How harmful are microplastics?

Microplastics are very small plastic particles generally less than 5 mm in size. There are significant levels of microplastics polluting the ocean, freshwater and land, and research is showing that animals including humans are eating these microplastics. For some animals, they mistake the particles as food, while others are ingesting them when they consume animals that have eaten them.

So are these microplastics harmful for us and other animals?

Animals and microplastics

Microplastics can be ingested by various animals, ranging in size from tiny creatures like zooplankton to sharks and whales. The likelihood of microplastics being eaten is influenced by the amount in the environment and how closely they resemble food. Laboratory studies indicate that microplastics can potentially transfer through the food web when marine, terrestrial and freshwater species that have previously ingested microplastics are preyed on by other animals.

Microplastics eaten by larger marine animals will generally pass through their bodies. However, research does show that microplastics can be retained in the gut for extended periods where they may cause abrasion and damage to internal tissues. Nanoplastics can pass through the gut wall and travel to different parts of the body, such as the lungs and liver, where they can cause damage. Further research is required to understand the potential health implications from ingesting microplastics.

Smaller animals including marine and freshwater invertebrates can suffer a range of effects from ingesting microplastics including reduced reproduction and growth. These effects are generally caused by physical damage including lacerations and inflammatory responses as well as reduced feeding behaviour when microplastics are consumed instead of digestible food. Filter feeders such as mussels and oysters are particularly vulnerable to ingesting microplastics as they filter high volumes of water while feeding. Effects vary between species and by the types of plastic and the concentration of microplastics.

Research released in 2021 by University of Waikato Master of Science student, Anita Lewis found extremely high levels of microplastics across the Bay of Plenty moana. Every area she tested showed evidence of microplastics, and high levels were found in shellfish.

Toxic chemicals associated with plastics

Various chemicals incorporated into plastics as raw materials or additives during manufacture can leach from plastics into the environment. Humans and other animals are also exposed to these chemicals when they ingest plastics or when these chemicals leach into food or are released into the atmosphere when plastics are burned. Some chemicals directly associated with plastics have been detected in humans and wildlife including birds and various marine species.

Pollutants that stick to microplastics

Some pollutants and heavy metals can also adsorb or stick to plastic surfaces. As a result, plastics can act like sponges in the environment, passively collecting chemicals onto their surfaces.

While plastics can remove some persistent organic pollutants (POPs) from the surrounding water, there is concern about what happens when plastics containing these adsorbed pollutants are ingested by animals. The ability of some POPs to bind to plastics is particularly concerning due to their toxicity at low doses. These toxic and persistent chemicals are widely distributed in the marine environment and are readily concentrated onto plastic surfaces at up to 1 million times the concentration than in the surrounding water. Studies have shown that these chemicals can transfer from ingested plastics to animal tissue where they can become concentrated within the animal and transfer through the food chain.

Do plastics affect human health?

Plastics are beneficial to human health through their use in medical applications and for protecting our food and beverages. Plastics have revolutionised healthcare through improving sterility by the use of disposable syringes, gloves, IV tubes and catheters and providing increased comfort with hypoallergenic medical devices, heart valves, and flexible prosthetics (artificial body parts). Plastic bottles and containers provide a way of distributing water that is safe to drink in locations where there are major issues of water contamination. Plastic packaging limits food and beverage spoilage through microbial contamination.

It is likely that we are ingesting some level of plastics in our diet. A rapidly growing body of research is showing that ongoing accumulation of toxins associated with plastics poses a risk to our food safety and public health. However, the levels of plastics and associated chemicals we are exposed to in our diet compared with other everyday activities has not been assessed.

Microplastics have been found in many seafood species, including various kinds of fish and shellfish. Research into commercial fish species in the South Pacific identified microplastics in up to 25% of fish collected from Auckland, Sāmoa, Tahiti and Rapa Nui, Easter Island. These plastics are often in the gut, which is removed before eating. However, they may still pose a dietary risk through plastic-associated contaminants transferring to other tissues.

Microplastics have also been detected in common table salt, tap water and up to 90% of bottled water. Scientists found on average 325 microplastic particles per litre of bottled water. Most of these particles were very small in size (6.5–100 µm), and only about 10 particles per litre were larger than 100 µm.

Recently, microplastics have even been found to be present in the air and in household dust. Most of us are likely to breathe in microplastics on a day-to-day basis. However, it is not yet known what degree of harm this exposure could be causing us or what happens to such microplastics within the human body.

Knowledge gaps

A better understanding about the use, disposal and effects of plastics in the environment is required to help mitigate harmful effects of plastic debris and better manage disposal of plastics and plastic pollution.

There are some knowledge gaps on microplastics:

• Where, how, how much and what types of plastic (particularly microplastic) enter the environment including into air, freshwater and terrestrial systems?
• How much leaching takes place from plastics (particularly absorbed chemicals) into animals and humans compared with other routes of chemical exposure?
• What level of microplastics do we encounter daily through our diet or airborne sources, and what are the potential health risks associated with these exposure rates?
• To what extent does the ingestion of microplastics actually harm individuals and populations in the real environment?

ESR scientist Dr Olga Pantos and Grant Northcott (Northcott Research Consultants) are co-leading a 5-year research study in New Zealand. The research aims to assess the extent of microplastic contamination and to advance research on the mediation of that threat. The research is being carried out in two primary case study sites in the Auckland and Nelson regions.