[hollerith]

Most plastic breaks down into microscopic pieces, which get everywhere including in the human brain in alarming amounts. They get into the human body through food and water.

You haven't seen any reports about this? "Microplastics" does not ring any bells?

>[plastic bags] don't really react chemically with anything in nature

Almost no one denies that "forever chemicals" are toxic to humans even in tiny concentrations even though they are very much chemically inert. By "forever chemicals" I refer to per- and polyfluoroalkyl substances (PFAS) (used in the production of Teflon, Gore-Tex, etc) or more precisely the chemically-stable compounds into which they break down. Just like forever chemicals, microplastics bioaccumulate.

[card_zero]

By what mechanism are PFAs harmful to health? Is it because they are not, in fact, chemically inert? Or else how.

[ben_w]

Nothing made of atoms is truly chemically inert, not even noble gases. It's just more or less reactive, and when/how.

But even if it was literally un-reactive, sometimes it's enough to just be in the way. Imagine folding a protein, or assembling a structure of RNA origami*, but some big lump of un-reactive molecule is in the middle — the ultimate shape is different, leading to different biochemical results. Grit in the gears.

Or even just heavy: deuterium is chemically identical to hydrogen, but still has a lethal concentration** because it is twice the mass.

* https://en.wikipedia.org/wiki/RNA_origami

** Replacing 50% of the hydrogen in a multicellular organism with deuterium is generally lethal, unless this is a widely believed myth that's about to get a bunch of debunking

[vharuck]

Not all harmful effects are caused by direct chemical reactions. For instance, asbestos causes health problems through the physical process of friction and piercing. Small particles that aren't removed by the body can do a lot of harm.