Early childhood memories are evoked by many triggers, of which one of the most powerful is a particular smell. All odors result from a blend of substances and their interactions. For example, over 165 volatile compounds have been measured and reported in odor from swine production.

Most odorous substances can be grouped into classes of chemical compounds, such as volatile fatty acids, phenols, nitrogen derivatives or reduced-sulfur compounds. Such chemical compounds stem from the degradation of plant fiber and protein as well as the anaerobic degradation of more complex compounds. This complexity at the source is matched by the complexity in human reaction to odor.

Human sense of smell

The human sense of smell is a primary factor in the sensation of comfort. Research shows that smell perception is unique to each person, and varies over time because of changes to the physical condition of the individual and their memory of past exposures to similar odors.

The world, as we know it, is filled with fragrances from sweet smelling foods and beauty products to the unpleasant smells of pollution and chemical supplies. How is it possible for humans to distinguish and discriminate between the millions of odors present?

To answer this question, one must start with an individual odor molecule. With such a diverse array of fragrances in the world, the shape of an odor molecule is unique to the emitting substance.

1. When inhaled, the odor molecule is absorbed in the nasal passage and binds to chemoreceptors in the olfactory epithelium, which are specific to certain odor molecules.

2. This binding initiates a change in the permeability of the sensory neuron, which creates a slow electric potential that travels to the olfactory bulb.

3. From the olfactory bulb, the transmitted signal is sent to the limbic system of the brain for further processing.

4. Recognition of the odor occurs in the limbic system when the signal is interpreted through a comparison to past experiences with the odor and relation of the smell to the emitting substance.

The human nose reacts to the odorous mix, not to a single compound in that mix, which has fathered an entire science of perfumes. The response to odors does not always correspond to intensity or concentration. Therefore nuisance odor is generally defined by the "FIDO factors:" frequency, intensity, duration and offensiveness.

• Frequency – How often an odor occurs,
• Intensity – The strength of an odor,
• Duration – The length of time the odor is encountered, and
• Offensiveness – The unpleasantness or character of the odor.

These four factors contribute to determining whether a given odor may or may not be a nuisance problem.

Threshold of smell and measurements of odors

As perceived by humans, odors have five basic properties that can be quantified:

1. Intensity
2. Degree of offensiveness.
3. Character.
4. Frequency
5. Duration

All of these factors can contribute to the neighbor's attitude towards the odor as well as the business generating the offense.

It is generally accepted that the extent of objection and reaction to odor by neighbors will vary. Most people will accept a strong odor for a short period of time, provided they do not have to smell it often. But people have a threshold for the frequency and duration of the odor, above which their tolerance is exceeded and they view the odor as a nuisance.

These thresholds, however, are person-specific. While the frequency and duration of an odor often triggers a nuisance complaint, odor measurement procedures typically focus on the first three traits (intensity, offensiveness and character). From a human health standpoint, exposure time is an essential measure in predicting negative effects that may occur and this encompasses frequency and duration as well as concentration (intensity). As a result, regulatory permits often include concentration, frequency and duration as part of the compliance protocol.

Odor threshold is a term used to identify the concentration at which animals respond 50 percent of the time to repeated presentations of an odorant being tested. Most often, however, odor threshold is used to describe the detection onset, which identifies the concentration at which 50 percent of a human panel can identify the presence of an odorant without characterizing the stimulus.

The recognition threshold is the concentration at which 50 percent of the panel can identify the odorant. Although the detection threshold concentrations of substances that evoke a smell are low, often times in the parts per billion (ppb) or parts per trillion (ppt) range, a concentration only 10 to 50 times above the detection threshold value often is the maximum intensity that can be detected by humans.

This is in contrast to other sensory systems where maximum intensities are multitudes above threshold intensities. For example, the maximum intensity of sight is about 500,000 times that of the threshold intensity and a factor of 1 trillion is observed for hearing. For this reason, smell is often concerned with identifying the presence or absence of odor rather than with quantifying intensity or concentration.

Odorants can act as additive agents, counteractants, masking agents, or be synergistic in nature. The combination of two odorants can have an odor equal to that of either one of the components, have an odor less than that of one of the components, have an odor equal to the sum of the components, or even have an odor greater than the sum of the components.

Health effects from odors

Health effects or symptoms from exposure to odors are usually traced either to the sensation of the odor or the odorant itself. Health effects or symptoms vary depending upon the frequency, concentration and duration of the odor. The most common complaints are eye, nose and throat irritation, headache, nausea, hoarseness, sore throat, cough, chest tightness, nasal congestion, palpitations, shortness of breath, drowsiness, and mood changes. Usually, these symptoms decrease within a short time after the odor ceases.

While many unpleasant odors in the environment may not be harmful, they may trigger more severe illnesses. These illnesses can result from a variety of physiological mechanisms including a worsening of underlying medical conditions, such as asthma and other respiratory diseases, depression, hypersensitivity, stress-induced illness, or possibly hormone interactions. The influence of odors on the health and comfort of individuals is difficult to evaluate. Unpleasant odors can result in social and behavioral changes, such as diminishing one's sense of well being, enjoyment of daily activities, or ability to perform various tasks.

Sewage waste and other industrial wastes near residential areas can be responsible for unpleasant odors. These odors not only smell, but also can be responsible for various diseases if not controlled. Treating odors released from sewage and other wastes is important, and many scientists and government organizations are working together to develop technologies to control these emissions. Among the major technologies currently used to control odor are:

• Biofilters,
• Activated carbon adsorbers,
• Wet scrubbers,
• Thermal oxidizers, and
• Ozone generators

Capture and identification of organic odors

Most often, the first sign of an odor control problem comes from neighbors' complaints. Local air chemistry and other factors, such as a presence of certain rare sensory phenotypes in the local population, can make it difficult for some industries to know they will produce an odor until their facility is under operation. Because people can become accustomed to odors, sometimes a single new neighbor moving in, a family visit or perhaps a new housing development can provide the trigger. Companies often bring in a panel from a sampling of locals to physically test smells.

The odorous air emissions from confined animal feeding operations (CAFOs), such as swine, poultry and dairy farms, increasingly are raising community complaints. Odorous emissions can result in health damages, psychological discomforts and adverse aesthetic effects in the community. However, these emissions have not been well characterized up to now, due to the lack of legislation, the limitations in sampling and instrumentation techniques, and the complexity of the emissions themselves.

Recently, efforts have been aimed at the development of a high-volume sampler and sorbet assembly to identify the odor-causing compounds from a dairy CAFO. The sorbet was custom designed to target the potential compounds that may exist in a dairy farm, and was validated in the laboratory with a synthetic odor from swine manure. The tests indicated that high-volume sampling could shorten sampling time from days to within four hours. VOCs and volatile fatty acids have been identified from the dairy farm, such as phenol, methylphenol, 4-ethyl phenol, indole, methyl indole, benzyl alcohol, hexanoic acid, valeric acid and iso-valeric acid, together with some nitrogen-containing compounds that have not been reported before.

An apparatus for the detection of odor consists of a dielectric sensor, a carrier gas for directing a substantially uniform flow into contact with the sensor, and a detector for measuring the dielectric potential of the sensor.

The characterization of complex odors such as those associated with composting, biosolids and agricultural activities may present challenges because of the variety of compounds present and the low odor thresholds of the contributing substances. Often an odor panel is used to evaluate odors from these facilities. However, in some situations the identification of the specific compounds contributing to the odor may be useful. One field trial utilized four methodologies in an attempt to determine the odorous constituents at a composting facility.

Odor interpretations

The sense of smell in mammals is based on a combinatorial approach to recognizing and processing odors. Instead of dedicating an individual odor receptor, the olfactory system uses an alphabet of receptors to create a specific smell response within the brain's neurons. As in language (or music), the olfactory system appears to use combinations of receptors (analogous to words or musical notes, or to the way computers process code) to greatly reduce the number of actual receptor types required to convey a broad range of odors.

When an odor excites a neuron, the signal travels along the nerve cell's axon and is transferred to neurons in the olfactory bulb. This structure, located in the very front of the brain, is the clearinghouse for the sense of smell. From the olfactory bulb, signals are relayed to the brain's higher cortex, which handles conscious thought processes, and to the limbic system, which generates emotional feelings.

Using this technique, it has been shown that

• Single receptors can recognize multiple odorants,
• A single odorant is typically recognized by multiple receptors, and
• That different odorants are recognized by combinations of receptors, thus indicating that the olfactory system uses a combinatorial coding scheme to encode the identities of odors.

Social health

One of the more significant social impacts of odor is the disruption of the quality of life for neighboring residents. More than an unpleasant odor, the smell can have dramatic consequences for communities where lives are rooted in outdoor activities. The encroachment of a coffee or chocolate processing facility near homes can be significantly disruptive to nearby residents. The highly cherished values of freedom and independence associated with life oriented toward the outdoors gives way to feelings of violation and infringement. Social gatherings when family and friends come together are affected either in practice or through disruption of routines that normally provide a sense of belonging and identity, such as backyard barbecues and visits. Homes are no longer an extension of or a means for enjoying the outdoors. Rather, the outdoors becomes a barrier that must be escaped.

Studies evaluating the impacts of CAFOs on communities suggest that such facilities generally attract controversy and often threaten community social capital^[1]. The rifts that develop among community members can be deep and long-standing.

Environmental injustice

Disproportionate location of odorous activities in ethnically populated areas or people with low incomes is a form of environmental injustice that can have negative impacts on community health. Several studies have shown that a disproportionate number of swine CAFOs are located in low-income and ethnic communities. These facilities and the hazardous agents associated with them are generally unwanted but are often thrust upon communities with the lowest levels of political influence.

Technologies

The major factors to be considered in odor control technology include effectiveness, capital cost, annual operating and maintenance cost, and other components used along with the technology.

Among the major technologies developed to control odor are biofilters, activated carbon adsorbers, wet scrubbers, thermal oxidizers, and ozone generators. PE