Three categories of engineering controls are generally considered: substitution, enclosure, and exhaust ventilation. Substitution may involve replacing the actual contaminant or replacing the source of the contaminant with one that produces lower concentrations. One consideration in substitution is to avoid introducing a new hazard when replacing the original hazard. Enclosure can mean either enclosing the source or the exposed individual. The source may also be remotely located so that little or no exposure results. Of the three engineering controls, however, ventilation is the most relevant to this document and is discussed in greater detail below.