Respiratory protection should be the last resort for controlling an exposure to a hazardous material (e.g. dust, mist, gas, fume). Engineering controls should be explored first to eliminate or reduce the exposure to levels less than occupational exposure limits. Examples of engineering controls include: local exhaust ventilation; process enclosure; wet methods; and product substitution. If engineering or work practice controls do not reduce the exposure, then selecting the proper respirator will be necessary. Air monitoring is necessary to determine the level of employee exposure to air contaminants to evaluate engineering controls and/or select the appropriate respirator based on protection factors of various types of respirators (half-face vs. full-face, etc.)