Biomarkers of susceptibility are indicators of the natural characteristics of an organism that make it more susceptible to the effects of an exposure to a chemical.  They can help define what sensitivities are more susceptible as well as critical times when exposures can be most detrimental.  For example, the exhalation strength of an asthmatic will indicate how susceptible that person would be to the respiratory effects of exposure to brevetoxin, the toxic compound produced during a red tide.