Plastic is undoubtedly a problem. It has countless uses and is inexpensive to produce, but it remains an environmental scourge and requires fossil fuels for its production. To make matters worse, a host of new studies are also finding that microplastics are becoming increasingly linked to a range of health issues.

In addition to seeking ways to reduce our reliance on plastics, especially single-use ones, scientists are exploring biodegradable alternatives. These are praised for their “environmental friendliness” because they break down more easily.

But a recent study by scientists at Southern Medical University in Guangzhou, China says even they may pose unforeseen health risks.

“Our investigation was inspired by the urgent need to replace fossil-derived plastics with biodegradable alternatives and the associated health implications,” explained Zhenlie Huang, the study’s lead author in an email. “While much focus has been on the environmental and economic benefits, the potential health risks of biodegradable plastics remain uncertain.”

Tina Bürki, group leader at the Swiss Federal Laboratories for Materials Science and Technology (Empa) and an expert in nanoparticle biological safety, who was not involved in the study, agrees.

“The human health impacts of micro- and nanoplastics are still poorly understood, and responses are likely to vary between tissues and organs,” she said. “A better understanding of the origin of adverse responses and the underlying toxicity mechanisms […] may help achieve a better understanding of adverse outcomes or potential for disease development from microplastic exposure.”

Do biodegradable plastics produce microplastics?  

A stark reality, say Huang and his team of researchers in their current paper published in Advanced Science, is that biodegradable plastics might still be contributing to a growing microplastics problem. These plastics can only be fully degraded back into their constituent molecules under specific conditions, requiring certain temperatures and microorganisms to complete the process.

This may still lead to the formation of micro- and nanoplastics if the biodegradable polymers don’t break down fully, says Huang, posing significant health risks.

To figure this out, the team investigated the effects of incomplete degradation of a specific biodegradable plastic polymer known as polylactic acid (PLA) — one of the first renewable plastics capable of competing with conventional plastics. “Particularly, given their potential to degrade into smaller, possibly more harmful oligomer nanoparticles within the body,” added Huang.

Rising rates of gastrointestinal illnesses, such as colon cancer in younger age groups, have been loosely linked to the ubiquity of microplastics in our environment. To explore this, Huang and his team studied what happens when the products of PLA’s incomplete degradation make it to the gut as well as how they get transformed and spread to other parts of the body.

PLA, when degraded properly, breaks down into lactic acid — a molecule generally considered safe. But incomplete degradation results in oligomers, which are shorter sequences of the repeating monomer units.

“The main differences between the degradation processes of PLA polymer microplastics and oligomer microplastics in the gastrointestinal tract are the extent of their degradation and the resulting bioavailability [the proportion in active circulation],” explained Huang. “PLA polymer microplastics undergo incomplete degradation, leading to the formation of oligomer nanoparticles, which increases their bioavailability and exacerbates their toxicity [due to their smaller size].”

PLA oligomers don’t fare much better. “Our experiments show that oligomer microplastics exhibit greater [cell toxicity] than polymer microplastics of the same size, indicating that the transformation into oligomers [during incomplete degradation] further enhances PLA toxicity.”

Parkinson’s-like neurotoxic effects

The PLA polymer and oligomer microparticles were found to share the same known toxic mechanisms. In their mouse model, the PLA microplastics caused Parkinson’s disease-like symptoms, which occurs by increasing a protein called MICU3 in their midbrains, leading to harmful calcium buildup in neurons.

“This overload can result in […] dysfunction and increased neurotoxicity, as it ultimately triggers harmful cellular processes in neurons,” said Huang. “The complete degradation of PLA oligomer microplastics into lactic acid in the gastrointestinal tract can reduce their bioavailability and toxicity, thereby mitigating the overall toxic effects associated with PLA microplastics.

This suggests that ensuring the complete degradation of biodegradable plastics could lessen their harmful impact on human health.

“The study provides important mechanistic insights into the biotransformation, biodistribution and health impacts of biodegradable micro-/nanoplastics, which is crucial to guide a fact-based plastics discussion, to develop interventions to mitigate potential harmful exposures, and to support the safe development and use of plastic products,” said Bürki.

But she adds that the results need to be replicated in humans before any concrete claims can be made. “Due to differences in physiology between species, it needs to be confirmed if similar neurotoxic effects and hallmarks of Parkinson’s-development (e.g. alpha-synuclein aggregation) from PLA microplastics exposure may also occur in humans,” she elaborated.

“Moreover, future studies should consider other relevant exposure routes, such as inhalation, and include more realistic secondary bioplastic particles reflecting real-world exposures (i.e. heterogenous mixture of different particle sizes, shapes, and additives.”

According to Huang, while the potential neurotoxic effects are significant, the potential toxic effects in the gut, liver, and other organs also needs further exploration. “Our biodistribution results show that oligomer nanoplastics, resulting from the incomplete degradation of PLA polymer microplastics, are found in nearly all major organs of mice,” Huang said.

“This indicates that oligomer nanoplastics [from biodegradable plastics] could potentially impact various human organs, highlighting the importance of controlling the environmental presence of discarded biodegradable plastics,” Huang concluded.

Reference: Zhenlie Huang, et al., Gastrointestinal Incomplete Degradation Exacerbates Neurotoxic Effects of PLA Microplastics via Oligomer Nanoplastics Formation, Advanced Science (2024). DOI: 10.1002/advs.202401009

Feature image credit: Nicolas Arnold on Unsplash