December 2016

Trigger action response plans for diesel exhaust exposures

  • By P G Knott, Occupational Hygienist; C Evans, Project Director; F Velge, Managing Director; and J Jeffery, Implementation Manager, Pinssar Pty Ltd

An approach to risk management that allows for the changing conditions of underground mining operations

In situations where levels of risk fluctuate, imperceptibly increase over time or exert their effect over a long time frame, the risk management approach can become clouded. As a result, the decisions and actions required by those involved may or may not adequately deal with the hazard. In a regulatory environment where risk is to be managed to ‘as low as reasonably practicable’, it is necessary for those making decisions about risk control to be given guidance as to what actions are required. Hopkins (2010) describes this approach as creating rule-based guidance for front line employees when they are confronted with complex risk management scenarios.

The concept of taking robust indicators of a hazard, increasing levels of risk associated with that hazard and creating rule-based guidance lies at the heart of trigger action response plans (TARPs). The Australian mining industry currently uses TARPs as a means of providing guidance on the management of primary hazards (Queensland Government, 2004) such as fall of ground, inrush or spontaneous combustion. Their effective application, as described by Cliff (2009), relies on a number of key criteria:

  • TARPs must be simple and robust
  • TARPs must be adequately resourced, both in terms of personnel and equipment
  • the focus of TARPs should be on prevention and control through early detection
  • setting triggers requires detailed knowledge of what is ‘normal’
  • TARPs need to be regularly reviewed and revised as necessity and experience dictates
  • TARPs should be based on high-quality mine environment monitoring
  • TARPs should be set based on the best available advice, both on-site and off-site
  • if a TARP mandates an action, that action must be carried out.

A TARP typically consists of three or four levels related to indicators of the level of risk posed by the hazard in question. Techniques for measuring the indicator should be reliable, and the trigger levels must be easily obtained, in a short timeframe, to enable prompt action. It is not feasible to wait for days to obtain a measurement. Hence, to be effective, TARPs rely on early detection.

  • Level 1 of a TARP is ‘normal’, where current procedures and practices maintain the risk at a level that is currently acceptable to the operation. However, this does not prevent any efforts to further reduce the level of risk.
  • Level 2 of a TARP is an ‘alert/investigate’ level, where the risk level indicator has deviated from normal. The level of the deviation is such as to cause increased investigation of the reason(s) for the change and an increased vigilance or control efforts. Actions at Level 2 are likely to contain the mobilisation of additional resources, increased monitoring and the deployment
    of additional controls to prevent increasing risk.
  • Level 3 is a ‘high alert/rectify’ level, where the indicator has reached a level at which the focus shifts from investigating the cause to managing the preparations for withdrawal. However, this does not mean that all efforts at control are abandoned. Indeed, more extreme control measures may be pursued.
  • Level 4 is the ‘withdrawal/removal’ of personnel from an environment that poses an unacceptable risk.

Each level requires actions for various participants. The participants and actions are determined through company risk management processes and are specific to the operation.

Application to diesel particulate exposures

Diesel exhaust exposures in mining have been of concern for over two decades. More recent epidemiological evidence (IARC, 2012) has indicated an increasing risk of adverse health effects from chronic exposure. A conventional occupational hygiene approach has been utilised by mining companies consisting of the identification of similar exposure groups (SEG), personal exposure monitoring within the SEG to submicron carbon particles averaged over a work shift (NIOSH, 2003) followed by comparison to exposure standards or guidelines to establish the level of risk. Controls to these submicron carbon particles are developed and implemented. Exposure to these contaminants is evaluated further to determine the subsequent residual risk. This approach is robust and provides statistically reliable estimates of group exposure. One downside is the time taken to obtain exposure results and the effectiveness of subsequent efforts to address controls in the event of individual results exceeding exposure standards. This is particularly difficultin many mining scenarios, where the configuration of the working environment and conditions can change daily or even hourly.

Regulators have been increasingly attentive to situations of potential overexposure to contaminants. This attention has resulted in laws requiring investigation of individual exposures exceeding exposure standards (United States Federal Register, 2005). While this approach provides additional legislative focus on air quality, it is inherently retrospective in its application of examining instances of overexposure after they have occurred.

The advent of direct reading instruments that are capable of measuring diesel particulate comparable to established methods (Noll and Janisko, 2013) or surrogate measures of diesel particulate (Kimbal et al, 2012; Stephenson, Spear and Lutte, 2006) now permit time-resolved analysis of exposure throughout a work shift and near real-time indications of airborne concentrations. This technology opens up the possibility of creating rule-based guidance to proactively mitigate risks from diesel particulate exposure. Currently, these instruments are typically deployed in an ad hoc manner for specific investigative projects as an adjunct to standard gas and ventilation measurements during surveys or as a result of complaints (ie reactive investigative purposes). The incorporation of such technology into a useful proactive tool for managing risk within shift (as is the case with portable gas monitors) is hampered by a lack of guidance as to what levels represent increasing risk.

As an example, respiratory protection is relatively easy to implement. Employees exposed to diesel particulate should wear respiratory protection. Some jurisdictions (New Brunswick, 1996) and many corporate guidelines require respiratory protection at 50 per cent of the exposure standard. In practice, it is difficult to consistently identify locations or situations that meet the 50 per cent criteria, apart from reliance on SEG-based full-shift exposure monitoring data. With only full-shift group data, what rule-based guidance could one provide?

Trigger levels

In relation to the levels of increasing risk, there is little in the way of specific written guidance on the taking of such measurements and their relationship to legislated exposure standards. It is essentially a risk-based decision. However, general guidance on the extent of excursions above an exposure standard is made in the SafeWork Australia guidance note on exposure standards (SafeWork, 2013):

Excursions above the 8-hour TWA Exposure Standard

During periods of continuous daily exposure to an airborne contaminant, the 8-hour TWA exposure standard permits short term excursions above the exposure standard provided they are compensated for by extended periods of exposure below the standard during the working day.

In practice, the actual concentration of an airborne contaminant arising from a particular process may fluctuate significantly with time. Even where the TWA exposure standard is not exceeded, excursions over the 8-hour TWA exposure standard should be controlled. A process is not considered to be under reasonable control if short term exposures exceed three times the TWA exposure standard for more than a total of 30 minutes per eight-hour working day, or if a single short term value exceeds five times the 8-hour TWA exposure standard.

In this respect, Level 4 TARP values should be a maximum of five times the TWA Exposure Standard time weighted value (ie 500 μg/m3 elemental carbon (EC)), despite there being no additional guidance from SafeWork Australia on the time base to be applied. This same philosophy is embodied in the guidance notes for threshold limit values (ACGIH, 2014) as well as regulatory instruments in other jurisdictions (Ontario Regulation, 1990; Quebec Regulation, 2014). McGarry et al (2013) has applied this principle to exposures to particles from nanotechnology processing operations. A parameter termed ‘Local Background Particle Concentration’ is used to describe particle concentrations during no processing operations. Levels during processing operations are compared to excursion guidance criteria to provide indications when emission or exposure controls are needed.

Determining ‘normal’ in relation to diesel particulate matter requires an appreciation of the variability of concentration throughout the operation. This is best conducted by taking multiple continuous measurements during normal operations throughout the operation and analysing the results to obtain an understanding of their distribution. Implicit in this analysis is the knowledge that as a continuous distribution, ‘normal’ levels will contain a small percentage of high concentration periods.

Initial trials of this concept have utilised a number of direct reading instruments positioned in various locations throughout operating mines. This process has involved selecting locations to represent a range of environments where people are engaged in work and includes stoping levels, development headings and workshops. Instruments are operated continuously for at least four weeks to collect a rich dataset covering different shifts, mining locations and equipment utilisation. All data is combined to create a whole of mine data set, although theoretically it is possible to create ‘normal’ levels for each location or similar locations.

The data obtained from this exercise is anticipated to be lognormally distributed (nonparametric) and may contain zero values. As such, the statistical approach to determining an upper limit of ‘normal’ concentrations requires an appreciation of these characteristics. We have used ProUCL, ‘Statistical Software for Environmental Applications for Data Sets with and without Nondetect Observations’, version 5.0.00 (from the United States Environmental Protection Agency) for these analyses. Originally developed for contaminated site testing, the program can calculate stable estimates of population parameters and decision-making statistics such as upper confidence limit of the mean, upper tolerance limit and upper prediction limit from a wide range of data variability, distribution, skewness and sample size.

Intermediate levels may be initially set following an examination of the distribution of ‘normal’ levels using a variety of statistical measures; alternatively, the three times TWA Exposure Standard guidance value may be used. There is some biological support for establishing an increasing risk trigger at 300 μg/m3 EC. In a review of published controlled human exposure studies to diesel exhaust, Ghio et al (2012) conclude ‘in healthy subjects, controlled human acute exposure to diesel exhaust incites lung and systemic inflammation with a threshold concentration approximating 300 μg/m3.’


The implementation of a TARP is the start of a constantly reviewed process, where new knowledge from experience and scientific endeavor is added to refine decisions and rules. It provides a vehicle to deliver practical guidance to those people who are most affected and who have the power to alter their day-to-day exposure. In its initial stages, a TARP may not comprehensively cover all eventualities, but it provides a base from which to start.


The authors would like to thank the management and staff from the various trial locations for their permission to install and test the concepts outlined in this paper.  

A version of this paper was originally presented at
The Australian Mine Ventilation Conference 2015
in Sydney.


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