Breathing zone

Proximity to Breathing Zone. Whereas all exposure measurement methods attempt to sample from air that is likely to be inhaled, some methods do so better than others. A sampler fixed some distance away from a breathing area is not usually accurate in measuring exposure. Even using mobile samplers that move with the worker, the few centimeters in distance from the nose and mouth to the position of the sampler, has been found to make a difference. 

Beeause eomplete eontainment is physieally impraetieable in many eases, loeal exhaust ventilation is often applied to remove eontaminants. The objeetive is to extraet pollutant as near as praetieable to its souree and before it enters, or passes through, a worker s breathing zone. Vents should lead away from personnel to a safe loeation, with sembbing/filtering as appropriate. Common examples are ... 

The conditions experienced by building occupants are best simulated by sampling air from the "breathing zone " away from the influence of any particular individual. However, if an individual sits at a desk all day (except for brief periods), samplers placed on the desk when the individual is elsewhere can provide a good estimate of that person s exposure. 

Sample open face in worker s breathing zone. 

This knowledge is also important in order to determine if air tests conducted by OSHA compliance officers are valid. For example, if threshold limit value in the health standard is an 8-hour time-weighted average, the air sample should be obtained by sampling over the entire shift in the employee s breathing zone. It cannot be measured by a few short term samples, even if spaced over the full shift unless the worker is in a relatively fixed location with no variation in his work procedure or in the process. Such an event is generally the exception rather than the rule. 

FiCURE 6.9 Concentrations of inorganic total dust in the breathing zone in conventional and benchmark factories. 

The company must decide the air quality target values for breathing zones in the work environment after discussion with industrial hygienists and ventilation engineers. 

Concentration in breathing zones lower than chosen target value... 

The definition of the breathing zone and of the zone of occupancy must be revised at each stage in the design process to make the climatization design effective and efficient. 

At a later stage in the design process, when more detailed information is available, the following parameters should be described as a function of time for each worker breathing zone position, activity level, and clothing value. 

One example of how such information can be presented is shown in Table 8.3. Breathing zone location and activity level for one specific worker during a typical workday are listed. 

Time period (h) Accumulated time (h) Work description Breathing zone position Activity level (met) Pulm V. rate (kg/h)... 

FIGURE 10.48 The air shower principle creates a zone of clean air around the worker and pushes the contaminant plume away from the breathing zone. 

Exterior hoods intended to capture contaminants should be placed as close to contaminant sources as possible. In actual practice, however, the hoods can not always be placed close to the source due to circumstances such as working conditions. In such cases, to enhance the exhaust efficiency of exterior hoods, it is useful to use a low-momentum air supply directed toward the exhaust outlet. The supply airflow, which functions to transport contaminants emitted from sources located at a distance from the exhaust outlet,. should be relatively low with a uniform velocity but high enough so that it is not disturbed by the. surrounding air motions. The advantages of using low-momentum supply with exterior hoods are that (1) a lower supply airflow rate to the workspace is possible, (2) a lower exterior hood exhaust flow rate is possible, and (3) it is possible to supply clean air to the breathing zone of the worker. 

The final example is shown in Fig. 10.86. Several workers are breaking gates off of castings on the conveyor by hand. Much dust is generated by this operation and the dust rises due to buoyancy. To remove the dust, an exterior hood was placed beside the conveyor and a supply inlet was placed above the workers. The supply airflow is blown toward the breathing zone of the workers and the dust source. In this case, as the workers and the dust source are located within the supply airflow, the airflow functions to supply the workers with clean air and to transport the dust toward the exhaust inlet. The velocity of supply air is relatively low, 1.1 m s , and the exhaust velocity at the hood face is 2.75 m s . The dimensions of the system are indicated in the figure, and the depth of the device is 6.0 m (compare with Sections 10.3.3 and 10.4.6). 

The low-momentum supply system could also be applied to operations inside booths. If a worker must be inside a booth, to protect the breathing zone, a supply inlet with a relatively wide area is placed above the worker and the low-momentum clean air is blown toward the worker. At the same time, the airflow could transport contaminants to the exhaust outlet. 

Workers could be either inside the supply airflow or outside. If a worker is inside the airflow, it is possible that the breathing zone of the worker is within the wake region and he or she may be exposed to the contaminated airflow. To reduce the exposure the worker should always be upstream of the source and should try not to have his or her back to the supply airflow. 

The standing or sitting worker is normally close to the contaminant source, which is on the working table. The relative location of the exhaust openings should be designed to ensure that contaminants are forced away from the cjperator s breathing zone. 

The influence of air disturbances on performance can be minimized by locating the supply air device as close to the operator s breathing zone as practical to improve protection and by using walls or side baffles near the contaminant source. 

The flow field created within the protection zone depends mainly on the density difference between supply air and room air (Fig. 10.90). With vertical flow the supply air should be isothermal or cooler than ambient air. If it were warmer, the extension of the controlled flow would be reduced due to buoyancy effects, resulting in the supply air not reaching the operator s breathing zone. As the. supply air cannot be used for heating, the operator s thermal comfort should be maintained, preferably with radiant heaters in cold environments. If the supply air temperature is lower than the room air, the denser supply air accelerates down to the operator, and for continuity reasons the supply flow contracts. Excessive temperature differences result in a reduced controlled flow area with thermal discomfort, and should only be used in special cases. 

In-use tests deal with the concentration measurements in the worker s breathing zone for various hood applications. The performance tests presented here are typically in-use tests, where normal work procedures are ongoing. 

---