Marine Plastic Pollution and Seafood Safety
The path from plastic pollution to chemical exposure through seafood is a long one, figuratively and often literally, and tracing all the individual steps in that theoretical journey is not the same as identifying human health effects, researchers say. Actual exposures, which are determined by innumerable variables along the way, including seafood consumption, still need to be quantified. Then these levels must be evaluated within broader contexts of consumer plastic use and environmental pollutant levels.
(Enlarge Image)
Different marine plastics resemble foods eaten at various trophic levels. These plastic bags look like the jellyfish eaten by turtles.
© Norbert Wu/Minden Pictures/Corbis
(Enlarge Image)
Plastic debris can travel far from its point of departure—this beach in Svalbard, Norway, for instance, is only about 600 miles from the North Pole. A 2014 study reported finding large quantities of microplastics frozen into Arctic ice.52 © Ashley Cooper/Corbis
Exposures to plastic debris have been clearly documented for marine organisms at all trophic levels (i.e., positions within the food chain), says Bradley Clarke, a lecturer at RMIT University in Melbourne, Australia. "What remains to be determined is whether this exposure increases the body burden of … marine organisms in the natural environment and if it does, by what magnitude," Clarke says.
There is a lack of controlled experimental work completed on the topic, Clarke adds, and it's very difficult to disentangle pollutant exposures and bioaccumulation via plastic versus food and environmental sources. Uncertainties also surround the transfer of plastic additives to marine organisms and resultant human exposures through seafood.
We do know that plastic has become nearly ubiquitous on the planet. It has washed up on the most remote beaches, amassed in distant gyres, and been discovered in the bodies of dead organisms from fish to birds to whales.
(Enlarge Image)
Ocean currents carry plastic debris into the five major ocean gyres. Thousands of tons of microplastics are estimated to bob in these gyres, but more than half of all plastic debris likely sinks upon reaching ocean waters. © Jane Whitney
Numerous efforts have sought to quantify the amount of plastics floating on or present throughout the ocean environment, and they've arrived at vastly different numbers. The 5 Gyres paper was preceded in July 2014 by a similar study suggesting that between 7,000 and 35,000 tons of plastic are floating on the ocean's surface.
Anna-Marie Cook, one of two EPA lead scientists investigating the potential health effects of marine plastics, believes that estimates calculated through the use of surface trawl nets, including both of the recent global studies, vastly underestimate the scope of the problem. "Slightly more than half of all plastic is negatively buoyant, meaning that it will sink upon reaching the ocean, either into the near-shore sediment environment or to the ocean floor," she explains. "Surface trawls do not account for the fraction of plastic in sediments, on the ocean floor, or suspended past the top few feet of the water column."
World plastics production has experienced almost constant growth for more than half a century, rising from approximately 1.9 tons in 1950 to approximately 330 million tons in 2013. The World Bank estimates that 1.4 billion tons of trash are generated globally each year, 10% of it plastic. The International Maritime Organization has banned the dumping of plastic waste (and most other garbage) at sea. However, an unknown portion of the plastic produced each year escapes into the environment—instead of being landfilled, incinerated, or recycled—and at least some of it eventually makes its way to sea.
Plastics that reach the ocean will gradually break down into ever-smaller pieces due to sunlight exposure, oxidation, and the physical action of waves, currents, and grazing by fish and birds. So-called microplastics—variably defined in the scientific literature and popular press as smaller than 1 or 5 mm in diameter—are understood to be the most abundant type of plastic in the ocean. The 5 Gyres authors found microplastics almost everywhere they sampled, from near-shore environments to the open ocean, in varying concentrations, and they estimated that particles 4.75 mm or smaller—about the size of a lentil—made up roughly 90% of the total plastic pieces they collected.
But the degradation of larger pieces of plastic is not the only way microplastics end up in the ocean. Nurdles—the plastic pellets used as a feedstock for producing plastic goods—can spill from ships or land-based sources, and "microbeads" used as scrubbing agents in personal care products such as skin cleansers, toothpastes, and shampoos, can escape water-treatment facilities and pass into watersheds with treated water. (In June 2014, Illinois became the first U.S. state to ban the manufacture and sale of products containing microbeads, which have been documented in the Great Lakes and Chicago's North Shore Channel.)
Due to their hydrophobic nature, persistent organic chemicals—including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), dioxins, and DDT—have been shown to preferentially sorb to plastics when they encounter them in the ocean. Potentially thousands of such chemicals exist in the environment, but researchers are limited to screening for compounds they can actually identify, Bradley says.
The extent and rate of sorption can vary widely depending on the chemical, plastic type, and other variables, but plastic particles recovered from the ocean have been found to contain pollutant concentrations orders of magnitude higher than the water from which they were collected.
Marine organisms throughout the food chain commonly consume plastics of various sizes. The tiniest microplastics are small enough to be mistaken for food by zooplankton, allowing them to enter the food chain at very low trophic levels. Some larger predators are thought to confuse nurdles (which typically measure less than 5 mm in diameter) with fish eggs or other food sources.
Once plastics have been consumed, laboratory tests show that chemical additives and adsorbed pollutants and metals on their surface can desorb (leach out) and transfer into the guts and tissues of marine organisms. Some researchers speculate that chemicals already present in the organism may also be able to travel in the opposite direction by sorbing to plastics in the gut, depending on the concentration gradients. Yet neither process has been proven to occur in the natural environment.
We already know that many chemicals of concern are present in the seafood we eat, particularly in higher-level predators such as tuna and swordfish. Research has shown that harmful and persistent substances can both bioaccumulate (or increase in concentration as exposures persist) and biomagnify (or increase in concentration at higher trophic levels) within organisms as they assume some of the chemical burden of their prey or environment. Yet again, no research has yet demonstrated the bioaccumulation of sorbed pollutants in the environment.
Three key questions remain to be determined. To what extent do plastics transfer pollutants and additives to organisms upon ingestion? What contribution are plastics making to the contaminant burden in organisms above and beyond their exposures through water, sediments, and food? And, finally, what proportion of humans' exposure to plastic ingredients and environmental pollutants occurs through seafood? Researchers are moving carefully in the direction of answers to these questions.
(Enlarge Image)
Small plastic pellets known as nurdles are used as a feedstock for producing plastic goods. In July 2012 Typhoon Vicente swept more than 165 tons of nurdles from a cargo ship off the coast of Hong Kong. © Nigel Cattlin/Science Source
(Enlarge Image)
Polyethylene microbeads (orange, shown with yellow flakes of silica) are used as exfoliants in many personal care products. In June 2014 Illinois became the first U.S. state to ban the manufacture and sale of products containing microbeads, which are small enough to slip through filters at wastewater treatment plants.
© Steve Gschmeissner/Science Source
Plastic Vectors
The path from plastic pollution to chemical exposure through seafood is a long one, figuratively and often literally, and tracing all the individual steps in that theoretical journey is not the same as identifying human health effects, researchers say. Actual exposures, which are determined by innumerable variables along the way, including seafood consumption, still need to be quantified. Then these levels must be evaluated within broader contexts of consumer plastic use and environmental pollutant levels.
(Enlarge Image)
Different marine plastics resemble foods eaten at various trophic levels. These plastic bags look like the jellyfish eaten by turtles.
© Norbert Wu/Minden Pictures/Corbis
(Enlarge Image)
Plastic debris can travel far from its point of departure—this beach in Svalbard, Norway, for instance, is only about 600 miles from the North Pole. A 2014 study reported finding large quantities of microplastics frozen into Arctic ice.52 © Ashley Cooper/Corbis
Exposures to plastic debris have been clearly documented for marine organisms at all trophic levels (i.e., positions within the food chain), says Bradley Clarke, a lecturer at RMIT University in Melbourne, Australia. "What remains to be determined is whether this exposure increases the body burden of … marine organisms in the natural environment and if it does, by what magnitude," Clarke says.
There is a lack of controlled experimental work completed on the topic, Clarke adds, and it's very difficult to disentangle pollutant exposures and bioaccumulation via plastic versus food and environmental sources. Uncertainties also surround the transfer of plastic additives to marine organisms and resultant human exposures through seafood.
We do know that plastic has become nearly ubiquitous on the planet. It has washed up on the most remote beaches, amassed in distant gyres, and been discovered in the bodies of dead organisms from fish to birds to whales.
(Enlarge Image)
Ocean currents carry plastic debris into the five major ocean gyres. Thousands of tons of microplastics are estimated to bob in these gyres, but more than half of all plastic debris likely sinks upon reaching ocean waters. © Jane Whitney
Numerous efforts have sought to quantify the amount of plastics floating on or present throughout the ocean environment, and they've arrived at vastly different numbers. The 5 Gyres paper was preceded in July 2014 by a similar study suggesting that between 7,000 and 35,000 tons of plastic are floating on the ocean's surface.
Anna-Marie Cook, one of two EPA lead scientists investigating the potential health effects of marine plastics, believes that estimates calculated through the use of surface trawl nets, including both of the recent global studies, vastly underestimate the scope of the problem. "Slightly more than half of all plastic is negatively buoyant, meaning that it will sink upon reaching the ocean, either into the near-shore sediment environment or to the ocean floor," she explains. "Surface trawls do not account for the fraction of plastic in sediments, on the ocean floor, or suspended past the top few feet of the water column."
World plastics production has experienced almost constant growth for more than half a century, rising from approximately 1.9 tons in 1950 to approximately 330 million tons in 2013. The World Bank estimates that 1.4 billion tons of trash are generated globally each year, 10% of it plastic. The International Maritime Organization has banned the dumping of plastic waste (and most other garbage) at sea. However, an unknown portion of the plastic produced each year escapes into the environment—instead of being landfilled, incinerated, or recycled—and at least some of it eventually makes its way to sea.
Plastics that reach the ocean will gradually break down into ever-smaller pieces due to sunlight exposure, oxidation, and the physical action of waves, currents, and grazing by fish and birds. So-called microplastics—variably defined in the scientific literature and popular press as smaller than 1 or 5 mm in diameter—are understood to be the most abundant type of plastic in the ocean. The 5 Gyres authors found microplastics almost everywhere they sampled, from near-shore environments to the open ocean, in varying concentrations, and they estimated that particles 4.75 mm or smaller—about the size of a lentil—made up roughly 90% of the total plastic pieces they collected.
But the degradation of larger pieces of plastic is not the only way microplastics end up in the ocean. Nurdles—the plastic pellets used as a feedstock for producing plastic goods—can spill from ships or land-based sources, and "microbeads" used as scrubbing agents in personal care products such as skin cleansers, toothpastes, and shampoos, can escape water-treatment facilities and pass into watersheds with treated water. (In June 2014, Illinois became the first U.S. state to ban the manufacture and sale of products containing microbeads, which have been documented in the Great Lakes and Chicago's North Shore Channel.)
Due to their hydrophobic nature, persistent organic chemicals—including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), dioxins, and DDT—have been shown to preferentially sorb to plastics when they encounter them in the ocean. Potentially thousands of such chemicals exist in the environment, but researchers are limited to screening for compounds they can actually identify, Bradley says.
The extent and rate of sorption can vary widely depending on the chemical, plastic type, and other variables, but plastic particles recovered from the ocean have been found to contain pollutant concentrations orders of magnitude higher than the water from which they were collected.
Marine organisms throughout the food chain commonly consume plastics of various sizes. The tiniest microplastics are small enough to be mistaken for food by zooplankton, allowing them to enter the food chain at very low trophic levels. Some larger predators are thought to confuse nurdles (which typically measure less than 5 mm in diameter) with fish eggs or other food sources.
Once plastics have been consumed, laboratory tests show that chemical additives and adsorbed pollutants and metals on their surface can desorb (leach out) and transfer into the guts and tissues of marine organisms. Some researchers speculate that chemicals already present in the organism may also be able to travel in the opposite direction by sorbing to plastics in the gut, depending on the concentration gradients. Yet neither process has been proven to occur in the natural environment.
We already know that many chemicals of concern are present in the seafood we eat, particularly in higher-level predators such as tuna and swordfish. Research has shown that harmful and persistent substances can both bioaccumulate (or increase in concentration as exposures persist) and biomagnify (or increase in concentration at higher trophic levels) within organisms as they assume some of the chemical burden of their prey or environment. Yet again, no research has yet demonstrated the bioaccumulation of sorbed pollutants in the environment.
Three key questions remain to be determined. To what extent do plastics transfer pollutants and additives to organisms upon ingestion? What contribution are plastics making to the contaminant burden in organisms above and beyond their exposures through water, sediments, and food? And, finally, what proportion of humans' exposure to plastic ingredients and environmental pollutants occurs through seafood? Researchers are moving carefully in the direction of answers to these questions.
(Enlarge Image)
Small plastic pellets known as nurdles are used as a feedstock for producing plastic goods. In July 2012 Typhoon Vicente swept more than 165 tons of nurdles from a cargo ship off the coast of Hong Kong. © Nigel Cattlin/Science Source
(Enlarge Image)
Polyethylene microbeads (orange, shown with yellow flakes of silica) are used as exfoliants in many personal care products. In June 2014 Illinois became the first U.S. state to ban the manufacture and sale of products containing microbeads, which are small enough to slip through filters at wastewater treatment plants.
© Steve Gschmeissner/Science Source
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