Introduction
Air monitoring can be either periodic or continuous and is the quantitative or qualitative assessment of the extent of pollutants in or around the workplace. It is used to ensure compliance with appropriate legislation and to evaluate control measures. There are numerous approaches to measuring dangerous substances in air ranging from simple passive sampling techniques to sophisticated remote sensing devices. A monitoring strategy should be in place taking into account the most appropriate methodology in terms of costs and practicability.
Under EU directive 89/391/EEC[1] concerning health and safety at work, there is a requirement for employers to prevent or control the exposure of workers to dangerous substances. For a large number of these substances the major route of exposure is by inhalation[2], although airborne substances can also lead to dermal and, indirectly, oral exposure.
Legislation
The legal requirement to control workplace exposure to dangerous substances is contained within EU Directive 98/24/EC[3] and concerns risks related to chemical agents at work. It states that the employer must determine whether any hazardous chemical agents are present at the workplace and assess any risk to the safety and health arising from their presence. As well as chemical fumes and vapours etc., this directive also considers dust.
Several EU directives deal with the protection of workers to a variety of dangerous substances and these are summarised in Table 1. The emphasis on monitoring in these directives varies. Concerning biological agents, 2000/54/EC[4] stipulates: ‘In the case of any activity likely to involve a risk of exposure to biological agents, the nature, degree and duration of workers' exposure must be determined in order to make it possible to assess any risk to the workers' health or safety and to lay down the measures to be taken’. For carcinogens, mutagens and reprotoxic substances in 2004/37/EC[5] it requires the 'use of existing appropriate procedures for the measurement of carcinogens, mutagens or reprotoxic substances, in particular for the early detection of abnormal exposures resulting from an unforeseeable event or an accident'.
Table 1: EU directives concerning protection of workers to airborne dangerous substances
| Dangerous substances | EU Directive |
| Chemical Agents | 98/24/EC[3] |
| Biological Agents | 2000/54/EC[4] |
| Explosive Atmospheres | 2014/34/EU ATEX 'product'[6]; 1999/92/EC ATEX 'workers'[7] |
| Asbestos | 2009/148/EC[8] |
| Carcinogens, mutagens and reprotoxic substances | 2004/37/EC[5] |
The European Commission has produced non-binding, practical guidelines[9] that relate to the 'chemical agents' Directive 98/24/EC. These guidelines provide information on measurement methods regarding air monitoring of chemical substances. Similar guidelines have been produced for he ATEX directive 1999/92/EC[10] and although mainly concerned with general provisions for explosion and fire prevention it does contain guidance on gas monitoring by means of gas alarms.
Standards
Standards are technical documents, which are intended to be the accepted method for doing something, in this case assessment of dangerous substances in air. They are consensus built, and bring together all stakeholders, e.g. manufacturers and regulators and address issues such as quality, efficiency and best practice.
The European Committee for Standardization (CEN) Technical Committee CEN/TC 137 prepares and provides European standards for the 'Assessment of workplace exposure to chemical and biological agents'. CEN/TC 137 standards both published and under development, cover workplace atmospheres as well as dermal exposure and are available from the CEN website[11]. A European standard automatically becomes a national standard in all the member countries. The national provisions communicated by the Member States for the chemical agents Directive 98/24/EC can be found on the EUR-lex website[12]
Exposure Limits
Competent authorities or institutions set Occupational Exposure Limit (OELs) values nationally. They are limits for concentrations of dangerous substances in workplace air and are important tools for risk assessment and management. Several binding upper limit values are set in the Directives listed in Table 1, especially in the CMR Directive (2004/37/EC[5]).
Furthermore, Directives based on article 3(2) of Directive 98/24/EC include lists of indicative occupational exposure limit values: Commission Directive 91/322/EEC[13], Commission Directive 2000/39/EC [14], Commission Directive 2006/15/EC [15], Commission Directive 2009/161/EU[16], Commission Directive (EU) 2017/164[17] and Commission Directive (EU) 2019/1831[18]. Indicative occupational exposure limit values (IOELV) are health-based, non-binding values, derived from the most recent scientific data available and taking into account the availability of reliable measurement techniques. For any chemical agent for which an IOELV has been set at European Union level, Member States are required to establish a national occupational exposure limit value taking into account the EU values.
Sampling
Ensuring a suitable sampling strategy and analytical methodology is essential to maintain the integrity of the measurements. Air sampling in the workplace is required when a risk assessment indicates that monitoring is necessary. The chemical agents Directive 98/24/EC Article 43 states that if it can be shown that existing preventive or control measures adequately reduce the risk then monitoring may not be required. In addition, it may be that the problem is obvious and e.g. fixing a leak may be sufficient. The two elements of sampling are strategy (what and when to sample) and methodology (how to sample).
Sampling strategy
CEN standard EN 689:2018[19] is the EU standard that provides a strategy to compare workers' exposure by inhalation with relevant limit values for chemical agents in workplace and measurement strategy.
Scope
Only a competent person, this being somebody with the skill, knowledge, practical experience and training to enable him/her to assess the risks arising from work activities involving substances hazardous to health should conduct sampling. Sampling should not interfere with the work being undertaken by altering the workers’ routine and the act of sampling should not alter the air.
The main reasons for sampling are to ensure compliance with legislation, to establish the levels of exposure and to demonstrate the effectiveness of control measures. As such sampling should be part of a planned approach, based on risk assessment. It is, however, often the case that air sampling is reactive, being undertaken due to prosecution or complaints. It is not possible to sample the whole of an environment and so air sampling is intended to capture a representative portion of that environment. In terms of occupational health and safety that environment is usually an indoor workplace but may also include samples taken outside.
An initial appraisal of the hazards and risks and the level of existing control are essential. If it is decided there is adequate control, routine monitoring may be suitable to ensure this control remains effective. If the initial appraisal suggests that controls may not be adequate, a more detailed survey should be undertaken to estimate a worker’s personal exposure. It should be noted that monitoring a hazard is not a substitute for safe working practices and maintenance.
Planning
The first stage in planning is to prepare an overview of the immediate workplace and surrounding areas, the work practices being undertaken and equipment and materials in use, as well as substances incidental to the work processes (fumes, dusts, mould, etc.). Useful information may be taken from operators’ manuals, site operating procedures, risk assessment documentation, and, for specific materials, safety data sheets (SDS). In the absence of an SDS it would be prudent to sample bulk material if safe to do so. This would allow a laboratory to determine the composition of the material in use. This step should have established the identity and physical characteristics of the hazard allowing the measurement and analytical methods to be selected. These methods should be chosen together to ensure they are compatible. This is often already done in the case of standardised methods but where no standard exists adapted or novel methods may be required.
The choice of a measurement method must be looked at in context. Other factors e.g. temperature or humidity may affect the sampling method. The requirements of the analytical method may not allow for one compound to be determined in the presence of another or may have limits of detection that require a longer sampling period than that available. Discussion between the person taking and the person analysing the samples is recommended.
The sampling time may be determined by whether the most appropriate limit is an eight hour time-weighted average (TWA) or a 15 minute short-term exposure limit (STEL). The choice here may be influenced by exposure being relatively constant over a shift or resulting from peaks in exposure at key points of a worker’s shift. Although the TWA and STEL are common, it does not exclude sampling times extending beyond a single shift. The number of samples to be taken should be sufficient to allow for variation and range of exposure and the statistical validity of these needs to be considered at this point.
CEN standard EN 482:2021[20] is the EU standard that gives the performance requirements for measurement methods concerned with the determination of the concentration of chemical agents in work atmospheres. It applies to all the procedures of measurement irrespective of the physical form of the chemical agent (gas, vapour, suspended matter) to the methods used, both sampling and analytical.
Measurement methods are available from many sources including the International Organization for Standardization (ISO) and the European Committee for Standardization (CEN).
Sampling approaches
There are four sampling approaches to be considered:
Personal sampling involves placing a sampler in the breathing zone of the worker, usually attached to the lapel. The breathing zone is defined as the hemisphere (or a similar volume) with a radius of 0.30 meters (30 cm) extending in front of the worker's face, centred on the midpoint of the line connecting the ears. This area should include the nose and mouth, ensuring that air samples reflect the air that the worker actually breathes [21]. The choice of lapel, left or right may produce differing results depending on the location of the contamination source. Personal sampling is typically used where the chemical exposure of a worker due to inhalation is of greatest concern and can be used to demonstrate which tasks performed during a shift are leading to the highest levels of exposure. It removes variation due to proximity to point sources of contamination by providing a more realistic measurement of actual exposure.
Area samples are positioned in the general area of the worker and/or operation of concern. This produces general or background measurements that can show the spread of contaminants or indicate when entry to an area may be considered safe, e.g. in a spray booth where adequate time is required to allow the local exhaust ventilation (LEV) to remove any isocyanates and/or other dangerous substance present. Area sampling can establish trends in air concentrations and provide information on exposure due to control or containment not being present or being inadequate as well as deposited material becoming re-suspended.
Source samples are samples taken adjacent to a source of the hazardous substance. It may miss out the effect of control measures such as LEV as it may be placed closer than a worker may be. This can be used to determine the effectiveness of controls.
Surface sampling, such as wipe tests, lift-off tape or more complex techniques such as X-ray fluorescence (XRF) can provide information on settled contaminants that may have deposited and not be air-borne during the sampling period.
Sampling methodology
Active sampling
Active sampling is a common method using a pump, typically a flow controlled, rechargeable pump. Standard EN ISO 13137:2022[22] gives the requirements and test methods for pumps used to sample chemical agents in workplace atmospheres and standard EN 1076:2009[23] gives the requirements and test methods for sampling for gases and vapours using pumped samplers.
For personal sampling the pumps are often attached to a belt with a tube passing to a sampling head on the worker’s lapel. A known volume of air is drawn through a sampling media. It is often worn for an entire shift to produce the 8 hour time-weighted average (TWA). According to EN 482:2021[20], however, an 8 hour TWA may be extrapolated from representative measurements taken over shorter time periods. Personal sampling is also used for comparison with the short-term exposure limit (STEL).
The sampling media may be a filter, a sorbent tube or impinger.
- A filter may simply trap the substance of interest, e.g. particles including dust and aerosol, fibres or semi-volatile organic compounds. The choice of filters is determined by the application e.g. glass fibre or cellulose fibre for gravimetric sampling and metal analysis. Gravimetric sampling is the collection of a substance and the subsequent quantification based on the mass of a solid. They may be coated with a reagent to stabilize and trap a reactive substance e.g. ISO method 16702:2007[24] for isocyanates using 1-(2-methoxyphenyl) piperazine. This method also employs an impinger.
- Sorbent tubes are used for gaseous hazards. Sampling tubes are also used in pumped sampling, these contain a sorbent such as (activated) charcoal, silica gel, Tenax®, Chromosorb®, molecular sieve.
- Impingers (bubble tubes for the collection of airborne substances into a liquid medium) can be used for both particulates and vapours. Impinger use is limited as they are breakable and can contain harmful and/or flammable liquids.
- Dust particles can be graded by size using devices such as cyclones.
Passive sampling
Passive sampling is a simple alternative to active sampling whereby contaminants in air are adsorbed onto a sorbent by diffusion. Many sorbents are inert polymers whilst others react to form a derivative. The surface area may be large as in the case of badges or cylinders, or small as for tubes. Radial type samplers allow for higher diffusion rates due to their radial geometry and provide a greater sensitivity to determine the concentration of the airborne compound[25]. The rate at which the contaminant is absorbed, the uptake rate, must be derived for each substance on each type of sampler and sorbent. The choice of sorbent is critical to effective sampling; this could be to ensure the retention of highly volatile substances or the stabilisation of very reactive ones.
EN ISO 16017-2:2003[26] and EN ISO 23320:2022[27] contains information on sampling and analysis of ambient, indoor and workplace air for volatile organic compounds (VOC) by diffusive sampling.
Real-time monitoring
There are several types of real-time or direct reading monitors[28].
- Gas detectors, both specific and non-specific, form an important part of safety systems to help protect users from explosion, fire or ill health (acute and chronic) arising from flammable, toxic or asphyxiant gases. They provide instant measurements of air exposure. Oxygen monitors allow for safe working in oxygen deficient atmospheres and/or confined spaces. Multi-Gas Monitors can detect multiple gases simultaneously providing comprehensive real-time monitoring of various contaminants.
- Real-time gas detectors monitors are predominantly used to trigger alarms if a specified concentration of gas is exceeded and measure workers’ exposure to gases. This can provide an early warning of a problem and help ensure worker’s safety and health. However, a detector does not prevent leaks occurring or indicate what action should be taken. It is not a substitute for safe working practices and maintenance.
- Real-time dust, aerosol and particle monitors are non-specific monitors used for several purposes including background sampling, site measurements, assessment of the effectiveness of control systems and measurement of indoor air quality. They are also used to visualise exposure to identify peaks in particulate levels due to poor work practice and in the investigation of control techniques[29]. The main advantage of these monitors is that they give an instantaneous measure of airborne particulate concentration, thereby reducing considerably the time and effort associated with standard gravimetric methods[30].
- A special case is PIMEX (Picture Mix Exposure)[31] [32], which allows the flow of work to be filmed with a video camera and simultaneously exposure data to be recorded by means of sensors and transducers attached to the workers. The exposure data are added to the video by means of special software. But, this method is more applied in the context of performing a risk analysis of a specific task or situation rather than for permanent real-time monitoring[33]. PIMEX videos have also been used to raise awareness and to train workers as well as students[34].
Biological sampling
The primary objective of bioaerosol sampling (biological agents such as microbial cells or spores in air), is to collect a representative sample from the air environment and transfer it to a sampling medium, and then prepare it for analysis. Most samplers utilise techniques like filtration, impaction, impingement, electrostatic precipitation, or a combination of these methods to capture the particles of interest[35].
- The impactor sampler is a device that pumps the air through either a perforated plate (sieve sampler) or a narrow slit (slit sampler). The air deposits the collected microbial matter onto a solid or adhesive medium such as agar plates. The agar plate can be removed and incubated to estimate the number of colony forming units in the sampled air. The most common instruments of this type are Andersen sampler and the Casella slit sampler[36].
- With impingers the impinger liquid can be cultured to estimate viable microorganisms.
- Filter samplers: Here the filter medium is incubated directly by transferring onto the surface of an agar or gelatine medium. Filtration methods are accurate and reliable but can lead to dehydration stress in the trapped microorganisms and a potential reduction in viability, especially amongst Gram-negative bacteria such as E. coli[29].
Analysis
Where measurement is required, samples should be returned to a suitably accredited laboratory. An accredited laboratory must be able to demonstrate it can produce precise and accurate test and calibration data. To gain accreditation evaluation of the following factors by external assessors is required: sample preparation, handling and transportation of test items; staff competency; laboratory environment; suitability, calibration and maintenance of test equipment; method validity and suitability; traceability of measurements; and quality assurance of test and calibration data.
The standards EN 482:2021[20], EN 1076:2009[23], EN ISO 23320:2022[27], EN ISO 21832:2020[37]and ISO/IEC 17025:2018[38] specify the general requirements for the competence to carry out tests and/or calibrations, including preparing the samples and calibration performed using standard methods, non-standard methods, and laboratory-developed methods. Between them they cover management requirements such as quality management systems, administrative and technical operations. Microbiology laboratories must comply with EU Directive 2000/54/EC[4] regarding biological agents at work.
Analytical methodology
Chemical analysis tends to be dominated by chromatographic techniques such as gas chromatography (GC) also known as gas liquid chromatography (GLC), high-pressure liquid chromatography (HPLC) and ion chromatography (IC). GC and LC methods are common for many organic chemicals whereas for inorganic samples the methods are often more specific to the analyte of interest and many standard methods are available. ISO 30011:2010[39] covers metals and metalloids by use of inductively coupled plasma mass spectrometry (ICP-MS). ISO 21438 parts 1 to 3[40] deal with inorganic acids by IC.
Analysis of fibres, including asbestos is usually conducted using phase contrast light microscopy (PCM). This method was also prescribed in Directive 2009/148/EC on exposure to asbestos at work[16]. However, the use of PCM alone does not give sufficient information to positively discriminate between respirable fibre types. The measuring of asbestos fibres in the air based on electron microscopy constitutes a significant improvement as it allows for the counting of thinner asbestos fibres. Therefore, when Directive 2009 was amended in 2023[41], the PCM method was replaced by electron microscopy.
Quality control
Quality control consists of two complimentary processes, internal quality control (IQC) and external quality control (EQC) also known as proficiency testing (PT).
IQC[42] is a set of procedures undertaken by laboratory staff to ensure consistency of results. It ensures that the factors affecting the degree of uncertainty of a result do not change significantly over a period of time. It does this by looking for bias in the data by using control samples and the precision by use of duplicate analyses and/or control samples. Control samples would consist of blank samples or, if no blank sample is available, a real sample fortified with known levels of a pure analyte. The use of independent standards or standard solutions and the analysis of reference materials are all examples of IQC. IQC procedures are limited to data produced internally by a laboratory and may have a serious bias that goes unnoticed and for this reason must be conducted along with EQC.
EQC allows for independent assessment of a laboratory’s methodology. This is most commonly done by participation in an accredited PT scheme. Suitable PT schemes can be found using the search facility on website of the European Proficiency Testing Information System (EPTIS). If this is not possible due to lack of a suitable scheme then it may be possible to arrange an exchange of samples with another laboratory or to have some samples re-analysed by another laboratory.
Uncertainty
Systematic and random errors can affect any analyses and introduce a positive or negative bias. This results in a degree of doubt in the result, which is termed uncertainty. This does not mean that the result is unreliable, but that quantifying the margin of doubt in turn increases confidence in the result. ISO/IEC Guide 98-3[43] and the Guide to the Expression of Uncertainty in Measurement (GUM) establishes general rules for evaluating and expressing uncertainty in measurement[44]. ISO 20988:2007[45] provides more comprehensive guidance and specific statistical procedures for uncertainty estimation in air quality measurements.
Interpretation
Decisions made on the basis of these measurements have consequences for workers’ health as well as the financial burden of the employer. In the case of real time monitoring these decisions are often made automatically with the sounding of an alarm leading to a pre-determined action.
The results from air sampling can be used to demonstrate compliance with existing limits, such as occupational exposure limits (OELs), regulations and guidance values. This is normally a simple task as the values presented are usually set values based on established scientific data, including epidemiology and toxicology data. This means they are health based values with a safety margin built in and exceeding them can be seen as harmful to workers as well as being out of compliance. Some limit values are not health based, and are instead based on analytical capability or achievable control. Failure to comply with these limits may demonstrate compliance but may not protect the health of workers. When comparing results with limit values it is important to take into account the accuracy and precision of the sampling and analytical methods as well as other relevant workplace information. The level of uncertainty of the results must be considered before concluding whether limits have been exceeded or not.
The absence of exposure limit values does not mean there is no health risk but may require further work to determine if acceptable exposure level have been achieved. This is likely to involve other occupational health specialists. The results of this may even lead to establishing an exposure limit.
Although meeting OELs and compliance with regulations and laws is important it is not the sole reason to undertake air monitoring. The overall purpose of monitoring is to assess and evaluate the risks from dangerous substances in the workplace air. This process should not be conducted in isolation but as an integral part of a health and safety program.
The results of air monitoring may demonstrate that control measures are effective in reducing a hazard to as low as is practicable and may justify the cost of installing control measures. As stated above, workplace measurement is often the result of a complaint and the results of this measurement may be used to settle a dispute. When the results indicate the limits have been exceeded or in the absence of limit values, that exposures are high, exposure must be reduced. This is often through improved controls but may involve changes to the substances or processes being used. A follow-up survey is then required to demonstrate exposure has been adequately controlled.
