Microplastics are increasingly found in drinking water, but there is no evidence so far that this poses a risk to humans, according to a new assessment by the World Health Organization.

However, the United Nations body warned against complacency because more research is needed to fully understand how plastic spreads into the environment and works its way through human bodies.

There is no universally agreed definition of microplastics but they are generally considered to be smaller than half a millimetre across.

Plastic production has grown exponentially in recent decades and is predicted to double again by 2025, said the report, which means more beads and threads are breaking down into minute particles and winding up in water supplies, pipes, cups, throats and bellies. Studies suggest bottled drinking water even contains minuscule elements of the polymers used in the container and cap.

This has prompted concerns that humans might be contaminated by the chemicals used in plastics or the pathogens that ride on the particles. More alarming still are suggestions that vital systems could be overwhelmed by the alien matter, conjuring up images of seabirds, fish and other wild animals with their innards choked with plastic waste.

These fears are not grounded in science, according to the WHO report, which summarises peer-reviewed research on the subject.

The WHO report says microplastics in the water supply are less of a risk than bacteria and viruses Photograph: a-ts/Alamy Stock Photo

Counterintuitively, the report said larger microplastics (those bigger than 150 micrometres – about the diameter of a hair) are of least concern because they pass straight through the human body. Smaller particles could potentially pass through the walls of digestive tracts and get stuck, but researchers believe they are unlikely to accumulate in harmful quantities. Not enough is known about the tiniest nanoplastics (those less than 1 micrometre) to be sure of their impact.

“Based on the limited evidence available, chemicals and microbial pathogens associated with microplastics in drinking water pose a low concern for human health. Although there is insufficient information to draw firm conclusions on the toxicity of nanoparticles, no reliable information suggests it is a concern,” the conclusion stated.

The authors said routine monitoring of microplastics in drinking water is not recommended because resources would be better spent on removing bacteria and viruses that are a far greater and proven risk. More than 2 billion people lack access to untainted drinking water.

“The overall conclusion is that consumers shouldn’t be too worried,” said Bruce Gordon, one of the authors of the study, though he urged more extensive research. “With the data we have, we believe the risk is low, but can’t say conclusively that there won’t be a risk in the future. We aren’t alarmed by any means.”

The best response to the problem, he said, was to reduce plastic pollution by phasing out single-use plastics and promoting recycling and the use of alternatives.