Cast metals contain porosity. Casting porosity occurs where the cast structure is not solid, i.e., it contains holes. Casting porosity is not detrimental below a level. Casting porosity can be related to insufficient liquid being fed to the mold, low liquid metal temperature, improper mold and gating design, etc.
All castings have observable porosity at some level of magnification. Casting porosity becomes critical when there is not sufficient solid metal present to resist the stress being applied to the casting.