Over 20 million tons of plastic are estimated to end up in the environment every year, with much of it breaking down into microplastics that are harmful to the health of humans and wildlife. Biodegradable and bio-based plastics made from organic material are often touted as more sustainable alternatives, but until now, scientists haven't had the tools to assess the impact of biodegradable plastics that are not disposed of properly.

A team of researchers from the Center for Industrial Ecology at the Yale School of Environment recently developed a valuable environmental impact assessment method to quantify the climate change and ecotoxicity impacts of biodegradable microplastics in the natural environment.

The study, published in Nature Chemical Engineering, was led by postdoctoral associate Zhengyin Piao and co-authored by Yuan Yao, associate professor of industrial ecology and sustainable systems, and doctoral student Amma Asantewaa Agyei Boakye.

Only 50% of bio-plastics are in fact biodegradable, and many biodegradable options are fossil-fuel based. Outside of the controlled conditions of a waste management facility, biodegradable plastics can have some of the same impacts as conventional plastics, including breaking down into small, problematic pieces. While they take less time to degrade, they also release greenhouse gases.

"There are a lot of people doing life cycle assessments for biodegradable plastics without being able to quantify impacts when those plastics enter nature," said Yao. "There just hasn't been any methodology available."

For the study, the team modified existing tools to model the fate of biodegradable microplastics in aquatic environments, creating a more dynamic method that can account for fluctuations in emissions as plastics degrade. They tested it using the five types of biodegradable plastics that dominate the global market, two of which were made from petroleum and three from organic material.

Yao noted that it is a common assumption that growing biomass absorbs enough carbon dioxide to offset emissions from disposed bio-based biodegradable plastics. However, the research team found that the release of methane as these microplastics degraded in the natural environment had greater global warming potential than the carbon uptake from biomass growth.

The study also showed that the tradeoff depends on degradation rate and microplastic size. Shifting from conventional options to alternatives that degrade faster may reduce ecotoxicity, but it could result in higher greenhouse gas emissions. The burden shifting did seem to disappear for smaller microplastics. For the smallest sizes tested—particles a million times smaller than an inch—the less biodegradable plastics had the highest emissions and toxicity.

"When plastic engineers try to design plastics, they often think higher biodegradability will definitely always be better," Yao said. "Our results show that it's not a linear relationship."

The researchers said they hope the study will inform the design of sustainable plastics and waste management systems moving forward. The team is now refining the model to scale it up for a global analysis.

"Doing this kind of large-scale analysis is really important to have a better vision of what strategies the plastic industry can take if it wants to reduce all these environmental impacts," Yao said.