1. Discussion
Odor, like taste, depends on contact of a stimulating substance with the appropriate human receptor cell. The stimuli are chemical in nature and the term chemical senses often is applied to odor and taste. Humans and other animals can avoid many contaminated or harmful foods and waters because of adverse sensory responses. These senses often provide the first warning of hazards in the environment.
Odor affects the acceptability of drinking water (and foods prepared with it) as well as the aesthetics of recreational waters.1 Water is a neutral medium, always present on or at the olfactory receptors. In its pure form, water is odor free. Domestic consumers and process industries such as food, beverage, and pharmaceutical manufacturers require water to be essentially free of tastes and odors.
Many organic and inorganic chemicals contribute taste or odor. These chemicals may originate from municipal and industrial waste discharges, from natural sources such as decomposition of vegetable matter or from associated microbial activity, and from disinfectants or their products. The ways that the sensory quality of water can be impaired has increased as a result of expansion in the variety and quantity of materials, demand for water disposal of captured air pollutants, and increased reuse of available water supplies by a growing population.
Some substances, such as certain inorganic salts, produce taste without odor and are evaluated by taste testing (Section 2160). Many other sensations ascribed to the sense of taste actually are odors, even though the sensation is not noticed until the material is taken into the mouth.2 Because some odorous materials are detectable when present in concentrations of only a few nanograms per liter, it is usually impractical and often impossible to isolate and identify the odor-producing chemical.3 The human nose is the practical odor-testing device used in this method. Odor tests are performed to provide qualitative descriptions and approximate quantitative measurements of odor intensity. The method for intensity measurement is the threshold odor test, based on a method of limits.4 This procedure, while not preferred by sensory experts,5 has been used to flag the occurrence of water having an objectionable odor.6
Sensory tests are useful as a check on the quality of raw and finished water and for the control of odor through the treatment process.4,5,7 They can assess the effectiveness of different treatments and provide a means of tracing a source of contamination.
Section 6040 provides an analytical procedure for quantifying several organic odor-producing compounds including geosmin and methylisoborneol.