Air Sampling for Dust: The Basics

| OSHA, USF Safety Florida

Written by Luis Silva, USF SafetyFlorida Health Consultant

Air quality is a serious issue in an industrial setting where dust can affect our sinuses, lungs, and entire respiratory system with potentially severe consequences. Understanding the differences in dust particles, the various types of filters available, and the latest dust measurement and sampling methods are essential to maintaining a safe and healthy working environment.

There are three basic categories of dust: respirable, thoracic, and inhalable (inspirable). Each type of dust exists in the air we breathe; the only difference between them is the diameter of the dust particle. Respirable dust particles are under 10 microns in diameter, thoracic dust particles are under 25 microns, and inhalable dust particles are under 100 microns in diameter. The dust sampling method varies depending upon the type of dust to be evaluated.

Another key factor to consider in particulate (dust sampling) monitoring is filter selection. There are two types of dust filters: pre-weighed and matched weight. They can be made of polyvinyl chloride (PVC) or mixed cellulose ester (MCE). Generally, PVC filters are used for gravimetric analyses; they are more durable. However, MCE filters are used especially when testing for metals during sampling.

Pre-weighed filter analysis starts when the filters are equilibrated (consistent temperature and humidity) and weighed, or “tared.” They are then placed in cassettes, sealed with color-coded bands, and labeled with a unique identification number. After sampling, they are again equilibrated, and post-sampling weight is determined. The difference between the initial weight and the final weight is the number reported (along with the air volume sampled and the resulting mg/m3 calculation).

Pre-weighed filters eventually expire after use. You should avoid using pre-weighed filters with a tare weight labeled more than one year old. Also, always try to return pre-weighed filters to the same lab that prepared them. Slight variations in analytical balances may affect your results. Blank filters in their cassettes should always be submitted when using pre-weighed filters. Blank filters are considered field blanks and help confirm the integrity of your dust samples during transport and storage.

Matched weight filters do not require the initial weighing, or “tare,” step.
A matched weight filter cassette contains two filters of equal weight, one on top of the other. During sampling, dust is collected on only the top filter; the bottom filter acts as a blank (equivalent to a preweight). Once back in the laboratory, both filters are weighed, and the difference between them is the number you see on your report.

Total (Nuisance) Dust Sampling

Total, or nuisance, dust sampling is performed by removing the plugs from the ports on the sampling cassettes to the air sampling equipment. NIOSH 0500 recommends sampling rates of 1 to 2 liters per minute (Lpm) for a maximum of 133 liters.

Respirable Dust Sampling

When respirable dust (50% cut point of 4 microns) is being evaluated, you will need to use a cyclone. This is a sampling device that excludes dust particles larger than the respirable size. Depending on the type of cyclone used, a two-or three-piece cassette may be needed, so make sure you check before ordering. NIOSH 0600 recommends sampling rates of 1.7 Lpm or 2.5 Lpm (depending on the type of cyclone used) for a maximum of 400 liters.

Inhalable (Inspirable) Dust Sampling

A different type of sampler called the IOM has been designed to collect inhalable (inspirable) dust (50% cut point of 100 microns) and offers a simplified method of sampling and analysis. The molded cassette has a 15-mm diameter inlet and is used with a standard personal sampling pump operating at 2 Lpm. Inside the tared cassette is a 25-mm PVC filter. An analysis is performed by post-weighing the entire cassette and filter, incorporating into the test results all particles collected during sampling, both on the filter and on the inside walls of the cassette.

Other tests can also be performed after the gravimetric determination. Silica exposures can be evaluated by infrared (IR) or X-ray diffraction (XRD) techniques. Metals analyses may also be important. Ideally, samples for metals analyses should be collected on pre-weighed MCE filters. These filters are easily digested in acid, whereas the PVC filters often fail to digest completely.

For more information regarding industrial hygiene air sampling and analytical methods, please visit https://www.osha.gov/chemicaldata/sampling-analytical-methods, or to request a no-cost, confidential consultation, please visit our website at www.usfsafetyflorida.com or call us at (813) 974-9962.

Luis Silva
Health Consultant
USF SafetyFlorida Consultation Program