How Much Microplastic Is in Bottled Water? What One Study Found

If you drink bottled water, you are almost certainly drinking plastic along with it. In a 2018 study that tested 259 bottles from 11 brands across nine countries, 93% of the bottles showed some sign of microplastic contamination.[Mason et al., 2018]

That headline number is startling on its own. But the more useful finding is quieter: the evidence points to the bottle and its cap as a source of the contamination, not just the water inside. This section walks through exactly what the researchers measured, how they measured it, and what the numbers do — and do not — mean.

What the study actually tested

The research team purchased 259 bottles spanning 11 brands and 27 different production lots, buying from 19 locations across nine countries.[Mason et al., 2018] The lineup mixed leading international brands — including Aquafina, Dasani, Evian, Nestlé Pure Life, and San Pellegrino — with major national brands such as Aqua (Indonesia), Bisleri (India), Epura (Mexico), Gerolsteiner (Germany), Minalba (Brazil), and Wahaha (China).[Mason et al., 2018]

To detect the plastic, the researchers used a fluorescent dye called Nile Red, which adsorbs to plastics but not to the naturally occurring materials you would expect in water, and glows under specific wavelengths of light.[Mason et al., 2018] They counted and photographed particles larger than roughly 100 micrometers, then confirmed the identity of many of those particles using FTIR spectroscopy. Smaller particles, down to 6.5 micrometers, were counted with custom software.[Mason et al., 2018]

259

Two numbers, two size ranges

The study reports two different contamination figures, and confusing them is easy. They correspond to two different particle sizes.

For the larger particles (over 100 micrometers) that could be individually confirmed as polymeric by spectroscopy, the average was 10.4 microplastic particles per liter.[Mason et al., 2018] Once the smaller “Nile Red tagged” particles (6.5–100 micrometers) were included, the average rose to 325 particles per liter.[Mason et al., 2018]

The contamination was highly variable. Seventeen bottles (about 7%) showed no contamination beyond possible lab background, while a single bottle exceeded 10,000 particles per liter.[Mason et al., 2018] Averaged by brand, Nestlé Pure Life and Gerolsteiner showed the highest densities (930 and 807 particles per liter), while San Pellegrino and Minalba showed the lowest (30.0 and 63.1 particles per liter).[Mason et al., 2018]

The evidence points at the packaging

The most consequential finding isn’t the raw count — it’s where the plastic appears to come from.

Three lines of evidence in the study converge on the packaging. First, polypropylene was the single most common polymer identified (54%), and polypropylene is a plastic commonly used to make bottle caps.[Mason et al., 2018] Second, fragments were by far the most common particle shape (66%), rather than the fibers that dominated an earlier study of tap water.[Mason et al., 2018] Third, about 4% of retrieved particles carried the chemical signature of industrial lubricants — the kind used in manufacturing equipment.[Mason et al., 2018]

The authors suggest fragments could be breaking off the cap, potentially even entering the water through the simple act of opening the bottle.[Mason et al., 2018]

Glass told a cleaner story

The most direct test of the packaging hypothesis came from one brand sold in both glass and plastic. Gerolsteiner draws from the same water source for both formats, which makes the comparison unusually clean.

The water bottled in glass contained far less microplastic than the same water bottled in plastic: 204 particles per liter versus 1,410.[Mason et al., 2018] The researchers read this as evidence that some contamination comes from the water source, but a larger share likely originates from the packaging itself.[Mason et al., 2018]

204 vs. 1,410

This is a single brand and a small comparison, so it should not be read as proof that glass is always cleaner. But it is a striking, controlled data point in a field where controlled comparisons are rare.

The honest limitations

A credible reading of this study has to include what its own authors caution.

The reported numbers are described as conservative and likely to be undercounts. The particle counts were reduced using lab blanks to subtract background contamination, the microscope photos did not capture the full filter surface, and the software’s smallest detectable particle was 6.5 micrometers — meaning anything smaller went uncounted entirely.[Mason et al., 2018]

There is also the matter of health. The study is explicit that the impacts of microplastic contamination on human health remain unknown.[Mason et al., 2018] Finding particles is not the same as demonstrating harm. What the study establishes is prevalence and a probable source — not a health verdict.

How bottled water compared to tap water

The study was partly a follow-up to earlier work on globally sourced tap water, which lets us line the two up. In the same larger size range, bottled water contained on average nearly twice the microplastic of tap water — 10.4 versus 5.45 particles per liter.[Mason et al., 2018]

The particle profile also differed. In the tap water study, 98% of particles were fibers; in bottled water, only 13% were fibers and the majority (65%) were fragments.[Mason et al., 2018] That difference in shape is part of what led the authors to conclude the two sources of contamination are different — with bottled water’s fragments pointing back toward the packaging and bottling process.[Mason et al., 2018]

What to actually take from this

For a reader trying to decide what to do, the study supports a modest, evidence-based conclusion rather than alarm.

The authors’ own recommendation is a reduction in the bottling and consumption of bottled water, especially in places where clean, safe tap water is already available.[Mason et al., 2018] That is a reasonable takeaway: if your tap water is safe, this research gives you one more reason to prefer it. The glass-versus-plastic comparison, while limited to one brand, also hints that the container matters as much as the water.

What this single study cannot tell you is whether the microplastics you ingest from bottled water will harm you. That question is still open. What it does tell you, clearly, is that the plastic is there — and that the bottle is part of how it gets in.