What is a common characteristic of the contaminants listed under the Safe Drinking Water Act?

The correct answer emphasizes that contaminants listed under the Safe Drinking Water Act can pose health risks when they are present at elevated levels in drinking water. This act aims to ensure safe drinking water for the public by regulating various harmful substances, which can arise from both natural sources and human activities. Many of these contaminants, such as certain heavy metals, microbial pathogens, and synthetic chemicals, become a concern primarily at higher concentrations, as their potential health impacts may include acute and chronic illnesses, including cancer.

The other options do not accurately represent the characteristics of these contaminants. Not all contaminants are natural; some are man-made. Many contaminants can be present at safe levels without being detectable by taste or smell, which means that relying solely on sensory detection is not practical or reliable for ensuring water safety. While some contaminants might require control measures, not all of them necessitate immediate action unless they exceed specific regulatory thresholds, thereby making this option less universally applicable.