reductions from clothing reuse, there needs to be a standardized carbon-accounting method for the entire textile industry, says Suzanne Greene, an environmental scientist at the Massachusetts Institute of Technology. And despite the fact that major clothing companies have pledged to aim for a 30 percent greenhouse gas reduction industry-wide by 2030, only two out of 45 top clothing companies have set sustainability goals that actually meet standards called for by the Paris Agreement, according to a recent report by the international environmental protection organization “The industry is doing a lot of work on reducing its environmental impact. But as a whole, measuring the impact of these actions against the environmental challenges we face, it’s clear that more needs to be done faster,” says Matthijs Crietee, secretary-general of the International Apparel Federation, which seeks to unite all apparel industry stakeholders to promote better supply chains.

The sheer number of variables that factor into the environmental toll of just one article of clothing—from growing or producing various fibers to transportation from factory to store—also means there are still gaps in the research that must be filled in order quantify how much the secondhand-shopping trend could potentially advance sustainability. The 2018 Journal of Cleaner Production review advised researchers to look at more impact categories such as water pollution, land use, packaging and transportation. Researchers such as Diddi and Greene have more specific lingering questions, including: Does buying secondhand clothing piecemeal versus using a monthly subscription service make a difference? How does the energy consumption involved in selling and distributing secondhand clothing compare with that of new clothing? And do differing laundry habits change secondhand clothing’s environmental benefits? “We know the impact is big and compounded with years,” Karpova says. “But it would be nearly impossible to measure all the numerous ways [secondhand shopping] affects the environment.”

As researchers work to understand the impact of policies—including those that encourage more secondhand shopping—to make the fashion trade more sustainable, the industry is poised to see major changes, says Rachel Arthur, chief innovations officer at Current Global, a fashion- and retail-trends consultancy. “Do I think we’re going to step away from fast fashion and the consumerist nature of our society entirely?” she says. “No. But I do think we’re going to have increasing choices [to encourage] consumers to shop more sustainably. And in doing so, that will make [companies] not addressing this quickly left behind.”