Nanoplastics Size vs Microplastics: How Small Is Nano, Really?

Last updated: June 5, 2026

The exact size definitions (and where they disagree)

There isn't one universally agreed nanoplastic size cutoff. The two definitions you'll see in the literature:

• The 1 µm cutoff. Used by most marine and food microplastic researchers, by GESAMP (the UN Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection), and by NOAA in its working definitions. A nanoplastic is anything below 1 µm.
• The 100 nm cutoff. Used by ECHA (European Chemicals Agency) and aligns with the broader EU definition of a “nanomaterial.” A nanoplastic is anything below 100 nm.

Both are defensible. The 1 µm cutoff is more practically useful because it marks the boundary where particles begin to behave differently in biology — crossing membranes, getting taken up by cells, evading the size-based filtration of the gut wall. The 100 nm cutoff is stricter and matches how regulators think about engineered nanomaterials.

Microplastics are easier — the field has converged on 1 µm to 5 mm as the consensus range. Above 5 mm is “macroplastic”; below 1 µm crosses into the nanoplastic category whichever cutoff you use.

Comparing nano to things you can picture

Why size matters biologically

The size of a particle determines which biological barriers it can cross. Three barriers matter most:

• The gut wall. Particles above ~150 µm pass through. Particles between 1 µm and 150 µm largely stay in the gut lumen, with some uptake into the lymphatic Peyer's patches. Particles below 1 µm — nanoplastics — can be taken up across the intestinal epithelium and enter circulation.
• The placenta. Ragusa et al. (2021), publishing in Environment International, found microplastics in human placental tissue at sizes down to 5 µm. Smaller nanoplastics likely cross more easily, although direct detection has been limited by instrument sensitivity.
• The blood-brain barrier. Animal studies (including the 2024 University of New Mexico work by Campen and colleagues) have detected microplastics in brain tissue. The accumulation is hypothesised to be dominated by the nanoplastic fraction, which can cross the BBB's tight junctions, whereas micrometre-scale particles cannot.

What human-tissue studies actually measured

The headline human-tissue studies of the last few years — often-quoted as “microplastics found in blood / placenta / plaque / breast milk” — typically detected particles in the low-micrometre range, not the nanometre range. This isn't because nanoplastics aren't there. It's because the detection methods used (FTIR, micro-Raman, scanning electron microscopy) have a practical lower limit of about 700 nm to 1 µm. Below that, instruments stop reliably identifying polymer chemistry.

• Leslie et al. (2022), human blood. Detected particles down to ~700 nm using py-GC/MS. Found microplastics in 77% of 22 healthy donors.
• Ragusa et al. (2021), human placenta. Detected particles 5–10 µm using Raman.
• Ragusa et al. (2022), human breast milk. Detected particles 2–12 µm using Raman.
• Marfella et al. (2024), NEJM, carotid plaque. Detected particles 1–10 µm using pyrolysis-GC/MS and electron microscopy. Patients with plaque microplastics had a 4.53× higher risk of MACE (heart attack, stroke, or death) over 34 months.

The implication: when you see a count like “microplastics detected in blood,” the actual particle burden is likely much higher than the published number, because the nano-fraction below detection is invisible to the instrument.

Why nanoplastics are likely more harmful than microplastics

Three mechanisms drive the toxicological concern around nano vs micro:

1. They cross biological barriers. Micrometre-scale particles are mostly trapped by the gut wall, the placenta, and the blood-brain barrier. Nanoplastics are not.
2. They have much higher surface area per unit mass. Surface area is where chemical migrants (BPA, phthalates, PFAS) adsorb and where reactive sites interact with cells. A given mass of plastic broken down into nanoparticles has thousands of times the surface area of the same mass as one large piece.
3. They are taken up by cells. Cell-uptake mechanisms (endocytosis) are size-dependent. Nanoparticles in the 50–500 nm range are particularly efficient at being internalised by cells, including immune cells, where they can trigger inflammatory cascades.

The blunt summary: micrometre-scale microplastics are mostly an exposure marker. Nanoplastics are the fraction that toxicologists believe is doing the actual biological damage.

The detection gap — and why it matters for the news you read

Most published microplastic counts in food, water, and tissue are undercounts. The reason: the methods used (FTIR, Raman, SEM-EDS, py-GC/MS) trade off between sensitivity and chemistry identification, and almost all of them lose reliability below ~700 nm to 1 µm. So when a study reports “5 particles per litre of tap water,” the nanoplastic fraction below the detection cutoff is excluded from that number.

A 2024 PNAS study by Qian et al. — using stimulated Raman scattering, which can resolve down to ~100 nm — reported that bottled water contains an average of 240,000 plastic particles per litre, roughly 90% of which are nanoplastics. That's 100× higher than previous bottled-water estimates and is the closest single number we have to the “true” particle load including the nano fraction.

See also our explainer nanoplastics vs microplastics for the broader comparison, and microplastics in bottled water for the Qian 2024 PNAS findings in context.