What Are Microplastics? Definition, Types, Sizes & Sources

A simple definition

Microplastics are solid plastic particles smaller than 5 millimeters, about the size of a sesame seed or smaller. The term was popularized by marine scientists in the 2000s, and the 5 mm cut-off is used by NOAA and most regulators. There is no agreed lower limit, but particles under 1 micrometer (µm) are usually called nanoplastics.

For scale, a human hair is about 70 µm wide and a red blood cell is about 7 µm. Most of the plastic particles that reach the bloodstream are thought to be at the small end of this range.

Primary vs. secondary microplastics

Scientists sort microplastics by how they were made.

Primary microplastics are manufactured small. Examples are microbeads in older scrubs and toothpastes, loose cosmetic glitter, plastic pellets ("nurdles") used to make products, and the polymer coatings on some fertilizers. Many of these are now banned in the US and EU. See cosmetics & personal care.
Secondary microplastics break off larger items through wear, heat, sunlight and washing. Examples are fibers from polyester clothing, dust from car tires, fragments of bottles and bags, flakes from scratched nonstick pans, and particles from food containers. Secondary microplastics make up most of what people are exposed to today.

An IUCN report estimated that synthetic textiles (about 35%) and tire wear (about 28%) were the two largest sources of primary microplastics reaching the oceans, followed by city dust. Strong

The plastics you will see named most often

Polymer	Code	Where you meet it
Polyethylene terephthalate (PET)	#1	Water and soda bottles, some tea bags, polyester clothing
Polyethylene (PE)	#2, #4	Milk jugs, bags, cling film, food pouches, cutting boards, cup linings
Polyvinyl chloride (PVC)	#3	Pipes, flooring, some cling wraps and toys
Polypropylene (PP)	#5	Food containers, baby bottles, kettles, bottle caps, cutting boards
Polystyrene (PS)	#6	Foam cups and takeout boxes, disposable cutlery
Nylon (polyamide)	#7 (other)	Clothing, "silky" tea bags, filters, fishing gear

The resin code on a container tells you the plastic type. It does not tell you whether the item sheds particles. Any plastic can shed, and heat, scratching and age make it worse.

How microplastics get into the body

Eating and drinking

Food and drinks pick up particles from packaging, processing equipment, kitchenware and the environment. A widely cited 2019 review estimated that Americans take in tens of thousands of particles a year from food alone, and many more if they mainly drink bottled water. Emerging These estimates were built from limited data and are likely too low, because they could not count nanoplastics.

Breathing

Indoor air carries fibers and fragments from carpets, upholstery, clothing and bedding. Because people spend most of their time indoors, household dust is a major and often overlooked route. See clothing & laundry.

Skin

Intact skin is a good barrier against particles. Skin contact with cosmetics is considered a minor route compared with eating and breathing, though products used near the mouth or eyes may be swallowed or inhaled.

Why they are hard to measure

Particles under about 10 µm are difficult to identify, and particles under 1 µm are at the edge of what current lab methods can see. Tests can confuse plastic with natural fats and proteins in human tissue, and plastic lab equipment can contaminate samples. That is why reported numbers vary by thousands of times between studies, and why this guide marks some findings as Contested. In 2026 the US government funded a $144 million program partly to build reliable measurement tools.

Frequently asked questions

What is the difference between microplastics and nanoplastics?

Size. Microplastics are under 5 mm. Nanoplastics are under 1 micrometer (some scientists use 100 nanometers as the cut-off). Nanoplastics are far more numerous and are more likely to cross the gut wall and enter cells, but they are much harder to measure.

Do microplastics break down in the body?

No. Common plastics do not biodegrade in the body. Some particles are thought to pass out in stool, and others may lodge in tissue. How long they stay and where they collect are open research questions.

Is biodegradable plastic free of microplastics?

Not necessarily. Many plastics labeled biodegradable only break down under industrial composting conditions. In soil, water or the body they can still fragment into small particles.

What are microplastics?