Suppressing ammonia smell in hair colouring products

However, covering the unpleasant ammonia smell of these products is a difficult challenge. In addition, the stinging effect of ammonia during the hair colouring process requires high performance fragrances for consumer acceptance. Consumer tests have revealed that users of hair colouring products expect good ammonia coverage together with a tangible sign of effectiveness: the odour should warn consumers that they are using a chemical that reacts with the hair, and therefore a low-level ammonia smell is accepted during the mixing phase of the pigmented base with the oxidant, followed by the application phase on the hair. The fragrance should therefore provide partial ammonia coverage while leaving an attractive smell on the hair. Ammonia is both an odourant and an irritant.1 The sensation of smell is mediated by the olfactory nerve and the sensation of irritancy by the trigeminal nerve. Both the odour detection threshold (ODT) and the irritancy threshold (also called the trigeminal threshold) have been investigated by various authors.1,2 Typically, the ODT appears to be in the ppm range. In comparison, the trigeminal threshold seems to be 15 to 30 times higher, according to lateralisation measurements.1 The first aim of this study was to determine the gaseous ammonia concentrations encountered throughout the hair colouring process: above the bottle of the colouring product at different vertical distances from the lid, on hair after application, on hair following rinsing, and in the ambient surroundings. The second aim was to investigate the psychophysical behaviour of ammonia, in particular its ODT and its dose-response plot. By combining these data, we should be able to select promising perfume ingredients and then to create fragrances with significant ammonia coverage and long-lasting properties.

Materials and methods

To determine the ammonia concentrations in the air surrounding the head at different stages of the hair colouring process, we used ammonia-specific Dräger tubes.3 We then generated ammonia fluxes in olfactometers, with concentrations corresponding to those determined in the hair colouring product. This technique is a common way to mimic realistic conditions in which fragrance ingredients can be tested in the presence of adequate concentrations of ammonia. Air-dilution olfactometers were used to determine the ammonia ODT and its dose-response plot, which corresponds to the plot of the perceived intensity (response) versus the gas phase concentration (dose).4 Experiments were set up by injecting precisely measured gaseous ammonia concentrations into an olfactometer (Fig. 1). The sniffing outlet delivered a continuous and constant odourised and humidified air flow. Experiments were similarly designed to study perfume ingredients. In this case, the studied ingredient was diluted in propylene glycol or in a mixture of propylene glycol and dipropylene glycol, injected, and then vaporised. Our method allowed us to determine the psychophysical characteristics by using an iterative process with a minimum number of experiments.5 The dose-response plot and the ODT determinations were combined in the same process. The perceived intensity was rated on a scale from 0 (no odour) to 6.25 (extremely strong). Twenty volunteers, all Firmenich employees, were involved in these evaluations.