cyccommute 

Not likely. Burning of waste isn’t a wide spread practice and even if it were, the products of incineration should be water and carbon dioxide if done properly. Microplastics is due to the break down on the plastics we put out in the world. UV light and oxygen work (i.e. weathering) to break the chemical bonds in plastics which causes the item to fall apart into smaller and smaller pieces. Eventually, given enough time, the pieces should break apart to the point where they are only carbon dioxide. The resistance of the microplastics to weathering are going to depend on the monomer used. Polyethylene terephthalate (PET) used in bottles is likely going to break down fastest because it has lots of reactive sites in the polymer. Polystyrene is equally as reactive. Polybutadiene used in rubber will break down slower but there is more of it used and more of it abraded by the road surface so there is much more of it around. Polyester will break down more slowly because it still has reactive sites but fewer and less energetic. Polypropylene is much less reactive than any of these having only carbon-carbon bonds which are difficult to break using just UV and oxygen.

PFAS or polyfluoroalkyl substances are called “forever chemicals” because they are almost noble…i.e. they don’t react with much of anything. That’s why we use them. You can destroy them by trying to burn them but they really don’t “burn”. They just decompose into really nasty stuff. Up to about 350°C (660°F), they are pretty much impervious to anything.

Micro plastics are defined as particle smaller than 5mm. That’s actually a rather large particle. Most of the microplastics found at high altitudes are the tiniest of particles…down to the micron (0.001mm) or submicron size…that can picked up by the wind and transported into the upper atmosphere. Some may even serve as nucleating sites for rain drops and/or snow flakes like dust particle do. The mass of these microplastics particles is in the range of nanograms. That’s 1 billionth of a gram. A single grocery bag could decompose to 6 billion microplastics particles. A single PET bottle could decompose to about 9 billion microplastics particles. You can start to see the problem. Even a little bit of plastic can result in a huge number of microplastics particles.

But those pale in comparison to what comes off tires. Tires put about 40 kg of rubber into the air per kilometer of road way per day. Per year, that 14.6 metric tons per km or 14.6 million grams or 1.4x10^16 nanogram sized particles per kilometer of road way. Given that there are 4.09 million miles of road in the US, that means there are 9.1x10^24 nanogram sized particles put into the air every year. That’s 98 million metric tons per year. Worldwide, we use 480 billion plastic bottles every year which, at 8 g per bottle, works out to 3.8 million metric tons per year and it takes quite a while for a bottle to break down to the nanogram sized particle. But that is worldwide. Worldwide rubber release is 980 million metric tons per year.

Consider how high that number would be if we still used bias ply tires on vehicles instead of radials. Bias ply tires heat up more than radials and wear faster by about 3 times. Those values above would be 3 times higher if bias tires were still the predominate tire.

Yea, it’s tires. People have proposed calling the current era the “Plastic Era”. Maybe we should call it the Rubber Era. But no PFAS was used or released by those tires.