Graveyards have been the subject of superstition since burial rites were first performed for the dead. In ancient cultures the rituals surrounding burial grounds were, of course, regarded as dutiful rather than superstitious and this particular belief may have originated from a blending of the practical with the spiritual. Although they had no notion of the way in which infectious disease could be passed through the air, the smell of decomposing bodies made people wary of breathing in the vapours emitted by a corpse, so covering the mouth became customary. On the spiritual level, early man associated the breath with the life force, or soul, as a result of simply observing that the spirit seemed to leave the body with the exhalation of the final breath.
The soul is linked with the breath linguistically too: pneuma is an ancient Greek word for breath and is translated in religious contexts as ‘spirit’ or ‘soul’. The English word spirit comes from the Latin word spiritus, which means breath, and in Hindu philosophy the word prana means breath, but also ‘life force’. It stood to reason in their minds that just as the spirit could be exhaled from the body of someone on their deathbed, that soul might then be inhaled into the body of a living person.
Over time, beliefs like these were incorporated into folklore and compounded by stories of ghosts and possession by evil spirits. Graveyards were believed to be full of the spirits of the dead, either returning to communicate with their loved ones or trapped in limbo as a result of their earthly sins. Since it was thought to be more common for the spirits of the sinful to loiter around graveyards (the good spirits being happily ensconced in Heaven), the chances of becoming possessed by an evil soul were greater and certainly worth holding your breath to avoid.