Which of the following best describes a characteristic of active ingredients in pesticides?

Active ingredients in pesticides are the specific chemicals that are responsible for the intended effect of controlling or killing pests. This means they are the part of the formulation that interacts with the target organism, leading to the desired pest management outcome. For instance, these ingredients may disrupt biological processes in the pests or inhibit their growth.

It's important to recognize that while the effectiveness of these ingredients is crucial, they can vary in their mechanism of action, persistence, and toxicity. Understanding that they are the driving force behind the pesticide's functionality helps in both application and regulatory considerations.

The other options do not accurately represent the nature of active ingredients. For example, while some may be non-toxic to humans at certain concentrations, it is not universally true as many active ingredients can pose health risks if not handled properly. Similarly, active ingredients can be effective at low concentrations, making it unnecessary for them to function only at high levels. Lastly, active ingredients often do interact with environmental factors, which can influence their efficacy and behavior in the environment.