Article www.lasemainedespyrenees.fr as of July 15, 2026: Nine hikers were airlifted to the Hautes-Pyrénées after drinking mountain water. 

Article www.ffrandonnee.fr dated July 18, 2026: In the Hautes Pyrénées, hikers intoxicated by the water of the torrent. 

Find out why some filter bottles do not protect against viruses and how to choose a truly effective filtration.

Why a filter bottle is not always enough. 

The water that flows from a mountain stream often appears to be of irreproachable purity. Clear, fresh, natural... it easily gives the impression of being drinkable.

However, in July 2026, several accidents were a reminder that this appearance can be deceiving.

In the Hautes-Pyrénées, the High Mountain Gendarmerie Peloton (PGHM) intervened twice in a few days to evacuate by helicopter nine hikers who were victims of violent digestive disorders after consuming water from torrents and high altitude lakes.

A few months earlier, in August 2025, a 17-year-old hiker had also had to be evacuated by helicopter after experiencing severe stomach pains, nausea and vomiting. The young man, rescued near Lake Uzious in the Pyrénées-Atlantiques, had previously drunk water from Lake Anglas, while using a filter bottle. (Source leprogres.fr)

These events are a reminder of an often unknown reality: Not all filter bottles protect against the same contaminants.

Clear water is not necessarily drinking water

In the mountains, crystal clear water is not necessarily safe water.

Even far from any home, various elements can contaminate a watercourse:

• wild animal droppings ;
• the herds present on the mountain pastures ;
• mountain huts ;
• heavy rains that carry contaminants into streams ;
• certain human activities.

These contaminations can be invisible to the naked eye while causing severe gastroenteritis.

What does a filter bottle actually filter ?

All filter bottles use different technologies.

Some are very effective against bacteria and parasites, but are not designed to kill viruses.

*Performance varies by model and technology used.

This difference is essential when taking water directly from a torrent, river or lake.

Why are viruses harder to eliminate ?

The answer is simple: Viruses are extremely small.

A bacterium usually measures between 0.5 and 5 microns.

A virus, on the other hand, only measures 20 to 100 nanometers, up to fifty times smaller.

Many filter bottles use a 0.1 to 0.2 microns, which is very effective against bacteria, but which is not always enough to retain viruses.

This is why the health authorities regularly remind people that filtered water is not automatically free of risks if the system has not been designed and tested for this use.

Why is NSF/ANSI P231 important? 

When a manufacturer claims that its system eliminates viruses, this claim must be verifiable.

The NSF/ANSI P231 standard is now one of the most recognized references for evaluating the performance of portable water purifiers against microbiological agents.

In particular, it measures the ability of a system to reduce viruses such as rotavirus, under controlled laboratory conditions.

This certification provides a much stronger guarantee than simple marketing arguments.

Virus-tested filtration

Cartridges Coldstream FTO+, used in gravity water purifiers Pure Filters, have been tested by independent ISO 17025 accredited laboratories to multiple NSF/ANSI standards.

The results show in particular:

• more than 99.99% reduction in viruses (rotavirus) to NSF/ANSI P231, up to 3,000 liters ; ;
• 99.9999% reduction in bacteria ; ;
• 99.9% reduction in cysts, microplastics and fine particles ; ;
• Fluoride reduction up to 97,5 %, without additional cartridge ;
• Reduction of many pesticides, heavy metals, pharmaceutical residues and other contaminants tested.

These performances are documented in Test reports carried out by independent laboratories.

A solution for the home... and a safer water supply before departure

Gravity water purifiers Pure Filters operate without electricity or grid connection.

Thanks to their cartridge Coldstream FTO+ Made of high-density ceramic combined with high-performance activated carbon, they allow water to be purified while preserving Naturally occurring minerals.

At home, they offer a simple solution to improve the Tap water quality.

Before a hike or bivouac, they also make it possible to prepare a reserve of filtered water to limit the risks associated with the consumption of water taken directly from nature.

For situations where it is necessary to filter water directly in the field, it is important to choose a system that has been tested and documented for virus performance.

In summary

Recent events in the Hautes-Pyrénées remind us that mountain water, as clear as it is, is not necessarily drinkable.

Before filling your water bottle in a torrent or lake, take the time to check what your system is actually capable of filtering. Bacteria are not the only microorganisms present in natural water: some viruses can pass through many conventional filtration systems.

Choosing a purifier whose performance is validated by independent tests is a real safety criterion, on hikes as during any activity in the great outdoors.

Are you still hesitating about the choice of a water filter? We've put together a comprehensive video guide to help you understand the different filtration technologies and choose the best solution for your needs, whether you're at home, traveling, or hiking.

Discover our Video guide to choosing the right water filter.