Papp and her co-author 鈥 Kimberly Oremus, a marine sciences professor at the University of Delaware 鈥 said they got the idea for their study after learning about beach, riverbank, and lakeshore cleanups organized by the nonprofit Ocean Conservancy. These volunteer cleanups go all the way back to 1986, and reports from each year document the number and type of plastic items collected across jurisdictions. In more recent years, participants have logged their item counts and types in a mobile phone app.
That standardized data could help fill an important research gap, Oremus said, on the connection between plastic bag restrictions and shoreline pollution. Prior scientific analyses had tended to focus on consumer behavior 鈥 for example, by counting the number of shoppers who emerge from a supermarket with plastic versus reusable bags. Some studies had focused on plastic bags clogging storm drains, since this can create a flooding hazard. 鈥淲hat we were missing was a direct measurement of the litter in the environment,鈥 Oremus said.
A small number of analyses looking at this had come from nonprofits, including the Ocean Conservancy, and had not undergone peer review, she added.
Papp and Oremus combined eight years of Ocean Conservancy鈥檚 data 鈥 constituting more than 45,000 cleanups across the US from between 2016 and 2023 鈥 with information on roughly 180 plastic bag bans and fees implemented between 2017 and 2023. They analyzed plastic bag collection in ZIP codes with and without plastic bag restrictions, and took into account differences in the bag policies, including whether they banned all bags or only certain kinds.
According to the analysis, plastic bags鈥 share of collected items increased over the study period: They represented a larger and larger fraction of all the pieces of plastic that volunteers picked up. But this increase was much slower in places covered by a plastic bag restriction, where volunteers collected 25 to 47 percent fewer plastic bags as a fraction of their total haul. The study showed the highest impact from state-level policies compared to local ones, and found that decreases in the share of plastic bags grew over time after bag policies went into effect.
The study looked at bags as a fraction of plastic items collected rather than the total number of plastic bags because this helped make the measurements more comparable between jurisdictions. 鈥淭his measure is not sensitive to the size and frequency of cleanups, fluctuations in overall litter, and other factors,鈥 Papp said.
The study also suggested that taxes 鈥 like a 10-cent charge per plastic bag 鈥 cause a greater reduction in shoreline litter than outright bans, though the researchers said this finding was inconclusive. Not a lot of jurisdictions have fees, Oremus said, so the sample size is small. And there could be explanations that extend beyond the fee itself: Washington, D.C., for example, uses revenue from its plastic bag fee to fund river and shoreline cleanups that might reduce the number of bags found by Ocean Conservancy volunteers. Oremus said it鈥檚 also possible that fees have greater coverage than bans 鈥 the latter sometimes apply to grocery stores but not to restaurants, for example 鈥 or that supermarkets and restaurants are less likely to flout a fee than a ban.
What is clearer, according to Papp, is that 鈥減artial bans鈥 aren鈥檛 as effective. These policies outlaw plastic bags below a certain thickness, on the basis that thicker bags can count as 鈥渞eusable鈥 or 鈥渞ecyclable鈥 and are less likely to become litter. Papp and Oremus鈥 study showed that jurisdictions covered by partial bag bans had the 鈥渟mallest and least precise鈥 effect on reducing plastic bag litter, potentially because consumers treated the thicker bags just like they had the thin ones.
Other analyses have shown that California鈥檚 partial bag ban led to an increase in the weight of plastic bags used per person between 2014 and 2021. The state closed this loophole last year by banning plastic bags outright, and Oregon followed suit with its own bag ban earlier this month. Lawmakers in other states, however, oppose bag bans altogether 鈥 at least 17 states have passed 鈥減reemption鈥 laws preventing their cities and counties from restricting the sale of plastic bags.
Susanne Brander, an ecotoxicologist and associate professor at Oregon State University, applauded the research, though she said it鈥檚 unfortunate that plastic bag bans have become so politicized that a scientific study is needed to back their effectiveness. 鈥淲e knew they were working, but this gives hard data to support that,鈥 she said.
Brander is also a member of the Scientists鈥 Coalition for an Effective Plastics Treaty, which is advocating for the international agreement 鈥 which will enter its sixth round of negotiations in August 鈥 to include legally binding limits on plastic production and the use of some types of plastic. One of the articles in the current draft of the treaty proposes restrictions on individual items like balloon sticks, plastic drink stirrers, and 鈥減lastic-stemmed cotton bud sticks.鈥 Brander said the new study makes 鈥渁 strong argument鈥 in favor of broader bans.
鈥淩ather than asking scientists to go and say you need to study Styrofoam containers separately, and study plastic takeaway containers separately, I think we should be able to apply these findings broadly to other bans of harmful products,鈥 she said.
Martin Wagner, a biology professor at the Norwegian University of Science and Technology who is also a member of the Scientists鈥 Coalition for an Effective Plastics Treaty, agreed with Brander. He also said the study could be used by U.N. member states to craft their own plastic reduction policies: 鈥淭hese political measures are often discussed in the absence of data 鈥 they just say, 鈥楲et鈥檚 ban some items,鈥欌 he said. Having concrete evidence that policies can reduce pollution will be 鈥渞eally helpful.鈥
Celeste Meiffren-Swango, state director of the nonprofit Environment Oregon, said the study in Science reinforces the recommendations of a report she co-authored last year. That report, 鈥淧lastic Bag Bans Work,鈥 estimated that five US policies in New Jersey; Vermont; Philadelphia; Portland, Oregon; and Santa Barbara, California, had prevented the use of 6 billion bags per year. Presumably, many of those avoided bags would have become litter.
鈥淭here are proven environmental benefits to passing plastic bag laws,鈥 she said. 鈥淚t鈥檚 not just that we want people to change their shopping habits.鈥