Technical Thinking By Mark Jones
Why the 7-gram microplastics study fell apart Susan, my friend, really put me on the spot. It was at an American Chemical Society national meeting, a session about microplastics. I was due to speak in 30 minutes. It was my first technical presentation on my microplastics research. The moderator announced a paper had been withdrawn and asked everyone to return on time for the next talk, my talk. Susan popped up and proposed a discussion. The moderator, looking a bit confused, asked, “What would we discuss?” Susan pointed to me and said “Mark told me last night that the 7 grams of plastic in a human brain study had to be wrong. Mark, why don’t you tell everyone what you told me?” A study finding microplastics in human brains was getting a lot of attention. CBS Mornings is what really got things rolling. The hosts holding black plastic spoons to their temples shined a bright light on the work. Total mass concentrations measured by pyrolysis GC-MS in 52 brain samples averaged 4,040 mg/kg with the highest measurements equaling about 7 grams per brain, about the weight of a plastic spoon. Microplastic levels were higher in 2024 brains than 2016 brains by about 50% and found to correlate with dementia. The only “good” news was that microplastics do not appear to shorten life. Polyethylene was measured as the most prevalent polymer. Polypropylene and PVC followed. I knew it could not be true.
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My argument centered on two main points. The first was that there are no credible studies of plastics exposure that could get 7 grams in a brain over a lifetime. The second was that the analytical chemistry must be flawed. Plastic particles have to get into the blood and then through the bloodbrain barrier to get into the brain. Only exceedingly small paricticles, nanoplastics, can get through. Google microplastic exposure and you may get 5 grams per week. That’s been discredited. It is more like 5 grams in 23,000 years. Seven grams of nanoparticles getting into the brain in a lifetime isn’t credible. Pyrolysis GC-MS is a technique that uses heat to blast apart a sample. Fragments are different for different polymers and measurement of the fragments gives the amount of polymer in the sample. Fragments can tell a lot about the sample but can also be confounded. The term of art is “matrix effects.” That is the technical way of saying that techniques that are accurate for a pure sample can get messed up when dealing with mixtures. Some polymers, like polystyrene, depolymerize and produce styrene. Detection of styrene guarantees the presence of a styrenic polymer. Polyethylene and PVC don’t cleanly depolymerize. Polyethylene falls apart to a range of oligomers, polymer chunks that are smaller than the starting polymer but still multiple
monomer units. PVC is even more complicated. Liberation of HCl causes a cascade of reactions. The standard pyrolysis method quantifies on naphthalene. The pyrolysis of lots of stuff makes naphthalene. The levels of PVC just had to be a red herring. The measured concentration was higher than other polymers that are more prevalent in the real world. The very high polyethylene levels had to be due to an interference. The same oligomers can form from polyethylene pyrolysis can come from lipids. The brain has lots of lipids. The method had to be counting lipids as polyethylene. Fast forward to today and it looks like I got it right. There is a growing literature arguing the shortcomings and false positives coming from pyrolysis GC-MS. A letter to the editor questions the validity of the original brain study. Questions about the metabolic load 7 grams of plastic would place on the brain have also been raised. The authors of the original study offered a rebuttal but I don’t find it compelling. Seven grams per brain isn’t correct. Calls for better validation of methods are growing as are calls to be suspect of pyrolysis GC-MS for microplastics analysis in biological samples. It looks like I got it right. DW
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3/2/26 11:46 AM