“There are always two aspects to risk: the exposure in the environment and the effects on organisms in nature,” explains Koelmans. The first question is how much microplastic is circulating in the sea, rivers or sediment and could get into organisms. Researchers could for example take a sample of water, run it through a sieve and determine in the lab how many microplastic particles are caught in the sieve.

“But that is where the problems start. After all, the size of the mesh in the sieve determines what proportion of the particles you measure,” says Koelmans. So if you get two different results, you can’t be sure whether that is because there are far more microplastics in the one place than in the other. It could also simply be because the sieves were different. This is just one of many issues affecting research measurements due to differences in equipment and methods. Koelmans developed ways of converting the existing datasets to make them comparable and drew up recommendations for new studies. His suggestions are increasingly being taken on board around the world.

The second question is what effect microplastics have on the health of various animals and of humans. One test is to expose small aquatic creatures to microplastics in the lab by adding plastics to an aquarium containing the creatures. But that approach is sometimes far from ideal, says Koelmans.

For instance, if you use microplastics of a consistent granule size for these tests, you are not taking sufficient account of the real-world situation. After all, microplastics in nature come from a range of sources, such as food packaging, cosmetic products, car tyres and clothing. “Partly for that reason, the microplastics in nature are very diverse. They are made up of particles of different kinds of plastics, in a variety of shapes and sizes. Even so, the mix of particles is quite similar across the different locations, as plastic becomes fragmented through roughly the same processes everywhere.” The idea is that eventually everyone everywhere will use the same mix in laboratory tests of the effects of microplastics.

The mix of plastic particles gave Koelmans another new idea. “If microplastics are so diverse, how can you make statements like ‘this animal will have problems if the number of particles exceeds ten’? Ten large particles or ten small ones? Ten rods or ten globules? That makes quite a difference, certainly for tiny creatures.” That put Koelmans on the track of what is probably the underlying problem: food dilution. “When the tiny creatures eat pieces of plastic, they end up consuming less of their favourite food, algae say. So the food is diluted. That made us realise that the volume consumed is a much more relevant indicator of the effect of microplastics for these kinds of animals than the number of particles.”

So new tricks and tools are helping researchers determine the risks from microplastics for animals more and more accurately. But how much risk exactly is there for humans? Koelmans: “An earlier study of the exposure in humans came up with an amount equal to one credit card per week, whereas our calculations gave an amount of less than one grain of salt. Far smaller, in other words. But this difference also shows how little we know for certain. The expectation is also that the amount will increase over the next few years.” Although that may only be because much of the plastic litter in nature has not yet broken down into microplastics.

Data on human exposure mainly relate to the products that we consume. For example, there is data for beer, honey, fish, mussels and salt but not yet for breakfast cereals, bread, vegetables, fruit and meat. Yet these food products are consumed in far greater quantities. Koelmans expects data will become available for many of the products over the next two years. Policymakers will have to wait slightly longer for well-founded statements on the distribution of plastic particles in the human body and the effect they have. That is because even less research has been done on this aspect so far.

Even so, policymakers will be able to take action using the new knowledge that has been acquired about microplastics in recent years. The state of California in the US has used Koelmans’ research as the basis for new standards and policy. The European Union is also taking steps. “We need a whole raft of measures to stop plastics leaking into the environment,” says Koelmans. “For example, you could use different materials to make the plastics, such as polymers that don’t break down at all or alternatively ones that break down very quickly. But I’m also thinking about enforcement, clean-up campaigns and raising awareness. We can’t afford to ignore any of these measures.”

It will be a long journey, but Koelmans is optimistic about the future. “I’m assuming we will soon have a much clearer picture of the risks associated with microplastics. Hopefully, we will also have got a lot further in finding solutions in five years’ time. I hope we’ll reach the point where we can say we’re seeing so many measures being implemented that our models show the amount of plastic in the environment is no longer increasing. Because if we don’t do anything, we know almost for sure there will be widespread, irreversible adverse effects in nature, such as an even greater decline in biodiversity, and probably in humans too. That won’t happen immediately, but it will in the next thirty to one hundred years.”