THE COHEN GROUP
  Environmental Health & Safety Consulting Services

The Cohen Group Newsletter - Volume 18  Issue 1, Article 4.  January 2007

Respirable Dust Sampling Basics
By Mark Golembiewski, CIH

With the recent increased attention on silica exposures in the construction industry, we have been fielding questions from some of our clients on the sampling procedures that are used to assess personal exposures to silica (and other respirable particulate contaminants) on the job. This article briefly outlines the basics of respirable dust sampling which is required to evaluate compliance with the Cal/OSHA standard for exposure to respirable silica.

Respirable dust sampling is conducted much like any other particulate air sampling, in that the objective is to collect a measurable mass of particulate on a filter that can be weighed in the laboratory. A sampling pump is used to draw air at a known rate through a filter contained in a sampling cassette that is either attached to a worker (personal sampling) or placed in a fixed position in the work area (area sampling). The sampling result is then expressed as a weight (mass) of particulate per unit of air sampled, such as milligrams per cubic meter (mg/M³).

 The additional component used when sampling for respirable particulates (i.e., those in the respirable size range) is a size-selective cyclone that is placed before the filter to separate the in-coming particle stream into respirable and non-respirable fractions (see figure at left). Air enters tangentially through a slot on the side of the cyclone cylinder and spins rapidly in a vortex inside. Through centrifugal forces, the larger particles exit the vortex and drop down to the base of the cylinder ("grit pot") while the smaller particles remain in air stream and are carried upward and through the attached filter. In this way, the respirable fraction of the incoming air stream is collected on the filter for analysis. Respirable particles are defined as those 10 micrometers and smaller. The respirable particle fraction, by definition, is characterized as having 50% of the particle mass represented by particles that have an aerodynamic diameter of 4 micrometers.

In measuring worker exposure to silica, a PVC filter contained in a 3-piece cassette is used. To assemble the sampling device, the top section of the filter cassette is removed and attached to the cyclone (a rubber O-ring around the top edge of the cyclone fits snuggly into the cassette opening and creates and air seal). The cyclone-cassette assembly is then secured in the holder (see figure) and the outlet hose connected to the rear port of the cassette. A length of Tygon^® tubing is used to connect the assembly to a battery-powered industrial hygiene sampling pump. The pump flowrate must be precisely set to the flowrate specified by the cyclone manufacturer. Be aware that not all cyclones utilize the same flowrate. The Cohen Group uses the aluminum cyclone made by SKC Inc., which specifies a flowrate of 2.5 Liters per minute (Lpm). An older type of respirable cyclone made of nylon (white) is operated at a flowrate of 1.7 Lpm, while other cyclones operate at flows of 2.0 and 2.75 Lpm.

Once the sampling train components are completely assembled, the cyclone-filter holder is attached to the workers shirt collar (or equivalent) in the breathing zone to collect a personal sample, just as one would do for any type of sampling media. Care should be taken to ensure that the assembly always hangs vertically on the worker and is not inverted at any time. Doing so would allow the large particles removed to the "grit pot" during sampling to fall into the vortex, be drawn into the filter and invalidate the sample. When recovering the samples after the sampling period, it is again important to avoid inverting the cyclone when handling the assembly and especially when detaching the cyclone from the cassette. Another important consideration when sampling is to ensure that the inlet slot on the side of the cyclone is not covered by clothing during the work shift (this is often problematic during colder weather when employees working outdoors may wear and change outer layers of clothing during the day).

In summary, respirable dust sampling is really no more complicated than any other type of employee exposure monitoring, except for the care that must be taken in assembling and disassembling the cyclone-filter cassette sampling device. If you have any questions, please contact The Cohen Group for assistance.