The data used for the experiments are available as .txt files. As many control measurements were carried out, there are often more data in the files than was used for analysis.  The data were measured using a BreathSpec (G.A.S. mbH, Dortmund, Germany). The data obtained from these measurements were analysed using VOCal software (version 0.2.9, G.A.S mbH) and saved as a .txt file. ). For quantification of butanone the volume above area minimum of the dimer peak was selected.