One way to think about how science works is to imagine an onion: With each new research study, more is learned, and another layer of the onion is peeled away leading scientists closer to the answer they seek.

Climate change is an example of a debate that has followed this trajectory. Throughout the 1960s and 1970s scientists couldn’t agree whether the world was warming or cooling. Researchers persevered, more studies were completed, and by 1988 the first international conference on global warming was hosted in Toronto.

In the same year, the Intergovernmental Panel on Climate Change (IPCC) was established, which continues to co-ordinate investigations by thousands of scientists around the world. The result of all this work? The scientific consensus that humans are the cause of global warming has now “grown to 100 per cent.”

Another large onion that the research community is in the process of peeling is that of microplastics. The science of microplastics is younger than that of climate change. In fact, the word itself was only coined in 2004.

With hundreds of millions of tonnes of plastic discarded into the environment over the past few decades, scientists are now learning that what is visible is only the tip of the plastic waste iceberg. Plastic bottles and bags, coffee cup lids, vehicle tires, and lint from polyester clothing gets broken down over time into tiny plastic particles that have become a permanent part of ecosystems globally.

Scientists have now shown that microplastics are cycling in global dust and water cycles, and can be found from the deepest ocean trenches to the highest mountain peaks, and from the most remote corners of the Arctic to the house dust in the corners of our homes. The threat is sufficiently clear that the United Nations has recently declared that plastics are “the second most ominous threat to the global environment, after climate change.”

With microplastics ubiquitous in the environment, and ubiquitous in food crops and meat, the next frontier of microplastics research is whether — and to what extent — microplastics are not just an environmental problem but also a concern for human health.

Research demonstrating measurable risk to wildlife offer some hints in this regard. Concentrations that can lead to adverse effects in wildlife range from about 10-520 particles per litre in freshwater ecosystems. Although many places have lower levels than this threshold, such concentrations are not unheard of in heavily urbanized areas and freshwater tributaries — including in Lake Ontario and its watersheds in the Greater Toronto Area.

As a result, legislation has been adopted, for example in California, to begin to quantify microplastics in drinking water and determine adverse effect thresholds for human health.

Because microplastics can be both ingested and inhaled, scientists are beginning to estimate human exposure from all sources, with a recent calculation pegging this at 39,000 to 52,000 particles a year. At the same time, the first studies have started to come out measuring microplastic levels in actual human bodies in Europe and Asia and in North America.

Microplastics have now been found to be released by baby bottles, they’ve now been found for the first time in human placentas and a study this month has shown that microplastics can disrupt the cells in human lungs.

Later this year, the federal government has served notice that it plans on banning a variety of single-use plastics, such as plastic grocery bags. Its January 2021 scientific justification for doing so is largely based on the demonstrated environmental harm of visible and invisible plastic to the environment.

The government notes that: “Humans may be exposed to microplastics via the ingestion of food, bottled water, and tap water, as well as through the inhalation of indoor and outdoor air. However, information on the human health effects of microplastics is limited.”