Just when you thought it was safe to drink bottled water, a new U.N. report raises concerns about what you may be consuming: tiny particles of microplastics.

The just-published 124-page report from the World Health Organization (WHO) is the first major international study to examine the potential human health risks caused by exposure to microplastics in drinking water — and it found there are more microplastics present in bottled water than in tap water.

"Every day we are ingesting tiny, often microscopic pieces of plastic known as 'microplastics' with our food, beverages and the air we breathe," the report warns.

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The adult human body, which is about 60% water, needs to be replenished with 2 to 3 quarts per day from drinking water and foods. But when water or food is packaged in plastics, and plastic waste gets into the environment, people end up with that in their systems, too.

"Lots of questions, very few answers"

Since 2018, public concern has escalated about the impact of these plastic particles in the environment and on people's health. The WHO launched its own study after previous, less conclusive research detected tiny plastic particles in several brands of bottled water.

Bruce Gordon, WHO's coordinator for Water, Sanitation, Hygiene and Health, told CBS News the study found "that in bottled water, plastic particle counts are slightly higher than tap water."

The presence of microplastics "appears to be at least partially attributable to the bottling process and/or packaging," the report says.

Plastic in bottled water

The report says the potential hazards associated with microplastics come in three forms: from the particles themselves, from chemicals that make them up and from "microorganisms that may attach and colonize on microplastics, known as biofilms."

But should we worry about these findings?

"We are not alarmed," WHO technical officer Jennifer de France said. "With the data that we have, we can say that we believe the risk to be low, but at the same time, we can't rule out conclusively that there might never be a risk in the future."

However, others saw cause for concern.

"The WHO is right to sound the alarm about this issue," said CBS News chief medical correspondent Dr. Jon LaPook, who is also a professor of medicine at NYU Langone Medical Center.

"It's not just the thought of what harm the associated chemicals might cause after being absorbed from your gastrointestinal tract," LaPook explained. "What about possible effects inside your gut? For example, what interactions might occur between plastics and our microbiome, the community of microorganisms that includes trillions of bacteria — hundreds of different species — accustomed to eating real food, not plastic."

LaPook said medical research increasingly suggests the microbiome has powerful effects throughout the body, including on the function of our immune system, metabolism and brain. "Could the bacteria metabolize plastic in a way that changes the composition of the microbiome and affects health? Could substances created by bacterial breakdown of plastic be absorbed into the bloodstream and lead to health problems?"

There are, he said, "lots of questions, very few answers."

What are microplastics?

The definition of microplastics is not completely standardized, but by any definition they're tiny — just a small fraction of the diameter of a human hair.

"A widely used definition describes microplastics as plastic particles smaller than 5 micrometers in length," according to the WHO, but particles in drinking water may be as small as 1 micrometer. Really small microplastics, smaller than 1 micrometer, are called nanoplastics.

Plastic production is predicted to double by 2025 and more than triple by 2050, according to the report, which says particles get into the environment through industrial run-off, wastewater and degraded plastic waste, and have become "ubiquitous" worldwide.

In fact, another alarming study published earlier this month reported the discovery of microscopic particles of plastic in snow in the remote regions of the Arctic. The study, published in Science Advances, suggests that winds are carrying them in the air and delivering them in snow around the world.

More microplastics found in Arctic sea ice

"By changing the way we do things, we can limit plastics pollution," said Dan Shepard, Information Officer at the U.N. Department of Global Communications, who has specialized in the environmental impact of climate change.

Food for thought on the human impact

The lead authors of the U.N. study include academics, non-government experts and policy makers from the U.K., Netherlands, Canada, Australia, Switzerland and an American from the Environmental Protection Agency (EPA). They call for additional research on microplastics throughout the water supply chain, and say an increased understanding is needed of the toxicological effects of microplastics following ingestion in the human body.

Dr. Stephanie Wright, a research fellow at the MRC Centre for Environment and Health at King's College London, told CBS News that she still believes what she said after the 2018 studies were published that plastic particles could stay within an immune cell in the gut lining, be passed into our lymphatic system ending up in the lymph nodes, or potentially enter the blood stream and possibly accumulate in the liver.

That, she said, "is plausible for plastic particles, and have been shown in rodents and other mammalian models in laboratory studies, predominantly using polystyrene beads."

"There's a lack of data on the toxicity of environmentally-realistic microplastics in human models to conclude whether there is a health hazard, but given the potential for exposure, it is an area we are addressing in the MRC Centre for Environment and Health," Wright said.

Dr. LaPook said that to him, it's not at all reassuring to read, "Based on the limited information we have, microplastics in drinking water don't appear to pose a health risk at current levels." He said the key phrases are: "limited information" and "appear."

"If you ask somebody whether they'd like their water with or without plastic, I doubt you'd get any answer other than, 'hold the plastic, please!'"