Bioaccumulation occurs when an organism absorbs a pollutant at a rate greater than it can eliminate it, resulting in increasing concentrations of the pollutant in the organism's body over time. Biomagnification occurs when concentrations of a pollutant increase at each trophic level in a food chain. Certain substances like mercury, DDT, and radioactive materials are prone to bioaccumulation and biomagnification due to their ability to dissolve in fats and persist in the environment. A classic example is the buildup of DDT in birds of prey in North America in the 1960s which impaired reproduction.