Instrument Performance Most instruments include filtering of readings near zero to reduce visible fluctuation. The extent of the filtering band is determined by the instrument manufacturer and is based on the properties of the sensor such as signal strength per ppm and the effects of ambient conditions such as humidity and temperature. Most instruments use unfiltered values to calculate peak or ceiling limits, short-term exposure limits (STELs), and time-weighted averages (TWAs).

NO2 is an oxidizing gas. The electrochemical detection reaction converts NO2 into NO and oxygen. NO accumulates in the sensor electrolyte as nitric acid (HNO3).

The sensor in the deployed instrument is systematically examined in accordance with the RTDS fact sheet “Reporting Specifications for Electronic Real Time Gas and Vapor Detection Equipment” (PDF). Because the instrument had sufficient resolution (0.1 ppm), minimum detection limit (0.3 ppm), and accuracy (plus or minus 15 percent), and due to other practical factors, the instrument is deemed fit for purpose. While readings of less than 0.3 ppm are not visible in the display, at 0.3 ppm and above the instrument is able to resolve step-changes in signal strength equal to 0.1 ppm.

Because the worker exposure profile also contains cross-sensitivities that can affect carbon monoxide and hydrogen sulfide monitoring results, analysis of data logs and event logs is determined to be required for proper interpretation. These known interferences are judged to be acceptable because historically collected data showed that these agents were rarely detected. Finally, the temperature and humidity ranges of the sensor cover the anticipated environmental conditions found at the mine.

Datalogging The instrument records data in an extractable data log. A separate events log records occasions of alarm. Calculated cumulative averages of the full-shift TWA and STEL are logged. The TWA is calculated based on the duration of the shift. The instrument also records discrete events whenever instantaneous values above the programmed alarm thresholds are exceeded. A review of historical logged data indicates that most exposures were below the 12-hour adjusted TWA and STEL values with only rare instances of exceedance.

Regulatory Basis The mining industry in this example is uniquely regulated by an authority that uses 1994 ACGIH NO2 values (1 ppm TWA and 3 ppm STEL), while the company seeks to follow the most current ACGIH TLV of 0.2 ppm TWA. ACGIH has not established an NO2 STEL. The TLV basis is reported as “lower respiratory tract irritation.” The current TWA limit value was established to protect asthmatics and their occasion in the workforce.

Toxicological Basis The toxicological basis of lower respiratory tract irritation was used to inform the appropriateness of adjusting the OEL values according to shift length. Adjusting OELs for extended work shifts ensures that the occupational hygienist considers the peak or cumulative body burden and metabolic clearance rate when determining if extended exposures are allowable. A variety of methods exist for adjusting OELs, ranging from simple and conservative (for example, OSHA adjustments in the lead standards and the Brief and Scala approach referenced in the ACGIH TLV documentation) to complex (such as toxicokinetic models requiring biological half-life data). Deciding which method to use is not as simple as choosing the model with the lowest adjusted limit.

For example, according to the Brief and Scala model, the adjusted exposure limit is calculated by means of the following formula, where h equals the number of hours worked per day:

According to this model, for a 12-hour shift, the adjusted TWA TLV limit for NO2 should be reduced to 0.1 ppm.

A different method proposed by Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST), a Canadian OEHS research organization, adjusts OELs for unusual work schedules according to categories determined by potential health impacts of excessive exposure. IRSST classifies NO2 is a Category IV agent, defined as a “substance that produce[s] effects following a short- or long-term exposure” and requiring adjustment by a factor of 0.67 based on a daily shift length of 12 hours and a 48-hour weekly cycle. According to this model, for a 12-hour shift, the TWA TLV limit for NO2 should be reduced to 0.134 ppm (0.2 ppm x 0.67).

Although this method results in a slightly higher adjusted exposure limit than the Brief and Scala model, the occupational hygienist believes it more accurately reflects the toxicological characteristics of the contaminant. There is no reason to use a more conservative model than the already very conservative IRSST guidance. Also, setting the alarms at an even lower value would be difficult given the capabilities of the client’s existing equipment, probably requiring a major capital investment in new equipment—a step the client is willing to take but only if necessary.

STEP 4: ESTABLISHING ALARM SETPOINT AND RESPONSE PROCESS After discussion, the company determines that the alarm value policy will be established as 95 percent of the shift-length adjusted OEL as a TWA, and one of two possible values: 95 percent of the published STEL or 95 percent of the ACGIH peak exposures guidance.

The company previously established the policy of enforcing an internal 5 percent “safety margin” to prevent exposures very close to the OEL. Following discussion of the adjustment factors, the company adopts the values shown in Table 2.

The high instantaneous alarm is chosen based on NIOSH guidance identifying a lowest observed adverse effect level (LOAEL) of 30 ppm for respiratory irritation and severe cough in volunteers following a 70-minute exposure.

STEP 5: SUMMARIZING THE ANALYSIS FOR EFFECTIVE STAKEHOLDER COMMUNICATION The analysis presented in steps 1 through 4 is documented in a summary report that is distributed to company and line management and forms the basis for a briefing with staff, blasting crews, and subcontractors. The transparency of the report is key for communication on worker protection and how to respond to alarms. The company also provides this report to the regulator as evidence of ethical and comprehensive examination of the topic given specific regulatory, technology, and field conditions that set the context.