How We Smell and Eliminate Odors

An odor or fragrance, commonly referred to as a smell, is caused by one or more volatilized chemical compounds generally at very low concentration, that humans or other animals perceive by the sense of olfaction. Humans can perceive an odor even at the low part per billion (ppb) concentrations. Most odor causing chemicals are perceived in the mid to high ppb range.

The sense of smell gives rise to the perception of odors, mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, a small patch of tissue in back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with air. The olfactory nerve is considered the smell mediator; the axon connects the brain to the external air. Odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia, initiating an electric signal.

When the signal reaches a threshold, the neuron fires sending a signal traveling along the axon to the olfactory bulb which is part of the limbic system of the brain. Interpretation of the smell begins, relating the smell to past experiences and in relation to the substance(s) emitted. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and projected through a pathway to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes.

Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant stimulus type is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the patterns of neuron signals helping to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix, not necessarily corresponding to concentration or intensity of any single constituent.

The widest range of odors consists of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part; the detection of stimuli by receptors in the nose. The stimuli are processed by the region of the human brain which is responsible for olfaction. Because of this, an objective and analytical measure of odor is impossible. Odor feelings are very personal perceptions and individual reactions are related to gender, age, state of health, and personal history.


In some industries like pharmaceuticals, neutraceuticals, perfumery, cosmetics etc., an olefactometer test is used to evaluate odor emanating from a product. You can also use olefactometry to determine if a facility has an odor or a process gives off an odor. An olfactometer is an instrument used to detect and measure ambient odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research to quantify and qualify human olfaction. The panel of human noses act as the sensors…the human’s perceptions is the measure of goodness.

A flow-olfactometer produces a constant heated and humidified flow of pure air. This air flow runs continuously to the subject’s nose. For the length of the stimulus pulse the continuous air flow is replaced by a bloc of odorized air. The new generations of dynamic dilution olfactometers quantify odors using a panel and can allow different complementary techniques:

  • odor concentration and odor threshold determination
  • odor suprathreshold determination with comparison to a reference gas
  • hedonic scale assessment to determine the degree of appreciation
  • evaluation of the relative intensity of odors
  • allow training and automatic evaluation of expert panels

The most recognized olfactometry standard is currently the EN13725 standard. Analyses performed by olfactometers are often used in site diagnostics (multiple odor sources) performed with the goal of establishing odor management plans.

In the air purification industry, the subjective elimination of odor is not commonly done using olefaction testing. The elimination of odor is subjectively determined by the client in each unique installation. How long it takes to “eliminate” an odor varies widely from client to client and as a function of the problem (odor source), treatment volume, air circulation etc. If a more quantitative measure of “odor” is required, an objective measurement of the reduction of volatile organic chemicals in air is done using a variety of analytic methods (gas chromatograph-mass spectrometer, Summa canisters, individual chemical detectors for ozone, ammonia etc.) that test samples of air collected during operation of an air purification device as a function of treatment time in a given volume of space under controlled conditions. Most of these analytical methods can measure levels of volatile organic chemicals (VOCs) as low as a few parts per billion. HGI Industries (HGI) systems are able to reduce a very broad range of VOC concentrations to the low ppb range with continuous use in a wide variety of treatment environments and conditions. That level of VOCs generally eliminates the perception of all odors.

HGI has published analytical data collected for its MDU™ Unit (two optics) by Columbia Analytical Laboratories which shows the reduction in ambient VOCs to very low ppb levels after treatment with its hydroxyl sanitizing system. HGI also provides various case studies where the elimination of different types of odors was perceived to be satisfactory by the customer. Many of these odors like smoke residue, meat rendering processing odors, animal odors, chemical odors etc. are very strong and persistent and have proven to be very difficult to eliminate by other sanitizing and deodorizing methods.

HGI’s systems work by generating hydroxyl free radicals (HO·) and related peroxy and oxy free radicals from the irradiation of ambient oxygen and water vapor by ultraviolet energy in the same way that the sun produces hydroxyl free radicals and their by-products outdoors. The sun’s ultraviolet radiation produces natural, atmospheric hydroxyl radicals continuously. There are, on average, two (2) million hydroxyls in each cubic centimeter of ambient outdoor air during daylight hours. They are the main driving force behind the daytime reactions with hydrocarbons in the troposphere and neutralize most natural and man-made pollutants including greenhouse gases like methane, hydrogen sulfide, ammonia, ozone and various nitrogen oxide compounds.

Odorox® systems recreate the balance of nature indoors by generating “natural” hydroxyls in the same concentrations that work exactly the same way as they do outdoor. They are powerful free radical oxidizing agents that abstract a hydrogen atom from nearly all other chemicals. They are very effective deodorizing agents because they target the locations in VOC molecules that cause odor. The odor causing atoms are generally highly polarizing and activate the atoms nearby, making them more susceptible to attack by free radicals.

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