defined by reduced fecundity, growth, or changed behavior in long-term 

 tests at subacute concentrations; (3) endpoints defined by chemical changes 

 in the body or changed physiological rates; and (4) endpoints defined by 

 behavioral responses. 



Acute Tests 



Acute tests resulting in death are performed for short periods of time 

 ranging from several hours to 30 days. Interpretation of results has been 

 accomplished by calculation of a tolerance limit or concentration of the 

 test solutions which will cause death to some proportion of the test 

 organisms within a specified time. The median tolerance limit (LC50, TLm) 

 or concentration killing 50% of test organisms has been employed most fre- 

 quently. Two broad uses have been made of acute tests. One use has been 

 to identify potentially toxic materials or deleterious effluents through a 

 "screen test." This test is usually a static test of short duration, nor- 

 mally not exceeding 24 or 48 hr, which provides a gross estimate of 

 probable toxicity. A second form of acute test is designated to determine 

 a short-term response which may be used as a base to estimate effects of 

 long-term exposures or to provide a criterion for monitoring and enforce- 

 ment of water quality standards. This type of test has conventionally been 

 of 96-hr duration, especially in the United States. More recently, longer 

 time periods have been employed to secure a better base for predictions 

 that will accommodate the differing speeds with which toxicants may act. 



The screen test is of short-duration and must permit calculation of a 

 median tolerance limit after 24-48 hr. The 96-hr test similarly depends on 

 calculation of median tolerance limits of concentration over this time 

 period, which presumably gives both a standard value and is long enough to 

 permit acute lethal effects to develop. Because many toxicants or 

 effluents do not effectively demonstrate their toxicity within the 96-hr 

 period, the asymptotic or time-independent test for acute response is being 

 used with increasing frequency. This test may run for up to 30 days and is 

 interpreted by the flattening of the toxicity curve. It is stopped when no 

 death occurs in test chambers for a period of 24-72 hr. This could be 

 described as a lethal threshold concentration (LTC) (Figure 1). 



Long-Term Test 



A second set of endpoints can be employed in tests with long-term expo- 

 sure of organisms to toxicants or effluents. The test concentrations will 

 be lower than the median tolerance concentrations (LC50), and in conse- 

 quence, the significant endpoints will usually be demonstrated by physio- 

 logical inhibitions such as reductions of fecundity, growth rate, or 

 ability to do work. Increased deaths in treatments over losses in the 

 control after long exposure may also be used as an endpoint. Interpreta- 

 tion of these long-term inhibitions at low concentrations cannot be arbi- 

 trarily assessed. Usually the toxicant level which does not result in 

 reduced growth rate, lessened fecundity, or lowered fertility is considered 

 to be a "safe level" and consequently an acceptable concentration of poten- 

 tially toxic materials. A satisfactory endpoint of a long-term test, 



37 



