Biodegradable plastics sound like the perfect eco-friendly alternative to landfills full of plastic containers, cups, and bags that last past eternity.

However, scientists at North Carolina State University have discovered that new materials like PLAs are a mixed blessing. They are definitely better than conventional plastics. But, they still have a negative impact on the planet.

When biodegradable plastics break down in landfills, they produce methane, a greenhouse gas that is substantially more potent than carbon dioxide. Only about one third of dumps capture methane and convert it into fuel. The majority let the gas escape into the environment, or burn it off as waste.

What’s more, the landfills that use methane capture equipment install it after garbage has been in a specified “cell” for two years. Most biodegradable plastics are designed specifically to break down in less than two years. The methane they produce is not contained even by the greenest waste facilities.

Scientists are quick to point out that this does not mean we should give up on biodegradable plastics. Instead, we should realize that like many new green technologies, they are a step in the right direction, but not a perfect solution. They suggest that we focus on biodegradable plastics that degrade more slowly, so that most of their methane release occurs after two years. Additionally, they highlight the need for more landfills to capture methane and convert it into fuel.