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Tap water or bottled water? The point has been debated for years, but a new study has added another dimension to the discussion. A recent study in the聽聽has created a bit of a panic with the suggestion that high levels of microplastics can be found in bottled water.

Scientifically, the World Health Organization refers to nanoplastics and microplastics in their official reports. Microplastics refer to pieces of plastic between one micrometre and five millimetres in diameter. Nanoplastics are smaller than one micrometre. While these are often lumped together in media reports, that might be problematic. There is more worry about the smaller particles than the larger ones. Larger particles are less likely to be taken up or absorbed into the human body. When you hear reports about microplastics being detected in blood or tissue samples, that usually refers to smaller particles.

The difficulty in deciding how much of a threat nanoplastics and microplastics pose to human health was detailed in聽. Much of the research has focused on larger particles, which are easier to measure, even though smaller particles are probably more clinically relevant. Different groups also use different measurement techniques, which makes it hard to compare studies.

Many studies have looked at occupational exposure to plastics, but it鈥檚 not clear if those would naturally translate to the environmental exposure most people worry about. The field has also been muddied by some high-profile retractions. In 2017, a聽聽claiming microplastics were affecting fish had to be retracted because of data fabrication and fraud.

When dealing with nanoplastics and microplastics, it is pretty clear that they are present in the environment, in the water supply and our food. It is also clear that you can detect traces of them in human tissue. But evidence that they cause disease in humans is still lacking, and the heterogeneity of the research data makes it unclear how much of a threat they pose.

While studies have looked at the prevalence of microplastics in sea life, most of that has focused on clams, oysters and mussels, which make up a minority of most people鈥檚 diet. Fish may not pose the same risk of ingestion since we tend to eat filets of fish and discard the guts, unlike oysters that people swallow whole. Also, how much meat, grains, fruits and vegetables contribute to microplastic ingestion is unclear.

Finally, there is the question of whether any of this affects human health. This is harder to answer. Studies where rats are fed high doses of microplastics may not be directly relevant to humans that ingest them because of environmental exposure.

The study in the Proceedings of the National Academy of Science聽聽because it found more nanoplastics and microplastics in bottled water than prior research. The researchers were developing a new and potentially better measurement technique. But much of the discussion of the paper goes beyond what was actually published. The presumption is that the detected nanoplastics leached out from the plastic bottle. While this may be, the research didn鈥檛 establish this. Ideally, you would want to compare the water before and after bottling to prove the point or compare the bottled water to tap water. To their credit, the researchers said that this is the focus of their next research project, but it wasn鈥檛 established in this paper.

Lest people accuse me of being an apologist for plastic bottle companies, we probably should be transitioning away from plastic water bottles. We generate too much plastic trash as a species and the聽聽will have to be dealt with sooner or later. But claiming the plastic bottles are bad for your health is a much harder question to answer.