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Safety

Chemical Exposure Monitoring

Chemical exposure monitoring is the quantitative assessment of hazardous substances in the laboratory atmosphere to ensure that employee exposure does not exceed limits established by the Occupational Safety and Health Administration (OSHA). While the Chemical Hygiene Plan provides the framework for safety, monitoring provides the hard data to validate that engineering controls (like fume hoods) and work practices are effective. This process is most critical in Anatomic Pathology, Histology, and Cytology, where high volumes of volatile organic compounds are used daily. The management of this monitoring program falls under the jurisdiction of the Chemical Hygiene Officer (CHO) and must be documented meticulously to avoid regulatory citations and long-term health liability

Key Exposure Definitions

To interpret monitoring results, the laboratory scientist must understand the specific regulatory benchmarks. These metrics determine whether a laboratory is “safe,” requires medical surveillance, or must implement immediate corrective actions

  • Permissible Exposure Limit (PEL): The maximum amount or concentration of a chemical that a worker may be exposed to under OSHA regulations. It is usually expressed as a Time-Weighted Average (TWA), calculated over a standard 8-hour workday. If the TWA exceeds the PEL, the lab is in violation of federal law
  • Short-Term Exposure Limit (STEL): The maximum concentration to which workers can be exposed continuously for a short period (typically 15 minutes). This ensures that even if the daily average (TWA) is low, the worker is not poisoned by a massive “burst” of exposure during a specific task, such as pouring a carboy or changing a tissue processor
  • Action Level (AL): This is the “warning light.” The Action Level is typically set at 50% of the PEL. If exposure monitoring reveals levels above the Action Level (even if they are still below the PEL), specific compliance activities are triggered, including periodic re-monitoring and medical surveillance programs. It acts as a buffer to prevent accidental overexposure
  • Ceiling Limit (C): A concentration that must never be exceeded during any part of the working exposure, even for an instant

Formaldehyde Monitoring (29 CFR 1910.1048)

Formaldehyde (and its aqueous solution, Formalin) is a known human carcinogen and a potent sensitizer. Because of its high toxicity, OSHA has a specific “vertical standard” dedicated entirely to formaldehyde, with stricter monitoring requirements than most other chemicals

The Limits

  • PEL (TWA): 0.75 parts per million (ppm) averaged over 8 hours
  • STEL: 2.0 ppm averaged over 15 minutes
  • Action Level: 0.5 ppm averaged over 8 hours

Monitoring Protocols

  • Initial Monitoring: Every laboratory using formaldehyde must conduct initial monitoring to determine a baseline. This monitoring must accurately reflect the exposure of employees with the highest potential exposure (e.g., the person performing “grossing” or changing the tissue processor)
  • Exemption: If the initial monitoring shows results below the Action Level (0.5 ppm) and below the STEL (2.0 ppm), no further monitoring is required unless there is a change in procedure, equipment, or personnel that might increase exposure
  • Periodic Monitoring
    • Action Level Range: If results are at or above the Action Level (0.5 ppm) but below the PEL (0.75 ppm), monitoring must be repeated every 6 months
    • Above STEL: If results exceed the STEL, monitoring must be repeated annually
  • Termination of Monitoring: Periodic monitoring can be discontinued only if two consecutive sampling rounds, taken at least 7 days apart, show results below the Action Level and STEL

Employee Notification

  • Management must notify affected employees of the monitoring results within 15 business days: of receiving the report. This notification must be in writing (either distributed individually or posted in a central location accessible to all)
  • If the results are above the PEL, the written notice must include a description of the corrective actions being taken to decrease exposure (e.g., “Installation of a new downdraft table”)

Xylene Monitoring (29 CFR 1910.1000)

Xylene is a central nervous system depressant and skin irritant used extensively in tissue processing and staining. Unlike formaldehyde, xylene does not have its own specific standard; it falls under OSHA’s “Air Contaminants” standard (Z-Tables). Consequently, the rules for monitoring are less prescriptive but equally vital for safety

The Limits

  • PEL (TWA): 100 ppm averaged over 8 hours
  • STEL: OSHA does not federally enforce a STEL for Xylene in Table Z-1, but the American Conference of Governmental Industrial Hygienists (ACGIH) recommends a STEL of 150 ppm. Many accredited laboratories adopt this stricter guideline as a best practice
  • Action Level: Generally considered 50 ppm (half of the PEL) for internal safety protocols, though not strictly defined in the Z-tables like formaldehyde

Monitoring Strategy

  • While periodic monitoring is not explicitly mandated by a vertical standard, the laboratory is required to ensure the PEL is not exceeded. Therefore, monitoring is typically performed:
    • Upon initial setup of the histology lab
    • Whenever there is a complaint (e.g., employees reporting headaches or dizziness)
    • When ventilation systems malfunction or are modified
  • Because xylene has a distinct odor threshold (~1 ppm), employees can smell it long before it reaches hazardous levels (100 ppm). However, olfactory fatigue: (loss of ability to smell the chemical) occurs rapidly, making “smell” an unreliable safety indicator. Quantitative monitoring is the only safe method

The Mechanics of Monitoring

Understanding how monitoring is performed is essential for the laboratory scientist to ensure valid results. The goal is to measure the air in the “Breathing Zone,” which is defined as a hemisphere forward of the shoulders within a radius of approximately 6 to 9 inches

Passive Dosimetry (Badges)

  • The most common method in clinical labs. These are small, lightweight badges clipped to the employee’s collar or lapel (closest to the nose and mouth)
  • Mechanism: The badge contains a charcoal or chemically treated pad. As air naturally diffuses across the badge membrane, the chemical is adsorbed onto the pad. No pumps or batteries are required
  • Duration
    • For TWA: The badge is worn for the full work shift (8 hours). The start and stop times must be recorded to the minute
    • For STEL: The badge is worn for exactly 15 minutes during the task with the highest expected exposure (e.g., during the changing of reagents on a tissue processor)
  • Handling: After the monitoring period, the badge is sealed immediately and shipped to a reference laboratory for analysis. Control badges (blank badges opened but not worn) are sent along to verify that exposure didn’t occur during shipping

Active Sampling

  • Involves a battery-operated pump attached to the employee’s belt, with a tube leading to a collection media (charcoal tube) clipped to the collar. The pump actively pulls air through the tube at a calibrated flow rate
  • This method is more accurate for low concentrations but is bulkier and more intrusive to the worker’s movement. It is typically used by industrial hygienists during comprehensive audits rather than for routine compliance checks

Response to Overexposure

If monitoring reveals that the PEL or STEL has been exceeded, the laboratory is legally required to implement a hierarchy of controls to reduce exposure. PPE is considered the last resort

  1. Engineering Controls: Fix the ventilation. This includes increasing the air exchange rate in the room, installing local exhaust ventilation (snorkels), or upgrading fume hoods and grossing stations
  2. Administrative Controls: Change work habits. This might involve rotating staff so that no single person spends 8 hours grossing, or capping containers immediately after use
  3. Respiratory Protection: If engineering and administrative controls fail to reduce exposure below the PEL, the employer must provide respirators. This triggers a Respiratory Protection Program, which requires annual fit-testing, medical clearance to wear a respirator, and training