Residue dries on a leaf, and the easy verdict arrives before the science does: poison. That verdict is lazy. Toxicity describes what a substance can do under defined conditions; risk asks whether a living target actually receives enough of it, by a route and for a duration that can produce injury. Possibility is not outcome.
Not chemistry alone, but quantity, decides the first half of the case. A compound may bind a receptor or disrupt an enzyme, yet biological effect follows a dose-response curve rather than a moral label. Thresholds matter here. The no-observed-adverse-effect level, paired with uncertainty factors, helps regulators establish reference doses; an application can suppress a pest at a rate that remains below levels associated with harm in a specified non-target population. Raise the internal dose, and the margin can vanish.
Exposure changes the verdict. Drift, dermal contact, inhalation, runoff, and dietary residues create different internal doses, while absorption, distribution, metabolism, and excretion can amplify or reduce the amount reaching vulnerable tissue. Targets are not interchangeable. A herbicide aimed at a plant enzyme, an insecticide aimed at an insect nervous system, and a fungicide aimed at fungal metabolism may affect pollinators, farm workers, fish, or mammals in sharply different ways. Risk assessment earns its name only when it joins hazard, exposure, and susceptibility. On the leaf, the residue remains still; the question is where it travels, and whom it reaches.
