Table salt and shellfish can contain plastic

Microplastics found in China may point to global problem

sea salt

Sea salt is extracted from salt water in many regions of the world. A large share of sea salt purchased in China was contaminated with microbits of plastic. 

LoggaWiggler / Pixabay CC0

Scientists have found tiny plastic bits, known as microplastics, in salts collected from supermarkets across China. The researchers analyzed 15 brands of salt. They turned up plastic bits in table salt extracted from seas and lake water. They also found plastic bits in rock salt mined from underground deposits. By far, however, sea salt contained the most plastic. In a second study, the same team found similar plastic fibers in shellfish.

The new findings should, perhaps, come as no surprise. For years, studies have reported finding microplastics in ocean water. And in 2011, scientists showed that laundering clothes made of nylon and other types of plastic shed bits of lint. The wash water carried that lint down the drain and eventually into rivers and the ocean. Plastic bits have since turned up in sea animals. But the new paper is one of the first to report microplastics in food to be eaten by people.

It found that sea salt had 550 to 681 particles per kilogram (2.2 pounds). Each kilogram of lake salts had 43 to 364 particles. Rock salts had seven to 204 particles per kilogram.

These new data suggest that sea salt may be dragging microplastics from tainted water to dinner tables, the scientists conclude. They reported their findings October 20 in Environmental Science & Technology.

“We’re finding plastics in stranger and stranger places,” says Kara Lavender Law. She is an oceanographer at the Sea Education Association in Woods Hole, Mass., where she studies plastics in the ocean. Plastics, she notes, are “in salts, in the oceans, in the air. They’re all around us.” Law was not involved in the new study. But she says the findings may not be isolated to salt in China. Microplastics could taint sea salts from other regions, she warns. For now, no one knows what threat, if any, eating plastic bits might pose.

Huahong Shi led the new study. He works at East China Normal University in Shanghai. As an ecotoxicologist, he studies how pollutants affect plants, animals and other organisms. He and his colleagues had suspected sea salts might contain microplastics. A growing number of studies had found them in seas around the world.

They hadn’t suspected, though, that rock salt would have any. Indeed, notes Lance Yonkos, “It is not possible that rock salt begins with microplastics.” Rock salts “are the result of ancient seas depositing salts millennia before we started dumping plastics in the oceans,” he explains.

Yonkos is an aquatic toxicologist at the University of Maryland in College Park. He studies how toxic chemicals can affect animals that live in water. He, too, was not involved in the new study. Because the Chinese team found microplastics in rock salt, he says, this contamination must have occurred when the salt was mined, milled or packaged. If true, those processes may also add additional plastic crumbs to sea salt.

“This is one more example of the truly global consequences of human activities,” Yonkos says. “We once trusted in the enormity of the oceans to simply deal with our waste,” he says. “Now we find the situation isn’t that simple.”

How much people might be eating

Shi and his colleagues estimated how many plastic bits a person might ingest from salt each year. They started with the World Health Organization’s suggested limit on salt. Adults should eat no more than five grams of salt (a little less than a teaspoon) each day. Many people eat far more. Based on the degree of sea salt contamination seen in China, a person eating the suggested limit of salt from sea salt could consume about 1,000 microplastic particles per year, the group concludes.

But salt may not be the only source of tiny plastics in a person’s diet. In the December 15 Environmental Pollution, Shi’s team reports data on microplastics in Chinese shellfish. The group surveyed nine local species of bivalves. They include mussels, scallops, oysters and clams. That analysis turned up 2 to 10 microplastics — mainly fibers — per gram of edible bivalve tissue. Eating lots of these animals tainted at this level could add 100,000 particles of microplastics to the human diet each year, Shi’s team estimates.

This value, they note, is much higher than the 11,000 particles per year to which the top shellfish-consumers in Europe may be exposed. Shi’s team suggests that more microplastics are found in Chinese bivalves. People who eat Chinese seafood may thus face a higher exposure to plastics in their diet.

The team also notes that many more people eat salt than shellfish. So, overall, more people may be exposed to plastics from salt than from shellfish.

One possible risk from eating microplastics may not be due to the plastic itself. Plastics act like a sponge for many chemicals that can prove toxic, note Sarah Gall and Richard Thompson of England’s Plymouth University. Such pollutants include hydrocarbons, pesticides and solvents. Bits of plastic may ferry such toxic water pollutants into the human diet, they argue March 15 in the Marine Pollution Bulletin.

Law says much more work needs to be done before anyone can say that eating microplastics in sea salt is a major health threat. The amounts remain quite small. Still, she concedes, “no one wants to eat plastic.”

Power Words

(for more about Power Words, click here)

aquatic An adjective that refers to water.

bivalve  An invertebrate that lives in a two-part, hinged shell. Most bivalves filter their food from the waters that slosh past them. Bivalves include clams, oysters, scallops and mussels.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

environment    The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

hydrocarbon Any of a range of large molecules created by chemically bound carbon and hydrogen atoms. Crude oil, for example, is a naturally occurring mix of many hydrocarbons.

microplastic   A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.

millennia   (singular: millennium) Thousands of years.

oceanography  The branch of science that deals with the physical and biological properties and phenomena of the oceans. People who work in this field are known as oceanographers.

organism  Any living thing, from elephants and plants to bacteria and other types of single-celled life.

pesticide  A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pet or livestock, or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

plastic  Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

solvent  A material (usually a liquid) used to dissolve some other material into a solution.

toxic  Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

toxicology  The branch of science that probes poisons and how they disrupt the health of people and other organisms. Scientists who work in this field are called toxicologists.

World Health Organization  An agency of the United Nations, established in 1948, to promote health and to control communicable diseases. It is based in Geneva, Switzerland. The United Nations relies on the WHO for providing international leadership on global health matters. This organization also helps shape the research agenda for health issues and sets standards for pollutants and other things that could pose a risk to health. WHO also regularly reviews data to set policies for maintaining health and a healthy environment.

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