Sampling Guide for Air Contaminants in the Workplace - Irsst

12 IRSST – Sampling Guide for Air Contaminants in the Workplace
an imaginary line joining the ears."
In the case of groups of workers, if the preliminary evaluation did not produce data on homogeneous
exposure, samples must be used to establish the variability of this exposure over time (day, night, seasons,
climatic conditions, during certain operations, etc.) and in space (different workstations or emission sources).
The result of a single sample that covers the complete 8-hour period can be compared directly to the TWAEV,
or if it covers a 15-minute period, to the STEV. In the case of unusual schedules, the single sample must cover
the total duration of the work shift and the result compared to the AMEV.
Consecutive samples covering the entire work period offer the same advantage as single samples regarding
comparison with the appropriate reference value. This strategy can also provide information on the variation
in the concentration of a contaminant during the work period and allows a sample contaminated voluntarily or
accidentally to be identified.
Multiple partial-period samples may be satisfactory, depending on the information on the homogeneity of the
exposure results. In general, with homogeneous exposure, the mean daily exposure (MDE) can be calculated
from multiple samples of a total duration of at least two hours or from 5 samples for the duration prescribed in
the reference method, and where these samples are distributed uniformly within an eight-hour time period in a
workday or for the duration of the work shift in the case of an unusual schedule. Several examples of
calculations of the TWAEV, one example of a calculation of the R m (sum of the fractions of the mixture
during daily exposure to several substances), and one example of an application of the excursion limit are
presented in section 1.2.1.
In certain cases, due to limitations in the methods or measuring instruments that require short-term samples, a
series of grab samples can be collected at random intervals during the entire work period or the period of
application of the standard. Grab samples are also used to compare the concentration of one contaminant to a
ceiling value. In this case, the minimum sampling period must take into account the analytical limitations of
response time, instrument stability, or others that are described in the IRSST's analytical and calibration
methods. Even in the case of ceiling values, result interpretation must take into account the precision and
accuracy of the technique, and establish the reliability of the comparison of the results and the limit value
using normal statistics.
Figure 7 summarizes the time characteristics of the different types of sampling in characterizing a TWAEV.
Several factors have an impact on the choice of one strategy over another. The availability and cost of the
sampling equipment, access to the workplace, variability in the processes, the precision and accuracy of the
methods, and the number of samples, are all factors to be considered in choosing a strategy. Of the four types
of samples described, the results most representative of the actual situation involve collecting several
consecutive samples over the entire work period. The second choice would be to collect a single sample over
the entire period. The interpretation of the results of partial-period samples, and the grab sample applied to the
TWAEV, AMEV or to the STEV, require a good knowledge about the homogeneity of the exposure and an
appropriate statistical analysis.
1.1.6 Frequency of environmental monitoring
Pursuing long-term objectives in the evaluation and effectiveness of means of control and elimination at
source, or obtaining exposure value results that are close to the reference value, raise the question of the
frequency of environmental monitoring (step ). In certain cases, a minimum frequency is provided in the
Québec regulation. For example, for asbestos, the ROHS specifies in section 43 that:
"In any establishment where workers are exposed to asbestos, the employer must at least once a year
measure the concentration of airborne asbestos dust and the concentration of respirable asbestos
fibres in the workers' breathing area. A sampling strategy may provide for more frequent measuring,
depending on the extent of the risk to the health, safety or physical well-being of the workers." The
same regulation sets the same frequency requirement for any establishment that employs 50 workers
or more, "… where the concentration of gases, dusts, fumes, vapours or mists at a work location
exceeds or could exceed the standards prescribed in Schedule 1…"