Computer experiments were performed on the effect of impurity atoms, hydrostatic pressure, uniaxial tension, and radiation attrition on void nucleus stability in alpha-iron. The critical void nucleus size was found to be a 44-vacancy octahedral (bipyramidal) void with (110) facets. Helium atoms, oversize metallic impurity atoms, and carbon-vacancy complexes served as strong centers for heterogeneous nucleation of voids. Carbon-vacancy complexes at void apexes and corners inhibited thermal dissociation. The amount and tenacity of vacancy trapping by carbon atoms increased with hydrostatic pressure up to 110 kilobars.