Chronic effects occur over a long period of time. The length of time termed "long" is dependent upon the life cycle of the organism being tested. For some aquatic species a chronic effect might be seen over the course of a month. For animals such as rats and dogs, chronic would refer to a period of several weeks to years.

Chronic effects can be either caused by chronic or acute exposures. Acute exposure to some metals and many carcinogens can result in chronic effects. With certain toxicants, such as cyanide, it is difficult, if not impossible, to cause a chronic effect. However, at a higher dosage, cyanide readily causes acute effects. Examples of chronic effects include pulmonary tuberculosis and, in many cases, lead poisoning. In each disease the effects are long-term and cause damage to tissues; acute effects generally result in little tissue reaction. Thus, acute...