Sampling Guide for Air Contaminants in the Workplace - Irsst

26 IRSST – Sampling Guide for Air Contaminants in the Workplace
Equation 15: Collection efficiency of a respirable dust sampler E r
E ( d ) = E *(1 − F(
x))
r
a
i
d a = aerodynamic diameter
x = Ln (da/Γ)Ln (Σ)
Γ = 4.25 µm (median diameter of the cumulative lognormal distribution)
Σ = 1.5
F(x) = cumulative probability function of the standardized normal variable x
2.3.2.4 Total dust
The term "total dust" has an experimental definition based on a sampling technique that refers to the quantity
of dusts collected on a 37-mm diameter filter placed in a closed cassette with a 4-mm opening. There is no
international consensus on this experimental definition of total dust. The choice of this sampling device is
based on practical considerations such as the preservation of the integrity of the sample, the ease of handling,
etc. A cassette with a 4-mm opening results in an underestimation of dusts whose aerodynamic diameter
exceeds approximately 20 µm. This way of sampling therefore does not provide an effective evaluation of the
health risks of an aerosol for the upper airways, namely the nose, mouth, pharynx and larynx. Theoretically,
these risks would be better evaluated by sampling the inhalable fraction as defined above. Total dust
corresponds historically to a cleanliness index rather than to a fraction that can be associated with a target
zone in the respiratory tract. For solid or liquid particles, the PEVs in the ROHS are expressed in terms of
total dusts.
2.3.3 Evaluation methods
Aerosol particles are generally sampled in the breathing zone or by stationary sampling using a personal pump
placed in series with a sampling device. The pump aspirates the aerosol through the sampling device, which
will collect the particles reaching it. When desired, a particle selector can be placed in series upstream from
the sampling device so that it collects only a specific fraction of the ambient aerosol. Despite the fact that
stationary sampling allows an easy comparison of various equipment or collectors, it is important to
remember that the PEVs in the ROHS apply to the worker's breathing zone.
The pump flow rate must be checked before and after sampling. All information relating to sampling and
necessary for the analysis, such as flow rate, sampling time, temperature, pressure, humidity and the
substances present in the workplace likely to interfere with the analytical method must be noted. Samples are
shipped to the laboratory as soon as possible for analysis. Although available, direct-reading instruments for
aerosols are not considered as a reference method.
2.3.3.1 Sampling devices
2.3.3.1.1 Cassettes for sampling total or respirable dust
Aerosols are generally collected by filtration on a membrane. The most
common sampling device consists of a 2- or 3-section cassette made of
plastic material, and a porous support on which a filter or membrane is
placed. The cassette sections are pressed together and a cellulose strip
seals the joints between the two or three sections. It is important that the
cassette be tightly sealed. If the different sections of a cassette can move,
the seal is not total. Such a cassette should not be used. Two diameters of
cassettes are available, namely 25-mm and 37-mm, with a 4-mm inlet
orifice. Cassettes with a 25-mm diameter are used mainly for asbestos
sampling or inside a mask during welding. Membranes or filters with
different porosities and compositions are available. Membrane selection
is based on the nature of the substance to be sampled and the analytical
method used.
On the sampling site, the plugs are removed and the cassette is connected
to the sampling pump by means of flexible tubing. At the end of the