The odor detection threshold is the lowest concentration of a certain

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

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,

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 quant ...

. 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 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 substan ...

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 is the concentration at which an aroma or taste can be detected (air, water and fat). * The recognition threshold or

arousal threshold Arousal is the physiological and psychological state of being awoken or of sense organs stimulated to a point of perception. It involves activation of the ascending reticular activating system (ARAS) in the brain, which mediates wakefulness, th ...

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 The human nose is the most protruding part of the face. It bears the nostrils and 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 bone ...

. 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 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. 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 Geosmin ( ) is an irregular sesquiterpenoid, produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step -dependent reaction. Geosmin, along with the irregular monoterpene 2-met ...

(OTV = 400 ppt) * Sotolon (OTV = 1 ppt) * Grapefruit mercaptan (OTV = 0.1 ppt) * Thioacetone

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 pha ...

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 (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This ...

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