Each smelter has literally hundreds of sources of emission, any of which might contribute to an employee's overall daily exposure. Because of limited technical and financial resources, it is often necessary to prioritize control needs and implement controls on a step-wise fashion. This requires planning so that controls can be applied on a true "worst-case first" basis.

Therefore, a systematic evaluation of emissions, exposures, contributors to exposure, optional controls, and costs of controls should be made prior to any attempt to install engineering controls. This will save time, money, and provide the most cost-effective application of controls and give the optimum control for the money spent.

The following is an inexpensive assessment procedure:

1. Employee exposures are measured using 8-hour time-weighted average, breathing zone sampling techniques (8-hour, TWA-BZ). Standard methods for sampling and analysis are available from OSHA's Salt Lake Technical Center at: Sampling and Analytical Methods

2. An emission inventory is conducted to identify all potential emissions to the work environment. This inventory qualitatively identifies emission sources which potentially contribute to employee exposure. The second plan view on the figure "Source Characterization" (Figure 1) demonstrates the approach.

3. Employee work practices, work locations, and air movements are studied to determine potential contact with emissions. See, for example, the third and fourth plan view on Figure 1.

4. Based on the available information, the industrial hygiene engineer ranks emission sources in order of contribution to exposure. If enough information is available, percentage contribution of emissions may also be estimated. Spot examples (short term) may be taken to confirm the estimate.

Major data inputs to these methods are:

• Employee exposure levels
• Area monitoring results
• Local emission source characterization
• Work practices and geographical location of employee throughout shift
• Employee interviews concerning emissions, work practices, and upset conditions
• Air flow patterns in the work area
• Plant engineering appraisals of the relative importance of emissions sources. (See Example Appraisal Form)

Sample Calculation:

If a yardman works 7 hours in the yard (average lead value from area samples: 60 µg/m³) and one hour in the bedding area (average lead value: 500 µg/m³), is the bedding operation the major contributor to exposure? To "quantify" the contribution, it is possible to calculate the expected exposure level and the relative contribution of each major source (420 hr × µg/m³ vs. 500 hr. × µg/m³ , or 45% from the yard and 55% from the bedding operation).

TWA = ^{(7)(60) + (1)(500)} / ₍₈₎ = 115 µg/m³

Now, to verify this estimate, we can imagine the monitoring records for the yardman. If the average TWA exposure level is near 115 µg/m³ , then we have some evidence to support our calculations. If the actual exposure level is higher, or lower, we should go back to the beginning and evaluate the initial assumptions (thereby possibly identifying other sources contributing to the worker's exposure, or making an adjustment in the initial exposure assumptions).

Having identified the major contributor, we can now make a judgment for control (i.e., should yard emissions be controlled? How much control of each emission source can be achieved? What control will give the greatest reduction of lead levels versus the number of employees exposed?).