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HOW WE GRADE A FINDING

Evidence Levels

Every key finding on this site carries a grade from A (consensus) to E (unknown), and factual records carry a separate mark, P. This page gives the rubric, worked examples, and how grades are assigned and revised. The levels are this site's editorial categories, not a universal scientific standard.

8 min readUpdated ReviewedAConsensusBWell supportedCLimitedDDisputedEUnknownPProvenance

A grade on this site belongs to a finding, not to a study, a journal or a topic. It answers one question: how much independent, concordant evidence stands behind this exact statement? Two published estimates of weekly plastic intake differ about a millionfold, from roughly 4.1 µg to 5 g (our calculation from1 and2). So the intake figure is graded D, while the fact that people take in plastic is not in question.

Method

A grade is per finding. A strong study can support a weak claim, and a single paper can contain an A finding and a D finding side by side. The grade of a study never transfers automatically to the claims made from it, and a broad finding's grade never transfers to a precise number.

The five levels and the provenance mark

The rubric is quoted exactly from the site's editorial brief3, each definition followed by what it means in practice.

A consensus: multiple independent studies or a systematic review plus a statement by a scientific body. We read it as two routes, several independent studies or a systematic review. Both also need a written statement by a scientific body, such as a national academy or a UN agency.

B well supported: several independent studies with concordant findings. Different teams, places or methods arrive at the same answer.

C limited: few studies, narrow conditions, or methodological caveats. It may be correct, yet rests on one or two studies, a laboratory setup, a small sample or a method with blind spots.

D disputed: conflicting results; both sides must be shown. Published work points different ways, and the page names who says what and why.

E unknown: no reliable evidence found; state the search scope. We looked and found nothing we could rely on, and we say where and when we looked.

P provenance: checked against the primary record. Since 3 November 2026, A–E apply to scientific findings only4. Bibliographic facts, legal texts, official metadata, definitions quoted from a standard, our own arithmetic and statements about this site's process carry P. A confirmed publication date is a checked fact, not consensus.

Each mark on the page is a black-and-white shape beside its letter; the level's name is always available as text. A is a solid circle, B a three-quarter-filled circle, C a quarter-filled circle, D a circle half solid and half hatched, and E an empty dashed circle. P is an outlined, unfilled square, so it cannot be read as a grade. D and E are not lower rungs below C: they mark conflict and absence of evidence.

The evidence levels of this site (rubric version 1.1) and the separate provenance mark

Table of six marks: A, B and C as one sequence of decreasing concordant evidence; D (conflict) and E (absence) set apart; P for checked factual records, outside the scale. Source:3,4 Site original (this site's own record)Nanoplastics ObservatorySite original
Data table
labelvalueunitgroupposition
A consensusmultiple independent studies or a systematic review plus a statement by a scientific bodyamount of concordant evidence1
B well supportedseveral independent studies with concordant findingsamount of concordant evidence2
C limitedfew studies, narrow conditions, or methodological caveatsamount of concordant evidence3
D disputedconflicting results; both sides must be shownconflict (not a lower rung)apart
E unknownno reliable evidence found; state the search scopeabsence (not a lower rung)apart
P provenancechecked against the primary recordfactual records, not a scientific gradeseparate

Two systems elsewhere are related, not copied. GRADE rates the certainty of clinical evidence from high to very low, starting from study design5. The IPCC combines the amount of evidence with the degree of agreement6. Our A–C sequence borrows the second idea; D and E have no direct counterpart.

Worked examples

The full catalog of graded findings lives on Evidence by Status.

Level A

Microplastics are consistently found in ocean surface waters worldwide. Grade A. Decades of surveys, a 20-year review7 and the European academies' evidence review8 agree. The counts are separate findings: more than 5.25 trillion floating particles (2014 estimate) and 82 to 358 trillion for 2019 are model estimates, graded C9,10.

Data are insufficient for a full human health risk assessment of micro- and nanoplastics. Grade A. The World Health Organization concluded this for drinking water in 2019 and for food and air in 202211,12. The European Food Safety Authority noted missing nanoplastic data in 201613; its page reviewed 21 October 2026 still calls knowledge limited, with an opinion due by the end of 202714. This finding is about missing evidence, not harm or safety.

Level B

Plastic particles travel through the air and settle in remote places. Grade B. Independent teams found them in a remote Pyrenees catchment15 and western US protected areas16. Others found them in snow from the Alps to Arctic sea ice17 and as nanoplastics in polar ice18. We found no scientific body's statement on remote deposition, so it stays at B. Single-study numbers, such as 132 particles per square meter per day16, are C.

Spreading sewage sludge on farmland adds microplastics to soil. Grade B. In 31 Chilean fields, the median count rose from 1.1 to 3.5 particles per gram of dry soil as sludge applications rose from one to five19. In 16 Spanish fields, sludge-amended soils held about 2.6 times more particles than unamended ones (our calculation)20. A European mass-flow estimate points the same way21. Each study's number alone is C.

Level C

Plastic detected in carotid plaque is associated with heart attack, stroke or death. Grade C. Marfella and colleagues detected polyethylene in plaque from 150 of 257 Italian surgical patients, with a hazard ratio of 4.53 over about 34 months22. It is one observational study; letters question contamination control and confounding23.

Sand filters retain model nanoplastics in drinking-water treatment. Grade C. In a pilot rapid sand filter with aged sand, 99.5% of labeled test particles stayed in the first 0.9 m during one nine-hour run24. A model of the full-scale treatment train gave more than 3-log removal, dominated by slow sand filtration25. One group, engineered particles, a modeled full-scale value.

Level D

How much plastic sits in the human brain is disputed. Grade D. Nihart and colleagues reported a 2024 frontal-cortex median of 4,917 µg/g (about 4.9 mg/g) by pyrolysis gas chromatography–mass spectrometry, against 3,345 µg/g in different people who died in 2016, a cross-sectional comparison26.

A Matters Arising of 13 November 2025 cites limited contamination controls and missing validation, arguing that lipid-rich tissue can mimic a polyethylene signal27. The authors reply that the lipid comparison mixes dry- and wet-weight percentages, that polymers without known lipid interference rose too, and that blanks held no detectable polymer28,29. A 2026 framework asks for at least two independent methods30.

The weekly mass of plastic a person takes in is disputed. Grade D. Senathirajah and colleagues estimated 0.1–5 g per week by ingestion of particles up to 1 mm, converting counts into mass2,31. Pletz argues that this conversion overestimates the mass by several orders of magnitude32. Mohamed Nor and colleagues modeled a median adult intake of about 4.1 µg per week from food and air, for 1–5,000 µm, graded C on its own1.

Level E

A regional or global mass of nanoplastics in farm soils is not established. Grade E. Method studies, crop-tissue studies and spiked-tracer experiments do not give native soil mass. The only field quantification we found is one 2026 study of polystyrene at one compost-amended site in France: 2.57 µg/g in the 200–1,000 nm fraction of the topsoil, about 28 times a reference plot33.

A 2024 isotope estimate at a second French field covers only the mobile fraction in soil water, about 0.25 µg/kg down to 55 cm34. Search scope: Crossref and web searches on 3 November 2026 for nanoplastic quantification in agricultural soil, 2024–2026, and this site's registry.

Long-term health effects of nanoplastics in people are not established. Grade E. We found no completed study following initially healthy people with a nanoplastic-specific exposure measure. Marfella's association (level C, above) concerns surgical patients and micro- and nanoplastics together22. A Beijing cohort registered in August 2026 expects results by the end of 202935. Search scope: ClinicalTrials.gov and a web search on 3 November 2026, WHO 202212 and two reviews36,37.

Mark P

The U.S. definition of a HEPA filter is at least 99.97% removal of 0.3 µm particles. P. A definition quoted from an agency page, not a measured result38.

One 1 mm sphere has the mass of a trillion 100 nm spheres of the same plastic. P. Our own arithmetic, explained on How to Read the Numbers.

How a graded key number reads: two published estimates of weekly plastic ingestion per person, 0.1–5 g and about 4.1 µg, from different models and size ranges, graded D (disputed). Data: Senathirajah et al. 2021; Mohamed Nor et al. 2021.Nanoplastics ObservatorySite original

How a grade is assigned and reviewed

Four roles touch every grade. The writer proposes a level and a rationale in the claims file. A verifier opens each cited source and marks the claim confirmed, contradicted, unsupported or unknown. A reviewer argues against the grade, and the editor decides. Details are on How We Work and Methods & Sources.

Every page is still a draft. A published grade will carry its date, sources and approving role; those fields are planned, not yet in the ledgers.

Decision flow for one finding: are there reliable sources, do they conflict, are they independent and concordant, has a scientific body made a statement. Conflicts route to D; nothing reliable routes to E; factual records route to P.Nanoplastics ObservatorySite original

What changes a grade

A grade is a snapshot, and five things move it. New independent studies that agree can lift a finding from C to B, and a scientific body's statement can then lift it to A. A correction, a retraction or a credible critique can move it down or to D. A new reliable measurement turns an E into a graded finding.

A mistake in how we applied the rubric, our arithmetic or a citation also changes a grade, without any new paper. We plan a public grade history with old level, new level, date and reason; it has no entries yet.

What grades do not mean

A grade is not a measure of risk. An A finding can be bad news, and an E finding can matter a great deal. A grade is also not a measure of importance or of the quality of one paper. One primary study supports at most C for its own result, because it cannot show independent replication.

The levels are not steps on one scale. A to C describe how much concordant evidence exists; D describes conflict, and E absence. So D must show both sides and E must state where we searched. P is not a grade. Unanswered questions are listed on Knowledge Gaps.

Is a C finding wrong?

No. It means the finding rests on few studies, narrow conditions or a method with known caveats. Many C findings will move up as studies accumulate; some will move down.

Why is a whole study not graded?

One paper often contains several findings of different strength. Marfella et al. 2024 reports detection in plaque and an association with outcomes; each is graded separately (level C,22).

Who decides that a scientific body has made a statement?

We count written statements by academies, UN agencies and national or European agencies, such as SAPEA 2019 or WHO 2022, and cite them by name (8,12).

What does P mean?

The record was checked against its primary source: a publication date, a legal text, a definition or our own arithmetic. It says the fact is right, not that a scientific question is settled.

Can I challenge a grade?

Yes, through Corrections & Contact. Send the finding, the grade you would give and your reason. A misapplied rubric or an arithmetic slip is a valid report without a new source.

Version and change log

Version 1.1, 3 November 2026: adds the provenance mark P; states how we read the A rule; limits the single-study rule to one primary study's own result. Version 1.0, 3 November 2026: first rubric and examples. The A–E wording matches the editorial brief verbatim. Any change to a definition raises the version and is listed here with its date.

Sources

  1. Mohamed Nor NH, Kooi M, Diepens NJ, Koelmans AA (2021). Lifetime Accumulation of Microplastic in Children and Adults. Environmental Science & Technology. doi:10.1021/acs.est.0c07384 journal article
  2. Senathirajah K, Attwood S, Bhagwat G, Carbery M, Wilson S, Palanisami T (2021). Estimation of the mass of microplastics ingested – A pivotal first step towards human health risk assessment. Journal of Hazardous Materials. doi:10.1016/j.jhazmat.2020.124004 journal article
  3. Nanoplastics planetary observatory (2026). Methods and production record of the Nanoplastics planetary observatory (project brief, content schema and tools, agent prompts, registry build notes, verification and review audits). Nanoplastics planetary observatory (site methodology record). Methods record on this site site record open access
  4. Nanoplastics planetary observatory (2026). Editorial decisions record and complete-solution criterion of the Nanoplastics planetary observatory. Nanoplastics planetary observatory (site editorial record). Methods record on this site site record open access
  5. Guyatt GH, Oxman AD, Vist GE, Kunz R, Falck-Ytter Y, Alonso-Coello P et al. (2008). GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. BMJ 336(7650):924–926. doi:10.1136/bmj.39489.470347.AD journal article
  6. Mastrandrea MD, Mach KJ, Plattner GK, Edenhofer O, Stocker TF, Field CB et al. (2011). The IPCC AR5 guidance note on consistent treatment of uncertainties: a common approach across the working groups. Climatic Change 108:675–691. doi:10.1007/s10584-011-0178-6 journal article open access
  7. Thompson RC, Courtene-Jones W, Boucher J, Pahl S, Raubenheimer K, Koelmans AA (2024). Twenty years of microplastic pollution research—what have we learned?. Science. doi:10.1126/science.adl2746 review
  8. SAPEA (Science Advice for Policy by European Academies), Koelmans B, Pahl S, Backhaus T (2019). A Scientific Perspective on Microplastics in Nature and Society (Evidence Review Report No. 4). SAPEA, Berlin. doi:10.26356/microplastics report open access
  9. Eriksen M, Lebreton LCM, Carson HS, Thiel M, Moore CJ, Borerro JC et al. (2014). Plastic Pollution in the World's Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea. PLoS ONE. doi:10.1371/journal.pone.0111913 journal article open access
  10. Eriksen M, Cowger W, Erdle LM, Coffin S, Villarrubia-Gómez P, Moore CJ et al. (2023). A growing plastic smog, now estimated to be over 170 trillion plastic particles afloat in the world’s oceans—Urgent solutions required. PLOS ONE. doi:10.1371/journal.pone.0281596 journal article open access
  11. World Health Organization (2019). Microplastics in drinking-water. World Health Organization, Geneva. www.who.int/publications/i/item/9789241516198 report open access
  12. World Health Organization (2022). Dietary and inhalation exposure to nano- and microplastic particles and potential implications for human health. World Health Organization, Geneva. www.who.int/publications/i/item/9789240054608 report open access
  13. EFSA Panel on Contaminants in the Food Chain (CONTAM) (2016). Presence of microplastics and nanoplastics in food, with particular focus on seafood. EFSA Journal. doi:10.2903/j.efsa.2016.4501 report open access
  14. European Food Safety Authority (EFSA) (2026). Microplastics and nanoplastics in food (topic page). EFSA, last reviewed 21 October 2026. www.efsa.europa.eu/en/topics/microplastics-and-nanoplastics-food web open access
  15. Allen S, Allen D, Phoenix VR, Le Roux G, Durántez Jiménez P, Simonneau A et al. (2019). Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nature Geoscience. doi:10.1038/s41561-019-0335-5 journal article
  16. Brahney J, Hallerud M, Heim E, Hahnenberger M, Sukumaran S (2020). Plastic rain in protected areas of the United States. Science. doi:10.1126/science.aaz5819 journal article
  17. Bergmann M, Mützel S, Primpke S, Tekman MB, Trachsel J, Gerdts G (2019). White and wonderful? Microplastics prevail in snow from the Alps to the Arctic. Science Advances. doi:10.1126/sciadv.aax1157 journal article open access
  18. Materić D, Kjær HA, Vallelonga P, Tison JL, Röckmann T, Holzinger R (2022). Nanoplastics measurements in Northern and Southern polar ice. Environmental Research. doi:10.1016/j.envres.2022.112741 journal article
  19. Corradini F, Meza P, Eguiluz R, Casado F, Huerta-Lwanga E, Geissen V (2019). Evidence of microplastic accumulation in agricultural soils from sewage sludge disposal. Science of The Total Environment 671:411–420. doi:10.1016/j.scitotenv.2019.03.368 journal article
  20. van den Berg P, Huerta-Lwanga E, Corradini F, Geissen V (2020). Sewage sludge application as a vehicle for microplastics in eastern Spanish agricultural soils. Environmental Pollution 261:114198. doi:10.1016/j.envpol.2020.114198 journal article
  21. Nizzetto L, Futter M, Langaas S (2016). Are Agricultural Soils Dumps for Microplastics of Urban Origin?. Environmental Science & Technology. doi:10.1021/acs.est.6b04140 journal article
  22. Marfella R, Prattichizzo F, Sardu C, Fulgenzi G, Graciotti L, Spadoni T et al. (2024). Microplastics and Nanoplastics in Atheromas and Cardiovascular Events. New England Journal of Medicine. doi:10.1056/NEJMoa2309822 journal article
  23. Correspondence (multiple letter authors) (2024). Microplastics and Nanoplastics in Atheromas. New England Journal of Medicine. doi:10.1056/nejmc2404154 journal article
  24. Pulido-Reyes G, Bianco C, Magherini L, Sethi R, von Gunten U, Kaegi R et al. (2023). Rückhalt von Nanoplastik-Partikeln bei der Wasseraufbereitung. Aqua & Gas (SVGW/VSA), issue 7/8, published online 6 July 2023. www.aquaetgas.ch/wasser/trinkwasser/20230707_ag7_8-r%C3%BCckhalt-von-nanoplastik-partikeln-bei-der-wasseraufbereitung/ journal article open access
  25. Pulido-Reyes G, Magherini L, Bianco C, Sethi R, von Gunten U, Kaegi R et al. (2022). Nanoplastics removal during drinking water treatment: Laboratory- and pilot-scale experiments and modeling. Journal of Hazardous Materials. doi:10.1016/j.jhazmat.2022.129011 journal article
  26. Nihart AJ, Garcia MA, El Hayek E, Liu R, Olewine M, Kingston JD et al. (2025). Bioaccumulation of microplastics in decedent human brains. Nature Medicine. doi:10.1038/s41591-024-03453-1 journal article
  27. Abdolahpur Monikh F, Materić D, Valsami-Jones E, Grossart HP, Altmann K, Holzinger R et al. (2025). Challenges in studying microplastics in human brain. Nature Medicine. doi:10.1038/s41591-025-04045-3 journal article
  28. Campen MJ, West AB, Garcia M, Gullapalli R, El Hayek E (2025). Reply to: Challenges in studying microplastics in human brain. Nature Medicine. doi:10.1038/s41591-025-04046-2 journal article
  29. Viola, Makdissi, Musacchio, Aliprandi, Franco Taveras, Mayer et al. (2026). Healthy mice show resilience to chronic nano- and microplastic exposure in the brain. Frontiers in Immunology 17:1917614. doi:10.3389/fimmu.2026.1917614 journal article open access
  30. Thomas KV, Belz S, Booth AM, Clift MJD, Cross RK, Davies G et al. (2026). Communicating Confidence in the Reliability of Micro- and Nanoplastic Identification in Human Health Studies. Environment & Health. doi:10.1021/envhealth.5c00671 journal article
  31. Dalberg Advisors, de Wit W, Bigaud N, WWF (2019). No Plastic in Nature: Assessing Plastic Ingestion from Nature to People. WWF International / University of Newcastle (Australia). www.newcastle.edu.au/news/2019/06/plastic-ingestion-by-people-could-be-equating-to-a-credit-card-a-week press open access
  32. Pletz M (2022). Ingested microplastics: Do humans eat one credit card per week?. Journal of Hazardous Materials Letters 3:100071. doi:10.1016/j.hazl.2022.100071 journal article open access
  33. Ryzhenko N, Dia A, Pattier M, Davranche M (2026). From compost-amended soil to plant tissues: field quantification of environmentally relevant polystyrene nanoplastics and limited translocation in Holcus lanatus L.. NanoImpact 44:100647 (online 19 September 2026). doi:10.1016/j.impact.2026.100647 journal article open access
  34. Wahl A, Davranche M, Dia A, Vilbert D, Khatib I, Pattier M et al. (2024). Nano(micro)plastic mobility in soil: Metallic additives and Sr isotopes as potential tracers. Journal of Hazardous Materials. doi:10.1016/j.jhazmat.2024.135153 journal article open access
  35. Beijing Anzhen Hospital (2026). Association of Micro-Nanoplastic Exposure With Coronary Microvascular Dysfunction and Subsequent Cardiovascular Outcomes: A Prospective Cohort Study (NCT07748767). ClinicalTrials.gov registry record. clinicaltrials.gov/study/NCT07748767 dataset open access
  36. Chartres N, Cooper CB, Bland G, Pelch KE, Gandhi SA, BakenRa A et al. (2024). Effects of Microplastic Exposure on Human Digestive, Reproductive, and Respiratory Health: A Rapid Systematic Review. Environmental Science & Technology. doi:10.1021/acs.est.3c09524 review
  37. Pan Y, Janjua TI, Thomas KV, Shepherd CE, Rauert C (2026). A critical review of micro- and nanoplastic permeation in the human body. Microplastics and Nanoplastics. doi:10.1186/s43591-026-00177-6 review open access
  38. US Environmental Protection Agency (2026). What is a HEPA filter?. US EPA. www.epa.gov/indoor-air-quality-iaq/what-hepa-filter web open access

Evidence levels and provenance

AConsensusUsed on this page
Multiple independent studies or a systematic review plus a statement by a scientific body.
BWell supportedUsed on this page
Several independent studies with concordant findings.
CLimitedUsed on this page
Few studies, narrow conditions, or methodological caveats.
DDisputedUsed on this page
Conflicting results; both sides are shown.
EUnknownUsed on this page
No reliable evidence found; the search scope is stated.
PProvenanceUsed on this page
Provenance: checked against the primary record. A separate category, not an evidence grade: used for bibliographic facts, legal texts, official metadata, definitions quoted from a standard, the site's own arithmetic and statements about the site's process.

Editorial categories of this site. A–E grade scientific findings, per claim, with a one-line rationale; D and E are not lower rungs of the A–C scale. P is not a grade: it marks facts checked against their primary record.