Microplastics are in effectively every kind of drinking water — tap, filtered, and bottled alike. In bottled water the biggest source of contamination isn't the spring or the treatment plant. It's the bottle and its cap. This is a summary of my master's thesis reviewing the literature on microplastic exposure through water and its effects on the human body: where the particles come from, how much of them we actually swallow, and what the evidence does and doesn't yet show about the harm they cause.
What counts as a microplastic
A microplastic is any plastic particle smaller than 5 mm (Li et al. 2020). Primary microplastics are manufactured at that size — the abrasives and exfoliants washed down drains from cosmetics and soaps (Schymanski et al. 2018). Secondary microplastics break off larger plastic as it degrades — like polypropylene (PP) shedding from a water bottle cap. Degradation speeds up when plastic is heated or stored for long periods (Mason et al. 2016). This turns out to matter a great deal for bottled water which is often stored for months in hot storage facilities.
Microplastics are in every kind of water
They've been found in bottled and fresh water worldwide, in sea salt, in aquatic life, in the air, and — in one study — in every single sample of human stool tested (Luo et al. 2019; Schwabl et al. 2019). Reported concentrations in water range widely, from roughly 10 to 300 particles per liter, largely because different labs use different detection methods. And when newer techniques go looking for the smallest particles, the counts climb: single-particle imaging of nanoplastics has found them present at orders of magnitude above microplastics, with some counts reaching almost 250,000 particles per liter (Qian et al. 2023).
The contamination happens after the bottle is capped
This is the finding at the center of the review. When researchers measured microplastic levels at every step from the well to the sealed bottle, raw well water came back with relatively low counts. Levels stayed low through filtering and even through filling. Then, after the bottles were capped, microplastic concentrations spiked (Weisser et al. 2021).
Two things drive that spike: caps whose interiors are never cleaned the way the bottle body is, and warm, long-term storage that lets plastic leach from the packaging into the water. Bottled water bought from Amazon consistently carried more microplastics than the same brands bought elsewhere — a strong hint that heat and transport conditions matter (Mason et al. 2018). The polymer fingerprint backs this up. In glass bottles, the largest share of contamination is polyethylene (PE) at about 35% — the material the caps are made from — with PET close behind (Weisser et al. 2021). In plastic bottles, PET makes up nearly 80%, matching the bottle itself (Schymanski et al. 2018). In other words, the material touching the water is the material you find in the water.
How much are we actually drinking
A meta-analysis of ingestion studies estimated the average American swallows about 60 microplastic particles a day from tap and bottled water combined — around 21,000 a year, roughly half of all the microplastics they consume from any source (Cox et al. 2019). A separate global analysis of salt, water, and beer found significantly higher contamination in developed countries (Kosuth et al. 2018). And these are conservative counts: when Mason et al. used Nile Red staining to catch particles down to 6.5 micrometers, they measured an average of 335 particles per liter in bottled water — pushing the daily estimate to nearly three times the meta-analysis figure (Mason et al. 2018).
Shape and size decide the harm
Not all microplastics behave the same way. Fibers are the most inflammatory shape (Gasperi et al. 2018), and they dominate tap water — 97–98% of the particles there (Mason et al. 2018; Li et al. 2020) — while fragments make up the bulk of what's in bottled water. Size may matter even more than shape for cytotoxicity (Hwang et al. 2019). Particles between roughly 0.1 and 10 micrometers can cross the cell membrane, and 4-micrometer particles are taken up most readily, because they sit in the sweet spot for both phagocytosis and pinocytosis in intestinal cells (Stock et al. 2019; Luo et al. 2019).
What microplastics do in the body
In model organisms and human cells, high microplastic exposure produces a consistent set of effects: oxidative stress, inflammation, and DNA strand breaks (Avio et al. 2015; Blackburn & Green 2022). In human immune cells, PP particles raised reactive oxygen species and triggered inflammatory cytokines (Hwang et al. 2019). Oxidative stress directly causes DNA mutations that lead to cell death and cancer. Polystyrene particles accumulated in the livers, kidneys, and guts of mice exposed through drinking water (Deng et al. 2018). And microplastics disrupt the gut microbiome — reducing its diversity, a change that cascades into metabolic problems — an effect shown not only with polystyrene but with common polyethylene as well (Qiao et al. 2019; Li et al. 2020; Lu et al. 2019). Microplastics can also act as carriers for chemical additives, though whether that transport happens at concerning levels is debated (Avio et al. 2015; Koelmans et al. 2016).
What the research still can't say
Nearly every study demonstrating harm used exposure levels far above what a person actually encounters — often by orders of magnitude — to force a measurable result. That means we can say with confidence that high microplastic exposure is damaging, but we cannot yet say what today's real-world exposure does over a lifetime. The most reasonable concern is slow accumulation in the liver and kidneys over decades leading to inflammatory or oxidative-stress effects (Luo et al. 2019). The research the field still needs chronic studies at realistic exposure levels, and accumulation studies in the gut, liver, and kidneys.
What this means for how water should be bottled
Two conclusions carry over directly to how you'd design a better bottle. First, source and filtering aren't the main problem — filters already capture upward of 99% of microplastics (Wolff et al. 2021), and they'll likely keep pace even as ocean plastic degrades. The problem is the packaging. Second, the fix follows from that: clean the inside of the cap the same way the bottle is cleaned, and store water cool to slow the leaching of plastic from cap and bottle into the water. The most reliable way to reach zero, though, is to remove the plastic contact points entirely — glass in place of PET, and a cap material that isn't shedding polymer into every liter.
Sources: Avio et al., Environmental Pollution (2015) · Blackburn & Green, Ambio (2022) · CDC, Water Treatment (2022) · Cox et al., Environmental Science & Technology (2019) · Deng et al., Journal of Hazardous Materials (2018) · Gasperi et al., Current Opinion in Environmental Science & Health (2018) · Hwang et al., Science of the Total Environment (2019) · Koelmans et al., Environmental Science & Technology (2016) · Kosuth et al., PLOS ONE (2018) · Li et al., Chemosphere (2020) · Lu et al., Science of the Total Environment (2019) · Luo et al., Environmental Pollution (2019) · Macleod et al., Science (2021) · Mason et al., Frontiers in Chemistry (2018) · Qian et al., PNAS (2023) · Qiao et al., Chemosphere (2019) · Schymanski et al., Water Research (2018) · Schwabl et al., Annals of Internal Medicine (2019) · Stock et al., Archives of Toxicology (2019) · Weisser et al., Water (2021) · Wolff et al., Water (2021).