If a gaseous emission sample is analyzed and found to contain water vapor and a pollutant concentration of say 40 ppmv, then 40 ppmv should be designated as the "wet basis" pollutant concentration.  The following equation can be used to correct the measured "wet basis" concentration to a "dry basis" concentration:
Thus, a wet basis concentration of 40 ppmv in a gas having 10 volume percent water vapor would have a dry basis concentration = 40 ÷ ( 1 - 0.10 ) = 44.44 ppmv.