I do not know what ostry is, perhaps a sharp/well defined value. Otherwise, median effective and median lethal concentrations.


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Median effective concentration (EC50) and lethal concentration (LC50) are used in ecotoxicology.
http://www.scienceinthebox.com/ec50-lc50-median-effective-co...
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The term half maximal effective concentration (EC50) refers to the concentration of a drug, antibody or toxicant which induces a response halfway between the baseline and maximum after a specified exposure time.[1] It is commonly used as a measure of drug's potency.
https://en.wikipedia.org/wiki/EC50

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A lethal dose (LD) is an indication of the lethality of a given substance or type of radiation.

In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 (lethal concentration and time) of a toxin, radiation, or pathogen is the dose required to kill half the members of a tested population after a specified test duration. LD50 figures are frequently used as a general indicator of a substance's acute toxicity. A lower LD50 is indicative of increased toxicity.

https://en.wikipedia.org/wiki/Lethal_dose#Median_lethal_dose

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LC50 Median Lethal Concentration--a statistically derived concentration of a substance that can beexpected to cause death in 50% of test animals. It is usually expressed as the weight of substance per weight or volume of water, air or feed, e.g., mg/l, mg/kg or ppm.

http://www.epa.gov/opp00001/reregistration/REDs/0097red.pdf