Microplastics: Where They Come From and Where They End Up

Every week, each of us ingests roughly 5 grams of plastic — the weight of a credit card. Microplastics have been found in human blood, placentas, Arctic snow, and the Mariana Trench. Here's the science of how they get everywhere — and why removing them is so hard.

1. What Are Microplastics?

Microplastics are plastic particles smaller than 5 mm. They are further classified by size:

Large microplastics: 1–5 mm — visible to the naked eye. Fragments from broken bottles, bags, fishing nets.
Small microplastics: 0.001–1 mm (1–1,000 µm) — require a microscope. Most common size class in ocean samples.
Nanoplastics: <1 µm — can cross cell membranes and the blood-brain barrier. Least studied, potentially most dangerous.

400Mtonnes plastic produced/year

11Mtonnes enter oceans/year

170Tparticles floating in oceans

5gplastic ingested/person/week

2. Sources & Pathways

Microplastics are classified as primary (manufactured small) or secondary (breakdown of larger plastics).

Source	Type	Contribution	Mechanism
Tyre wear	Secondary	28%	Friction releases 20–100 g/tyre/year as particles
Synthetic textiles	Secondary	35%	Each wash cycle releases 700,000+ microfibres
Pellet spills (nurdles)	Primary	3%	Pre-production pellets lost during shipping/handling
Personal care products	Primary	2%	Microbeads in scrubs, toothpaste (now banned in many countries)
Paint degradation	Secondary	10%	Marine anti-fouling paint, road markings
Single-use packaging	Secondary	15%	UV + mechanical breakdown of bottles, bags, wrappers
Agricultural films	Secondary	7%	Mulch films fragment in soil over seasons

3. Transport & Accumulation

Once released, microplastics follow complex pathways through water, air, and soil:

Rivers: Carry ~80% of ocean microplastics. The top 20 rivers (mostly in Asia) contribute the majority. Particles travel in the water column or settle in sediments near estuaries.
Ocean gyres: Surface currents concentrate floating plastics in five subtropical gyres. The Great Pacific Garbage Patch covers 1.6 million km² — twice the size of Texas — but most plastic is microplastic, not visible debris.
Atmospheric transport: Microplastics are found in air samples from cities (~1,500 particles/m²/day in Paris) and remote mountains. They act as condensation nuclei for rain droplets, cycling back to Earth in precipitation.
Vertical sinking: Marine biofouling (algae colonisation) increases particle density above seawater, causing them to sink. Deep-sea sediments contain 10,000+ particles/litre in some locations. Marine snow aggregates carry particles to the seafloor.
Food chain: Zooplankton → small fish → large fish → humans. Bioaccumulation is limited for the particles themselves, but adsorbed pollutants (PCBs, PAHs, heavy metals) concentrate up the food chain.

4. Detection & Measurement

Identifying microplastics is difficult because they are small, diverse in polymer type, and mixed with natural organic matter.

Visual sorting (stereomicroscope): Fast but unreliable below 300 µm. High false-positive rate — natural fibres look similar.
FTIR spectroscopy: Identifies polymer type by infrared absorption spectrum. Gold standard for particles 20 µm–5 mm. Micro-FTIR images entire filter surfaces automatically.
Raman spectroscopy: Better spatial resolution than FTIR (down to 1 µm). Identifies polymer and some additives. Slower, fluorescence interference can be a problem.
Pyrolysis-GC/MS: Burns the sample and analyses combustion products. Identifies polymer type and mass concentration, but destroys the sample and cannot count individual particles.
Nile Red staining: Fluorescent dye binds to plastic surfaces. Enables rapid automated counting under UV light. Prone to false positives from natural lipids.

Counting problem: There is no universal standard for sampling, extraction, or reporting microplastics. Studies often report in different units (particles/L, particles/kg, mass/L), making comparison difficult. Harmonisation efforts (ISO, GESAMP) are ongoing.

5. Health & Ecological Effects

Marine life

Over 700 marine species have been documented ingesting plastics. Seabirds (90% of species), sea turtles (100%), and filter feeders (mussels, oysters) are most affected.
Physical effects: gut blockage, false satiation (animals feel full but starve), inflammation, reduced growth rates in fish larvae exposed to nanoplastics.
Chemical effects: plastics adsorb hydrophobic pollutants (PCBs, DDT, PAHs) from seawater at concentrations 100–1,000,000× ambient levels. These desorb in the organism's gut.

Human exposure

Ingestion: Drinking water (bottled: 325 particles/L; tap: 5.5 particles/L), seafood, salt, beer, honey. Estimated intake: 0.1–5 g/week depending on diet.
Inhalation: Indoor air contains 1–60 fibres/m³ (mostly polyester from textiles). Outdoor air: lower concentrations but more varied polymer types.
Human tissue: Detected in blood (2022, Vrije Universiteit), lung tissue (2022, Hull York), placenta (2020, Rome), and breast milk (2022, Catania). Most common polymers: PET, PE, PP, PS.
Health effects: Limited direct evidence in humans so far. Animal studies show inflammation, oxidative stress, endocrine disruption (leached BPA, phthalates), and gut microbiome changes at high doses. Long-term effects of chronic low-dose exposure are unknown.

6. Modelling Microplastic Spread

Computational models simulate how microplastics move through the environment:

Lagrangian particle tracking: dx/dt = u(x, t) + v_s \cdot ẑ + D_turb where: u(x,t) = ocean current velocity field (from HYCOM/NEMO) v_s = settling velocity (Stokes' law, modified for shape) D_turb = turbulent diffusion (random walk term) ẑ = vertical unit vector Settling velocity (sphere approximation): v_s = (2/9) \cdot (ρ_p - ρ_w) \cdot g \cdot r² / μ ρ_p = polymer density (PE: 950, PVC: 1,400 kg/m³) ρ_w = seawater density (~1,025 kg/m³) r = particle radius μ = dynamic viscosity (~1.08 \times 10⁻³ Pa\cdots)

Key modelling challenges include biofouling (which changes particle density over time), fragmentation (which creates new smaller particles), and the "missing plastic" problem — models predict far more surface plastic than is observed. Most plastic likely sinks, is ingested, or is washed ashore.

7. Mitigation & Solutions

Source reduction: Most effective. Extended producer responsibility (EPR), microbead bans (UK 2018, EU 2023), single-use plastic bans, tyre wear regulations (Euro 7 proposes limits).
Laundry filters: External filters (Filtrol, PlanetCare) capture 80–90% of synthetic microfibres. France mandated filters on all new washing machines from 2025.
Wastewater treatment: Conventional WWTPs remove 95–99% of microplastics, but effluent volumes are huge — a large plant still releases billions of particles daily. Tertiary treatment (membrane bioreactors, sand filtration) pushes removal to 99.9%.
River interceptors: Passive barriers (The Ocean Cleanup's Interceptor) and bubble curtains guide floating plastics to collection points. Effective for macroplastics; limited for microplastics.
Bioremediation: Some bacteria (Ideonella sakaiensis) and fungi can break down PET, but degradation rates are slow (~6 weeks for a thin film at 30°C). Enzymatic recycling (Carbios) accelerates this for industrial recycling but doesn't address environmental microplastics.
Alternative materials: Biodegradable polymers (PLA, PHA, PBS) degrade under specific conditions (industrial composting at 58°C), but often persist in marine environments similar to conventional plastics.

The scale challenge: Removing microplastics from the open ocean is impractical — the volume of water is too vast and concentrations too low. Prevention (stopping plastic entering the environment) is orders of magnitude more effective and cheaper than cleanup.