Explainer: what is a microplastic, and how do we know they’re there?

You may have seen worrying headlines about “microplastics” popping up on our beaches, oceans, in the air, or even in our blood.

But what, exactly, is a microplastic – and how are they identified? What’s the real environmental impact of microplastics? Cosmos investigates.

What is a microplastic?

Microplastics are tiny fragments of plastic. How tiny? That’s a little difficult to decide.

“As a rule of thumb, it’s from one micrometre to five millimetres,” says Professor Sophie Leterme, from Flinders University’s Institute for Nanoscale Science and Technology. A micrometre is a thousandth of a millimetre, or about the same size as an E.coli bacterium.

But definitions range depending on context. Many studies only look at microplastics bigger than 10, 50 or 200 micrometres (for example), because it’s harder to detect and identify smaller plastics.

Alternatively, plastic can certainly fragment into amounts smaller than one micrometre. These fragments, sometimes called nanoplastics (for nanometre) are even harder to spot with conventional microplastic-finding methods.

So, while we’re mostly looking at things bigger than a micrometre and smaller than five millimetres, there’s emerging research that looks at smaller things.

Where do microplastics come from?

Hand on beach holding microplastic nurdles
Microplastic nurdles. Credit: UniversalImagesGroup / Getty Images

Microplastics can get into the environment at every point in the production cycle: from nurdles, which are used to make plastic products, to fragments from plastic rubbish. They also break off plastic products when they are in use – even tearing open a chip packet can shed microplastics.

Two big sources that might surprise you, however, are cosmetics and clothing.

Cleansers and other cosmetics sometimes have plastic beads in them – which are washed down the sink, and are too small for conventional filters to take out of our waterways. These beads have become problematic enough that most countries, including Australia, have started phasing them out.

Another major source is the plastics in textiles, which can be shed from garments while you’re wearing them, and particularly, while they’re being washed.

How are they identified?

Spotting microplastics starts with collecting samples from the environment – be it sand, soil, water, or something else.

Once collected, the sample has to be sifted through to take out the things that are – or might be – microplastics. This can be done in a few different ways – from sifting on location, to filtration in a lab.

Leterme and her colleagues collect water samples, and take them straight back to the lab to pass them through a series of filters.

“Filtering out one micron [micrometre] is not possible because there is a lot of detritus in the water that will clog the filter,” she says. They do their analysis for microplastics that are 20 micrometres and above – and even at this level, they need microscopes to find the smallest fragments.

“A lot of other studies use a net – so you can collect more plastic in in one go, but it means that you are limited by the mesh size of the net,” says Leterme.

Read more: Mapping microplastics on Australian beaches

Once potential microplastics have been spotted, they have to undergo a chemical analysis to show they’re actually plastic.

A technique called spectroscopy can tell us this, and even identify which plastic it is – polyethylene, polypropylene, polystyrene, or one of the dozens of other types floating in our environment.

Spectroscopy revolves around shining certain types of light onto a substance, which makes the molecules vibrate and send out very precise signals. If you know all of the signals that plastic molecules are going to emit, you can figure out whether or not a fragment is plastic by putting it on a detector and firing a tiny beam of light onto it.

Hand on beach holding sand and microplastics
Credit: pcess609 / Getty Images

“There’s two general methods that are used, and both rely on looking at the vibrations in the different polymers,” says Dr Jason Gascooke, also at Flinders University.

These two methods are called infrared spectroscopy and Raman spectroscopy.

“Raman is not foolproof,” says Gascooke. “Whereas something like infrared would work 100% of the time, but it takes 10 minutes per microplastic.”

Even when Raman spectroscopy works, it takes a minute or two per fragment – which, given researchers are working with thousands of fragments in samples – makes it hard work.

“It is a slow and tedious process,” says Gascooke.

What effects do microplastics have on the environment?

Because researchers have so many different standards for spotting microplastics, it’s hard to make wide-ranging statements about their impact on the environment.

“It’s impossible to do a true comparison, because samples are collected in so many different ways,” says Leterme.

It’s also not a particularly well-funded field of research – given the amount of work needed to trace microplastics.

“There’s very large interest from the public, but it’s very hard to obtain funding to do large scale studies,” says Leterme.

That said, there is mounting evidence that microplastics can damage animals and plants – particularly in marine environments.

“There has been a focus on plankton,” says Leterme. Microplastics can be a similar size to zooplankton prey, meaning they get eaten and get lodged in their stomachs, taking up space needed for food.

“It means they are starving, or they can’t reproduce properly,” says Leterme. This problem could get magnified up the food chain.

Read more: Ocean microplastics captured using sound

There are also concerns that microplastics can leach toxins into the environment, and super-tiny nanoplastics can get into cells – where they have the potential to cause more trouble.

Microplastics might not – yet – be in high enough concentrations to worry about. Without better standards for counting them, and better resources for tracking them, it’s hard to judge their overall impact.

But with 11 million tonnes of plastic flowing into the ocean each year, even if they’re not a huge problem now, there’s every chance they will be at a later date.

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