Editing Toxicology (section)

== Dose response complexities ==
Most chemicals display a classic dose response curve&nbsp;– at a low dose (below a threshold), no effect is observed.<ref name=Ottoboni/>{{rp|80}}  Some show a phenomenon known as sufficient challenge&nbsp;– a small exposure produces animals that "grow more rapidly, have better general appearance and coat quality, have fewer tumors, and live longer than the control animals".{{sfn|Ottoboni|1991|pp=83-85}}
A few chemicals have no well-defined safe level of exposure.  These are treated with special care.  Some chemicals are subject to bioaccumulation as they are stored in rather than being excreted from the body;<ref name=Ottoboni/>{{rp|85–90}} these also receive special consideration.

Several measures are commonly used to describe toxic dosages according to the degree of effect on an organism or a population, and some are specifically defined by various laws or organizational usage.  These include:
* LD50 or LD<sub>50</sub> = [[Median lethal dose]], a dose that will kill 50% of an exposed population
* NOEL = No-Observed-Effect-Level, the highest dose known to show no effect
* NOAEL = [[No-observed-adverse-effect level|No-Observed-Adverse-Effect-Level]], the highest dose known to show no adverse effects
* PEL = Permissible Exposure Limit, the highest concentration permitted under US [[Occupational Safety and Health Administration|OSHA]] regulations
* STEL = Short-Term Exposure Limit, the highest concentration permitted for short periods of time, in general 15–30 minutes
* TWA = Time-Weighted Average, the average amount of an agent's concentration over a specified period of time, usually 8 hours
* TTC = The Threshold of Toxicological Concern concept<ref>{{cite journal | vauthors = Patlewicz G, Worth A, Yang C, Zhu T | title = Editorial: Advances and Refinements in the Development and Application of Threshold of Toxicological Concern | journal = Frontiers in Toxicology | volume = 4 | pages = 882321 | date = 2022 | pmid = 35573274 | pmc = 9096208 | doi = 10.3389/ftox.2022.882321 | doi-access = free }}</ref> has been applied to low-level contaminants, such as the constituents of [[tobacco smoke]]<ref name="TalhoutSchulz2011">{{cite journal | vauthors = Talhout R, Schulz T, Florek E, van Benthem J, Wester P, Opperhuizen A | title = Hazardous compounds in tobacco smoke | journal = International Journal of Environmental Research and Public Health | volume = 8 | issue = 2 | pages = 613–628 | date = February 2011 | pmid = 21556207 | pmc = 3084482 | doi = 10.3390/ijerph8020613 | doi-access = free }}</ref>