By now everyone is aware that plastic pollution is a huge problem and environmental hazard. However, the most pervasive and widespread plastic pollution may be invisible. Tiny pieces of plastic with a diameter of 5 millimeters or smaller, termed microplastics, have been found virtually everywhere, and inside countless species — and that includes humans.

Just this year, scientists have found microplastics for the first time in human blood and lungs. How they got there is anyone’s guess, but given their tiny size, microplastics can easily contaminate us through the food we eat and the air we breathe, as well as through water sources. And, yes, this includes bottled water.

Although neither the short-term nor the long-term effects of microplastic exposure on human health are known, we shouldn’t take any chances. This is why researchers in Switzerland from Eawag Aquatic Research and Zurich Water Works launched a project in 2019 to investigate whether local water treatment plants were up to this 21st-century challenge — and they have good news to share.

Writing in the Journal of Hazardous Materials, the researchers found that biologically active slow sand filters retained over 99.9% of plastic particles, even those smaller than a thousandth of a millimeter across, the so-called nanoplastics.

“Third-party analyses failed to detect microplastics at the depths at which the lake water was collected (around 30 meters), so we assume that almost no nanoplastics will exist there either,” Dr. Ralf Kägi from the Department of Process Engineering at Eawag said in a statement.

The research team first performed small-scale experiments in the lab using sand filters, then moved to the lake water treatment facility at the Zurich Water Works from which drinking water is funneled into homes, hospitals, and restaurants. In both instances, researchers could barely find any trace of polymers.

In order to identify nanoplastic particles hundreds of times smaller than the width of a strand of hair, the researchers took inspiration from medicine, where cancer cells are tagged to monitor their distribution across the human body. By labeling a bunch of nanoplastic particles at the source, the researchers could track their route through the water treatment process until their final location using a mass spectrometer.

The researchers also investigated the microplastic filtration efficacy of ozonation, in which   water is chemically infused with ozone in order to treat and disinfect it. However, this process hardly affected the transport of nanoparticles. The most effective removal of the tiny polymers was obtained using aged, biofilm-coated sand.

This means that some of our water treatment plants, which are already in place to decontaminate other types of pollution, also work against microplastic and nanoplastic particles. So, at least as far as nanoplastics are concerned, your tap water is safe for now.