Scents and Sensitivity - Part II

Environmental Health Perspectives Volume 106, December 12, 1998

Article by Brandy E Fisher
Part I / Part II

Research on Fragrances and the Sense of Smell

Extensive research has been conducted on the allergic effects of fragrances on skin, and many fragrance materials have been shown to cause dermal allergic reactions. RIFM conducts most of its research on the dermal effects of fragrances, rarely testing the effects of inhaling fragrance chemicals. Roberts says, "It has always been the scientific opinion of the industry that the skin is the primary route of exposure [for fragrances]." However, he says the industry "continues to think about and look at" the issue of respiratory testing.

Mind-body connection. Some researchers believe the proximity of the olfactory bulb to the limbic system in the brain is responsible for the strong link between emotion and the sense of smell, and may contribute to chemical sensitivity illnesses.

Not only is it difficult for nonindustry researchers to identify and quantify the actual components of fragrances, it is also challenging to study how inhaling these chemicals impacts human health because very little is known about the olfactory system, and very little research has been conducted on the passage of fragrance molecules into the body via this system. There is a strong link between the sense of smell and emotion; many researchers believe this is due to the proximity of the olfactory bulb to the limbic system, which popular media have dubbed "emotion central." The nasal passage offers a unique route of exposure for chemicals, which can proceed directly into the brain because of the proximity of these systems. "The olfactory/limbic tract is the most direct connection between our brains and the air we breathe," says Miller. "There is no blood-brain barrier." Studies have shown that in rodents, chemical molecules can move through the nose directly into the brain, passing through only one or two synapses. Miller says research indicates that molecules follow this same route in humans.

Another problem in studying fragrances, according to Dennis Shusterman, an associate clinical professor in the division of occupational and environmental medicine and director of the Upper Airway Biology Lab at the University of California at San Francisco, is the assumption that the only property of a fragrance chemical is its ability to stimulate the olfactory nerve and produce the sensation of smell. "In fact, [such chemicals] can stimulate both the olfactory and the trigeminal nerve, which mediates irritation," Shusterman says. Stimulation of receptors in the trigeminal nerve results in the perception of irritancy or pungency, causing sensations such as stinging, burning, piquancy, prickling, freshness, and tingling. This process is referred to as sensory irritation and can result in a localized neurogenic inflammation.

Many researchers believe that exposure to fragrance and other chemicals can indeed cause irritation, which can mimic the symptoms of allergies. James Wells, a professor of medicine at the University of Oklahoma Health Sciences Center in Oklahoma City, recounts that in his private practice as an allergist, he has encountered many patients who complain of reactions to specific perfumes or fragrances. He has observed that in a vast majority of the cases, the reaction to the fragrances is one of irritation, not allergy. Wells says the reactions to irritants are less responsive to treatment than allergies, and that avoiding the offending chemicals appears to be the only effective solution. Wells stresses that he has not conducted research, but that in his clinical experience, he has found that these patients also react to other irritants, such as detergents, cleansers, and deodorizers that emit volatile chemicals into the air.

Despite the similarity of the symptoms, though, Shusterman says existing studies indicate that the process behind chemical-induced irritation is a different phenomenon from allergies altogether. Shusterman adds that many studies have indicated that people who have preexisting nasal allergies such as hay fever either perceive or react more strongly to irritant chemicals.

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