During a recent National Institute for Occupational Safety and Health field evaluation of a newly designed filtration and pressurization system for an enclosed cab of a surface drill, it was determined that the drill operator's respirable-dust exposure was significantly increased during time periods when the cab door was opened to manually guide another section of drill steel into place to achieve additional cutting depth.

Although no dust cloud was visible when the door was opened to perform this task, the drill operator's exposure was documented (over a three-day test period) to be nine times higher than when the cab door was closed. Even at this higher respirable-dust concentration, the drill operator's exposure remained well below the permissible exposure limit governed by the federal dust standard; however, an alternative method is being investigated to guide the drill steel in place in order to eliminate this increased exposure.

BACKGROUND

Enclosed cabs are used on mining, construction and agricultural equipment to protect operators from safety and health hazards. One primary concern is overexposure to respirable dust. When equipment is new, the operator's dust and noise exposure normally is at acceptable levels. As the equipment ages and components such as gaskets and seals deteriorate, the level of protection can degrade to a point that is no longer acceptable. A compromised level of protection can become a serious issue when respirable dust is involved, especially when the dust contains crystalline silica.

In an effort to improve the air quality in older enclosed cabs on mining equipment, NIOSH has been working with the mining industry and manufacturers of cab filtration and pressurization systems. This cooperative research has identified two critical components necessary for an effective system.

The first critical factor is an effective cab filtration system, which should be composed of both a recirculation and clean (outside) make-up air system. Approximately 75% of the air inside an enclosed cab should be re-circulated through a high-quality filter and mixed with the make-up air before being delivered back into the enclosed cab. The make-up air is pulled from outside the cab and should also be filtered before being mixed with recirculated air. The make-up air is what creates the positive pressure inside the enclosed cab.

The second critical factor for an effective system is cab integrity, which is necessary to achieve some level of pressurization. Testing has shown that installing new door gaskets and seals, and then plugging and sealing cracks and holes in the shell of the cab, has a major impact on achieving positive cab pressurization.

NIOSH has been working with Vulcan Materials Co. and Sy-Klone International to retrofit one of Vulcan's surface drills with a new filtration and pressurization system that uses a uni-directional airflow design. This new airflow design is better than current systems because all clean, filtered air is brought in at the cab roof, while all dust-laden recirculated air is drawn from the floor. NIOSH believes this is a superior design when compared with most current designs that deliver the filtered air, as well as exhaust the recirculated air, at the roof of the cab. In addition, since recirculated air is removed at the floor of the cab in the new design, it eliminates the dust-laden air being drawn over the equipment operator's breathing zone.

In an effort to test this concept, NIOSH entered into a cooperative working relationship with Sy-Klone International to design and build a filtration and pressurization system capable of the uni-direction flow pattern. Vulcan Materials agreed to work with NIOSH to evaluate this new system at one of its sites.

A baseline study originally was performed to determine respirable-dust concentrations inside the enclosed cab of one of Vulcan's rotary percussion drills. After this baseline testing was completed and the new uni-directional filtration and pressurization unit was installed, the exact same dust analysis testing was performed to evaluate the effectiveness of the new system.

While performing this analysis, it was noted that the drill operator repeatedly opened the cab door. Upon closer examination, it was determined that the operator was doing this so he could manually guide the next drill steel into place each time an additional section was needed. Since no drilling was occurring and no dust cloud was visible as the operator opened the cab door, the impact of respirable-dust concentrations in the cab was thought to be minor. However, when this issue was investigated by analyzing the instantaneous dust monitor results inside the enclosed cab, a large increase in respirable-dust concentrations was noted during periods when the door was open.