Bioplastics: Chemicals of Concern

Hardly a week goes by without a press release warning about plastics as a threat to human health.

We’ve been told to throw away black plastic household spatulas that will poison us with recycled flame retardants [1]. Or that the typical person ingests a bowlful of microplastics a month [2]. Or that in terms of chemicals of concern, bioplastics are as toxic as regular plastics. Even though several of these widely noted conclusions, including the black spatula warning, have been toned down or even debunked because they were drawn from over-interpreted, inaccurate or poorly analyzed scientific data [4,5], the sentiments have taken root. A recent survey in the U.S. and Canada [6] showed that up to 40% of shoppers are walking away from purchases in stores over worries that plastics are toxic and unsustainable. With conventional plastics raising such concern, this is time to promote sustainable plastics as a safer option, both in terms of environmental well-being and human health.

As for the health risks of plastics, a summary of plastic additives was recently published in Nature [7], which, while critically important for alerting the plastics industry about the wide-ranging use of additives, drastically overstates the health risks of bioplastics. Specifically, a background report by this team that was utilized for this Nature study concluded that a “comparison with conventional plastics indicates that bioplastics and plant-based materials are similarly toxic [8]”. How can this be??.

The Nature Review [7], “Mapping the chemical complexity of plastics”, with corresponding authors, L. Monclús and Martin Wagner from the Norwegian University of Science and Technology, summarizes a significant PlastChem report [9], “State of the science on plastic chemicals – Identifying and addressing chemicals and polymers of concern”.

This is a highly ambitious and important step in outlining the risks of chemicals that are associated with plastics, especially those used in day-to-day packaging. In it, over 16,000 plastic-associated chemicals are nominally identified, starting with the polymer backbone, and then including plasticizers, colorants, stabilizers, antioxidants, flame retardants, and preservatives. It also includes unreacted monomers, residual processing aids, contaminants (!) and non-intentionally added substances (NIASs) such as impurities and reaction by-products. Most noteworthy, a subset of the chemicals were termed as “chemicals of concern” as defined under four very broad hazard criteria: persistence, bioaccumulation, mobility and toxicity.

However, cataloging chemicals of concern in this way has resulted in overstated and erroneous conclusions, especially around biopolymers. Overstated conclusions start with poly(lactic acid), PLA, where “22 of 35” of the molecules detected were cited as chemicals of concern [7]. This kind of overreach is akin to treating all criminal suspects the same – from jaywalkers to serial killers – and locking everyone in the same jail cell.

The confusion begins with using a very simplified methodology to define toxicity, a methodology that deems lactic acid and lauric acid as “toxic”. And when people hear words like “toxic”, they automatically jump to “dangerous” and “scary”, rather than thinking of ingredients that you might find in your morning yogurt or that naturally occur in your muscles during your morning exercise.

How are these naturally occurring fatty acids deemed toxic? Toxicity was determined via a very simple test called a Microtox assay. In their Microtox protocols [8], off-the-shelf commercial packaging plastics were emptied of their contents, rinsed with water, cut into small (0.5–0.8 × 2 cm) pieces, and soaked in organic solvents to extract polymer-associated chemicals, and, of course, any contaminants carried by that polymer matrix. The solvent extractants were then mixed with culture media and fed to the bioluminescent gram-negative bacterium Aliivibrio fischeri in 96-well plates. Any changes in luminescence emanating from these gram-negative bacteria (relative to controls) meant that bacterial viability was affected and the extractant was deemed “toxic”.

Okay, lactic acid will be found in poly(lactic acid), PLA (see Table 1, a breakout listing of chemicals of concern derived from Nature’s supplemental data [7]). And, not surprisingly, lactic acid will fail a Microtox assay because it is a very effective natural antibiotic that acts specifically well against gram-negative bacteria. In fact, it is marketed as a natural preservative in organic foods to preserve meat, for example, so that the meat will still retain its USDA “organic” designation.

More bluntly, it is a gross disservice to define lactic acid as “toxic” when dieticians see lactic acid as a probiotic nutraceutical not as a hazardous chemical! According to WebMD [11]: “Lactic acid is a natural preservative often found in foods like yogurt, baked goods, and pickled vegetables. Along with making your food last longer, it can boost your health by strengthening your immune system”.

Lauric acid is also on the list (Table 1). Besides being a primary ingredient in coconut oil, it is used in polymer processing as a food-grade antistatic or mold release agent. Similar to lactic acid, lauric acid is a short-chain fatty acid that is seen by many, such as those following a keto diet, as a beneficial dietary aid. Interestingly, most of the bioplastics that this team tested previously including PHA fared poorly in the reported Microtox assays [8], likely because most bioplastics will release a range of short-chain fatty acids that would affect bacterial viability.

The next layer of uncertainty comes from the random sampling employed. According to reported protocols [10] they “…purchased the products in local retailer stores and confirmed their polymer types (most contained a recycling code) using Fourier transform infrared spectroscopy…”. First, they relied on recycling codes, which for bioplastics would be a “7”, the catch-all category. This means that they cannot rely on the recycling code to identify the bioplastic. And FTIR analysis is hardly considered a robust method for identifying biopolymers except for the simplest, pure samples. For example, in digging into the background spectra, several labelled as “PLA” appear to be missing the fundamentally critical reflection at 1750 cm-1. FTIR fails at characterizing blends.

For removing contaminants, the, “…content was removed from packaging samples, and the products were rinsed thoroughly with ultrapure water until residues were completely removed [10]”. However, it’s not that simple. This less-than-robust protocol of simply rinsing in water (even pure water) will not separate between an intentionally added additive versus a contaminant picked up somewhere along the line! Most bioplastics are not ideal moisture barriers. They are, at best, dynamic, ever-changing barriers that allow significant water permeation. Small molecules within a polymer matrix are not fixed but ebb and flow in and out of the polymer. Yes, a small molecule could be an intentionally added additive, but, just as well, it could be a contaminant picked up during processing, transportation, or, more likely, from the packaged goods themselves.

If a bioplastic is used to package cosmetics, detergents, shampoos, or even drinking water, the bioplastic could take up their ingredients, which include preservatives, UV stabilizers, surfactants, PFAS, and other contaminants from the goods inside. And there is no guarantee that they will be removed by merely rinsing them in pure water. Many products (especially cosmetics!) contain an array of “chemicals of concern”. To be blunt, the last two rows of Table 1 highlight an array of additives that reputable producers of sustainable plastics would not intentionally add to their packaging. Nine of these chemicals are phthalates and polyphenols that are more widely associated with polyvinylchloride pipes, fittings and water tanks, not food packaging.

Unexpected and interesting contaminants would definitely arise from packaged foods and beverages. For example, a biodegradable coffee pod would invariably take on the array of polyphenolic compounds from coffee, which contains hundreds of polyphenolic tannins. This includes a range of chlorogenic acids, all of which would migrate in and out of the biopolymer throughout its packaging lifetime. Clearly, before drawing conclusions from Table 1, a more robust study is required to analyze the source of plastic contaminants and not automatically assume they are derived directly via plastic packaging.

Interestingly, in an earlier report [9] virtually all of the bio-based and plant-based samples tested were deemed as containing “chemicals of concern”, including polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), PLA, and PHA, as well as starch- and cellulose-based plastics. In the Nature report [7], it appears that only PLA is cited for its “chemicals of concern”. What changed? Are the updated improvements due to (1) the randomness of their sampling protocols, (2) variations in their testing procedures, (3) more accurate bioplastic identification beyond FTIR and recycling codes, or (4) actual changes in the sustainable products that were tested? Or is it something else?

The team behind the Nature Review [7,8] recognizes their less-than-robust analytical analyses, and the uncertainties that arise. They report that 16,000+ chemicals are detected, but then note that, for “3,667 substances (23% of the inventory), defined chemical structures and properties are lacking in the public domain. These include substances of unknown or variable composition, complex reaction products or biological materials (UVCBs), mixtures and polymers [9]”. For bioplastics, it is likely that samples labelled as a specific bioplastic are likely complex blends that do not necessarily represent “the industry”. Let us hope that policymakers, regulators and consumers recognize this uncertainty before acting on the results of these studies, especially around bioplastics.

Also noted throughout the Nature review [7] are chemicals they deem as chemicals of concern but are recognized by regulators and the food industry as food-safe, GRAS [12]. This includes citrate esters, a class of plasticizers and additives which are allowed by the US FDA and EU regulators for food contact as well as for use in medical implants.

Finding safe and effective plasticizers has been a challenge for the sustainable plastics industry, with some formulators landing on citrate esters as viable plasticizers. They are generally plant-based, degradable and non-hazardous. An expert panel of independent toxicologists in 2014 concluded that 12 inorganic citrate salts, and 20 alkyl citrate esters are safe in the present practices of use and concentration [13]. However, more recently, one or two citrate acid esters have been shown to be “of concern” because they exhibited endocrine-disrupting activity in mice-feeding studies [14]. Specifically, acetyl tributyl citrate (ATBC) and, to a lesser extent trihexyl O-acetylacitrate (ATHC), were shown to exhibit some endocrine-disrupting behavior in mice. Notably, in that same study other citrate ester plasticizers including triethyl 2-acetylcitrate (ATEC) did not exhibit any negative impacts and were recommended as safe alternatives to bis(2-ethylhexyl) phthalate (DEHP), a well-established endocrine disruptor.

These latest results confirm that the field needs to constantly update its safety standards. However, while ATBC may now be a “chemical of concern”, this concern has not risen to the level encountered in some conventional synthetic plastics (let’s try to avoid all the criminal suspects being lumped into the same jail cell). A recent global review by Professor Leo Trasende at New York University’s Grossman School of Medicine reveals that DEHP and Bisphenol A, BPA, synthetic plasticizers that have been used in everything from food packaging to medical tubing, killed more than 356,000 people worldwide in 2018 alone [15].

With health concerns around microplastics and the global risks of plastics, it is time for the sustainable plastics industry to treat this as a “wake-up call”. Responsible packagers need to be as clear and open about the relative safety of their products as possible, avoiding “short cuts” that could allow rogue molecules to enter end-products. Due diligence is needed around all additives, similar to the due diligence that allowed the industry to become essentially PFAS free in just a few years.

Certification can play a role in safeguarding plastics. Manufacturers can enhance trust by maintaining the highest production standards and can take it a step further with protocols that include GreenScreen Certifications [16], Cradle to Cradle [17] as well as the EPA’s Recommendations of Specifications, Standards, and Ecolabels for Federal Purchasing. Where applicable BPI [18] or TÜV- Austria Belgium [19] can help ensure further bioplastic sustainability.

Conclusions from the Nature Review [7] warp the perspective of how relatively safe sustainable plastics are. Reputable suppliers of bioplastics produce thousands of safe, healthy items every day. Consumers should not lose access to healthier alternatives due to less-than-robust science or overstated conclusions. This is the time to let consumers know that healthier plastic options are available, healthier for both the planet and for our well-being.