Which one of the following poses virtually no health threat in terms of inorganic mercury?

Ingestion of inorganic mercury poses virtually no health threat primarily because the human digestive system does not efficiently absorb inorganic mercury. When inorganic mercury compounds are ingested, they tend to be poorly absorbed through the gastrointestinal tract. Instead, much of it is excreted unchanged in the feces, leading to a significantly lower risk of systemic toxicity compared to other routes of exposure.

In contrast, inhalation of mercury vapors or absorption through the skin can lead to more serious health effects as these routes allow for more direct entry into the bloodstream. Inoculation, or injection of mercury directly into the body, also presents a significant health risk due to the immediate and concentrated exposure to the tissues and bloodstream. Therefore, ingestion is considered the least harmful route regarding inorganic mercury exposure.