Which of the following is NOT a major route by which toxic materials enter the body?

The correct answer indicates that radiation exposure is not a major route by which toxic materials typically enter the body. The primary routes of exposure for toxic substances are inhalation, skin absorption, and ingestion, each of which allows substances to enter the bloodstream and affect bodily functions.

Inhalation occurs when harmful substances are breathed in, allowing them to directly affect the respiratory system and be absorbed into the bloodstream. Skin absorption refers to toxic materials penetrating the skin, which can also lead to systemic effects depending on the substance. Ingestion involves consuming toxic substances, where they enter the digestive system and can cause significant harm.

Radiation exposure, while harmful and associated with various health risks, operates differently than the other three methods. Radiation can lead to cellular damage primarily through its ionizing effects but does not constitute a route for toxic substances themselves to enter the body. Therefore, it stands apart in terms of how substances lead to toxicity and is why it is identified as the correct answer to this question.