The Occupational Safety and Health Act of 1970 (Public Law 91-596) assures, insofar as possible, safe and healthful working conditions for every working man and woman in the Nation. The act charges the National Institute for Occupational Safety and Health (NIOSH) with recommending occupational safety and health standards and describing exposure concentrations that are safe for various periods of employment, including but not limited to the concentrations at which no worker will suffer diminished health, functional capacity, or life expectancy as a result of his or her work experience. Under that charge and by a 1974 contract, NIOSH and the Occupational Safety and Health Administration jointly undertook the evaluation of sampling and analytical methods for airborne contaminants to determine if current methods met the criterion to produce a result that fell within 25% of the true concentration 95% of the time. In 1995, that protocol was revised. This document expands the 1995 method development and evaluation experimental testing methods to direct-reading monitors for gases and vapors. It further refines the previous guidelines by applying the most recent research technology and giving additional experimental designs that more fully evaluate monitor performance. These Components are provided for laboratory users, consensus standard setting bodies, and manufacturers of direct-reading instrumentation and are compatible with American National Standards Institute/International Society of Automation guidelines. They provide more simplified procedures to estimate the precision, bias, and accuracy of a monitor; to evaluate a monitor relative to the 25% accuracy criterion; and to demonstrate that an atmosphere is relatively safe. This document provides discussion of the physical, operational, and performance characteris-tics for direct-reading monitors. Guidance is provided for experiments to evaluate response time, calibration, stability, range, limit of measurement, impact of environmental effects, interferences, and reliability of direct-reading monitors. Also included are evaluation criteria for the experiments and details for the calculation of bias, precision and accuracy, and monitor uncertainty."