Most UNL buildings have mechanical heating/cooling systems. That means that fresh air is “pumped” into the building. In passive/natural ventilation systems, there is a means to heat/cool, but the only fresh air supplied to the building is that which is acquired through infiltration and opening/closing of doors and windows.
Regardless of whether a building is passively or mechanically ventilated, all laboratory spaces must be provided with mechanical ventilation. So, in a few of the older buildings, you’ll find that there is a passive system for non-hazardous areas (e.g., offices, assembly, hallways, etc.) and one or more mechanical systems for laboratories where hazardous chemicals are used (unless an exception has been granted because of extremely limited quantities of chemicals that are not toxic by inhalation and volatile).
In mechanically ventilated buildings, a portion of the exhaust air from offices, hallways, classrooms, and similar spaces is mixed with fresh air and re-circulated to the space. This results in greater energy efficiency since the system is not trying to temper 100% outside air. However, the amount of air that is re-circulated must be carefully controlled so that there is not a build-up of indoor air pollutants and irritants. This is why ventilation standards will require a specific amount of fresh air be provided to a space based on occupant load. Often, this works out to about 20% fresh air and 80% re-circulated air.
However, re-circulation of air in a laboratory is not allowed. This is to prevent the build-up of fugitive emissions of hazardous chemicals and exposure of non-laboratory personnel to chemical vapors emitted from laboratory operations. The air from a laboratory is said to be “single pass” – 100% fresh air and 100% is exhausted from the building.
When to cite:
- This checklist item would be cited for general exhaust if the space was not mechanically ventilated. You can tell if it is mechanically ventilated by the presence of air supply diffusers (Exhibit A) and exhaust ducts (Exhibit B). Diffusers can have different appearance, but something like Exhibits A & B are the most common.
- This checklist item would also be cited if it appeared that the general exhaust was not functioning properly. The most common problem here is “balancing.” Laboratories should be slightly negative to adjacent spaces with respect to air pressure differentials. This is so that fugitive emissions in the laboratory are not “sucked” into adjacent spaces, resulting in exposures to non-laboratory personnel. If laboratory air is “sucked” into non-laboratory spaces, it will be circulated throughout a building- remember, only 20% or so fresh air is introduced into non-laboratory spaces.
- There are a couple of ways to tell that the general ventilation may be ineffective- odors from the laboratory detectable in other spaces outside of the laboratory, doors that lead into the laboratory which are hard to open because of air resistance, or a detectable rush of air around or under doors from the laboratory that can be felt when the door is closed (indicating positive pressurization relative to the hallway).