Six abbreviations are used to describe the administered doses reported in the literature. These terms indicate whether the dose caused death (LD) or other toxic effects (TD) and if the dose was administered as a lethal concentration (LC) or toxic concentration (TC) in the inhaled air.
In general, the term "Lo" is used where the number of subjects studied was not a significant number of the population or the calculated percentage of subjects showing an effect was listed as 100. These are the used definition:

TDLo--Toxic Dose Low: The lowest dose of a substance introduced by any route, other than inhalation, over any given period of time and reported to produce any toxic effect in humans or to produce carcinogenic, neoplastigenic, or teratogenic effects in animals or humans.

TCLo--Toxic Concentration Low: The lowest concentration of a substance in air to which humans or animals have been exposed for any given period of time that has produced any toxic effect in humans or produced a carcinogenic, neoplastigenic, or teratogenic effect in animals or humans.

LDLo--Lethal Dose Low: The lowest dose (other than LDS0) of a substance introduced by any route, other than inhalation, over any given period of time in one or more divided portions and reported to have caused death in human or animals.

LD50--Lethal Dose Fifty:--A calculated dose of a substance which is expected to cause the death of 50% of an entire defined experimental animal population. It is determined from the exposure to the substance by any route other than inhalation of a significant number from that population. Other lethal dose percentages, such as LDI, LDi0, LD30, and LD99, may be published in the scientific literature for the specific purposes of the author. Such data would be published in the Registry if these figures, in the absence of a calculated lethal dose (LDS0), were the lowest found in the literature.

LCLo--Lethal Concentration Low: The lowest concentration of a substance in air, other than LCS0, which has been reported to have caused death in humans or animals. The reported concentrations may be entered for periods of exposure which are less than 24 hours (acute) or greater than 24 hours (subacute and chronic).

LC50--Lethal Concentration Fifty: A calculated concentration of a substance in air, exposure to which for a specified length of time is expected to cause the death of 50% of an entire defined experimental animal population. It is determined from the exposure to the substance of a significant number from that population.