Although the time weighted-average concentration provides the most satisfactory, practical way of monitoring airborne agents for complication with the established limits, there are certain substances for which if is inappropriate. These include substances which are predominantly fast-acting. Threshold limits for such substances are appropriately based on the responses to them; and they are best controlled by a ceiling (“C”) limit that should not be exceeded. Obviously, sampling to determine compliance with a “C” limit will differ from sampling to determine compliance with a time weighted-average limit. A single brief sample is adequate for a “C” limit but not for a time-weighted limit, which requires a certain number of samples throughout a complete cycle of operations or throughout a work shift.