In a reply to a review of NIOSH's Criteria for a Recommended Standard -- Occupational Exposure to Waste Anesthetic Gases and Vapors which criticized NIOSH's analytical methodology, NIOSH's analytic methodology is now defended. Part of NIOSH's standard protocol when designing sampling and analytic methods for gases and vapors using solid sorbents is to screen several sorbents for their ability to trap the contaminant of interest. The sorbent of choice is then subjected to breakthrough studies to determine the capacity of the sorbent for a particular contaminant. In Hertlein's critical review, he states that a known concentration of contaminant in air is not necessary, but only a known quantity has to be evaporated and drawn over the charcoal. Unfortunately, this oversimplifies the situation since it is reported that in fact, breakthrough varies with air concentration. Merely pulling a known quantity, regardless of concentration, of contaminant onto a sorbent is not a true test of capacity. Data on well over 100 compounds which demonstrate that the NIOSH determination of desorption efficiency produces good results is cited. Another NIOSH publication reporting the successful collaborative testing of seven solvents using the NIOSH charcoal tube method, once again establishing the validity of the desorption efficiency procedure, is noted.