Plastics in the world’s oceans are set to treble in the next 10 years, according to a new government report. They are also contributing to a rubbish heap in the Pacific Ocean that is estimated to be as big as France. These are the latest instalments of one of the most prominent environmental concerns of recent years.

It’s not surprising this has become a cause célèbre. Unlike many other human pollutants in the environment, plastic debris is very visible.

Images of birds or fish entangled in plastic are highly emotive – as is the idea that we could be harming ourselves by eating seafood containing tiny pieces of the stuff.

To be sure, this is a big problem. Plastics degrade the environment and we are certainly finding them in increasingly large quantities in our seas and oceans.

This may indeed harm marine life and their ecosystems, but when you look closely at the evidence, it turns out that we are far less sure than it might appear.

There are important gaps in our understanding about plastics. It’s not unreasonable for people to fill these with speculation to some extent – funding for research is limited and we cannot wait for scientific research to provide complete answers before taking action. On the other hand, unsupported speculation can lead to scarce resources being misdirected when they could be better spent on other environmental issues.

Certainly we produce large amounts of plastics each year. They continually end up as waste in the environment, and the polymers they comprise decompose extremely slowly. Large particles fragment into smaller pieces known as microplastics – technically 5mm in diameter or less. These are now recognised as one of the most prevalent human-made pollutants in marine environments across the world.

Plastic waste across the world: in pictures Show all 15 1 /15 Plastic waste across the world: in pictures Plastic waste across the world: in pictures A father and son on a makeshift boat made from styrofoam paddle through a garbage filled river as they collect plastic bottles that they can sell in junkshops in Manila. The father and son team earn some three US dollars a day retrieving recyclables from the river. AFP/Getty Plastic waste across the world: in pictures A composite image of items found on the shore of the Thames Estuary in Rainham, Kent. Tons of plastic and other waste lines areas along the Thames Estuary shoreline, an important feeding ground for wading birds and other marine wildlife. Getty Images Plastic waste across the world: in pictures Children collect plastic water bottles among the garbage washed ashore at the Manila Bay. According to the United Nations Environment Programme, at current rates of pollution, there will likely be more plastic in the sea than fish by 2050. AFP/Getty Plastic waste across the world: in pictures Plastics and other detritus line the shore of the Thames Estuary. In December 2017 Britain joined the other 193 UN countries and signed up to a resolution to help eliminate marine litter and microplastics in the sea. It is estimated that about eight million metric tons of plastic find their way into the world's oceans every year. Once in the Ocean plastic can take hundreds of years to degrade, all the while breaking down into smaller and smaller 'microplastics,' which can be consumed by marine animals, and find their way into the human food chain. Getty Plastic waste across the world: in pictures A dump site in Manila in 2013. The Philippines financial capital banned disposable plastic shopping bags and styrofoam food containers, as part of escalating efforts across the nation's capital to curb rubbish that exacerbates deadly flooding. AFP/Getty Images Plastic waste across the world: in pictures Children swims in the sea full of garbage in North Jakarta, Indonesia. Getty Plastic waste across the world: in pictures An Indian woman holds a jar filled with Yamuna river water polluted with froth and toxic foam to be used for rituals at the river bank in New Delhi, India. The Yamuna River, like all other holy rivers in India, has been massively polluted for decades now. The river that originates in a glacier in the pristine and unpolluted Himalayas, and flows through Haryana, Delhi and Uttar Pradesh before merging with the Ganges River in Allahabad, once used to be the lifeline of the Indian capital. Currently, it is no more than a large, open sewer that is choking with industrial and domestic discharge that includes plastic, flowers and debris and has virtually no aquatic life. EPA Plastic waste across the world: in pictures Plastic waste is washed up on South Troon beach in Scotland. Recent reports by scientists have confirmed, plastics dumped in the world oceans are reaching a dangerous level with micro plastic particles now being found inside filter feeding animals and amongst sand grains on our beaches. Getty Plastic waste across the world: in pictures Children collect plastic to be sold and recycled, in a polluted river in suburban Manila. The city's trash disposal agency traps solid waste floating down waterways that was thrown into the water by residents of slums along riverbanks upstream. AFP/Getty Plastic waste across the world: in pictures View of the Carpayo Beach in La Punta, Callao, some 15 km of Lima. In 2013, the NGO VIDA labeled the Carpayo Beach as the most polluted in the country - 40 tons of trash on each 500m2. AFP/Getty Plastic waste across the world: in pictures Trash from Kamilo Beach in Hawaii. Gabriella Levine/Flickr Plastic waste across the world: in pictures A scavenger collects plastic cups for recycling in a river covered with rubbish near Pluit dam in Jakarta. Reuters Plastic waste across the world: in pictures Rubbish fills Omoa beach in Honduras. Floating masses of garbage offshore from some of the Caribbean's pristine beaches are testimony to a vast and growing problem of plastic pollution heedlessly dumped in our oceans, locals, activists and experts say. AFP/Getty Plastic waste across the world: in pictures A man climbs down to a garbage filled river in Manila. Plastic rubbish will outweigh fish in the oceans by 2050 unless the world takes drastic action to recycle the material, a report warned in 2016. AFP/Getty Plastic waste across the world: in pictures Garbage on East Beach, Henderson Island (Pitcairn Islands), in the south Pacific Ocean. The uninhabited island has been found to have the world's highest density of waste plastic, with more than 3,500 additional pieces of litter washing ashore daily at just one of its beaches. EPA

Microplastics could be accumulating in some places to levels that somehow compromise ecosystems. Deep-sea regions are a probable candidate, for example, though they are also the areas where we have the least information about quantities and effects. We need to do more work to say with confidence whether this is a serious problem.

On the question of how much damage microplastics cause to marine life, we certainly know these particles are readily transported throughout our seas and oceans and there is considerable evidence that organisms ingest them. However, the polymers that make up plastics are of minimal toxicity to marine life.

The question is whether they may cause harm in other ways. It could be that organisms absorb these particles and they accumulate in internal tissues, though it’s not clear whether or not that might be harmful to them. Microplastics may also accumulate in the gut and potentially interfere with processes like nutrient uptake or the passage of waste – or they may just be expelled without any negative effects.

A few studies have shown microplastics being absorbed by marine life in very small amounts, but other studies have found the opposite. We don’t even know whether very small nanoplastics with diameters of less than 1,000 nanometres can be absorbed. The studies that do exist on nanoparticles suggest that such absorption is minimal. In short, the jury is still out on absorption.

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If microplastics are not appreciably absorbed, their potential to accumulate in tissues and cause problems is very low. It would also mean they can’t be passed on in any significant way to a predator who eats that organism. If so, it puts microplastics in a different category to toxic substances that end up in the food chain after accumulating in the internal tissues of fish – mercury, say.

There is considerable evidence to suggest that plastic particles are readily released from the gut of organisms without negative effects – and note that researchers have tended to test for concentrations in considerably higher amounts than are found in the environment.

Certainly, questions do remain. Perhaps of greatest importance is whether specific shapes of microplastics – fibres, for example – present particular difficulties for waste moving through the guts of some organisms.

Another concern is around toxic substances like DDT or hexachlorobenzene sticking to microplastics and potentially ending up in places they wouldn’t otherwise reach. Scientists have already found considerable evidence of this. Some people are alarmed that these substances could end up being ingested by marine organisms and harming them as a result.

In one study, the levels of toxic substances in the tissues of marine birds were actually lower when they had ingested plastics (Shutterstock)

Yet most studies have shown that toxicants associated with plastics are either at concentrations too low to be toxic – or that the substances stick too strongly to the plastics to be released into organisms and cause problems.

In one study, the levels of toxic substances in the tissues of marine birds were actually lower when they had ingested plastics. The investigators suggested the toxic substances already present within the bird tissues were sticking to the plastics and being removed. If so, toxic substances attached to plastics might be less of a concern for toxicity to marine organisms than is feared.

Then there are microplastics and the human food chain. We were intrigued by this possibility and conducted an experiment to check. While we cooked in our kitchens, we left open petri dishes with sticky tape to collect dust fallout in the surrounding air.

We compared the amounts of plastic fibres in this dust with the quantities we found in mussels collected around the Scottish coast. The results suggest that while a regular UK consumer might ingest 100 plastic particles a year from eating mussels, their average exposure to plastic particles during meals from household dust is well over 10,000 per year.

In sum, the evidence about the dangers of plastics and microplastics in the marine environment is far from conclusive. There are important gaps in scientists’ knowledge that need to be filled, particularly where plastic particles are likely to accumulate in large amounts over long periods and how this potentially affects ecosystems.

We must avoid undue speculation and overstating risks, and instead engage with the actual evidence. Otherwise it will detract from our ability to manage plastic pollution in the most effective way and have a clear sense of the right priorities.