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What is a science-based medicine approach to potential public health risks? We write a lot about such risks here, trying to put them into perspective and cut through the hype and sensational headlines. We all have more than enough to worry about without adding unnecessarily to this burden. At the same time, humans have transformed our environment with industry, potentially introducing new hazards and risks.

A relatively recent addition to the list of potential health risks to worry about is microplastics – or more precisely micro and nano plastics (MnPs). There are a few uncontroversial statements we can now confidently make about MnPs, the first being that they are everywhere.

We (also uncontroversially) have a plastic pollution problem in the world. Plastic is an extremely useful modern material. It’s cheap, light, strong, versatile, easy to form, and great for sterile applications. But it’s too good, in that it does not break down easily. There are actually many different types of plastic with different properties, but these are common features.

What happens to many plastics in the environment is that they break down into smaller and smaller pieces, but never go away entirely. They become micro (smaller than 5 mm) and nano (smaller than 1 µm) sized and are now essentially ubiquitous in the environment. We have found MnPs at the tops of the highest mountains and the bottoms of the deepest oceans.

We also know that MnPs get into living things, including people. They have been found in placentas, lungs, hearts, and every fluid type we can sample. This is not surprising since MnPs are in the air, in our food, and in our water.

That is just about where the uncontroversial hard evidence ends, leaving us with a big question – what are the health implications of MnPs? Here the research is in its infancy. This is also a good point at which to remind readers of some basic scientific principles that we use to address such questions. The first is the different between hazard and risk.

Hazard means that something is potentially harmful. Fire is a hazard, because it can burn tissue and cause harm. Risk is a measure of the probability of harm. So while fire itself is a potential hazard, the risk of getting burned can be minimized with good safety practices. We don’t have to ban fire because it is a hazard, we can manage the risk.

Another important concept is the precautionary principle – erring on the side of caution in the face of the unknown. However, in practice, the precautionary principle is difficult to apply, because unknowns cut both ways. It is easy to abuse the precautionary principle by applying it selectively and unreasonably. It is literally impossible to prove zero risk for anything, and someone might argue that until we prove zero risk the precautionary principle should prevail. This is essentially the argument of the anti-vaccine movement and the anti-GMO movement (and not surprisingly there is large overlap between these groups).

We often encounter two basic approaches to potential harm. There is a hazard based precautionary principle approach, and there is an evidence-based risk vs benefit approach. At SBM we prefer the latter as a more rational and pragmatic approach. This is because hazard does not necessarily mean risk, and because we need to consider all the effects of decisions, especially at a public health level. Banning GMOs out of a misguided precautionary approach would deprive the world of a powerful technology that can address many non-theoretical problems, for example.

Another critical concept is that dose maketh the poison. Everything is a potential hazard, at a high enough dose. But what we really want to know is the risk of harm at any given dose. This way we can set a generous safety limit to be sure human exposure is below the level of measurable risk.

With MnPs, we essentially don’t have this data. We don’t know that they are in fact causing any specific harm, and at what exposure levels the risk of such harm becomes significant. A rational use of the precautionary principle is to argue that therefore we need to research this issue. I also think it’s reasonable to consider if there is any low-hanging fruit – ways to minimize human exposure to MnPs that would not themselves introduce some real risk – by forgoing a useful technology or switching to other options which may have their own risks.

What data do we have on MnPs? A recent review highlights some preliminary evidence that MnPs may be linked to inflammation, which can exacerbate many chronic illness. Inflammation also appears to increase the uptake of MnPs, so may create a positive feedback loop. But we have to point out that much of this data is preclinical (there are lots of studies in zebrafish). This puts us at the “potential hazard” level but without any clear data on actual risk.

But again – “potential hazard” is useful for generating hypotheses and pointing the way toward future research. What does all this mean for current policy?

This is a tricky question because plastic is so embedded in our modern society. But it seems there are some clear priorities. First, we need to conduct a lot more research. Which kinds of plastics are more likely to create MnPs in the environment and in living things? Can we formulate newer plastics that are designed to break down completely (there is already a lot of progress here)? What are the actual risks for animals and people? It’s possible that MnPs are simply excreted, and a low level steady state in our bodies is below the level of any significant risk. That would be nice, but we can’t assume this is the case.

We need to address the plastic pollution problem we have anyway, so shifting away from single-use disposable plastic is a good idea even without a health risk – but this may add one more incentive.

One thing is clear – further research is definitely a priority.

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Posted by Steven Novella