Foreign substances refer to any material, substance or particle that is not typically present in a particular environment, organism or system. There are several synonyms for foreign substances such as contaminants, impurities, pollutants, toxins, allergens, and pathogens. Contaminants are foreign substances that are accidentally introduced into the environment, food or water. Impurities, on the other hand, are typically natural organic materials that are present in substances that are chemically refined. Pollutants are foreign substances that result from human activities such as industrial discharge and pesticides. Toxins are poisonous substances that can be of chemical or biological origin, while allergens are substances that can cause an allergic reaction. Pathogens refer to microscopic or infectious organisms capable of causing disease.