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 and chronic effects are frequently unrelated, and yet it is often necessary to predict chronic toxicity based on acute data. Acute data are more plentiful and easier to obtain. To illustrate the possible differences between acute and chronic effects, consider the examples of halogenated solvents, arsenic , and lead.
In halogenated solvents, acute exposure can cause excitability and dizziness, while chronic exposure will result in liver damage. Chronic effects of arsenic poisoning are in blood formation and liver and nerve damage. Acute poisoning affects the gastro-intestinal tract. Lead also effects blood formation in chronic exposure, and damages the gastrointestinal tract in acute exposure. Other chronic effects of exposure to lead include changes in the nervous system and muscles. In some situations, given the proper combination of dose level and frequency, those exposed will experience both acute and chronic effects.
There are chemicals that are essential and beneficial for the functions and structure of the body. The chronic effect is therefore better health, although people generally do not refer to chronic effects as being positive. However, vitamin D, fluoride, and sodium chloride are just a few examples of agents that are essential and/or beneficial when administered at the proper dosage. Too much of any of the three or too little, however, could cause acute and/or chronic toxic effects.
In aquatic toxicology , chronic toxicity tests are used to estimate the effect and no-effect concentrations of a chemical that is continuously applied over the reproductive life cycle of an organism; for example, the time needed for growth, development, sexual maturity, and reproduction. The range in chemical concentrations used in the chronic tests is determined from acute tests. Criterion for effects might include the number and percent of embryos that develop and hatch, the survival and growth of larvae and juveniles, etc.
The maximum acceptable toxicant concentration (MATC) is defined through chronic testing. The MATC is a hypothetical concentration between the highest concentration of chemical that caused no observed effect (NOEC) and the lowest observed effect concentration (LOEC). Therefore, Furthermore, the MATC has been used to relate chronic toxicity to acute toxicity through an application factor (AF). AF is defined as follows:
The AF for one aquatic species might then be used to predict the chronic toxicity for another species, given the acute toxicity data for that species.
The major limitations of chronic toxicity testing are the availability of suitable test species and the length of time needed for a test. In animal testing, mice, rats, rabbits, guinea pigs, and/or dogs are generally used; mice and rats being the most common. With respect to aquatic studies, the most commonly used vertebrates are the fathead (fresh water) and sheepshead (saltwater) minnows. The most commonly used invertebrates are freshwater water fleas (Daphnia ) and the saltwater mysid shrimp (Mysidopsis ).
[Gregory D. Boardman ]
Manahan, S. E. Toxicological Chemistry. 2nd ed. Ann Arbor, MI: Lewis Publishers, 1992.