Microplastics: What We Know, What’s Uncertain, and What It Means

A plain-language resource by Scott Coffin, PhD, environmental toxicologist. This page reflects my scientific understanding and does not represent the official position of OEHHA, CalEPA, or any other institution.

What are microplastics?

Microplastics are tiny plastic particles — typically defined as particles smaller than 5 millimeters. They come in many shapes: fragments, fibers, films, beads, and more. They vary in size from particles visible to the naked eye down to nanometer-scale particles too small to see without a microscope.

The term covers a wide range of materials, sizes, shapes, and chemical compositions. This diversity is one of the reasons microplastics are scientifically challenging to study and regulate.

Where do microplastics come from?

Microplastics enter the environment through two main pathways:

Primary microplastics are manufactured at small sizes and used directly — in products like microbeads in personal care products (now banned in many places), plastic pellets used in manufacturing, and synthetic fibers from textiles.

Secondary microplastics result from the breakdown of larger plastic items — bottles, bags, fishing gear, tires, and other plastic products — through weathering, UV exposure, and physical fragmentation.

Common sources include:

  • Tire and road runoff
  • Synthetic textile fibers (released during washing)
  • Fragmentation of single-use plastics
  • Degradation of plastic waste in the environment
  • Agricultural plastics (mulch films, etc.)

Where are microplastics found?

Microplastics have been detected in virtually every environmental compartment studied:

  • Drinking water (tap and bottled)
  • Surface water, groundwater, and ocean water
  • Seafood and freshwater fish
  • Air
  • Soil and agricultural land
  • Snow and ice, including remote Arctic and Antarctic regions
  • Human blood, lung tissue, placenta, and other tissues

The detection of microplastics in so many environments reflects both their widespread distribution and improving analytical methods — we are getting better at finding them.

Are microplastics in my drinking water?

Yes — microplastics have been detected in tap water and bottled water in studies across the world, including in California. Concentrations vary widely depending on the source water, treatment, and measurement method.

California became the first government in the world to require testing for microplastics in drinking water under Senate Bill 1422. That monitoring program is now underway, and results will help characterize what Californians are drinking and at what levels.

What do we know about health effects?

What is known:

  • Humans are exposed to microplastics through ingestion (food and water), inhalation, and possibly dermal contact.
  • Microplastics have been detected in human tissues, including blood, lungs, and the placenta — confirming that particles can reach and persist in the body.
  • In laboratory studies, certain types of microplastics and associated chemicals have shown biological effects at high doses, including inflammation, oxidative stress, and disruption of cellular processes.

What remains uncertain:

  • The doses at which health effects occur in humans — most laboratory studies use concentrations far above what is typically found in the environment.
  • Whether the physical presence of particles or the chemicals associated with them (additives, adsorbed pollutants) drives potential effects.
  • The long-term consequences of chronic low-level exposure, which is the exposure scenario most relevant to real-world conditions.
  • Which particle sizes, shapes, and polymer types matter most for human health.

What this means for decisions:

Current evidence does not establish a clear causal link between real-world microplastic exposures and specific human health outcomes. However, the ubiquity of exposure, the presence of particles in human tissues, and precautionary considerations have motivated California and other jurisdictions to act — developing monitoring frameworks, health-based guidance, and risk assessment methods — even while the science continues to develop.

TODO: Scott, please review and confirm this framing reflects your current assessment.

What about ecological effects?

Extensive research has been conducted on the effects of microplastics on marine and freshwater organisms. Effects documented in laboratory studies include:

  • Reduced feeding, growth, and reproduction in fish, invertebrates, and other organisms at elevated exposures
  • Physical blockage and inflammation in digestive systems
  • Transfer of adsorbed chemicals from microplastics to tissues

Risk assessments — including work I’ve led or contributed to — have found that microplastic concentrations in some environments (including parts of California) may approach or exceed levels associated with adverse effects in the most sensitive species. However, significant uncertainty remains, and risk varies greatly by location, particle type, and species.

What is being done?

In California:

  • The world’s first regulatory definition of microplastics in drinking water was developed under SB 1422 (definition adopted 2020).
  • The world’s first statewide monitoring program for microplastics in drinking water launched under SB 1422 (monitoring began 2023).
  • A health-based risk assessment framework for microplastics in drinking water was published in 2022.
  • SB 1147 (signed 2024) directs OEHHA to assess the human health effects of microplastics in drinking and bottled water — the work I am currently conducting.

Globally:

  • Multiple international bodies have initiated assessments of microplastics in drinking water, food, and the environment.
  • Negotiations for a global plastics treaty are ongoing.
  • Research on monitoring methods, health effects, and risk thresholds is expanding rapidly.

Learn more

This page is a living document. If you have questions or notice something that should be updated, please contact me.