Practice question

Many pharmaceuticals are consumed, incompletely metabolised, and excreted in urine or faeces. Sewer systems carry the parent compounds and active metabolites to treatment plants, where conventional processes may not remove them completely; treated effluent or untreated sewage then reaches rivers and lakes. Improper disposal can add to this route. Aerosols, fertilisers, and forest runoff are not the dominant general pathway. Because conventional polymers persist, fragmentation redistributes plastic into smaller pieces rather than removing its mass from the ecosystem. Trophic transfer can occur when predators consume contaminated prey, although evidence for consistent biomagnification of particle numbers remains system-dependent. Standardised sampling and contamination controls are essential because airborne fibres can enter samples during collection and laboratory processing. Particle size, shape, polymer type, weathering state, and associated chemicals all influence uptake and biological response, so microplastics are not a uniform toxicant. Laboratory effects must be interpreted alongside environmentally realistic concentrations, while field detection demonstrates exposure but does not alone prove causation.