• Elimination of the presence or magnitude of the hazard (not always possible if the material or condition is essential to the activity objectives but sometimes possible in the case of objectives that can be achieved by methods such as computer simulation), • Substitution of a less hazardous material or procedure (sometimes possible, such as through the use of materially similar surrogates or the use of less dispersible materials or less energetic processes; however it is prudent to remember that “regrettable substitution” is a term that applies to situations in which assumptions about the risk-reduction advantages of the substitution turned out to be wrong, • Modification of the material or procedure to reduce hazards or exposures (sometimes considered a subset of the substitution option but explicitly considered here to mean that the efficacy of the modification for the situation at hand must be confirmed by the user), • Engineering controls to prevent exposures (includes a variety of physical containment and ventilation strategies), • Warnings to indicate the need for and status of control (explicitly considered in the pyramid formulation to be a distinct hierarchy option to clarify the details of any warnings being used and to emphasize the growing capabilities and availability of real time sensors and monitors; whereas in other systems, warnings are sometimes considered part of engineered controls and sometimes part of administrative controls), • Administrative and work procedures to prevent exposures and confirm protection (an approach that relies highly on training and compliance), and finally, as the last barrier to exposure,