Final answer:
The compound that occurs naturally in low concentrations and is normally found in healthy adults is arsenic. While essential at trace levels, it can be highly toxic in larger doses, as illustrated by the groundwater contamination event in Bangladesh.
Step-by-step explanation:
The compound that occurs naturally in very low concentrations and is normally distributed in healthy adults is arsenic. Although arsenic is often regarded as a toxic element, it is actually a naturally occurring element found in the environment, including in water and soil. Some studies suggest that arsenic might be an essential trace element for growth in animals, and by extension, it might be required in minuscule amounts for human health as well. However, it can become extremely toxic at higher concentrations, which can be seen in cases like the groundwater contamination in Bangladesh, leading to the largest mass poisoning of a population in history.
Other elements like mercury, lead, and cadmium are also found naturally but are known to be toxic heavy metals that can accumulate in the body and cause various health issues, especially when present above safe levels. These metals can enter the environment through natural processes or through industrial activities such as mining and manufacturing.