Microplastics in Spanish Bottled Water: What PET Bottles Reveal

If you drink bottled water because you assume it is cleaner than what comes out of the tap, a 2024 analysis of Spain’s best-selling brands complicates that assumption. Researchers measured a microplastic mass concentration of 1.61 µg/L in single-use PET bottles — a level two to three orders of magnitude higher than what the same lab had previously found in municipal tap water.[Scientific Reports, 2024]

That is the headline. The nuance is more interesting: the same study concluded that the actual health risk from drinking that water is probably negligible. Both things can be true, and understanding why is the whole point.

What the study actually measured

The team purchased water from the five top-selling still, low-mineralization brands sold in 1.5 L single-use PET bottles — together roughly 40% of the Spanish market share for that format. They sampled 45 litres per brand, 225 litres in total, all bottled at their respective springs.[Scientific Reports, 2024]

Crucially, this study reported mass concentrations, not just particle counts. That distinction matters more than it sounds. As the authors note, obtaining mass concentrations “is not a trivial issue and can lead to severe errors” if a method ignores the size and shape of every particle.[Scientific Reports, 2024] Counting particles tells you how many; weighing them tells you how much you are actually ingesting — which is the number that matters for any real exposure estimate.

1.61 µg/L

Alongside microplastics, the researchers tracked a less-studied pollutant: artificial cellulosic particles, or ACPs. These are cellulose fibres that show evidence of industrial processing — think dyed or treated textile fibres. They averaged 1.04 µg/L. Interestingly, there were more ACP particles by count (a median of 1.70 per litre versus 0.73 per litre for microplastics), but microplastics still weighed more overall because the cellulosic particles tended to be smaller.[Scientific Reports, 2024]

The particles matched the packaging — mostly

Of the 81 particles positively identified as plastic, the composition was dominated by polyester and polyethylene: 64 polyester particles and 12 polyethylene particles, with a handful of polyamide, polystyrene, polypropylene, and polysiloxane.[Scientific Reports, 2024]

This lines up neatly with the packaging. PET bottles are a polyester, and every cap in the study was made of polyethylene. Nearly all the polyester fragments were white or transparent — consistent with PET from the bottle — and 5 of 11 polyethylene fragments matched the colour of the caps.[Scientific Reports, 2024] The sampling protocol deliberately mimicked normal consumer handling: each bottle was opened three times and recapped twice, replicating the abrasion of everyday use.[Scientific Reports, 2024]

But the packaging was not the whole story.

The median size of the microplastics was 93 µm (interquartile range 76–130 µm), and the ACPs were slightly smaller at about 77 µm.[Scientific Reports, 2024] These are large enough to see under a microscope but far too small to notice in a glass of water.

The comparison that matters: bottle vs. tap

The most actionable finding comes from comparing these results against tap water. In an earlier study, the same research group measured microplastics in municipal drinking water from several Spanish cities and found an average of 12.5 particles per cubic metre and a mass concentration of 45.5 ng/L.[Scientific Reports, 2024]

That tap-water figure is two-to-three orders of magnitude lower than what they found in PET bottles. The authors stress that the two datasets are genuinely comparable because both were analysed in the same laboratory, with essentially the same methodology, over a similar particle size range — removing the methodological differences that usually make bottled-water studies impossible to compare.[Scientific Reports, 2024]

45.5 ng/L

The authors are direct about the implication: tap water is “not only cost-effective and environmentally friendly, reducing plastic waste and carbon footprint, but also rigorously monitored by health authorities.”[Scientific Reports, 2024] Where safe municipal water is available, this study is a data-backed argument for the tap.

Why “more plastic” does not automatically mean “more danger”

Here is where honest reporting has to slow down. A higher plastic concentration is not the same as a demonstrated health harm — and the study is careful about this.

The researchers calculated an Estimated Daily Intake from bottled water of 4 ng/kg/day for adults and 18 ng/kg/day for children (rising roughly tenfold for someone drinking 2 litres of bottled water daily). Their conclusion: exposure to plastics through bottled water “probably represents a negligible risk to human health,” because it sits several orders of magnitude below the concentrations shown to cause damage in laboratory cell models.[Scientific Reports, 2024]

That is genuinely reassuring — but the authors do not stop there, and neither should we.

For that reason the authors invoke the precautionary principle: even if the measured particles appear harmless, caution is warranted for the fraction we cannot yet measure.[Scientific Reports, 2024]

Context: the air around you counts too

One underappreciated finding puts bottled-water contamination in perspective. The researchers estimated how much airborne fibre and plastic fallout a 250 mL glass of water would collect if left uncovered for just 20 minutes indoors, using published household deposition rates. The result: roughly 0.75–1.71 particles per litre — a range strikingly similar to the microplastic and ACP levels they measured in the bottled water itself.[Scientific Reports, 2024]

In other words, the indoor air in your home may deposit a comparable load of anthropogenic particles into an open glass. This does not excuse bottled water’s higher plastic content, but it frames microplastic exposure as a broad environmental issue, not a bottled-water-specific one.

The practical takeaway

Based on this study, a reasonable reading is:

• Where safe municipal tap water is available, it is the lower-contamination choice — by two-to-three orders of magnitude in this direct, same-lab comparison.[Scientific Reports, 2024]
• The measured risk from bottled water is low. At 4–18 ng/kg/day, the estimated intake is far below levels linked to harm in laboratory models.[Scientific Reports, 2024]
• The open questions are about what we cannot measure — chronic exposure and sub-micron nanoplastics — not about the particles this study did quantify.[Scientific Reports, 2024]

The strongest case for tap water here is not fear. It is that it delivers less plastic, costs less, generates less waste, and is monitored by public health authorities — a combination bottled PET water cannot match.[Scientific Reports, 2024]