Which factor can lead to illegal residues in milk or meat?

The presence of illegal residues in milk or meat can often stem from the consumption of contaminated forage by livestock. When animals graze on or consume feed that has been treated with pesticides, those chemicals can be absorbed into their systems. If the pesticide levels exceed the permissible limits specified by regulatory agencies, this can result in the presence of illegal residues in the products derived from these animals, such as milk and meat.

Improper storage of pesticides could indeed lead to accidents or misuse, but it does not directly involve the contamination of animal feed or forage. Pesticide application near water sources primarily deals with environmental contamination, which might affect aquatic life or water quality but does not necessarily impact livestock feed directly. High humidity conditions could affect pesticide application efficacy and degradation rates, but this factor is not directly related to the build-up of residues in animal products. Therefore, the pathway from contaminated forage to illegal residues in animal products makes this option the most directly connected to the question.