What Are Microplastics? A Plain-English Guide

Microplastics are plastic particles smaller than 5 millimetres in their longest dimension. The term was first used in a 2004 paper by Richard Thompson and colleagues at Plymouth University, published in Science¹⁷. Two decades of subsequent research have transformed them from an obscure marine science curiosity into one of the most studied environmental health concerns of our time²0.

The definition

The scientific definition describes a microplastic as any synthetic polymer particle measuring between 1 nanometre and 5 millimetres. Nanoplastics — below 1 micrometre — are a subset of particular concern because their tiny size allows them to cross biological barriers more readily¹².

A 2024 review in Science, marking 20 years since Thompson's paper, concluded that microplastics are now "recognized as a highly diverse set of globally important contaminants" detected in "1,300 aquatic and terrestrial species, from invertebrates at the base of the food web to apex predators."²0

Primary vs secondary microplastics

Primary microplastics — manufactured small from the start:

• Microbeads in cosmetics and toothpaste (now banned in rinse-off products in the EU, UK, and USA)
• Plastic pellets (nurdles) used as raw material in manufacturing
• Synthetic textile fibres shed from clothing during washing
• Industrial abrasive particles

Secondary microplastics — form when larger plastics break down:

• Plastic packaging fragmenting in the environment
• Tyre rubber worn onto roads (estimated to account for approximately 28% of ocean microplastic content)²¹
• Fishing gear degrading at sea
• Plastic bottles and containers weathering in landfill and waterways

The most common polymer types found in humans

Studies consistently find these polymers in environmental and human samples¹²:

• Polyethylene terephthalate (PET) — water bottles, synthetic clothing (most common in blood samples¹)
• Polyethylene (PE) — packaging bags, cutting boards
• Polypropylene (PP) — food containers, bottle caps
• Polystyrene (PS) — disposable packaging, food containers
• Nylon (polyamide) — fishing line, clothing
• Polyester — the dominant source of airborne fibres from textiles

Where microplastics have been found in humans

As of 2026, microplastics have been confirmed in human:

• Blood — detected in 77% of healthy adult volunteers tested¹
• Lungs — confirmed in surgically removed tissue samples
• Testicles — detected in all human testicular samples examined in a 2024 study⁷
• Semen — detected in the majority of samples in multiple 2023–2024 studies⁸
• Placenta — confirmed on both maternal and fetal sides³
• Breast milk — detected in 76% of samples in the first study to test for them⁴
• Arterial plaque — confirmed using pyrolysis–GC/MS and electron microscopy²

How much are people consuming?

A 2019 systematic analysis published in Environmental Science & Technologyestimated that the average adult in the United States ingests between 39,000 and 52,000 microplastic particles per year through food and drink, rising to 74,000–121,000 when inhalation is included¹⁶. People who drink only bottled water rather than filtered tap water consume significantly more.