Air intake (and exhaust) grilles shall not be located within an architectural screen or mask unless it is demonstrated to be acceptable. Any architectural structure that protrudes to a height close to the stack-top elevation (i.e., architectural structure to mask unwanted appearance of stacks, penthouses, mechanical equipment, nearby buildings, trees, or other structures) shall be evaluated for its effects on reentrainment. This may require wind engineering calculations or flow simulation studies; see 'Building Discharge and Wind Engineering' section of this chapter for information about wind engineering.
ANSI Z9.5 5.3.5
In laboratories maintained with a negative pressurization and directional airflow into the laboratory, supply air volume to the lab shall be less than the exhaust from the laboratory. In laboratories maintained with a positive pressurization and directional airflow supply, supply air volume shall be more than the exhaust from the laboratory.
Return air from nonhazardous areas (offices) may be used as makeup air in laboratories, but air exhausted from laboratories may not be returned as supply air to any space (see the 'General Laboratory Ventilation Design Issues' section of this chapter).
ANSI Z9.5 5.2.1
Room air currents at the fume hood should not exceed 25 fpm to ensure fume hood containment.
ANSI Z9.5 suggests that air velocities up to 50 fpm are acceptable, but lower room air velocities around hoods cause less interference with the operation of the hood. Makeup air should be introduced at low velocity through an opening with large dimensions to avoid creating jets of airflow. An alternative is to direct air towards the ceiling that will allow the air velocity to decrease by the time it approaches a hood.
Prudent Practices in the Laboratory, Section 8.C
ANSI Z9.5 5.2.2
Locate hoods away from activities or facilities that produce air currents or turbulence, e.g., high pedestrian or vehicular traffic areas, air supply diffusers, doors. Air supplied to a laboratory space shall keep temperature gradients and air turbulence to a minimum, especially near the face of the laboratory fume hoods and biological safety cabinets. The air supply shall not discharge on a smoke detector, as this slows its response.
Makeup air shall be introduced at the opposite end of the laboratory room from the fume hood(s), and flow paths for room HVAC systems shall be kept away from hood locations, to the extent practical.
Air turbulence defeats the capability of hoods to contain and exhaust contaminated air.
NFPA 45, Chapter 6-3.4 and 6-9.1
NIH Research Laboratory Design Policy and Guidelines D.7.7
Makeup air shall be introduced in such a way that negative pressurization is maintained in all laboratory spaces and does not create a disruptive air pattern.
Cabinetry or other structures or laboratory equipment shall not block or reduce the effectiveness of supply or exhaust air.
Supply system air should meet the technical requirements of the laboratory work, and the requirements of the latest version of ASHRAE, Standard 62, Ventilation for Acceptable Indoor Air Quality. Nonlaboratory air or air from nonhazardous building areas adjacent to the laboratory may be used as part of the supply air to the laboratory if its quality is adequate.
ANSI Z9.5 5.2.3 and 5.3.6