Basics

This is a measure of what concentration of a substance is needed in air in order for it to be smelled
Not only does it vary from chemical to chemical, but it also varies person to person, and with each person's age/health (Loss of Taste and Smell is a symptom of COVID-19, and even congestion can effect the Odor Detection Threshold)
Also one can experience Olfactory fatigue (aka "Nose Blindness" ) and thus gradually loose detection of the hazard
Despite this there is use to the estimate to get an idea of about when it should be detected
An issue is for some chemicals the Median Lethal Dose and/or the Lower Explosive Limit can be below the Odor Detection Threshold, thus waiting to smell the chemical before evacuating etc may not work

Internal Links

Gas Odorization (The use of an Odorant to make a gas with no scent have an artificially high Odor Detection Threshold, so that leaks can be detected)
Portable Gas Detector / Portable Gas Monitor (These are small units that fill a role akin to a Dosimeter / Noise Dosimeter but for various gases, typically they measure either a single gas, "4 Gas Detectors" that measure Oxygen, Carbon Monoxide , Hydrogen Sulfide , (Hydrocarbon) Combustible Gasses (And typically calculate a LEL
- As long as these are Calibrated they are far more effective than scent, and a must for Confined Space work, as well as Refineries etc