Between 2010 and 2015, the popularity of soft hydration flasks skyrocketed, and today it’s nearly impossible to find a bib for an ultra-trail or marathon without encountering one. Runners love them, and it’s easy to see why: they are lightweight (less than 50 g when empty for many models), easy to squeeze for quick hydration, offer less water sloshing than rigid bottles, and are sold at incredibly competitive prices. For less than 10 euros, you can find one on Amazon, and major sports retailers like Decathlon also offer them at very attractive prices.
The vast majority of these flasks are made of thermoplastic polyurethane (TPU), a synthetic elastomer derived from petrochemistry that gives them their elasticity. This material, also used in the medical field for catheters and infusion tubing, is known for its relative safety. In toxicology, the term “relative safety” implies that a zero-risk scenario doesn’t exist and contact between the liquid and TPU is not completely harmless.
The dilemma faced by runners is whether they choose to run healthily or drink from a plastic flask. Anyone who has used a soft flask is familiar with the lingering and persistent aftertaste it can leave on the palate. Despite efforts to clean it with baking soda or vinegar, and even after a 24-hour stint in the freezer, that taste remains. The culprit is the TPU or more precisely, the additives that give the flask its resistance and flexibility.
TPU on its own isn’t enough to make a flask; manufacturers incorporate additives like waxes and gels (also derived from petroleum) to make it easier to shape and mold. These substances can then partially migrate to the liquid stored in the flask, which ultimately ends up in your stomach.
Researchers have pointed out that the migration of additives from TPU to the liquid depends on three variables: temperature, the nature of the liquid, and the duration of contact. Unlike a cake mold that only accumulates one variable (temperature) and is used occasionally, a soft flask accumulates all these variables over hundreds of uses.
This raises questions about the total safety of TPU when the body comes into contact with it during regular exercise. While TPU is free of bisphenol A and phthalates, making it compliant with European regulations on plastics in contact with food, the variability of additives among manufacturers and their toxicological profile are not always well-documented or published.
Additionally, a soft flask degrades with each use. When excessively twisted or compressed, the polymer matrix weakens, potentially causing the detachment of particles. This process is exacerbated by heat, which is why most manufacturers set a maximum threshold of 40°C for the liquid, a temperature easily reached by a dark flask left in the sun.
It’s this question of usage that fuels concerns over the complete safety of TPU if the body is in contact with it during regular physical activity. While exempt from BPA and phthalates, the additives in TPU can vary between manufacturers, and their toxicological profile may not be well-documented or published.
Meanwhile, the microplastics, consisting of pieces smaller than 5 mm, have been at the center of a health scandal since the early 2000s. These particles are omnipresent in our environment, including our food supply and our bodies, such as blood, lungs, liver, placenta, urine, and even the brain.
While their toxicity is still under extensive research and several studies have highlighted biological effects and associations with various disorders, solid causal links in humans are not always clear. For example, a meta-analysis of 141 scientific studies published in PubMed in September 2025 established a link between chronic microplastic ingestion and various health issues.
In conclusion, while there are concerns about the safety of soft flasks, they are not necessarily more hazardous than other common plastics. However, proper maintenance is crucial to avoid bacteria, which pose a more immediate risk to health than the TPU material. Consider the bigger picture and other potential sources of exposure when evaluating the risks associated with using soft flasks.
