Plant-based fabrics may be more toxic than polyester is to earthworms

Those bamboo sheets you splurged on may not be as environmentally sound as their label claims. A recent study finds that microplastics from biobased fibers like viscose and lyocell are more toxic to earthworms than those from petrochemical-based polyester (Environ. Sci. Technol. 2024, DOI: 10.1021/acs.est.4c05856).

There’s a push to switch from polyester to what might be perceived as more environmentally friendly or sustainable alternatives, like fibers made from the cellulose in plants. But there’s currently limited evidence to prove that these biobased materials live up to that perception, says Winnie Courtene-Jones, a marine biologist at Bangor University and the paper’s lead author.

“These fibers are entering the soil ecosystem primarily through the application of sewage sludge,” Courtene-Jones says. Used in clothing, bedding, and sanitary products, the microfibers get into sewers when they shed off materials during laundering as well as through inappropriate flushing of disposable wet wipes and similar items.

The multi-institutional team of researchers conducted two sets of experiments: one to determine lethal concentrations of each fiber type and another to study their effects over a longer period of time at environmentally relevant concentrations. The aim was to determine the ecotoxicity of biobased microfibers on Eisenia fetida, an earthworm species commonly used in such assessments. These worms, found around the world, are critical to soil health because of their role in organic decomposition, nutrient turnover, and aeration of soil along with the movement of water through it.

The first set of experiments showed that a 72 h exposure to high concentrations of polyester microfibers resulted in a mortality rate of about 30% for the earthworms. But for lyocell and viscose, the numbers went up to 60% and 80%, respectively.

In the second set of experiments, earthworms exposed to environmentally relevant concentrations of viscose for 28 days showed decreased reproduction rates compared with those exposed to polyester fibers. In earthworms exposed to lyocell, the researchers found reduced growth rates and more burrowing and churning activity.

Viscose and lyocell are two of the most popular biobased polymers that have emerged as alternatives to plastic over the past decades. Both are made from cellulose fibers derived from wood pulp, but they’re processed industrially in different ways. For the study, the scientists obtained undyed, untreated fibers directly from manufacturers and manually processed them in the laboratory to generate microfibers.

“It’s reasonable to assume that in the natural environment, we might see similar responses to these that we saw in the lab,” Courtene-Jones says.

One of the limitations of their study, she adds, is that it looks only at the toxicity on earthworms, not at how long the fibers persist in the environment. Plus, biobased microfibers present in the environment, like other kinds of fibers, have various additives and dyes incorporated into them, which may have other toxic effects.

Carolin Völker, a researcher at the Institute for Social-Ecological Research who wasn’t involved with the study, says, “In general, this paper shows that focusing solely on plastic as a harmful material is not sufficient, as alternatives also cause environmental effects, including ecotoxicological ones.”