13/08/24, 10:30
Biodegradable plastics remain a hidden source of harmful microplastics
by Victoria Corless | Aug 8, 2024
There is an urgent need to replace fossil-derived plastics with biodegradable alternatives
but do they solve microplastics’ health issues?
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
Biodegradable plastics remain a hidden source of harmful microplastics “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.