Over the past year, Amazon has reduced the portion of shipments it packs in its cardboard boxes in favor of lightweight plastic mailers, which enable the retailing giant to squeeze more packages in delivery trucks and planes.

But environmental activists and waste experts say the new plastic sacks, which aren't recyclable in curbside recycling bins, are having a negative effect.

"That Amazon packaging suffers from the same problems as plastic bags, which are not sortable in our recycling system and get caught in the machinery," said Lisa Sepanski, project manager for King County Solid Waste Division, which oversees recycling in King County, Washington, where Amazon is based. "It takes labor to cut them out. They have to stop the machinery."

The recent holiday season, the busiest for e-commerce, meant ever more shipments — creating a massive hangover of packaging waste. As the platform behind half of all e-commerce transactions in 2018, according to eMarketer, Amazon is by far the biggest shipper and producer of that waste — and a trendsetter, meaning that their switch to plastic mailers could signal a shift across the industry. Other retailers that use similar plastic mailers include Target, which declined to comment.

The problem with the plastic mailers is two-fold: they need to be recycled separately, and if they end up in the usual stream, they gum up recycling systems and prevent larger bundles of materials from being recycled. Environmental advocates say Amazon, as the industry giant, needs to do a much better job of encouraging consumers to recycle the plastic mailers by providing more education and alternative places to bring that plastic for recycling.

"We are continually working to improve our packaging and recycling options, and have reduced packaging waste by more than 20 percent globally in 2018," said Amazon spokeswoman Melanie Janin. She added that Amazon provides recycling information on its website.