Iron powder combustion presents a promising green energy alternative, relying on millions of tiny microreactors that burn in a fundamentally different way than conventional fuels. To fully harness this potential, a deeper understanding of the combustion process at the level of individual iron particles is crucial. Our research uncovers new insights into the mechanisms driving iron powder combustion, revealing that while oxygen transport is a key factor, unexpected mass loss occurs through vaporization, even at temperatures below the boiling point. This finding challenges previous assumptions about iron's zero-emission potential and expands the possibilities for sustainable energy.