Systemic effects from pesticide exposure occur when:

Systemic effects from pesticide exposure occur when the substance is absorbed into the body's bloodstream and subsequently circulated to various tissues and organs. When pesticides are absorbed, they can enter the systemic circulation, leading to effects that are not localized but rather affect the entire organism. This is significant because it allows for the potential for more severe health effects, as the pesticide can interfere with various biological processes in different parts of the body.

Inhalation of a substance typically leads to respiratory effects and may not always result in systemic toxicity unless the pesticide is absorbed into the bloodstream from the lungs. While skin exposure can also lead to systemic effects if the substance penetrates deeply and enters circulation, it is more commonly associated with localized reactions unless specific conditions are met. Exposure through ingestion may also lead to systemic effects, but the key here is the absorption and distribution throughout the body, which is best encapsulated in the correct answer.