Pamela Dalton
BioCycle November 2003, Vol. 44, No. 11, p.26
What is fascinating about human olfaction, our sense of smell, is how much we underappreciate it. We often fail to recognize just how much information we obtain from our environment by using our noses, even when we are not consciously aware of it. Smell is the oldest and most sophisticated sensory system that mammals have for detecting information about their environment at a distance. The senses that we now consider most dominant for humans – vision and hearing – evolved much later. Even single celled organisms possess the ability to “smell,” or sense chemicals.

When we enter an environment, smell can bring us information about whether it is safe to stay, whether we should approach something as a food source, as a possible friend, or move away because there is a predator or other danger there. Although humans largely rely on vision and hearing, we are still remarkably responsive to odor cues to guide our judgments about our environment and the risks that surround us. At the same time, different people often react differently to the same odorant at the same concentration. In addition, a given person can react differently under different circumstances.
How and why people react to odors is obviously critically important in managing and recycling organic materials. Odors, and the risks that people associate with them, are major concerns whenever large volumes of organic materials are present. This article discusses the basic factors that influence people’s perception and reaction to odors. Based in part on research conducted at the Monell Chemical Senses Center in Philadelphia, the article suggests some of the reasons why one neighbor might cry danger in response to the odor of a composting facility while another merely considers it an occasional nuisance to deal with and, at the same time, an operator at the facility finds no objectionable smell at all.
ODOR NOISE
It is interesting that people seem to have become more sensitive to environmental odors. By sensitive, I mean more reactive and more intolerant. Ironically, this phenomenon actually coincides with a decrease in ambient odors and an improvement in indoor air quality.
For example, consider the history of environmental odors in Philadelphia. If you read the descriptions of the city at the time our founding fathers were writing the constitution, you quickly realize that trash and human wastes were simply dumped into the streets. You can clearly imagine the odors that surrounded people all the time. In addition to the background of environmental odors, people also would have experienced a much higher level of body odor, because washing, shampoo and cleaning ourselves on a daily basis simply wasn’t the norm then.
However, environmental clean-up came slowly, but surely. By 1908, Philadelphia established its first wastewater treatment plant, which coupled with indoor plumbing, began to stem the streams of waste running down city streets. It wasn’t until 1949 that the city established the Bureau of Air Pollution (now known as Air Management Services). The 1950s and 1960s brought further improvements in odors from landfills, incineration and wastewater treatment and finally the 1970s brought the arrival of the federal Clean Air Act and the Environmental Protection Agency (EPA). In fact, judged by our standards for air quality today (pollutants and odors), in 1962, more than 200 days would have been considered to have unhealthful air quality. We’ve also managed to reduce our indoor odors, as people use hygienic products to clean, minimize and mask body odors and other odors inside their homes.
Interestingly, although we have improved air quality and reduced odor from a variety of sources, many people seem increasingly intolerant of odors. This apparent paradox is related to a concept from the field of cognitive psychology called a “signal detection.” As we have reduced the odor background level (or what in Signal Detection Theory would be called the noise level), the presence of an intrusive odor signal becomes more apparent. The consistently low odor background can render us more discriminating and better able to detect transient low level odors. Moreover, from a psychological perspective, extensive media coverage of environmental pollutants also has rendered us more sensitized to the presence of chemicals and industrial emissions and their associated odors.
THE ODOR-HEALTH ASSOCIATION
Why do people react at such a primordial level to odors? What causes people to move away from, almost without conscious awareness, what they consider to be a bad odor? In many respects, this reaction seems to center around the relationship of odors to health. Long before we understood that germs were the basis of disease transmission, there was a concept, called the “miasma theory,” that it was the odors associated with sickness and disease that were causing people to become ill. By extension, people believed if they could remove those odors, by burning incense or dousing themselves with perfumes, then they could actually remove the source of disease. This hypothesis was the result of a simple co-occurrence of disease and bad odors in which people attributed causality to the odors because they were perceptible, while germs were not. Perhaps not surprisingly, this association between odors and disease persists. Many people believe that if it smells bad, it is likely to have a negative health impact.
Odors have a high propensity to become associated with anything. Think of your own odor memories – maybe the smell of the cologne that your first boyfriend wore, or the smell of the soap that your grandmother used. Those smells bring back vivid memories. Even more frequently, however, odors become associated with negative events or situations. Odors related to a natural vector of disease such as fecal matter, rotten food or decaying organic material often acquire all of the aversive characteristics of that vector. So when people smell the odor of feces, they are primed to think about the source and its potential hazards and do not realize that the odor molecules can be devoid of any pathogens or disease-producing materials present in the source. In this way, the odor itself becomes a surrogate for the source and its impact. Several years ago, the U.S. military approached the Monell Center to learn whether there were any odors that were universally repellant. They were wondering whether it was possible to disperse an odor anywhere in the world that would be sufficiently aversive to keep people out of a restricted area. After years of smelling some of the worst odors that anyone could imagine, and testing them on individuals from a diverse range of cultures, we found that odors associated with fecal or human waste products were uniformly averse to everyone we tested. We learned that not only are these odors highly recognizable, but that they are universally repellant and believed by many to be a potential source of disease. While that may be bad news to biosolids managers, the good news is that the government may be interested in buying some sludge to use in nonlethal weapons applications!
It is also important to recognize that some individuals believe that they are susceptible to odors from many sources, apart from the disease association of the sources. Certain individuals believe that the health risks from any odor may pose a special problem to them. Finally, in our current political climate, concerns about biological and chemical terrorism have made many people hyper-vigilant about the meaning of odors in the environment.
PERCEIVING ODORS
The science of olfaction, how we smell, is fascinating, though it should be acknowledged that some of the process remains a mystery. When we breathe or sniff, airborne molecules or ‘odorants’ enter our nose. What we call “odor” is the sensation that results from the stimulation of odor receptors in our nose. Most odorants are hydrophobic organic compounds, with molecular weights less than 300. Many of the odorants that are associated with organic decomposition can be smelled in extremely small concentrations, such as one part of odorant in a billion parts of air. Consequently, a very small amount of organic matter can produce a very potent odor.
Odor perception begins when the odor molecules enter our nose and are swirled and channeled by bony projections called turbinates (Figure 1). This is a critical process because the part of the nose containing odor receptors, called the olfactory epithelium, is located very far in the upper region of the nose and is quite protected. It is estimated that only ten percent of the air that we breathe through our nose actually reaches the olfactory epithelium. After reaching the epithelium, odorant molecules must migrate through a watery mucus layer in order to reach the olfactory receptors and then bind to hair-like cilia on the receptors. Then, through a process that is incredibly elegant and complicated, a cascade of biochemical reactions sends the signal from the odor receptor in the nose up to the olfactory bulb. Here, the signal is translated into a pattern of activation that allows one to recognize the difference between the smell of a rose and pizza.
Many chemical molecules are capable of producing two different effects in the nose through the activation of two separate sensory systems, each capable of picking up and detecting airborne chemicals. The first system is the olfactory system, which recognizes sensations of odor – qualities like floral, fruity, musky, sulphurous or putrid. The second sensory system, the trigeminal system, detects sensations of pungency or irritation, For instance, high concentrations of ammonia not only generate an odor, but also a tingling sensation in the nose and eyes. Although the odorant is stimulating two independent sensory systems, our brain tends to bind them together into a single perception of ammonia. As far as we know, odor sensation itself doesn’t produce a physiological impact but trigeminal sensations can produce ocular and nasal symptoms (e.g. running nose, red eyes).
As the odorant concentration increases, the sensory effects occur in a typical sequence (Figure 2). At low concentrations, the odor can be detected although typically not identified. The recognition threshold for an odorant is typically three times the detection threshold. With increasing concentrations, an undesirable odor will often bring about annoyance and/or intolerance. Direct physical effects from an odorant do not occur until the concentration increases to the point beyond trigeminal or sensory irritant activation. Nevertheless, people often report physical symptoms and irritation at concentrations much lower than the irritant threshold. In our research, we frequently focus on understanding and differentiating between the subjective report of perceived irritation and actual irritation or somatic irritation, which occurs at higher concentrations.
Although the presence of an odor is a signal of chemical presence, the potency or hedonic nature of an odor sensation does not correlate with its toxic potential. Because many malodors can be smelled in minute concentrations, simply being able to smell the malodor does not signify that it is present in a harmful concentration.
Why do odors cause people to feel unwell? There are several possible reasons:
1) The chemical may be present at concentrations capable of eliciting sensory irritation and/or other health symptoms.
2) The presence of an odor may cause an individual to alter their breathing patterns. For example, in our laboratory chamber studies, the infusion of a malodor into the chamber will cause research volunteers to hold their breath, breathe shallowly or through their mouth rather than their nose. This can result in a sensation of light-headedness or throat irritation. These responses to avoid smelling an odor, involuntary or voluntary, can possibly lead to symptoms themselves.
3) The propensity to associate odors with negative outcomes can elicit a learned or conditioned response to an odor, based on someone’s own personal experiences or information gained from others in the community or the media.
Often the odors associated with composting, land application of biosolids, or manure management become significant community issues. Several may claim, with full sincerity, that the odors are making them ill. Research has shown that environmental malodors are typically present at concentrations greater than those capable of generating odor perception but short of those concentrations capable of generating sensory irritation or other acute health effects. The challenge then is to identify the reasons behind community reactions to odors and to understand whether the volatile odor chemicals (i.e. odorants) elicit health symptoms through direct physiological mediation or through psychological or stress mechanisms?
One way to think about the sensory determinants of what makes an odor annoying is the acronym FIDO – the Frequency of an odor, the Intensity at which it occurs, its Duration and its Offensiveness. The first three characteristics can be measured analytically with instruments. However, to understand offensiveness, we have to measure people’s reactions. This requires understanding not only sensory attributes of an odor, but nonsensory attributes, or the cognitive and emotional factors that can produce heightened odor awareness and annoyance.
COGNITIVE FACTORS, OR NON-SENSES
Research has shown that people’s reaction to odor and their beliefs about the effects from odor are influenced by a diverse set of factors, including personality traits, personal experience and information or social cues from the community and media. These factors can increase, or in some cases decrease, a person’s sensitivity and awareness of environmental odors. We have conducted several studies that demonstrate how the perception of the risk from odors actually changes a person’s sensory perception of odor levels and character.
In one particular study, we exposed three different groups of people to an odorous chemical and supplied each group with a different expectation about the chemical. Each group was exposed to 20 ppm of n-butyl alcohol, which doesn’t have a pleasant odor but is not a sensory irritant at that concentration. Before entering our environmental chamber, the volunteers were connected to electrodes to record their heart and breathing and they were asked about their perceptions. Prior to exposure, we told the volunteers one of three things about the odorant: (1) that it was a natural extract from a South American plant (positive bias); (2) that it was a standard odorant that we used in our research (neutral bias); or (3) or that it was an industrial chemical that was used to degrease machinery (negative bias).
Figure 3 shows the reported symptoms for nose, eye and throat irritation of the three groups of subjects. As you can see, the reported symptom levels are significantly different, higher for the group that believed that the odorant was a solvent. Nothing differed among the groups except what they were led to believe about the chemical. It is not that the people in this group were sicker but, I would argue, that they were actually monitoring symptoms that they expect might occur from exposure to an industrial solvent. Therefore, they were more sensitive or more biased to report irritations.
In another experiment, we looked at the influence of social cues on odor perception and effects from an odor. Again we exposed a group of volunteers to an odorant in our environmental chamber (in this case acetone or nail polish remover). However, in this study, we included what we call a confederate subject – a paid actor, unknown to the other volunteers, who was given a script to respond to the odor either positively (e.g., helps breathing, increases alertness) or negatively (irritates eyes and nose, causes coughing). Figure 4 summarizes the results. Typically, people adapt to a constant odor level (such as we were presenting) and this leads to a significant decrease in sensitivity and perceived odor intensity following even a short amount of exposure. However, in this experiment, volunteers in the negative bias condition did not adapt. In fact, these individuals rated the odor as being stronger after 20 minutes than they did when they walked into the room, even though the concentration had not changed. When we asked them about their symptoms afterwards, people who were in negative condition (in which the confederate actor was complaining) claimed significantly more eye and nose irritation, nausea and drowsiness. Volunteers in this condition even reported coughing although none of them actually did!
What does this mean? We interpret the results of these studies to indicate that the reaction that people have to odors is not simply due to the sensory impact but is also shaped by the attitudes and expectations that an individual brings to an odor experience. Obviously, this has strong implications for building relationships and understanding with neighbors in communities where odorous emissions are sometimes present.
This should not be construed as minimizing the importance of remediating the sensory impact of any odor emission for reducing the level of community annoyance and complaints. Reducing the concentration and altering the quality of the odor being emitted to areas surrounding wastewater treatment plants or landfills remains the first line of defense against odor problems. However, because even small amounts of odorous molecules can generate odors, reductions of 70 to 80 percent in odor concentration can still result in complaints if neighbors are concerned about the health impact of the emissions. Thus, working with communities and neighbors to provide them with information and to help them understand the nature of the odors, what they represent and their known effects can be a powerful tool to modify the cognitive factors that often guide and influence community reaction to odor emissions.
Pam Dalton is an associate member of the Monell Chemical Senses Center in Philadelphia. She can be reached at pdalton@ pobox.upenn.edu.