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What are microplastics?

Microplastics are plastic particles smaller than 5 millimeters. They enter the environment from many sources. One of the largest is the washing of synthetic clothing — polyester, nylon, and acrylic garments shed plastic fibers with every wash cycle, too small for water treatment to fully capture.

How clothing releases microplastics

When a synthetic garment is washed, friction between fibers and water causes tiny plastic fragments to break off. A single wash cycle releases between 100,000 and 6.3 million microplastic fibers per kilogram of synthetic textile. The range varies more than 850-fold depending on the fabric — polyester softshell sheds the most, at 6.3 million fibers per kilogram.

Tumble drying adds to this: polyester fleece releases up to 1,700 mg of microplastic fibers per kilogram of textile in a single drying cycle.

Source: Environmental Science and Pollution Research, Springer, 2021 (PMC7969573); De Falco et al., Scientific Reports, Nature Publishing Group, 2019

Where microplastics end up

Water systems
Microplastic fibers shed during washing are 360 to 660 micrometers long and 12 to 16 micrometers in diameter — small enough to pass through most wastewater treatment processes. A single treatment facility can still release up to 65 million microplastic fibers per day into receiving water bodies. Combined U.S. and Canadian laundry releases an estimated 878 tonnes of synthetic microfibers into the environment annually, after treatment.
Murphy et al., ACS Environmental Science and Technology, 2016; PLOS ONE, 2021
Marine life
Chronic exposure to synthetic microplastic fibers causes aneurysms, surface erosion, and serious gill damage in fish, impairing their ability to survive in natural environments. Laboratory testing found polyester microfibers toxic to sea urchin embryonic development at concentrations found in current marine environments.
PMC11510891, peer-reviewed, 2024; ScienceDaily reporting on peer-reviewed research, 2020
Drinking water
The World Health Organization has identified microplastics in both tap and bottled drinking water globally. Current evidence is insufficient for a definitive health risk assessment, but microplastics are present throughout the drinking water supply.
WHO, Microplastics in Drinking-Water, 2019
The human body
Microplastics have been detected in 8 of 12 human organ systems. A 2023 study found nine types of microplastics across five types of cardiac tissue in patients undergoing heart surgery. A 2025 Nature Medicine study detected microplastics in human brain tissue at concentrations substantially higher than liver or kidney tissue.
Journal of Global Health, 2024 (PMC11342020); ACS Environmental Science and Technology, 2023 (PMID 37440474); Nihart et al., Nature Medicine, 2025

What the research does not yet show

The presence of microplastics in human tissue has been confirmed across multiple independent studies. What has not yet been established is a causal relationship between microplastic exposure and specific health outcomes. Research is active and growing. The findings above are detection-based — they show microplastics are present, not what level causes harm.

The Plain Thread approach

Every brand in this directory uses natural fibers as their primary material. Natural fibers do not contain synthetic polymers and do not shed microplastic particles. Learn about natural fibers or read how we verify brands.

Sources

Environmental Science and Pollution Research, Springer, 2021 — fiber shedding during washing and drying
De Falco et al., Scientific Reports, Nature, 2019 — microfiber shedding measurements and fiber size
PLOS ONE, 2021 — North American annual microfiber release estimate
WHO, Microplastics in Drinking-Water, 2019
Journal of Global Health, scoping review, 2024 — microplastics in 8 of 12 human organ systems
ACS Environmental Science and Technology, 2023 — microplastics in cardiac tissue
Nihart et al., Nature Medicine, 2025 — microplastics in human brain tissue