I think atomic bombs is a good analogy for why it's alarming, actually. Humanity had a great and brief time putting radium in all sorts of things until they realized that it was doing something that penetrated deeply into and through the body at a micro scale.

Humanity has also had a great time making different kinds of plastics, only to discover that they are also penetrating our body at a microscopic scale.

While we don't have the smoking gun yet (i.e. radium jaw), it's at least scary to me, because platics are so pervasive across the population that if a smoking gun DOES emerge, it might be too late. I think the article itself brings up another good example: asbestos.

I get all the jokes about dihydrogen monoxide killing people who breathe it, i.e. drowning, but this is fundamentally different, because we are discovering a thing that is interacting with our bodies in a way that was not historically present throughout evolution. Presumably if we'd evolved in the mist of nano plastics there wouldn't be much cause for worry, because a priori we'd have survived and thrived in its presence.