The odor detection threshold is the lowest concentration of a certain odor compound that is perceivable by the human

sense of smell The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

. The threshold of a chemical compound is determined in part by its

shape A shape or figure is a graphical representation of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture, or material type. A plane shape or plane figure is constrained to lie ...

polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of ord ...

partial charge A partial charge is a non-integer charge value when measured in elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+. Partial charges are c ...

s, and

molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

. 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 isomer In chemistry, a molecule or ion is called chiral () if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (). The terms are d ...

s 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 chromatograph Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...

s. 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 Critical value may refer to: *In differential topology, a critical value of a differentiable function between differentiable manifolds is the image (value of) ƒ(''x'') in ''N'' of a critical point ''x'' in ''M''. *In statistical hypothesis ...

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 An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are b ...

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

milligram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. ...

s per 1000

tonnes The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States c ...

, which is less than a drop in an

Olympic swimming pool An Olympic-size swimming pool conforms to regulated dimensions that are large enough for international competition. This type of swimming pool is used in the Olympic Games, where the race course is in length, typically referred to as "long cour ...

. 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: * (Z)-8- tetradecenal (OTV = 0.009 ppb in water) * p- vinylguaiacol (OTV = 10 ppb) * Geosmin (OTV = 400 ppt) *

Sotolon Sotolon (also known as sotolone) is a lactone and an extremely powerful aroma compound, with the typical smell of fenugreek or curry at high concentrations and maple syrup, caramel, or burnt sugar at lower concentrations. Sotolon is the major a ...

(OTV = 1 ppt) *

Grapefruit mercaptan Grapefruit mercaptan is the common name for a natural organic compound found in grapefruit. It is a monoterpenoid that contains a thiol (also known as a mercaptan) functional group. Structurally a hydroxy group of terpineol is replaced by the th ...

(OTV = 0.1 ppt) *

Thioacetone Thioacetone is an organosulfur compound belonging to the -thione group called thioketones, with a chemical formula (CH3)2CS. It is an unstable orange or brown substance that can be isolated only at low temperatures. Above , thioacetone readily co ...

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 vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...

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 Hyperosmia is an increased olfactory acuity (heightened sense of smell), usually caused by a lower threshold for odor. This perceptual disorder arises when there is an abnormally increased signal at any point between the olfactory receptors and th ...

among the latter. People with Multiple Sclerosis have been found to have lower olfactory thresholds.

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