The odor detection threshold is the lowest concentration of a certain

odor An odor (American English) or odour ( Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive ...

compound that is perceivable by the human sense of smell. The threshold of a chemical compound is determined in part by its

shape A shape is a graphics, graphical representation of an object's form or its external boundary, outline, or external Surface (mathematics), surface. It is distinct from other object properties, such as color, Surface texture, texture, or material ...

, polarity, partial charges, and

molecular mass The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...

. The olfactory mechanisms responsible for a compound's different detection threshold is not well understood. As such, odor thresholds cannot be accurately predicted. Rather, they must be measured through extensive tests using human subjects in laboratory settings. Optical isomers can have different detection thresholds because their conformations may cause them to be less perceivable for the human nose. It is only in recent years that such compounds were separated on gas chromatographs. For raw water treatment and waste water management, the term commonly used is Threshold Odor Number (TON). For instance, the water to be supplied for domestic use in Illinois is 3 TON.

Values

* The threshold value is the concentration at which an aroma or taste can be detected (air, water and fat). * The recognition threshold or arousal threshold of olfactory neurons is the concentration at which you can identify an odor (air, water and fat). * The odour activity value is the concentration divided by the threshold. * The flavor impact is the value the rate of change in perception with concentration. * The flavor contribution of an aroma component in a mixture to the total profile can be calculated from the total odor units and the number contributed by that aroma chemical.

Odor detection value

''Odor threshold value (OTV)'' (also ''aroma threshold value (ATV)'', ''Flavor threshold'') is defined as the most minimal

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

of a substance that can be detected by a

human nose The human nose is the first organ of the respiratory system. It is also the principal organ in the olfactory system. The shape of the nose is determined by the nasal bones and the nasal cartilages, including the nasal septum, which separates ...

. Some substances can be detected when their concentration is only few milligrams per 1000

tonnes The tonne ( or ; symbol: t) is a unit of mass equal to 1,000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...

, which is less than a drop in an Olympic swimming pool. Odor threshold value can be expressed as a concentration in water or concentration in air. Two major types of flavor thresholds can be distinguished: the absolute and the difference threshold. The odor detection threshold and the odor recognition threshold are absolute thresholds; the first is the minimum concentration at which an odor can be detected without any requirements to identify or recognize the stimulus, while the second is the minimum concentration at which a stimulus can be identified or recognized. The odor threshold value of an odorant is influenced by the medium. Examples of substances with strong odors: * Grapefruit mercaptan (OTV = 0.1 ppt) * Sotolon (OTV = 1 ppt) * (Z)-8- tetradecenal (OTV = 9 ppt in water) *

Geosmin Geosmin ( ) is an irregular sesquiterpenoid with a distinct earthy or musty odor, which most people can easily smell. The geosmin odor detection threshold in humans is very low, ranging from 0.006 to 0.01 micrograms per liter in water. Geosmin, a ...

(OTV = 10 ppt) * strawberry furanone (OTV = 40 ppt) *

Trimethylamine Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a trimethylated derivative of ammonia. TMA is widely used in industry. At higher concentrations it has an ammonia-like odor, and can cause necrosis of mucous membranes ...

(OTV = 0.37 - 1.06 ppb) * p- vinylguaiacol (OTV = 10 ppb)

Variables

Threshold in a food is dependent upon: * The threshold of the aroma in air. * Concentration in the food. * Solubility in oil and water. * Partition coefficient between the air and the food. * The pH of the food. Some aroma compounds are affected by the pH: weak organic acids are protonated at low pH making them less soluble and hence more volatile. * Number and functionality of odorant receptors in the observer's nose. The concentration of an odor above a food is dependent on its solubility in that food and its

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

and concentration in that food.

Variation by individual

A 2014 study found no significant differences between men and women, and between non-pregnant and pregnant individuals, despite the existence of anecdotal reports of hyperosmia among the latter. People with

Multiple Sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

have been found to have higher olfactory thresholds. In scientific research, this is often represented by a ''lower threshold score'', i.e. reversing the scale. Olfactory function is more impaired in patients with primary progressive MS than that in relapsing-remitting MS.

Variation among species

Some species can detect odors that others cannot. It is widely believed that animals such as dogs and rodents have a superior sense of smell overall, however a 2017 paper disputed that, saying that "the absolute number of olfactory neurons is remarkably consistent across mammals".

References