Intricacies of Odor Response to Chemicals in Mixtures

By Dr. Daniel Gallagher, Associate Professor of Department of Civil Environmental Engineering , faculty member of Water IGEP, presented at 11th IWA Symposium on Tastes, Odours & Algal Toxins in Water in Sydney Australia on February 2017.

Chemical spills that contaminate drinking water are often mixtures of chemicals. Each component in a mixture has unique characteristics such as concentration, volatility, aqueous solubility, octanol-water coefficients, toxicity, and odor properties that can result in significant differences to human risk exposures. Of the ten cyclohexanes in the 2014 crude (4-methylcyclohexyl)methanol (4-MCHM) spill that contaminated drinking water in Charleston, WV, USA, 1H nuclear magnetic resonance and chromatography determined that the mixture was comprised of 34% cis- and 60% trans-4-MCHM isomers and 0.7% cis- and 0.3% trans-methyl-4-methylcyclohexanecarboxylate (MMCHC) isomers. While very minor components, MMCHC isomers were large contributing factors to odors associated with the spill. At typical 40°C showering temperature, Henry’s Law Constants were experimentally determined to be 1.50×10-2 and 2.23×10-2 for cis- and trans-MMCHC, respectively, which is 20-50 fold higher than for 4-MCHM isomers. Odor thresholds concentrations (OTCs) were 1.83 and 0.02 ppb-v, air for cis- and trans-MMCHC, respectively, with both described as predominantly sweet. The OTCs for cis- and trans-4-MCHM, respectively, were 120 and 0.06 ppb-v, air; the trans-isomer had a dominant licorice odor. Application of the shower model revealed that trans-4-MCHM contributed about 82% of odor, but because of trans-MMCHC’s higher volatility and lower OTC, it contributed to 18% of the odor. This study of human sensory and varying OTC responses to exposure of the crude MCHM spill reaffirms the need to assess chemical, physical, and biological properties of all components within the mixture of chemicals spilled.