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<br /> Our 12 million"smell cells"and their many million more receptors allow us to discern
<br /> some 10,000 scents.S5 Smell is difficult to research because any given odor may be made up
<br /> of hundreds of different chemicals.59
<br /> Genes may control which smells we like and dislike.60 Olfactory abilities vary widely
<br /> among individuals—some people can smell things when no one else can, some people don't
<br /> notice unpleasant odor when most others do. Odors are usually described as pleasant or
<br /> unpleasant,but this is influenced by the type of molecule that is present but also the subjective
<br /> state when the odor is encountered. For example,food and cooking odors may be pleasant
<br /> when you are hungry, but may be unpleasant when you have just had a large meal and feel
<br /> very full.
<br /> Odor threshold is inexact and varies by individual sensitivity. It is not uncommon for
<br /> reported odor threshold values of some chemical compounds to range over three or four
<br /> orders of magnitude. As an example,the reported odor threshold values for n-butyl alcohol
<br /> range from 0.1 to 180 p.g/L.61 Factors that affect threshold measurement include differences in
<br /> human olfactory response(which demonstrate high inter-individual variability), age and
<br /> characteristics of volunteer,purity of the chemical compound,presence of other chemicals, as
<br /> well as experimental methodologies.62 The strength of an odor is not directly correlated to its
<br /> ability to induce health effects. As an example,the chemical most commonly used as a
<br /> natural gas tracer,tertiary butyl mercaptan, has a very strong,objectionable odor yet does not
<br /> cause toxicity at the concentrations associated with a leak. The pungent odor of this
<br /> compound is evident at 0.00009 ppm,yet the inhalation LC50(the concentration of the
<br /> chemical in air that kills 50%of the test animal,here, in rats) is 22,200 ppm,three hundred
<br /> million times higher.63 Neither documented exposure nor odor detection necessarily dictates
<br /> adverse responses to any chemical. The dose of a chemical determines whether that chemical
<br /> is toxic or nontoxic.64
<br /> Microorganisms produce a wide range of alcohols, ketones, aldehydes,esters, carboxylic
<br /> acids, lactones,terpenes, sulfur and nitrogen compounds, and aliphatic and aromatic
<br /> hydrocarbons in their metabolism.65 Many factors influence microorganisms microbial
<br /> 58 Marieb,E.N.and Hoehn,K.(2007).The special senses.Human Anatomy and Physiology,7th ed.Pearson
<br /> Benjamin Cummings,San Francisco.; Shier,D. et al. (1999). Somatic and special senses.Hole's Human
<br /> Anatomy and Physiology, 8th ed. WCB;McGraw-Hill,Boston.
<br /> 59 Marieb,E.N.and Hoehn,K.(2007). loc. cit.
<br /> 60 Shier,D.et al. (1999). loc. cit.
<br /> 61 AIHA.(1989). Odor Thresholds for chemicals with established occupational health standards.ACGIH,
<br /> Akron,OH.;US EPA.(1992). Reference guide to odor thresholds for hazardous air pollutants listed in the Clean
<br /> Air Act Amendments of 1990,EPA/600/R-92/047.U.S.EPA,Washington,D.C.
<br /> 62 AIHA.(1989)./oc. cit.;US EPA.(1992).loc. cit.
<br /> 63 Fairchild,E.J.and Stokinger,H.E.(1958).Toxicologic studies on organic sulfur compounds.I.Acute toxicity
<br /> of some aliphatic and aromatic thiols(mercaptans).American Industrial Hygiene Association Journal. 19(3):
<br /> 171-189.;Wilby,F.V.(1969).Variation in recognition odor threshold of a panel.J Air Pollut Control Assoc.
<br /> 19(2):96-100.
<br /> 64 Montgomery,M.R.and Reasor,M.J.(1994).A Toxicologic Approach for Evaluating Cases of Sick Building
<br /> Syndrome or Multiple Chemical Sensitivity.Journal of Allergy and Clinical Immunology.94(2):371-375.
<br /> 65 Pasanen,A.L.et al. (1996).Volatile organic metabolites associated with some toxic fungi and their
<br /> mycotoxins.Analyst. 121: 1949-1953.
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