Sampling Guide for Air Contaminants in the Workplace - Irsst

IRSST – Sampling Guide for Air Contaminants in the Workplace 17
Part 2: Sampling equipment and techniques
Introduction
Once the measurement strategy for the chemical substances or biological agents in the workplace has been
chosen, measuring equipment, techniques and methods for meeting the objective must be selected.
Measurements can be direct, using portable instruments, or indirect, by sampling on a collecting medium and
subsequent laboratory analysis. These direct or indirect techniques are described briefly in relation to the type
of contaminant: gases and vapours, aerosols (liquids, dusts and fumes), and microorganisms. The list of
equipment required for sampling or analysis is presented in Part 3. Other sections complete this part of the
guide by providing information about process samples and sampling using pumps.
2.1 Concentration units for PEVs in the ROHS (ppm and mg/m³)
The objective in using sampling equipment and techniques is to compare the concentration measurement
results for an air contaminant to a reference value, mainly the PEVs in the ROHS. It is therefore important
that the result of a measurement, whether expressed in ppm, mg/m 3 or in fibre/cm 3 , be comparable to the
value of the PEV in the ROHS and be consistent with the definition of the units. For this, the definitions in the
Definitions and Notes section of Schedule I of the ROHS are:
"mg/m³: milligram per cubic meter (milligram of substance per cubic meter of air)
ppm: parts per million (parts of gas or vapour per million parts of airborne contaminants per volume
measured at 25°C and 101.3 kPa)."
Consequently, one must conclude that the PEV values expressed in ppm are established for specific conditions
of temperature and pressure while the PEV values when expressed in mg/m³ or in fibres/cm³ in the ROHS are
invariable in relation to environmental conditions. This reasoning is based on two premises described in
greater detail by Stephenson et al. (9), and by the ACGIH ® in 2012 (10):
• The volume of air breathed in by a worker during his work day is not significantly different in relation
to moderate variations in the ambient conditions of temperature and pressure;
• Absorption of a gas or vapour is related to the partial pressure of the substance involved (11).
This approach is also used by other organizations proposing reference values, such as OSHA's Permissible
Exposure Limits (PELs) (12), the ACGIH's ® Threshold Limit Values (TLVs ® ) (10), and NIOSH's
Recommended Exposure Limits (RELs) (13). The aim of this approach is to limit the quantity of contaminant
that can be inhaled by a worker, and is therefore similar to a permissible dose approach. It is important to
emphasize that some countries such as Belgium (14) and Australia (15) have PEVs based on a permissible
concentration approach. The reader must therefore be careful when comparing the literature from different
countries because the manner in which compliance with a standard is verified may be different and lead to
different conclusions.
To expose a worker, inhaling a certain volume of air for a certain period of time at a given dose and under
different conditions, the PEVs in ppm must be adjusted in relation to the molar volume under the conditions
of the site where the worker is located, or as a corollary, the concentrations in ppm for conditions other than
NTP (normal temperature and pressure) must be corrected to "ppm at NTP" units before comparing them to
the PEV expressed in ppm.
In Québec, for the very great majority of workplaces, the molar fraction that compares the molar volume
under certain conditions to the molar volume at NTP (24.45 L) is close to unity. In this case, the permitted
ppm concentrations will be practically equivalent to those at NTP. However, for extreme and rather rare
conditions (for example for depth work in mines), the molar fraction can differ significantly from unity.
The following guidelines must be applied to verify compliance with PEVs: