The benefits and pitfalls of monitoring workplace exposure to dust are well documented and with the use of more precise terminology, visibility can be given to the different types of real-time monitoring and the associated characteristics and strengths and weaknesses.
Different types of real-time monitoring
Real-time monitoring should be considered an overall term for three different types of continuous monitoring which includes real-time quantitative (compliance) monitoring, real-time qualitative monitoring and indicative alarm monitoring. In the UK, currently real-time monitoring for respirable dust and respirable crystalline silica (RCS) must be considered as qualitative since there is no formal acceptance of real-time monitoring for quantitative compliance. Qualitative monitoring has value in itself, giving visibility to the dust and RCS exposure in variable processes, providing feedback on the operation of dust control equipment and procedures, and estimating dust exposure over time especially at low levels. In the future it is likely that with technical advancement and the development of appropriate standards that real-time quantitative monitoring for compliance measurement will also be available as a monitoring tool (as has happened in the emission monitoring and air quality world where approved quantitative particle instruments are readily available).
The other form of real-time monitoring is ‘indicative alarm’ monitoring where a surrogate for what is of interest is measured and changes in this surrogate are used to activate an alarm.
Personal monitors (portable or wearable) or fixed wall monitor
The different levels of real time quantitative, qualitative and indicative alarm monitoring apply to both personal monitors and fixed wall mounted instruments. Compliance sampling for occupational health purposes is also personal.
In personal monitoring the device is carried either as a portable or worn by an individual as a wearable, with the measurement sample being taken within a person’s breathing zone (ie < 300mm of the nose). Both portable and wearable personal monitors are used to monitor and control what an individual is being exposed to, independent of where the individual is located.
Fixed wall mounted monitors as their name suggests are mounted in fixed locations in a workplace. These are often used to assess if engineering controls including filtration equipment are working correctly. These may provide quantitative or qualitative results.
A fuller description of each type of personal monitoring is given below
- Compliance sampling (periodic exposure monitoring)
- Real-time quantitative measurement – compliance
- Real-time qualitative monitoring – non-compliance
- Real-time indicative alarming – non-compliance
It should be noted that the real-time methods and technology for monitoring RCS is different to that for measuring respirable dust, however the advantages and disadvantages of each type of monitoring in controlling the exposure of the substance of concern is similar.
Compliance sampling
This is a standardised sampling method from which the average exposure for a work shift can be calculated.
Advantages
- Standardised method for taking a sample over an individual’s working day.
- The calculated average dust concentration has known uncertainty allowing intercomparison between measurements
- Typically, a gravimetric technique (where change in weight of a filter is determined) is used but the collected sample can also be analysed for specific dust type. In the case of RCS, the sample is taken with an inlet which provides a respirable size cut. The sample is assessed by XRD (X ray powder diffraction) in the laboratory.
Disadvantages
- Results are retrospective
- Unlikely to be cost-effective for frequent measurements or warning on changes in well controlled processes since manpower intensive (setting up sample and analysis)
- Cannot be applied to intermittent tasks or where there are short transients in dust levels since insufficient sample taken for analysis
- No time or spatial resolution to give understanding of exposure risk in different locations and inform dust reduction priorities
Real-time measurement
Quantitative measurement – compliance
This is where a continuous measurement method has been calibrated to give results which equate to the compliance method
If the results are to be used for compliance purposes, then there must be confidence in the correlation between the real-time results and existing compliance sampling methods. In the field of occupational hygiene there are not yet standards for defining what uncertainty is allowable unlike the emission monitoring market where EN 13284-2 defines performance of continuous monitoring devices. This shortcoming will need to be addressed for continuous measurements to be accepted as compliance measurements in the UK.
Advantages
- Can be used for ensuring continuous compliance between periodic samples making it valuable for
- understanding dynamics of dust levels that an individual is exposed to,
- intermittent processes since there is sufficient resolution to measure where sampling is not
- Providing feedback on any change in performance of engineering controls to reduce exposure (ie filters leaking or seals have dislodged)
- Minimum detection level can be less than reference methods
- Immediate results and no need for delay in sending results
Disadvantages
- Technology for real time quantitative monitoring of RCS or particulate is being developed and is improving but has not yet been proven to show equivalent results to sampling
- For when the technology is at the right quality level, there are not yet agreed upon standards or approvals mechanisms for defining compliance acceptance
- Continuous Measurement methods do not correlate exactly with sampling results (due to difference in optical and inertial definition of particle size) so it is important to define acceptable levels of correlation to give validity to the results
- Periodic calibrations are necessary with reference method to ensure ongoing quality of results and no drift in instrument performance
- High quality instruments may be expensive
Real time qualitative monitoring
Real-time qualitative monitors provide a continuous result with higher uncertainty than real-time quantitative monitors which cannot be used for compliance measurements. They may be used for monitoring changes in arrestment plant performance, giving an indication of exposure during intermittent processes, showing compliance when dust levels are below Workplace Exposure Limits and providing alarms on changes in an individual’s exposure. The results are also useful for seeing changes in normally well controlled environments and provide a qualitative measurement of personal exposure on a day to day basis rather than intermittently. Alarms are set at conservative levels to compensate for uncertainty in the measurement.
Advantages
- Can monitor changes in exposure and triggering action
- Preventative actions are triggered immediately hence reducing exposure risk
- eg wear RPE, fix broken filter, move away from area
- Can instantly monitor changes in levels associated with changes in engineering or administrative controls
- Variations in dust exposure can be tracked to understand sources of dust and inform mitigation plans
- Risk of exposure is reduced. Instantaneous measurement with uncertainty can be more important than periodic sampling (typically every year) with lower uncertainty in well controlled processes
- Can be cost effective for daily use
Disadvantages
- Should not be used for compliance assessment where dust levels are close to or above WELs since uncertainty be significant
- Standards currently not used which give visibility to a monitoring device’s uncertainty
- Require periodic calibration checks for ongoing quality assurance
Continuous Indicative Alarms
In cases where it is costly or technically challenging to measure a parameter with real-time quantitative or qualitative measurements, there may be value in continuously measuring a surrogate of that parameter and alarming when there are changes in the surrogate value. For example, while innovation has led to the development of real time qualitative respirable crystalline silica monitors, these may be too expensive for personal use. As an alternative the qualitative monitoring of respirable dust can be a useful indicator of RCS in applications where the silica content is predictable. In this scenario indicative alarm monitoring provides an alternative method for monitoring the effectiveness of engineering control to reduce and control exposure to RCS. Real-time indicative alarm monitoring is a tool for helping minimise exposure in scenarios where dust is normally well controlled and minimised by warning of changes in dust exposure
Summary
There are three types of real-time monitoring to complement dust exposure sampling: real-time quantitative, real-time qualitative and continuous indicative alarm monitoring. These provide valuable options for organisations managing dust exposure risk with engineering and administration controls.
| Types of monitoring | Compliance exposure sampling | Real-time quantitative monitoring | Real-time qualitative monitoring | Real-time indicative alarm |
| What measured | mg/m3 | mg/m3 – continuously | Approx. mg/m3 over time | Alarms on change in exposure risk |
| Data frequency | One average for sampling period | Continuous | Continuous | Continuous |
| Standard/ approval | MDHS 14/4 (particulate) MDHS 101/2 (RCS) | None currently available | <50% uncertainty recommended | Not required |
Functionality
| Types of monitoring | Compliance exposure sampling | Real-time quantitative monitoring | Real-time qualitative monitoring | Real-time indicative alarm |
| Compliance with WEL | Yes | N/A yet | Yes if well below Work Exposure Limit | No |
| Compliance with WEL at low dust | Yes – but resolution is limited to method detection limit | Yes | Yes – any uncertainty in measurement is less significant | No |
| Ongoing compliance with WEL | Yes but only at time of sampling | Yes | Yes if dust is well below WEL | Tool for ensuring ongoing compliance (only if complimented with periodic sampling and well controlled process) |
| Understanding of exposure risk in different locations | Yes but no time or spatial resolution | Yes | Yes | No |
| Visibility to dust exposure in variable processes | No insufficient resolution | Yes | Yes | No |
| Warning on changes in well controlled processes | No (too much time delay between samples) | Yes | Yes | Yes |
| Visibility to dust elimination | No – insufficient resolution | Yes | Yes | Yes |