Molecular and particulate contaminants are the focus of this guideline. There are many contributing factors involved in the degree of filtration needed in these facilities; however, the recommendations provided are considered best practice. In order to maintain the degree of filtration needed, an active maintenance program is essential. Problems arising from poor filter maintenance can create a degradation of artifacts in a short period of time. Facility surroundings can change. New construction nearby or occupant changes in nearby existing buildings could affect the environmental air. As these changes occur, the HVAC system will need to be re-evaluated and possibly updated. Filter types and certain design parameters, including outside air requirements, may need some adjustments.