As poison, Magical Barrier Particles have thus far been only employed by Etherious. They take the form of a thick black (dark purple in the anime) mist that quickly spreads in its environment, feeding on the Ethernano in the atmosphere and contaminating it, thus causing Mages who breath it in to become lethally poisoned and weakening their Magic (due to that, they are also known as "Anti-Ethernano," and Etherious are generally unaffected by them because of their Curse Power); this may become visible as follows: the veins around the victim's eyes puffing up, and then the victims violently expectorating blood before, presumably, dying of contraction, all in mere moments; Dragon Slayers, however, because of their special lungs, have a far greater resistance against them than normal in case of inhalation. That aside, the particles also contain very trace amounts of iron along with the poison, as mentioned upon the confrontation between Bloodman and Gajeel, which allowed the latter to enter Dragon Force upon consuming much of them, as well as not die (or even fall unconscious for that matter) afterwards. Finally, the toxic particles can enter through the skin over prolonged periods of time.
Such poison aside, Magic barriers like Jutsu Shiki barriers are also comprised of what has been called Magical Barrier Particles, which are the same as, or at least of similar nature to, said poisonous particles, as the both of them were at one point shown being nullified together, whilst being not referred to distinctly.