About two decades ago we got a “compostable” cup at a party for an environmental organization, with green writing about its being compostable, so we happily took it home and put it in our backyard composter. As of last summer, it was still intact, except for the green writing, which was mostly gone.
What are bioplastics? We need to start with a few definitions.
Biodegradable means capable of being broken down, especially into innocuous products, by the action of living things, such as microorganisms. The term can be misleading or confusing because of its lack of specificity (e.g., time frame and environment in which they can degrade).
Compostable refers to a product that can disintegrate into nontoxic, natural elements. It also does so at a rate consistent with similar organic materials. Compostable products require microorganisms, humidity, and heat to yield a finished compost product (CO2, water, inorganic compounds, and biomass).
Bioplastics or bio-based plastics are polymers produced from natural or renewable sources and may be biodegradable or nonbiodegradable. They are polymers derived from living things.
Bioplastics include a great variety of plastics made from biological materials (plants, microorganisms, lobster shells) instead of from fossil fuels, which are the base for conventional plastics. All plastics are polymers, meaning they are made up of repeated units of a smaller molecule — for example, many ethylene molecules (from fossil fuels) are strung together to make polyethylene. Among bio-derived plastics from plants is polylactic acid (PLA), which is one of the most common types. Other common bioplastics include polyhydroxyalkanoates (PHA) and polyhydroxybutyrate (PHB), made from carbon that bacteria have extracted from plants.
In addition to the basic polymer, many additional chemicals (“additives”) are always included in plastics to make them more malleable or durable, or to alter their characteristics in other ways, such as color, strength, water resistance, or heat and light resistance. More than 16,000 chemicals (many of which have never been tested for toxicity) can be added to bioplastics. Among them are metals, e.g., lead and zinc (which can be neurotoxic), and persistent organic pollutants such as bisphenols, flame retardants, PFAS, and phthalates, which are endocrine disruptors that may affect reproduction, embryo development, and immune systems.
How well do bioplastics decompose? While they all claim to be compostable, many of them, including the most common one, PLA, require industrial high-temperature composting, and do not degrade in home composters, soil, or water (explaining that long-lived cup in our compost bin). Most bio-based plastics never get to the necessary high-temperature industrial facilities because many areas (including New York and New Jersey) do not have such facilities or collection programs for bio-based plastic. Furthermore, most commercial and municipal composters in this country don’t even accept compostable packaging (46 of 173 report they do).
Thus, consumers are fooled into thinking that they are using something “green” and beneficial for the environment. Some facilities remove all plastic wrapping, including bioplastics, from food waste and landfill it, thus negating the supposed benefit of using compostable materials in the first place.
There are some bioplastics, however, that do degrade in environmental conditions. In one study, polyethylene (conventional plastic) was compared with “compostable” plastic bags (made of polybutyl terephthalate) in marine conditions. The compostable bags lost weight and tensible strength, some fragmenting in the aquariums, while the polyethylene bags did not change. But light breaking up the polymers can lead to new toxic chemicals being created, so that compost containing compostable bioplastics could be a source of environmental pollutants, potentially putting wildlife and human health at risk.
If or when bioplastics eventually decompose, how toxic are the products they decompose into compared to conventional plastics?
Some studies have shown that the breakdown products of bioplastics are about as toxic as those of conventional plastics, although some studies found bioplastic degradation products had either higher or lower toxicity. These differing results are a result of scientists investigating different bioplastics, comparing them with different conventional plastics, testing toxicity with different species, and examining different effects.
In addition, some studies used large pieces of plastic while others used thin films, microplastics, or leachates from the plastics. Therefore, it is hard to conclude whether bioplastics are more or less toxic than conventional ones, but they seem to be just as problematic.
However, the majority of the toxicity studies, examining effects on plants, small freshwater or marine invertebrates, worms, fish, etc., seem to indicate greater toxicity of bioplastics. Because the commonly used plant-based PLA, PHA, and PHB are brittle and lack durability, they require more additives and probably leach more than conventional plastics because they contain more chemicals. A study in 2020 found that chemical mixtures in bioplastic products caused stress in a variety of organisms. The researchers extracted 43 different bioplastics and found that two-thirds of the samples induced baseline toxicity, 42 percent produced stress, and 23 percent were endocrine disruptors, impairing male hormones. They found 41,395 different chemicals, including 343 “compounds of concern.”
Bioplastics are more likely than conventional plastics to degrade into microplastics and release their additives. Scientists found that the rougher surface of bio-based microplastics makes them more likely than conventional microplastics to fragment into smaller particles, and thereby release more additives, thus causing increased toxicity. In addition, chemicals in the environment (metals, PCBs, and other toxic chemicals in the water or air) adhere to the surface of plastics. These chemicals, as well as those within the plastic, can then be transferred into the animals that eat or breathe the microplastics, as the plastic is a “vector” for moving these chemicals into organisms and food webs.
In terms of being vectors for environmental chemicals, bio-based microplastics seem to be more effective than conventional microplastics, with a greater ability to accumulate and transfer pollutants in the environment. Another recent review had similar conclusions: that bioplastics tend to deteriorate into bio-based microplastics more rapidly than conventional plastics, and the bio-based microplastics have comparable or greater toxic effects than conventional microplastics.
The terms “compostable” and “biodegradable” cause consumers to think that a product is safe and is truly compostable in their backyards. But bio-based or bioplastics are not “green” or compostable at normal temperatures, and their breakdown products are often more toxic than conventional plastics. They are more expensive than conventional plastics and do not provide a solution to plastic pollution, but instead just add to the problems.
Judith S. Weis is a professor emerita of biological sciences at Rutgers University. She has a house in Springs.