Gas, Vapors, Mists
Sampling and Analysis of Airborne Contaminants

Although skin disorders, via direct contact and subsequent irritation, represent the majority of reported occupational diseases, inhalation of airborne contaminants is still considered the major route of entry for systemic intoxicants.

Passive, dermal exposure to airborne contaminants is usually quite small in comparison to the potential dose received via inhalation. Thus evaluating and controlling airborne contaminants is always an important part of any occupational health program, according to National Safety Council’s “Occupational Health & Safety,” Third Edition.

Sampling and analyzing airborne contaminants is the function that is perhaps most well known as the definitive function of the industrial hygienist. While it is the responsibility of the hygienist to interpret the results of such measurements, measurement alone contributes to the awareness of hazards, as well as to their evaluation.

The NSC says, recent developments in instrumentation enable field measurements to be made of extraordinarily low concentrations of airborne chemicals, such that previously unsuspected contamination is being discovered. Examples include the discovery of measurable amounts of organic solvent vapors in the air of clean office buildings, where no significant chemical use takes place.

In some cases, these measures, coupled with evaluations of health status of those exposed, have led to discoveries of connections between relatively low levels of airborne contaminants and health effects. The field of indoor air quality is one such general case. The determination of exposures to occupants of buildings (office workers) historically has not received substantial attention by industrial hygienists in the past, but apparently real health effects have been found at concentrations of contaminants well below those for which occupational standards have been established.

In addition, standards typically have been lowered, as both our ability to discern clinical effects and our expectations of no detectable health effects have been heightened. An example is found in the concerns regarding asbestos in buildings. It is generally accepted (from the standpoint of avoiding “fault” in litigation) that no avoidable exposure to asbestos should be tolerated, since it cannot be shown with certainty that a threshold exists below which all harmful effects can be ruled out.

Further, background measurements of asbestos are common as part of asbestos abatement projects. Thus, measurements of asbestos concentrations down to and including ambient levels have become relatively commonplace.

There are two major general sampling approaches to determining airborne contaminant levels. In the first (personal or breathing zone sampling), the industrial hygienist places a collection device near the breathing zone of the worker whose exposure is to be evaluated.

The collection devices can be active – requiring that air be drawn through the collection device, or passive, requiring no pump or other suction source.

The second approach (area sampling) employs sampling stations in the work area.

Personal breathing zone sampling is ordinarily the more desirable of the two approaches, since exposures are measured at the point nearest to the accrual entry of airborne contaminants, and the sampling system moves with the worker. Thus the measurements made are more likely to represent actual exposures. In addition, breathing zone sampling is ordinarily required to show compliance with the OSHA Permissible Exposure Limits (PLEs).

There are disadvantages to this approach, however. First, the volume of air sampled is limited by the capacity of the battery-operated pumps used (or the diffusion coefficient of a passive collection device). Thus trace contaminants can be difficult to measure.

Secondly, when complex evaluations are required, the number of collection devices may be too cumbersome for practical installation in the worker’s breathing zone. In these circumstances, or when direct-reading instruments (usually larger and often requiring line power) are to be used, area monitoring can be used. Fixed monitoring stations can also be used to measure sources, background concentrations, or concentrations in several areas simultaneously to evaluate the effectiveness of controls.

Determining the time-weighted-average (TWA) exposure levels is most important. A thorough understanding of the process is important; and the potential variability of concentrations with time and different operating conditions should be identified before beginning the sampling. This ensures that all periods when employees can be exposed to a hazardous substance will be appropriately sampled for concentrations of the substances.

Time-weighted-average exposure determinations should be made for the entire period of work to be evaluated. (OSHA requires measurement for at least seven hours out of an 8-hour workday if the potential exposure lasts the entire day.) In a continuous process environment or an assembly line type process, the period of exposure will usually be the entire work shift. The TWA exposure level is usually required to determine compliance with relevant standards and also can be useful for comparing exposures at various points within the facility.

Gas and Vapor Sampling

Gas and vapor sampling is accomplished by any of several methods. These can be divided into five major categories. The first is active collection by drawing a measured volume of air through a collection system, which is subsequently analyzed. Second, air can be pulled through a device in which a color change in the collection medium is proportional to concentration of the contaminant, and which can be read directly.

A third type of device is a passive dosimeter, which collects gas or vapor molecules by diffusion from the atmosphere, with the collection device subsequently being analyzed.

A fourth type of collector is an evacuated container used to carry a sample of air to a convenient site for analysis. Finally, direct-reading instruments sensitive to one or several atmospheric gases or vapors can be used. Direct-reading instruments are particularly useful to evaluate gases and vapors, especially when the consequence of overexposure can be death such as entry into confined spaces – and when the delay required for laboratory analysis is not tolerable.

Particulate Material Sampling

Airborne particulate material can either be solid (dusts or fumes), liquid (mists, fogs or droplets) or biological (bacteria or fungi). The particles of health concern are usually those that are small enough to be inhaled (less than 10 microns), and are usually both small enough to remain airborne for long periods of time, and too small to be readily seen with the unaided eye.

Airborne particulate contamination can be measured either by collecting integrated samples with subsequent analysis, or by direct- reading instruments. Integrated sample collection and analysis is by far the more common modality of evaluation, both because of certain inherent difficulties associated with direct-reading instruments for particles, and because of the greater precision associated with laboratory analysis.

Modern particle sampling for solid or liquid particles is ordinarily done with filters that are subsequently analyzed for the concentration of specific analytes. In the case of materials, such as asbestos, where the number concentration of asbestos fibers is the most important factor affecting the toxicity of the environment, the number of fibers on the filter is counted by microscopic techniques. In some cases, sampling is done with the aid of a size-selective sampling device preceding the filter upon which the material is to be collected for analysis.

Only those particles small enough to penetrate to and be retained within the respiratory space of interest will pass through the selective device and be captured on the filter for analysis. Sampling for microorganisms is a specialized area, and requires specialized techniques and instruments.

In some complex environments, it is appropriate to use combined particulate and gaseous (or vapor) collection devices. This can be the case where a material exists in particulate form in the atmosphere, but has an appreciable vapor pressure so that substantial amounts may evaporate following collection on a filter.

In this case, a vapor-absorbing material would be used following the filter to assure complete collection. Such a combined sampling is often used for effective collection of pesticides. FSM

Source: National Safety Council’s “Occupational Health & Safety,” Third Edition.