Glass, aluminum, paper? What to know about alternatives to plastic.

The list of plastic substitutes seems to be growing longer by the day as companies come up with novel products such as cling wrap made from potato waste, seaweed-based food wrappers, and cassava starch bags.

That's in addition to efforts to package more products in everyday alternative materials, such as glass, metal and paper.

Yet, the world's plastic pollution problem has continued to worsen.

Work is underway to create the first global treaty to reduce plastic pollution. But experts say achieving that goal will probably involve, in part, developing better substitutes — a challenge that has appeared to vex many environmentalists and sustainability researchers.

That's because it hasn't been easy to replace plastic, a ubiquitous material that's inexpensive, robust and versatile.

"Plastics need to get fixed," said Michael Shaver, director of the Sustainable Materials Innovation Hub at the University of Manchester. "But doing that by simply switching to another material without considering the consequences of that is where that's dangerous."

There are 21,000 pieces of plastic in the ocean for each person on Earth

Conventional plastics are made from fossil fuels. But the problem with plastics, Shaver said, is less about the material and more about what is done with them at end of life.

"We haven't treated them with care," he said. "The lack of waste management of those materials is what creates the problem."

The little-known unintended consequence of recycling plastics

Much of the plastic that is produced does not get recycled. "That's not because people aren't putting the right thing in their bins," said Melissa Valliant, communications director for the Beyond Plastics advocacy organization. "It's because so much of our plastic products just cannot be recycled."

In the United States, recycling facilities typically can only effectively process No. 1 and 2 plastic. One peer-reviewed study of a recycling facility in the United Kingdom also found that 6 to 13 percent of the plastic processed there could end up being released into water or the air as microplastics.

However, other packaging materials can also come with recycling challenges, and some have disadvantages when compared to plastic.

"It's not that any of those solutions is bad, but there's not a panacea," Shaver said. "There's not a single solution which works for everywhere."

Here's a look at how some common plastic alternatives measure up:

Glass is made of natural materials such as sand, soda ash and limestone that are melted at high temperatures. Unlike plastic, experts say, glass is often easily reused and can be recycled many times without degrading in quality.

But glass is heavy, so moving it over long distances can drive up transportation costs, said Muhammad Rabnawaz, an associate professor in the School of Packaging at Michigan State University. The material can also be more prone to breaking than plastic, aluminum and paper.

And making and recycling glass are both energy-intensive processes, experts say. "Until we can couple that glass recycling to renewable energy, we’re at a risk of trading a waste problem for an energy problem," Shaver said.

Glass could, however, be the preferred choice in refill systems where transportation distances are short, he added.

Making virgin aluminum, which involves mining minerals such as bauxite, can be environmentally destructive and energy-intensive. But it has the benefit of being lightweight and recyclable.

"Aluminum is very difficult to make from the raw materials, so you must recycle it; otherwise there is no benefit," Rabnawaz said. Recycling of aluminum cans, for example, is estimated to save 95 percent of the energy required to make the same amount of aluminum from its virgin source.

But aluminum recycling, which involves melting down the material, can have its complications. Like glass, it can be recycled many times and still maintain its integrity, but aluminum cans are typically manufactured with a thin plastic coating on the inside that acts as a protective lining, Shaver said.

"What happens to that is that when you melt the aluminum down, it gets burned, so we’re actually burning the plastic bit and then we’re recycling the container," he said.

Paper, which is recyclable, is generally thought of as one of the most environmentally sustainable materials, said Laszlo Horvath, an associate professor and director of the Center for Packaging and Unit Load Design at Virginia Tech.

But recycling paper is an extremely environmentally damaging process, Horvath said. "It requires a lot of chemicals, it requires a lot of energy, a lot of water," he added. Similar to plastic, it can be challenging to maintain the quality of paper after it's been recycled, Shaver said.

Though a growing number of companies are finding more ways to use paper to package their products, experts say the material can fall short in some areas compared to plastic or aluminum. When it comes to packaging liquids, in particular, paper often isn't a good alternative material, Horvath said.

It's also difficult to recycle paper-based beverage containers, Rabnawaz added.

First, experts say, it's critical to understand what these terms mean. Using the label "bioplastic" or "biopolymer" typically indicates a material's source is something biological, which can include food products, food waste or agricultural waste, Shaver said.

"Bioplastics do not necessarily mean biodegradable or compostable," he said.

For consumers, it can also be hard to tell whether products marketed as biodegradable or compostable really are, he said.

"Many things are industrially biodegradable or industrially compostable, not biodegradable in the environment or in the ocean or home composting," he said. And because there can be different accreditations for products, that increases the risk of greenwashing, he added.

Regardless of the material, the key, Shaver said, is to think about what happens to packaging after people are done with it.

"It does not matter if something is recyclable, if it's not recycled," he said. "It does not matter if something is biodegradable, if it is not biodegraded. It does not matter if something is reusable, if it is not reused."