The industrial hygienist, field engineer, and office staff stood mesmerized by the data streaming onto the computer screen from the ISEEE test belt located in the Hitachi EX3600 shovel, which was moving piles of coal and creating a dust storm. The sensors detected the levels of CO2, cab pressure, and, most notably, coal dust, in the cab. Initially, dust levels were high due to the operator entering the cab with dusty shoes and getting positioned in his seat. However, as soon as the machine started, the high-efficiency particulate air (HEPA) recirculation filtration system kicked on, and the dust dropped to almost zero. The industrial hygienist could not believe what he was seeing.
NIOSH research has identified recirculation as a significant contributor to improving air quality within the cab. The recirculation filter sees many times more airflow than the intake air filter. Particulate brought into the cab by the operator is constantly stirred up by the operator's movement and the cab's ventilation system. This creates a constant source of particulate pollution in the cab. The recirculation filter is the engineering control used to remove particulate already in the cab. The most highly efficient intake filter cannot remove any particles from the air already in the cab. Therefore, the recirculation filter is required to remove particles from the recirculating airflow.
The graphs below show the importance of the recirculation filter is rapidly removing (decaying) particles out of the air. The recirculation filter rests with the ability to remove the dust and restore the cab to very low dust levels. When dust collects in the cab the recirculation system keeps the dust airborne, putting the operator into an unhealthy environment.
NIOSH uses a method to determine the quality of the air, called the protection factor. A protection factor of two, for example, means that the air inside the cab has half the particle count. Positive numbers indicate better air quality inside the cab than outside the cab. In the case of a machine running with low efficiency or no recirculation filter, the protection factor is negative, meaning that air quality is worse than the air outside the cab.
There remains a belief in the heavy equipment world that running a machine in general use environments without a recirculation filter is acceptable. The research as demonstrated above does not support this conclusion. All environmentally controlled operator enclosures should have efficient recirculation filters. When the data demonstrates the critical importance of efficient recirculation filtration. How can we standardize on anything less?
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