Between 2010 and 2015, flexible hydration flasks have become widespread at an astonishing rate, and today, it is impossible to find a bib for an ultra-trail or marathon without encountering one. Runners love them, and for good reason: they are lightweight (less than 50 g when empty for many models), easy to press for quick hydration, reduce water sloshing compared to rigid bottles, and are sold at incredibly competitive prices. For less than 10 euros, one can be purchased on Amazon, and major sports retailers like Decathlon also offer them at very attractive prices.
The overwhelming majority of these flasks are made of thermoplastic polyurethane (TPU), a synthetic elastomer derived from petrochemistry that gives them their elasticity. A material also used in the medical field for catheters and infusion tubes, known for its “relatively” harmless nature. An adjective that, in toxicology, implies that zero risk does not exist and that contact between the liquid and TPU is not completely harmless.
Running healthily or drinking plastic? The runner’s dilemma
Anyone who has already used a flexible flask knows the unpleasant and persistent aftertaste it can leave on the palate. Even after rinsing with baking soda or vinegar, and a 24-hour stint in the freezer, nothing seems to remove the taste, attributed to the TPU, or rather, the additives that give it its resistance and flexibility.
TPU in its pure state is not sufficient to make a flask; manufacturers incorporate additives (waxes, gels, also petroleum-derived) to facilitate shaping and molding, which may migrate partially to the contained liquid, ending up in your stomach.
Xavier Cousin, a researcher in physiology at INRAE, confirmed to our colleagues at “L’Équipe”: “The smell and taste of plastic are certainly due to the presence of these additives.” Jean-François Gérard, deputy scientific director at CNRS Institute of Chemistry, compared the phenomenon to silicone molds for cakes. The comparison is valid from a chemical standpoint: although they do not contain TPU, in both cases, cooking or polymerization residues can escape.
The chemical migration of an additive into a liquid depends on three variables: temperature, liquid nature, and contact duration. A cake mold only accumulates one of these variables (temperature) and is used punctually. A flexible flask accumulates all variables over hundreds of uses.
The question of usage is what fuels concerns regarding the total safety of TPU if the organism is in contact with it during regular sporting activities. Although it is free from bisphenol A and phthalates, which allows it to be listed among plastics authorized for food contact by European regulations, the variability of additives in TPU from different manufacturers means their toxicological profile may not always be documented or published.
In addition, a flask degrades with each use. Xavier Cousin explained, still in the columns of “L’Équipe”: “The other risk factor is wear or friction. If you excessively use it, twist it to get the last drop, you can weaken the plastic and absorb microplastics.” By squeezing the bottle, the polymer matrix erodes, promoting the detachment of particles.
This process is accelerated by heat, so most manufacturers set a maximum threshold of 40°C for the contained liquid, a limit easily reached by a dark flask left in the sun in July. Add a slightly acidic isotonic drink instead of water, and the three migration factors – temperature, acidity, contact duration – converge.
Microplastics: Unwelcome guests
Xavier Cousin mentioned the troublesome term: “microplastics,” fragments smaller than 5 mm, which have been at the center of a health scandal since the early 2000s. Present in our environment, they are also found in our food and bodies: blood, lungs, liver, placenta, urine, and even our brains.
Their toxicity is still being researched; several studies have demonstrated biological effects and associations with various disorders, but solid causal links are not consistently established in humans. For instance, a meta-analysis of 141 scientific studies published in PubMed in September 2025 established a link between chronic ingestion and various health issues, including respiratory problems, hormonal disturbances, neurotoxicity, reproductive toxicity, and a potential carcinogenicity – a worrisome scenario.
In March 2024, another Italian study published in the New England Journal of Medicine observed an association between the presence of micro and nanoplastics in carotid atherosclerotic plaques and an increased risk of heart attack or stroke. Although it did not establish causality, other researchers pointed out a possible bias due to the plastic particles in the surgical environment where samples were taken, making it challenging to confirm the source of microplastics. However, the presence of microplastics in human arterial tissues has been confirmed; the question of their role in cardiovascular risk remains uncertain.
Consensus around microplastics is now solidly established: these substances are not meant to be ingested by humans.
So, if you have one or more flexible flasks, should you get rid of them? In theory, we could say yes if you want to avoid all risks. But let’s be realistic: you probably consume more microplastics if you regularly drink water from PET plastic bottles. Your flask is likely not more dangerous than other common plastics, but it requires proper maintenance to avoid bacterial contamination.
In any case, the scientific consensus on microplastics is that they should not be ingested by humans.
Therefore, take action to prevent your exposure: avoid leaving the flask in a hot car or excessively squeezing it, opt for a well-known brand rather than a cheap option of unknown composition, and most importantly, clean it thoroughly – bacteria and mold are a more immediate danger compared to TPU.
