Sampling Guide for Air Contaminants in the Workplace - Irsst

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14 IRSST – Sampling Guide for Air Contaminants in the Workplace Equation 11: Calculating the R m (sum of the fractions of the mixture) C = Weighted concentration of each of the substances in the air (MDE) T = TWAEV 1,2,...n = Indication of each of the substances C1 C2 C R m = + + ... + T T T 1 2 n n When R m exceeds unity, the permissible concentration of the mixture is exceeded and the exposure does not comply. In the case of an unusual schedule, the TWAEV (T, 8 hours) must be replaced by the AMEV (T a , work period). • Example 1 An operator works for 7 hours and 20 minutes at a task in which he is exposed to a substance listed in Schedule I of the ROHS. The concentration measured during this period is 0.12 mg/m 3 . What is his mean daily exposure (MDE)? 7.33 hours at 0.12 mg/m 3 0.67 hours at 0 mg/m 3 (no exposure) namely: MDE = ((0.12 x 7.33) + (0 x 0.67)) / 8 = 0.11 mg/m 3 • Example 2 An operator works for 8 hours on a process in which he is exposed to a contaminant for which the ROHS gives a TWAEV. During this period, he is exposed to a concentration of 0.15 mg/m 3 . What is his mean daily exposure? MDE = (0.15 x 8) / 8 = 0.15 mg/m 3 • Example 3 An operator works for 8 hours during the night on a process that exposes him intermittently to a regulated substance. Knowing his work schedule and his exposure during these different tasks (Table 2), what is his mean daily exposure? Table 2- Evaluation of the exposure of worker Y Work schedule Task Exposure values (mg/m 3 ) Sampling time in hours 22:00 - 24:00 Shop assistant 0.1 (from exposure values of a group of full-time workers performing this task) 2 24:00 - 01:00 Office work 0 1 01:00 - 04:00 Cafeteria work 0 3 04:00 - 06:00 Cleaning in shop 0.21 (measured) 2 The exposure was established to be zero for the office and cafeteria work, and consequently his mean daily exposure will be: MDE = ((0.10 x 2) + (0.21 x 2) + (0 x 4)) / 8 = 0.078 mg/m 3 • Example 4 Workers are exposed to solvents under the conditions described in Table 3. Are the workers overexposed to these solvents, which all have effects on the central nervous system?
IRSST – Sampling Guide for Air Contaminants in the Workplace 15 Table 3- Exposure of a worker to a mixture of solvents Solvent Concentration (ppm) Exposure time (hours) TWAEV (ppm) Toluene 25 33 12 4,0 1.5 2.5 50 Acetone 225 560 6,0 2.0 500 Trichloroethylene 20 40 60 5.0 2.0 1.0 MDE toluene = [(25*4) + (33*1.5) + (12*2.5)] / 8 = 22.5 50 MDE acetone = [(225*6) + (560*2)] / 8 = 309 MDE trichloroethylene = [(20*5) + (40 * 2) + (60 * 1)] / 8 = 30 R m = 22.5 50 + 309 500 + 30 = 1.67 50 and unity (1) has been exceeded and the situation is noncompliant. 1.2.2 Application of the excursion limit The ROHS defines the excursion limit for substances with no STEV as follows: "Provided the time-weighted average exposure value is not exceeded, excursions in exposure levels may exceed 3 times that value for a cumulative period not exceeding a total of 30 minutes during a workday. Notwithstanding the foregoing, none of those excursions in exposure levels may exceed 5 times the timeweighted average exposure value during any length of time whatsoever." In the case of an unusual schedule, the excursion limits are calculated as a function of the AMEV rather than the TWAEV. Example 5 presents an idealized application of the excursion limit for each of the cases. • Example 5 Figures 8 and 9 give examples of the two possibilities of the excursion limit being exceeded in the case of exposure of a worker to a solvent for which the ROHS gives a TWAEV of 100 mg/m³ without specifying the STEV. A direct-reading instrument sampling in the worker's breathing zone records the concentrations over a period of slightly less than two hours. On each of the graphs, an arrow indicates the point when the excursion limit is exceeded. Note that the weighted average exposure of this worker was measured and that it was below the TWAEV.
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Sampling Guide for Air Contaminants in the Workplace - Irsst

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