Bottled water has long been marketed as a safe drink—if it doesn't come straight from a mountain spring, it's at least clean and free of chemicals．But a new study demonstrates that bottled water may not be so safe when it comes to microscopic plastic pollutants capable of passing into the bloodstream．

For years scientists have raised the alarm over microplastics, which form when plastics break down into increasingly smaller particles（微粒）, ranging from five millimeters down to one micrometer．Previous studies have found that a liter of bottled water can contain tens of thousands of identifiable plastic particles．But they stopped at the one micrometer level due to technological restrictions．

The study used a new technology to find even smaller particles, and detected an average 240,000 plastic particles per liter of bottled water．About 90% of the particles were considered nanoplastics, smaller than one micrometer. Unlike microplastics, they are capable of passing through some organs like lungs into the bloodstream．From there they can stay in the heart muscle and other organs, pass into the brain, and even into the bodies of unborn babies．

So far, little research shows what nanoplastics exactly do once they enter the bloodstream．But there is much evidence that chemicals used in plastic production are bad for human health．Even if nanoplastics themselves are not deleterious, they can serve as carriers for the dangerous chemicals, increasing the risk of cancer and impacting key organs such as the heart．

"There is a huge world of nanoplastics to be studied," said the researchers．Even if nanoplastics make up 90% of the number of plastic particles found in bottled water, they make up far less in mass, they said．However, this fact provides little comfort: It's the numbers rather than mass that matter, and the smaller things are, the more readily they can get inside us．