Exposure to near-ubiquitous microplastics, long a concern for marine life, could also cause health problems for humans ranging from inflammatory lesions to potential increased cancer risk, according to an unprecedented review of academic literature.

In a new paper for Science of The Total Environment, appearing in Volume 757 in February, researchers published the first scoping review of microplastics, defined as plastic particles less than 5 millimeters in size. 

Although the effects of the plastic bits, found commonly in hygiene and other products, have been well-documented, there have been as of yet few studies on how the materials affect human health, whether by ingestion, inhalation or dermal contact. 

“Almost nil, actually,” said Atanu Sarkar, one of the paper’s authors, an associate professor of environmental and occupational health in the Faculty of Medicine at the Division of Community Health and Humanities at Memorial University in St. John’s, in Canada’s Newfoundland and Labrador province.

“We always talk about microplastics affecting the various animals,” he said. “We human beings are also at risk due to exposure to microplastics, in multiple ways.”

Working with colleagues at the University of Calgary in Alberta, Canada, and the Environmental Institute in Kos, Slovakia, Sarkar initially selected nearly 17,000 articles somehow linked to microplastics and health for the review, ultimately narrowing it down to 129 that can be closely tied to potential human health risks. 

Extrapolating from research in the evolving field of study involving animal studies and in vitro testing, as well as that documenting humans’ inevitable exposure to microplastics, the team detailed how the particles can cause, for example, toxicity through oxidative stress, inflammatory lesions and compromised immune cells. 

Reproductive toxicity was a noted potential health risk, as was translocation to distant tissues and organs through the circulatory system, which reportedly caused a host of other issues including pulmonary hypertension. Several studies also noted potential for metabolic disturbances, neurotoxicity and higher risk of cancer in humans. 

Microplastics can be bits of larger plastics that have broken down, or they can be deliberately produced for products like soft furnishings and cosmetics. The latter are called primary microplastics; microbeads, known for appearing in facial cleansers and shower gels and banned in products in countries including the U.S. and U.K., are considered microplastics when they are small enough.

Most human exposure to microplastics occurs through the ingestion of contaminated food and water. For instance, microplastics have been found in bottled water — one study reported that people who drank only bottled water ingested more than 22 times as many additional particles as those who drank just tap water — and filter feeders like mussels can digest microplastic pollution from the ocean. 

People could also inhale the particles, both indoors and outdoors, that are shed from synthetic clothes, building materials and landfill incineration, for instance. Microplastics can make contact via the skin, too, when contained in cosmetic products such as toothpaste. 

Nanoplastics add another layer of risk — so much so that journal editors suggested explicitly calling them out in the final paper title, which initially was focused on microplastics generally. These ultra-tiny particles, a subset of microplastics, are more invasive, and can actually pass through the dermal barrier and enter into circulation. In animals, they’ve been shown to move beyond the intestine and cause additional inflammation and adverse effects in cells.

“More and more evidence is coming up that nanoplastic is actually more dangerous,” Sarkar said. “That itself is really a matter of grave concern.” 

Despite the lack of direct evidence gathered for the review, there is now growing researcher interest in the health effects microplastics can have on people. Many scientists are taking inventories of how humans are exposed through their diets, for example.

“More and more publications are coming up in microplastics,” Sarkar said. “I am hopeful that in one or two or three years, some path-breaking research will come up and will show the impact of microplastics on health — more direct evidence, or more compelling evidence.”

Still, Sarkar says, the United Nations should host a special convention on microplastics. The body’s international stature, existing environmental program and relationships with other agencies make it an appropriate venue for countries to come together and establish clear guidelines on defining microplastics and nanoplastics, assessing human health risks and mitigation strategies, he said. That will require looking at larger plastics that contribute to microplastics, too, and addressing the fact that no one country alone can fix the borderless issue.

“You look at the climate change thing. So many countries initially just simply ignored it,” Sarkar said. “I think there will be initially some resistance, blame game. But when people look at it, and when scientists develop more and more evidence, I am sure the government has no option but to accept the fact that it is an issue.”

In the meantime, some are taking action. Several European Union member states have enacted or floated national bans on the deliberate use of microplastics in consumer products. The bloc’s European Chemicals Agency last year proposed a broad restriction on microplastics intentionally added to products, which is expected to halt the release of 500,000 tons of the particles over the course of two decades. That proposal is under consideration and could be adopted as soon as 2021.

The study "Potential human health risks due to environmental exposure to nano- and microplastics and knowledge gaps: A scoping review," available Dec. 3 in Science of The Total Environment, was authored by Arifur Rahman, Memorial University; Atanu Sarkar, Memorial University; Om Prakash Yadav, Memorial University; Gopal Achari, University of Calgary; and Jaroslav Slobodnik, Environmental Institute.