4.7 Grab sampling

If a series of grab samples (e.g., detector tubes) is used to determine compliance with either an 8-h TWA limit or a ceiling limit, consult with an industrial hygienist (ARA) regarding sampling strategy and the necessary statistical treatment of the results obtained.

4.8 Saes—exposure to chemical mixtures

Often an employee is simultaneously exposed to a variety of chemical substances in the workplace. Synergistic toxic effects on a target organ are common for such exposures in many construction and manufacturing processes. This type of exposure can also occur when impurities are present in single chemical operations. New PELs for mixtures, such as the recent welding fume standard (5 mg/m³), addresses the complex problem of synergis­tic exposures and their health effects. In addition 29 CFR 1910.1000 contains a computa­tional approach to assess exposure to a mixture. This calculation should be used when components in the mixture pose a synergistic threat to worker health.

Whether using a single standard or the mixture calculation, the SAE of the individual constituents must be considered before arriving at a final compliance decision. These SAEs can be pooled and weighted to give a control limit for the synergistic mixture. To illustrate this control limit, the following example using the mixture calculation is shown. The mix­ture calculation is expressed as:

E_m = (Q/L, + C₂/L₂) + . . . Q/LJ where

• E_m = equivalent exposure for a mixture (E_m should be < 1 for compliance)

• С = concentration of a particular substance

• L = PEL

For example, to calculate exposure to three different, but synergistic substances:

Material 8-h exposure 8-h TWA PEL (ppm) SAE

Substance 1 500 1000 0.089

Substance 2 80 200 0.11

Substance 3 70 200 0.18

Using Equation I: E_m = 500/1000 + 80/200 + 70/200 = 1.25

Since E_m > 1, an overexposure appears to have occurred; however, the SAE for each substance also needs to be considered:

• Exposure ratio (for each substance): Y_n = C_n/L_K

• Ratio to total exposure: Rj = Yj/E_ml. . . R_n = Y_n/E_m

• The SAEs (95% confidence) of the substance comprising the mixture can be pooled by:

(RS_t²) = [(R*) (SAE^) + (R₂²) (SAE₂²) + . . . (R_n²) (SAE_n²)]

The mixture control limit (CL) is equivalent to 1 + RS_t.

—If E_m < CLj, then an overexposure has not been established at the 95% confi­dence level; further sampling may be necessary. —If E_m > 1 and E_m > CLj, then an overexposure has occurred (95% confidence).

Using the mixture data above:

= 80/200	Y3 = 70/200
= 0.4	Y3 = 0.35
= 0.32	R, = 0.28

Y, = 500/1000 Y, = 0.5

• (RS_t)² = (0.4²)(0.089²) + (0.32²)(0.11²) + (0.28²)(0.18²)

• RS_t = [(RS_t)²)](l/2) = 0.071

• CL = 1 + RS_t = 1.071

• E_m = 1.25

Therefore E_m > CL and an overexposure has occurred within 95% confidence limits. This calculation is also used when considering a standard such as the one for total welding fumes.

CHAPTER 5

Chemical Risk Assessment

Real-world examples portray the decision logic needed to conduct chemical sampling when assessing risk. This chapter includes a troubleshooting section/checklist to assist samplers in either choosing a consultant or appraising in-house sampling methodology.

Chemical risk assessment is a twofold process. One part occurs off-site as known chem­ical information is assessed and calculations based on accepted formulas are done. The EPA baseline risk assessments (BLRAs), screening assessments, and remedial investigation studies rely on a body of knowledge accumulated over the last 20 years. Decisions about supportive air monitoring and actual on-site monitoring required during sampling events should also be made at this time.

The second stage is the actual accumulation of data during which workers must be pro­tected against airborne hazards, including those resulting from their sampling efforts, including disturbance of the on-site medium (soil, water). Decision-making concerning personal protection and engineering controls may require air monitoring of personnel, area of influence, and the site perimeter.

In order to understand the context under which air monitoring protocols are devel­oped, an understanding of chemical risk assessment for these sites is necessary. Keep in mind that the term site is an all inclusive one for this section and may include active indus­trial and/or construction sites.