Microplastics databaseExplainerChecked 2026-10-01

Why do microplastic numbers differ between studies? Four points about measurement

Smallest particle size counted, identification method, count vs. weight, and blanks: how to read the numbers

The answer

Microplastic numbers change by orders of magnitude depending on (1) how small a size was counted, (2) which method confirmed the particles, (3) whether the result is a count or a weight, and (4) how contamination during the experiment was subtracted. For table salt, the identification method alone made a 10- to 600-fold difference.

When you see a number, checking whether these four are stated helps you avoid misreading it.

1. How small a size was counted

Smaller particles are far more numerous, so lowering the smallest size counted raises the count a lot. An analysis that pooled salt studies found a correlation between the smallest size counted and how high the results were (Lee et al. 2021).
A European salt survey that counted down to 0.005 mm says that counting only particles of 0.1 mm or more would have caught less than 25% of the particles found (Thiele et al. 2023).
The EU set a measurement method for drinking water (2024): particles of 0.02 mm to 5 mm, filtered from at least 1,000 L and confirmed by FTIR or Raman. This is a move to make methods consistent.

2. Which method confirmed the particles

Method	Rough lower limit	What it does	Caution
Visual inspection and staining (Nile Red, etc.)	Depends on the study	Counts particles by shape and color under a microscope	Cannot confirm whether a particle is plastic. By eye, only 48 to 50% of fibers were judged correctly (Catarino 2018).
FTIR (infrared spectroscopy)	About 0.003 to 0.02 mm	Checks the plastic type of each particle by how it absorbs light	Weak on small particles
Raman spectroscopy	About 0.001 to 0.002 mm	Checks each particle, down to smaller particles than FTIR	Slow, so often only some of the particles are examined
Py-GC/MS (pyrolysis gas chromatography–mass spectrometry)	— (by weight)	Breaks the sample down with heat and works out the plastic type and weight from what comes off	Gives no particle count or size. Hard to tell body fat from polyethylene (Rauert 2025).
SEM-EDX (electron microscope plus elemental analysis)	Down to very small particles	Looks at particle shape and elements	Cannot identify the plastic type (Oßmann 2019)
NTA (nanoparticle tracking)	Down to nano size	Measures the number and size of particles in a liquid	Does not distinguish what the particles are made of

3. Count or weight

FTIR and Raman give “how many particles,” and Py-GC/MS gives “how many μg.” A particle’s weight changes with the cube of its diameter, so counts and weights cannot be converted into each other or compared. This is one reason why studies disagree on whether sea salt or rock salt has more (→ Choosing salt).

4. Blanks and contamination

Plastic particles also get in from air, clothing and equipment. So a “blank” is measured, running the same procedure on, say, water with nothing added, and subtracted.
The bottled water figure of “about 240,000 per liter” (2024) drew criticism that the ultrapure water used as a blank was as contaminated as the bottled water and was not used, and that against it every sample would fall at or below the blank. The authors reply that the ultrapure water itself is contaminated and not suited to be a blank.
The tea bag figure of “11.6 billion per cup” (2019) counted short plastic chains (oligomers) that come off when the extract is dried as if they were particles; Germany’s BfR assesses the real figure as 2 to 3 orders of magnitude lower.

Checking the quality of studies

A study that scored 50 water studies on 9 items (0 to 2 points each) found only 4 with no item scored 0, and the reported concentrations spanned 10 orders of magnitude (Koelmans et al. 2019).
A 2026 study that checked 101 studies of the human body on 14 items found none that met all of the essential criteria. What was missing included sample representativeness, positive controls and records of blanks.

Sources

Last checked 2026-10-01. Research and regulations change over time. This page is a translation of our Japanese page; if they differ, the Japanese page is the reference.

This page lists facts from public agencies' documents, laws and peer-reviewed papers. Methods for measuring microplastics differ between studies, so numbers often cannot be compared directly. Public bodies such as WHO, FAO and EFSA do not currently recommend that individuals avoid any particular food because of microplastics.

Other explainers

← Back to the microplastics database