EBC consists of droplets of airway lining fluid, diluted during condensation by gas phase water vapor, into which water-soluble volatiles are also absorbed. EBC is a body fluid, not a biomarker in and of itself. It is a matrix in which biomarkers clearly can be identified. EBC biomarkers drastically vary in chemical characteristics, as one might expect given that discovered biomarkers include small ions, gaseous acids, gaseous aldehydes, low-molecular-weight compounds such as leukotrienes, and small to large proteins. Each biomarker has certain preferred collection and storage criteria, which differ among the biomarkers of interest.4 Each biomarker has certain biologic and nonbiologic variability in EBC and may have different stability and reactivity and variations in the accuracy of the available assays. For all of these reasons, one should not determine the suitability of EBC for a given use but, rather, the suitability of a given biomarker (as measured in EBC) along with the quality of the available assays. If all that adds up well, then the said biomarker in EBC may be suitable for a given purpose.