When you hear the term “plastic pollution,” what do you think of? If you’re like me, you might picture a pile of garbage on a beach, or microplastic beads inside a fish. Most of us are aware that plastic pollution can be found everywhere. But you might be surprised to learn that “everywhere” in this case includes our very cells as well. For example, a research team from New Zealand and Australia recently demonstrated that when polystyrene nanoplastic particles interact with human cells grown in the lab, the particles can cross cell membranes and reach the nuclei.¹ Other work with mollusk epithelial cells has demonstrated that these plastics are able to cross those cell membranes as well.²

If you’re like me, however, when you first hear about nanopastics you might wonder a couple things: how are they different from the microplastics that we have heard so much about over the past decades? And why haven’t we heard about nanoplastics before now? Never fear! In this post, we will explore the origins of these nanoscale particles, why we haven’t detected them before now, and the potential impacts these particles could have on biology.

Where do nanoplastics come from?

Plastics enter the ocean everyday through litter, wastewater, and rivers. The ocean is a harsh environment, and plastics in this environment can readily degrade into smaller plastic particles. This degradation can originate from processes that are physical (e.g., erosion), chemical (e.g., high energy UV sunlight breaking down chemical bonds), and/or biological (e.g., plastic-eating bacteria). These processes occur repetitively, breaking plastic particles in to smaller and smaller pieces, until eventually the particles reach the nanoscale (less than 100 nanometers). And that’s how we get nanoplastic particles.

What are nanoplastics made of, and how do they compare to other engineered nanoparticles?

A recent study found that nanoplastics in the ocean were made up of mostly polyvinylchloride (70%), plus some poly(ethylene teraphthalate) (17%), polystyrene (9%), and polyethylene (4%).⁴ As nanoplastic particles are typically formed viadegradation of larger plastics rather than through laboratory or commercial processes, it is important to recognize a few important distinctions compared to other engineered nanoparticles pollution:⁵

• Nanoplastics are not “clean”: Because of how nanoplastics are formed, they are typically found alongside small molecule industrial pollutants (for instance plasticizers).⁵ The hydrophobic nature of the polymers in the nanoplastics also makes them ideal for trapping other hydrophobic pollutants they might encounter. The disordered, polymeric, nature of nanoplastics allows them flexibility of shape and function that many traditional engineered nanoparticles are missing.
• Nanoplastics are unique: Unlike engineered nanoparticles, which are designed to have very specific sizes and shapes depending on their purpose, nanoplastics are very heterogeneous.⁶ Nanoplastics are essentially plastic polymers that have been broken and degraded randomly and repetitively until they reached the nanoscale. This, and the wide range of environmental conditions found in the ocean (different depths, animal and plant species, nearby pollutants, etc.) that can vary over time can lead to an incredibly large number of possibilities. This leads to significant heterogeneity not only in nanoplastic composition, but also in nanoplastic polymer chain lengths (how long they are), crosslinking (how tangled they are) and size.
• Nanoplastics are numerous: Research teams have estimated that about 5 billion tonnes of plastic have been released to the environment since 1950.^5,7 These plastics have had decades in the ocean to break down into nanoplastics, and are thought to significantly outnumber nanoplastics that have been intentionally engineered. 

Which brings us to the question…

How do nanoplastics interact with biology?

Nanoplastics can have significantly different interactions with biological systems than microplastics. For most marine wildlife, the primary threat posed by microplastics is related to ingestion – it’s bad for the wildlife to eat. However, recent studies have demonstrated that the size difference between microplastics and nanoplastics can lead to significantly different interactions with biology. But why? (If you’ve read a lot of our other posts, you probably have some ideas already.)

• Nanoplastics are really small: Nanoplastics range in size from 1 nanometer up to 100 nanometers. For reference, the average mammalian cell ranges from 10 to 100 micrometers, so while microplastics are typically around the same size, or slightly larger, than many cells, nanoplastics are anywhere between two to four orders of magnitude smaller than many cells they may encounter.

• Their small size allows them to go where microplastics cannot: The large size of microplastics means they can’t cross cell membranes using the same mechanisms as things like nutrients. In contrast, nanoplastics do not suffer this impediment. A recent study demonstrated that nanoplastics were able to cross epithelial cell membranes in sea scallops.² Similar studies have demonstrated that nanoplastics can cross cell membranes,^1,8–11 and can even aggregate in the cell nucleus and other organelles.⁷

• While their presence is understood, their interactions with biology are not: While scientists agree that nanoplastics can be internalized by cells, there is not yet consensus on how the particles cross cell membranes. Furthermore, we don’t yet fully understand any negative impacts of nanoplastics. Computer simulations have demonstrated that nanoplastics can alter protein shape, similar to the effect of some diseases which lead to plaque formation on neural tissues.^12,13 Experiments have also demonstrated that polystyrene nanoparticles can form protein coronas, induce immune responses,¹ and can lead to macroscopic changes in fish brain tissue.¹⁴

Why am I only hearing about these particles now?

A long-standing problem in environmental science has been the mass-balance problem of ocean-bound plastic: it doesn’t look like there’s as much plastic in the ocean as we’d expect.¹⁵ As of 2015, researchers estimate that there should be about 117 to 320 megatons of plastic in the ocean, of which 57-160 megatons should be plastics that are less dense than water (thus should be floating at or near the surface).¹⁶ Until recently, however, observations of plastic at the ocean surface account for less than 2% of the expected surface plastic.¹⁷ While scientists suspect that some of this missing plastic may be found by looking below the ocean surface (one research group found a substantial amount of plastic nearly 200 feet below the ocean surface ¹⁸), other recent studies have investigated whether micro- and nanoscale degraded plastics like I’ve described above could account for this missing plastic.

To some extent, microplastics have accounted for some of the plastic mass imbalance. It is now estimated that microplastics in ocean sediment alone account for nearly 14 megatons of the missing mass. A recent study by Bergmann and co-workers collected microplastics trapped in arctic ocean sediment and analyzed them for particle size and composition.¹⁹ Significantly, they noted that 80% of the particles detected were smaller than 25 μm, and that plastic concentration continued to increase significantly as microplastic size decreased towards the nano-scale (the smaller the size, the more particles there were). And even more microplastic is free floating in the ocean.¹⁸

Which brings us to the nanoplastic detection problem. Nearly all ocean-bound techniques for measuring small plastic particles rely on micron-sized filter meshes like the one shown above.²⁰ That means for now, what proportion of this missing plastic is made up of nanoplastic is yet to be discovered. But within the last few years, researchers are starting to develop new techniques and equipment for detecting nanoplastic particles from ocean samples.⁴ Finer meshes are being developed for collection of nanoplastics from environmental sources, and mass spectrometry techniques are being developed to characterize nanoplastics without needing costly pre-concentration and pre-treatment.¹⁶

While there’s a lot we still don’t know about the nature and prevalence of these nanoplastic particles, it’s clear we need to learn more. Expect to hear more about these particles in the near future, and in the meantime, we can all do our part by reducing the amount of plastic we use, recycling as much as possible, and advocating for policies that reduce industrial plastic.

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References

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