Many of the laboratories at HKUST have local exhaust ventilation provisions, such as fume hoods, canopy hoods, equipment exhausts, or other forms of air extraction devices. The extracted air goes up to the rooftop of the Laboratory Block and is emitted through exhaust stacks. The exhaust stacks on top of the Laboratory Block were all designed in accordance with recognized standards to provide for adequate dispersion of the air emission. In fact, a mathematical modeling and a wind tunnel study using a miniaturized model of the campus were conducted before our campus was built. However, actual air dispersion is much more difficult to predict because of the complex three-dimensional outline of a building, and the chaotic nature of wind on a localized scale.

From 1998 to 1999, a study has been conducted on campus to investigate the actual dispersion of laboratory emission, and its potential impacts to rooftop workers and the general public. A non-reactive gas not commonly found in the atmosphere was released in one of the laboratory fume hoods, and air samples were taken on the rooftop of the Laboratory Block and various downwind locations on the ground, and submitted to a laboratory for analysis. This kind of tracer gas study was repeated several times over the year under different weather conditions. Smoke release studies were also performed a few times for visualization of the dispersion conditions under different meteorological conditions.

Results indicated that the dilution factor between concentration at the source and measurements on the rooftop of the Laboratory Block is usually 10,000 times or more under favorable weather conditions. Considering the normal chemical use patterns and the allowable exposure limits of common laboratory chemicals, the dispersion of laboratory emission can be considered satisfactory. The impact to the rest of the campus is negligible. However, there are times when the weather conditions are not so favorable, i.e. when the wind speed is low, and the atmosphere is stable, the chemical concentration on the rooftop may accumulate to a level that requires protection for people working up there. Besides, if there is an accidental release of chemicals, the concentration on the rooftop can be even higher.

In view of these findings, several interim measures have already been implemented for protection of rooftop workers. These include scheduling regular maintenance work to avoid certain time of the year when the weather conditions are unfavorable, and the use of personal protective equipment, mainly respirators. In addition, a set of rooftop audiovisual alarms was installed, so that when the laboratory Emergency Ventilation system is activated, which usually means there is an accidental chemical release, rooftop workers can evacuate at the same time.

In the long run, an improvement of the laboratory emission is necessary to further reduce the risk of occupational exposure to rooftop workers. In 2000, the University was successful in securing a special funding from the UGC to improve the exhaust ventilation and emission dispersion for our "high impact" laboratories. These laboratories either use more hazardous gases, such as Microelectronic Fabrication Facility Phases I and II, or are connected to stack clusters that have relatively high impact on particular rooftop areas.

The improvement work essentially involves replacing the existing exhaust fans, which have been in service for 10 years, with a different type of fans, sometimes called "Virtual Stacks" These new fans utilize a unique design to create an air stream to push the laboratory emis- sion to tens of meters higher than before, without using a physical stack. The enhanced air stream of these virtual stacks is achieved partially by pooling air flows currently going to separate conventional exhaust fans, and by drawing extra air from the rooftop. The increased plume rise allows much better dispersion, and is expected to significantly reduce the rooftop chemical concentration. On the other hand, the short stack height allows us to retain the aesthetic outlook of the Laboratory Block.

The first shipment of the Virtual Stacks has already arrived. The current schedule is to install them throughout the rest of this year, up to March 2003. During this period, fume hoods and other local exhaust ventilation systems in some of the laboratories will be interrupted for a few days for disconnecting them from the old fans and reconnecting to the new ones. All possible preparation work will be done beforehand to minimize the interruption to laboratory users. EMO/Laboratory Services will coordinate the improvement work schedule, similar to the regular exhaust fan maintenance shut-down. We hope all affected laboratory users will understand the importance of this work, and will bear with the short-term inconvenience in the coming future.

Fume exhaust clusters