Median tolerance limit

Tonogai, Y., Ogawa, S., Ito, Y., and Twaida, M. Actual survey on TL (median tolerance limit) values of environmental pollutants, especially on amines, aromatic nitrogen compounds and artificial dyes, J. Toxicol. Sci., 7 193-203, 1982. 

Median tolerance limit (TLM) A statistically calculated concentration of a substance or chemical where 50% of the exposed population survive a specific period of time, (e.g., TLM 24 hours, TLM 72 hours). 

In general, most of the herbicides are only moderately to slightly toxic to fish, having median tolerance limits (Tim) generally above 1 p.p.m. A few, however, exhibit Tims well below 1 p.p.m. When fed to dairy and meat animals at dietary concentrations of several parts per million, the herbicides, in general, are not excreted in milk and do not accumulate in tissues. While this seems to be the case for the herbicide per se, the metabolites of many of the herbicides have not been fully investigated. 

TLm Median Tolerance Limit at which approximately 50% of the exposed species will show abnormal behavior including death under the conditions of concentration and time given. 

During the last decade parathion has been the most used organo-phosphorus insecticide. It has been proved to be valuable in crop protection 27). However, using this compound so much has also resulted in numerous accidental intoxications, and many have been lethal 28). In aquatic environments parathion hydrolyzes to yield p-nitro-phenol or oxidizes to yield paraoxon (25, 26). Baker (29) has shown that substituted phenols aflFect the odor quality of drinking water. p-Nitrophenol may be chlorinated at a water treatment plant to produce an odorous product. The U. S. Public Health Service has adopted 1 /xg/liter as a limit for phenolic compounds in water (10). Paraoxon is more toxic to insects and mammals than the parent compound parathion (27). The lethal dose (LD50) for male white rats is 14 mg/kg for parathion while that determined for paraoxon is only 3 mg/kg (30). Bioassay studies with fathead minnows indicated a Median Tolerance Limit (TLni) (96 hours) for parathion of 1.4 mg/liter and 0.3 mg/liter for paraoxon. 

Aquatic toxicity data is usually expressed in terms of the median tolerance limit. TLm, which is defined as that concentration of a substance that it lethal to 50 percent of the test population in an arbitrary time period. Table VIII. lists the TLm values obtained for sodium silicate. 

Acute toxicity unit For a given species and a single toxic substance, the 96-hr median tolerance limit (TLm). For a mixture of toxicants, any combination of concentrations that would be expected to kill half the individuals of the same species in 96 hr. 

Median tolerance limit (TLM) The concentration of a toxic substance or level of stress that is tolerated by (does not kill) 50% of a group of oiganisms after a specified exposure period. 

The equation describing this interchange can be written NH3 + H+ NH4. Although NH is nontoxic, NH3 is highly toxic. The NH3 96-hr median tolerance limits for a variety of fi eshwater fish lie in the range 0.08-4.6 ppm. 

The 24 and 48 h median tolerance limits of Pb(C2H5)4 for fresh-water fish are 2.0 and 1.4 ppm (as Pb), respectively, and the safe concentration in water is 0.20 ppm [129]. Fish kept in water with varying concentrations of Pb(C2H5)4 contained virtually identical amounts of lead at the time of their death the life span depended on the concentration [155]. 

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