Smell is a particularly acute sense, with a strong psychological component.
During the middle ages, people draped posies around their necks to ward off plague.
In the early 19th century, Napoleon commissioned a “bad smell map” of Egypt to protect troops against airborne disease.
Researchers have found that people often report symptoms from harmless odors. Almost all odors are immediately liked or disliked.
There may be individual differences in affective response, but there are more differences for pleasant odors than for malodors.
There are numerous substances that are highly toxic that do not have any odor at all, like carbon monoxide.
Humans acquire the disgust or pleasure of odors through experience.
All pleasant odors become malodors at very high concentrations and the pleasantness or malodor depends largely on its context.
Only odors which are associated with common experiences shared by people in the same culture can be trusted to have the same effect on people.
This is why the odor of methyl salicylate associated with a drink such as root beer is appreciated by Americans but not Europeans.
Smell is the least understood of the senses.
As yet there is no exact science of smell, but this has not prevented chemists from finding solutions to the odor problems.
Odor perception is subjective.
What goes unnoticed by one person may really disturb another.
One odor molecule can carry a pleasant scent of flowers, while an almost identical shaped molecule has the stench of rotting garbage.
Finding the source
Detection is sometimes difficult.
Odors may come from a point, area or line sources and they may be constant or intermittent.
Not all molecules that are volatile have a characteristic smell. Our noses have evolved to ignore water, natural gas, and gases of the air.
When it comes to odor control, the most reliable and influential arbiter of smell is still the nose.
Numerous deodorizing products from various companies are available, with no guarantees that the labor and expense will solve the odor problem.
Only when there is the lack of malodor detected by the nose can it be claimed with some amount of certainty the odor has dissipated.
Deodorization products such as natural oils, fragrances, adsorbent polymers and activated carbon are being used to neutralize and mask nuisance odors.
They provide a cost-effective alternative to more capital-intensive remove-and- replace methods.
Meeting the criteria
An odor can be perceived only if all of the several criteria are met.
- The substance has to be in vapor form.
- There must be an available odor site.
- There must be sufficient specific resident enzyme in the olfactory system of the brain to react with odorant.
- Some odors like human sweat can be detected by 65 percent of people, while ethyl mercaptan — which is added to natural gas to make it noticeable — is detected by 95 percent of people. For some materials, humans rapidly lose the ability to smell, such as how hydrogen sulfide, a toxic and poisonous gas with a smell of rotten eggs, becomes rapidly unnoticeable.
- There must be no interference with the pattern of signal generated by the reaction, and the concentration of the odorant must be high enough to create a signal strong enough to be perceived.
- People can discriminate more than 10,000 odors, even though there are only about 1,000 odor receptor enzymes (proteins) in the brain.
- There is no standard for measuring the intensity of odors, such as decibels for sound and lumens for light. Odor control has often been less successful because perception is very subjective. The concentration of odor compounds is low and is usually 1-100 parts per million (ppm).
- Odor sensations are often the result of a complex interaction of many, sometimes hundreds of chemical compounds on the sensory organs of the nose. The smell in a modern office building is a “cocktail” made up of the smells of more than a thousand substances (sweat, tobacco, carpeting, cleansers, plants, ink, etc.).
- The total smell perceived is often different from, and sometimes stronger than, the sum of its parts.
- Bad smells can cause health effects, such as headache, nausea and sleeplessness.
- Bad odor compounds are generally not poisonous, at least not in the concentrations at which they begin to cause an odor nuisance (although our nervous system often reacts as if they were).
- The most striking malodors arise from the decomposition products of meat or fish protein, containing sulfur, nitrogen and oxygen. But odors can also arise from fires, (incomplete combustion), fats, chemicals, etc. The smells that humans react to most strongly are associated with food going bad, when invading bacteria give off molecules like dimethyl sulfide, methyl mercaptan and ammonia.
- Some odor compounds (e.g., triethylamine and hydrogen sulfide) are quite soluble in water; others such as methyl mercaptan and some lower fatty acids, for instance, are somewhat less so; still others (like indole and skatole) are quite insoluble.
Methods of odor abatement
The chemical nature of the odor compounds can be changed to make them less odorous or the odorous compounds can be removed, or other compounds can be added to neutralize the effect on the human nose.
Malodors must interact chemically or physically with the deodorant to be effective. One should determine how combinations of compounds affect odors.
Are mixtures synergistic or antagonistic? The rotten-egg smell of hydrogen sulfide, for example, can be virtually eliminated when hydrogen sulfide is mixed with certain amines.
Chlorine bleach will form chloramines from odorous amines and chloramines have their own unpleasant odor.
The major treatments are: Airing the contents, cleaning and scrubbing, thermal oxidation (incineration), adsorption, absorption with chemical oxidation, and biological oxidation.
Odor control systems can be very complex or quite simple.
Airing out the contents
Sometimes simple airing out the contents, especially in sunlight, can dissipate some odors. At times, lowering the humidity in an enclosed space can eliminate odors like musty odors.
Cleaning and scrubbing
Water-based detergent cleaning can remove water-soluble malodors and to some extent the non-water-soluble malodors, but the surfaces should be able to tolerate water-based cleaning.
All odorous organic compounds can be completely oxidized to water, carbon dioxide, sulfur dioxide, nitrogen oxides and other oxides if exposed to a high temperature long enough.
This method is highly efficient for all organic malodors, but is not practical where contents have to be deodorized and saved.
Common inorganic agents, such as sodium hypochlorite, hydrogen peroxide, potassium permanganate and ozone can readily oxidize most of the usual odor compounds.
In general, the cheapest of these is sodium hypochlorite (chlorine bleach).
Industrially, hydrogen peroxide has been used for years to deodorize, disinfect and neutralize hazardous pollutants.
Ozone is a very powerful oxidizing agent. It is produced on site. Ozone in water decomposes to oxygen and hydroxyl radicals, each of which has a higher oxidation potential than either ozone or chlorine.
The activity of the hydroxyl radical is enhanced by a higher pH.
Ozone can oxidize odorous organic and inorganic compounds in the presence of water.
Also, in the presence of moisture, it is a powerful germicide. It can also directly oxidize odorous compounds by attacking double bonds or a reactive site.
While sterilization kills all microorganisms, more practical sanitation is obtained through the use of chemicals that are sanitizers or germicides.
Many germicides are specific regarding destruction of certain microorganism and not others. An ideal compound that will destroy all microorganisms, without causing any residual toxic effect, has not yet been developed.
Hundreds of disinfectant chemicals are now available.
These include oxidants, phenolics, anilides, pine oil, metal compounds, quaternaries and gases.
These germicides are effective on biological odors and not on chemical odors.
Adsorption and absorption
Most odor-causing compounds are organic in nature; hence, granulated activated carbon (GAC) is very effective on organic odors.
One drawback is the fouling of carbon with solid or liquid particles.
However, a large number of other materials can be used as well. Peat has an acidic pH, and compounds such as ammoniated molecules are easily fixed.
Other materials are: Compost, sawdust and baking soda. Zeolites are materials under extensive investigation, though their use as odorous compound adsorbents is recent.
Clays are used as support material for activated carbon. For silica gel, only laboratory experiments have been performed.
Synthetic adsorbents are used as adsorbents for odor removal. One such product is a resin adsorbent called Ambersorb. It is a partially pyrolysed copolymer of sulfonated styrene-divinylbenzene.
More hydrophobic than GAC, Ambersorb boasts greater adsorption capacities.
Strongly cationic resins (ion exchange resins) are effective in odor removal against ammonia-or amine-containing odors and also against hydrogen sulfide, methyl mercaptan, etc.
Microorganisms occurring naturally in many soils can destroy odors.
Some of these bacteria and fungi have been commercially isolated and selectively adapted for destruction of odors, like urine- based odors.
The disadvantage is that the substrate has to be kept moist and warm for the bacterium to be effective on odor and, those in turn are the right conditions for mold growth.
These are proprietary formulations of essential oils.
The essential oils form a thin charged film around each droplet.
Odorous molecules, attracted to the film, attach to the droplets where they are captured and neutralized by the essential oils.
The oil is dispersed through a simple nozzle-and-fan apparatus. Sometimes ingredients, such as glycols, betaines, salts of organic acids and amine compounds are employed
Signals to the brain result from the act of binding of odor molecule.
The molecule has to unbind before a second stimulation can take place.
Odorants can be added that can tightly bind with the receptors, thus blocking the malodors. Some odorants like ionones, some aldehydes, and ketones can block all perception.
Sometimes the use of masking agents is not advisable because it does not mitigate the health effects of the odor (like hydrogen sulfide).
Cyclodextrins are carbohydrate-derived materials obtained by starch fermentation. Cyclodextrins can reduce malodors on textile surfaces like carpets and upholstery.
They trap or encapsulate the odor molecules in the fibers thereby neutralizing the odor.
If the odor has penetrated a surface, sometimes it is advisable to block the surface using a sealer like epoxy, shellac, polyurethane, varnish, etc.
Sometimes receptor sites can be closed off by mechanical means using masks, filters or nostril plugs.
Space or air deodorization
Effective approaches are aerosolizing, fuming, misting, adsorption and absorption.
Surface or contact deodorization
Effective approaches are cleaning, scrubbing, chemical and biological methods.
Wrapping it all up
Biocides are effective against biological odors like fecal odors, musty odors, etc.
Oxidizers like hydrogen peroxide are effective against mercaptan odors like skunk, etc.
Adsorption is effective against organic odors like solvents, and other volatile matter.
Microorganisms are effective against fecal or urine odors.
Encapsulents are effective against tobacco odors.
Odor neutralizers, especially glycol- based, are effective space deodorization.
Ozone is a good space deodorizer.
Incineration is effective against organic odors.
Most satisfying odors mimic fresh air and natural scents.
Also, keep in mind these final points:
- Your deodorization method must not damage the area deodorized.
- Use common sense, thinking about the cause and source of the odor.
- Do not panic — remember that successful odor control can take time.
- Speed of action — eliminate the source of the odor as soon as it has been detected.
Aziz Ullah, Ph.D., MBA, is president of Fabpro Manufacturing, and is a leading formulator of top-quality carpet and upholstery cleaning products. He is a member of the American Chemical Society, senior member of the American Association of Textile Chemists and Colorists, member of The Textile Institute (UK). He can be reached at www.fabpro.com.