5.2 Airborne Sampling, Air Monitoring & Direct-Reading Instruments

Airborne Sampling and Monitoring Strategies

Once chemical hazards are identified in the workplace, their concentrations must be measured to evaluate worker exposure and ensure compliance with occupational exposure limits (OELs). This process involves air sampling and monitoring, which utilizes a variety of specialized instruments and techniques.

Active vs. Passive Sampling

There are two primary methodologies for collecting air samples: active and passive.

Active Sampling

Active sampling utilizes a mechanical device—a sampling pump—to actively pull a known volume of air through a collection medium (such as a filter, sorbent tube, or impinger).

• Advantages: Allows for precise volume calculations, can collect larger sample volumes, suitable for a wide range of contaminants (particulates, gases, vapors), and can be used with size-selective devices like cyclones.
• Disadvantages: Requires bulky equipment, batteries, and rigorous calibration.

Passive Sampling (Diffusive)

Passive samplers (often in the form of badges) do not use a pump. Instead, they rely on the natural principle of unassisted molecular diffusion. The contaminant molecules move across a concentration gradient into the sampler, where they are trapped by a sorbent material.

• Advantages: Lightweight, unobtrusive for the worker, easy to use, requires no pump calibration, and generally less expensive.
• Disadvantages: Sensitive to environmental factors like air velocity and temperature; primarily limited to gases and vapors, not particulates.

Personal Sampling vs. Area Sampling

• Personal Sampling: The sampling device is attached to the worker, and the collection medium is placed in the worker's breathing zone (a 10-inch radius around the worker's nose and mouth). This is the only acceptable method for determining an individual's compliance with OSHA PELs, as it accurately reflects what the worker is actually inhaling as they move through different tasks.
• Area Sampling: The sampling device is placed in a fixed location within the workplace. Area samples are useful for identifying emission sources, mapping hazard distribution, or evaluating the effectiveness of engineering controls (like ventilation), but they do not accurately represent an individual worker's exposure.

The Personal Sampling Train

When conducting active personal sampling, the setup is referred to as a "sampling train." It typically consists of:

1. Collection Medium: The filter cassette or sorbent tube that captures the contaminant.
2. Connecting Tubing: Flexible Tygon tubing connecting the medium to the pump.
3. Sampling Pump: A battery-operated pump calibrated to pull air at a specific flow rate (e.g., liters per minute).

Size-Selective Particulate Sampling: Cyclones

When sampling for dust, we are often concerned only with the particles small enough to penetrate deep into the lungs (the alveolar region). These are called respirable particulates (generally less than 10 micrometers in aerodynamic diameter).

To sample specifically for respirable dust (such as crystalline silica), a cyclone is attached to the sampling train ahead of the filter. The cyclone acts as a centrifuge; air enters tangentially, spinning inside. Larger, heavier particles (non-respirable) hit the walls and fall into a grit pot at the bottom, while the smaller, respirable particles are carried upward and captured on the filter.

Direct-Reading Instruments (DRIs)

While traditional sampling requires sending media to a laboratory for analysis (yielding delayed results), Direct-Reading Instruments (DRIs) provide immediate, real-time measurements of airborne concentrations. They are crucial for emergency response, confined space entry, and initial site characterizations.

PIDs and FIDs

Two of the most common DRIs used for detecting volatile organic compounds (VOCs) are PIDs and FIDs.

• Photoionization Detector (PID): Uses an ultraviolet (UV) lamp to ionize gas molecules. When the molecules are ionized, they generate an electrical current proportional to the concentration of the gas. PIDs are highly sensitive to aromatics and many VOCs but cannot detect methane or low-molecular-weight compounds that require higher ionization energies than the lamp provides.
• Flame Ionization Detector (FID): Uses a hydrogen flame to ionize organic compounds. FIDs respond to almost all organic compounds (including methane, which PIDs miss) and have a wider linear range than PIDs. However, they require a hydrogen fuel source, making them heavier and slightly more complex to operate.

Other DRIs

• Colorimetric Detector Tubes (e.g., Draeger Tubes): A glass tube containing a chemical reagent is attached to a hand pump. A specific volume of air is drawn through the tube. If the target gas is present, the reagent changes color, and the length of the color stain indicates the concentration. They are simple but generally less accurate (often ±25% error margin).
• Electrochemical Sensors: Commonly used in multi-gas meters for confined spaces to detect specific toxic gases like carbon monoxide (CO) or hydrogen sulfide (H₂S).

Calibration

For any sampling result to be scientifically and legally defensible, the equipment must be properly calibrated. Calibration verifies the airflow rate of the pump.

• Pre-calibration: Must be performed before the sampling event.
• Post-calibration: Must be performed after the sampling event to ensure the flow rate did not drift significantly (typically, a deviation of more than 5% invalidates the sample).

Calibration is performed using a Primary Standard. A primary standard is a device whose accuracy relies on direct, physical measurement of volume and time.

• Traditional Primary Standard: The soap bubble meter (inverted burette). The pump draws a soap bubble up a graduated cylinder, and the time taken to traverse a known volume is recorded.
• Modern Primary Standard: Electronic primary dry calibrators (e.g., DryCal). These use near-frictionless pistons and optical sensors to precisely measure flow rate without liquid, making them standard in modern industrial hygiene practice.