Carbonyl Iron is a highly pure (97.5% for grade S, 99.5+% for grade R) iron, prepared by chemical decomposition of purified iron pentacarbonyl. It usually has the appearance of grey powder, composed of spherical micro particles. Most of the impurities are carbon, oxygen, and nitrogen.

Much of the concern with respect to health impacts will hinge on air emissions. Given uncertainties about the exact effects of many chemicals, and the undisputed toxic nature of many of the substances released, it is not possible to say that emission levels are safe.