Chemicals end up in the environment in many different ways, intentional and unintentional. Plant protection products, hereinafter referred to interchangeably as pesticides, make up the quantitatively largest share of harmful substances which are used deliberately. Chemical pesticides are used to protect crops and plant products from harmful organisms and the diseases they cause, and contain one or more chemical and usually synthetically manufactured active substances. Pesticides combat animals acting as pests, e.g. insects (insecticides) or rodents (rodenticides), offer protection from diseases such as fungal infestations (fungicides) or simplify production methods and decrease the related costs through reducing the number of hours worked. Pesticides also include herbicides which are used to control weeds. Other applications include plant growth regulation and the conservation of plant products.

Alongside climate change, major changes to global nutrient cycles, the destruction of habitats and other factors, pesticide contamination plays a significant role in the concept of planetary boundaries, which suggests that crossing critical thresholds leads to profound disturbances in the Earth system processes.