1. A Plymouth experiment: some bags still carried loads after 3 years, while others vanished in 3 months
In 2019, a research team at the University of Plymouth in the United Kingdom placed bags labeled "biodegradable" in soil and the sea to observe what happened [10]. After more than three years, some of these bags were still strong enough to carry a full load of groceries. This suggests that a label alone does not guarantee quick disappearance.
On the other hand, bags labeled "compostable" disappeared from a marine testing device within 3 months. However, when those same compostable bags were placed in soil, they were still present after 27 months. "Compost" refers to a system where microorganisms turn food waste into compost [10].
Even bags with similar names had very different results depending on where they were placed. It is important to note that this was a single test. We cannot say for certain that "biodegradable bags always remain," but these results provide a starting point to verify what the term promises.
2. What "biodegradable" means: plastics that break down only under specific conditions
According to a roadmap compiled by the Japanese Ministry of the Environment and other agencies, biodegradable plastics are materials that function as plastics but also break down into carbon dioxide and water through the action of microorganisms in nature, provided certain conditions are met [1].
The key phrase is "under specific conditions." The definition does not say they break down anywhere, at any time. In Japan, the Japan BioPlastics Association (JBPA) sets the standards for labeling biodegradable plastics. The cover of their standard document notes the first edition was published in April 2000 [3].
For a plastic to be certified as compostable, all organic materials making up 1% or more of the weight must show a biodegradation degree of 60% or more in a specified test [3]. This 60% is a benchmark for the "degree of breakdown" measured in the test. It does not mean the plastic only breaks down 60% and stops there.
3. Why plastic made from plants does not always break down
Plastics are classified by their raw materials or by their properties after use. "Biomass plastics" use renewable resources like plants as raw materials. "Biodegradable plastics" have the property of being broken down by microorganisms. Both are collectively called "bioplastics" [1].
Because the raw material and the breakdown property are separate issues, some plant-based plastics do not break down. For example, polyethylene made from plants (Bio-PE) or PET made from plants (Bio-PET) may not biodegrade. Conversely, some plastics like PBAT are made from petroleum but do biodegrade [1].
In Japan, about 70% of biodegradable plastics distributed were biomass-derived in fiscal year 2018, based on shipments of products with identification marks. The rest primarily use fossil resources like naphtha [1]. This 70% figure applies to Japan, not the whole world.
Biomass plastics that do not break down can remain in composting or biogas-making processes, negatively affecting them. Therefore, "made from plants" does not automatically mean "returns to nature" [1].
4. How breakdown differs between hot industrial compost and ordinary soil
Biodegradability is considered separately for each environment, such as industrial compost, home compost, soil, and the ocean. Industrial compost uses specialized facilities that maintain high temperatures for fermentation. Home compost has lower fermentation temperatures and smaller quantities [1]. The national roadmap advises choosing biodegradable plastics that match the specific environment where they will be disposed of.
Breakdown tests are also conducted under set conditions. The test for biodegradation in compost (ISO 14855, JIS K 6953) involves a high temperature of about 58°C (about 136°F). The degree of breakdown is calculated from the amount of carbon dioxide produced, as explained by testing agencies [5].
For example, polylactic acid (PLA) is known to break down in conditions like compost, but hardly breaks down in ambient temperature environments, the sea, or the deep sea. However, the National Institute for Environmental Studies reports that yeast capable of breaking down PLA at ambient temperature has been found in rice leaves [7].
Combining materials or adding plasticizers and additives can also hinder biodegradability [1].
5. Which plastics broke down and which did not in the deep sea and the ocean
A research team from the University of Tokyo and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) placed biodegradable and general-purpose plastics at five locations on the deep seabed, at depths of 757 to 5,552 meters (about 2,484 to 18,215 feet), for 3 to 14 months. They reported in January 2024 that biodegradable plastics other than polylactic acid were broken down by microorganisms, while general-purpose plastics and polylactic acid were not [8].
This shows that not all biodegradable plastics break down even in the deep sea; polylactic acid is an exception. Whether breakdown occurs depends on the combination of material and environment.
Standards for the ocean have also been created. The JBPA requires that organic materials making up 1% or more of the weight show a biodegradation degree of 90% or more in a specified marine biodegradability test. This is a stricter standard than the 60% for compost, and the first edition of this system was published in July 2023 [4].
Kaneka’s PHBH received certification from a Belgian agency in September 2017, stating it reaches a biodegradation degree of 90% or more within 6 months in seawater at 30°C (about 86°F). This is a result from a laboratory test at 30°C, not a guarantee that it will disappear in the same time in the actual ocean [9].
Meanwhile, a 2015 report by the United Nations Environment Programme (UNEP) pointed out that complete biodegradation of plastics rarely occurs under conditions met in the sea. It concluded that even if "biodegradable" labeled products spread, the amount and danger of plastic waste entering the ocean would not decrease significantly [6].
Safety Note: The UNEP report notes that complete biodegradation requires conditions rarely met in the sea, such as industrial composters or sustained temperatures above 50°C (122°F). Do not assume labeled items will disappear in nature. Do not litter.
6. What biodegradable plastics are meant for and why they are no excuse for littering
The national roadmap envisions uses for biodegradable plastics in products that might unintentionally enter the environment, such as collection bags for food waste turned into compost or biogas, mulch films that can be plowed into farmland, fertilizer coatings, and some fishing gear [2]. The idea is that if food waste and bags can be processed together, waste can be reduced. However, as of 2018, there were only 101 facilities nationwide in Japan that compost or turn food waste into biogas [2].
Global plastic production was about 360,000 thousand tons per year in 2018. Of this, biomass plastics (non-biodegradable) had a production capacity of about 1,198 thousand tons, and biodegradable plastics about 912 thousand tons, totaling about 2,110 thousand tons. This is about 0.6% of the total, or roughly 1/170 of the whole [1]. This percentage is calculated from the figures above and is an older statistic from 2018. In Japan, bioplastics made up about 0.5% of domestic plastic input in 2018.
The national roadmap warns that consumer misunderstanding—"biodegradable plastics break down naturally, so it is okay to litter"—must not encourage littering. Regardless of whether a plastic is biodegradable, you must not litter [1]. Additionally, if biodegradable plastics mix with other waste, they can currently become contaminants in recycling.
7. How you can investigate biodegradable plastics yourself
You can start by looking at bags and containers at home or in stores. Search for words or marks like "biodegradable," "compost," or "biomass." Check if the label refers to the raw material or the breakdown property. If it refers to breakdown, see if it specifies the location where it breaks down.
You can also check your town’s waste sorting guide to see which category these bags and containers fall into. Even if you find them, do not bury them in soil or throw them in the sea or river. Dispose of them according to local rules.
To verify definitions, you can read the identification standards from the Japan BioPlastics Association (JBPA) [3] and the roadmap from the Ministry of the Environment [1].