LD50 and LC50 are a standard measure of the toxicity of the surrounding medium that will kill half of the sample population of a specific test-animal in a specified period through exposure via inhalation (respiration). LC50 is measured in micrograms (or milligrams) of the material per liter, or parts per million (ppm), of air or water; lower the amount, more toxic the material. Used in the comparison of toxicities, LC50 values cannot be directly extrapolated from one specie to the other or to humans. Also called median lethal concentration or population critical concentration 50. Written also as LC50.

In toxicology and eco-toxicology, dose descriptor is the term used to identify the relationship between a specific effect of a chemical substance and the dose at which it takes place. The dose descriptors will be used later for deriving the no-effect threshold levels for human health (i.e, DNEL or reference dose RfD) and the environment (PNEC). Dose descriptors are determined in the toxicological studies on the hazards of the substance and are usually expressed as LC50, LD50, NOAEL, NOAEC, T25, BMD, EC50, NOEC, DT50, etc. They are used for GHS hazard classification and risk assessment.


dose descriptor

Reference Book

PCt50, HCt50, LCt50 don't appear to be related to LD50 and LC50:

LCt50: The amount of liquid chemical agent expected to kill 50 percent of a group of exposed, unprotected individuals.

PCt50: Parasite clearance rate - the time which (malaria) parasite levels decline in a patient's blood after treatment

HCt50: Hematocrit levels are the percentage of packed red blood cells height inside a centrifuged blood tube.

Why are you asking about this terminology?

It is hard to answer briefly. Recent revision of a local code in my country stated that "threshold dose" in exposed territory shall be evaluated and this parameter influences SIL, reliability level etc. and by this way CAPEX. The problem is with "threshold dose" - there is no clear definition what exactly it means.

There is no an exact system for toxicity in my country. Some local codes exist but they are contradictory. Symbols LC50, PCt50 etc. are used so I considered that these local codes are some bad translation of international documents. There are some "threshold doses" of toxic chemicals:
PCt50 - 50% threshold dose in mg*min/m3
HCt50 - 50% threshold dose in mg*min/m3
LCt50 - 50% lethal dose in mg*min/m3
LC50 - 50% lethal dose in mg/m3
LD50 - 50% lethal dose in mg/kg

As you can see many figures can be implemented as "threshold dose" and by this way CAPEX.

Moreover there is no a reliable database of threshold doses in my country.