Book excerpt: Explosion hazards involving mixtures of different states of aggregation continue to occur in facilities where dusts, gases or solvents are handled or processed. In order to prevent or mitigate the risk associated with these mixtures, more knowledge of the explosion behavior of hybrid mixtures is required. The aim of this study is to undertake an extensive investigation on the explosion phenomenon of hybrid mixtures to obtain insight into the driving mechanisms and the explosion features affecting the course of hybrid mixture explosions. This was accomplished by performing an extensive experimental and theoretical investigation on the various explosion parameters such as: minimum ignition temperature, minimum ignition energy, limiting oxygen concentration, lower explosion limits and explosion severity. Mixtures of twenty combustible dusts ranging from food substances, metals, plastics, natural products, fuels and artificial materials; three gases; and six solvents were used to carry out this study. Three different standard equipments: the 20-liter sphere (for testing lower explosion limits, limiting oxygen concentration and explosion severity), the modified Hartmann apparatus (for testing minimum ignition energy) and the modified Godbert–Greenwald (GG) furnace (for testing minimum ignition temperature) were used. The test protocols were in accordance with the European standard procedures for dust testing for each parameter. However, modifications were made on each equipment in order to test the explosion properties of gases, solvents, and hybrid mixtures. The experimental results demonstrated a significant decrease of the minimum ignition temperature, minimum ignition energy and limiting oxygen concentration of gas or solvent and increase in the likelihood of explosion when a small amount of dust, which was either below the minimum explosion concentration or not ignitable by itself, was mixed with gas or solvent and vice versa. For example, methane with minimum ignition temperature of 600 °C decreased to 530 °C when 30 g/m3 of toner dust, which is 50 % below its minimum explosible concentration was, added. A similar explosion behavior was observed for minimum ignition energy and limiting oxygen concentration. Furthermore, it was generally observed that the addition of a non-explosible concentration of flammable gas or spray to a dust-air mixture increases the maximum explosion pressure to some extent and significantly increases the maximum rate of pressure rise of the dust mixture, even though the added concentrations of gases or vapor are below its lower explosion limit. Finally, it could be said that, one cannot rely on the explosion properties of a single substance to ensure full protection of an equipment or a process if substances with different states of aggregate are present.