Make sure you sample first to clear the air

Organisations have a legal requirement to protect their employees from hazardous substances such as dust and fumes, and should carry out air sampling to address this. Gary Noakes outlines the various methods available

Many people are exposed to harmful substances at work, such as chemicals, dusts and fumes, but the symptoms can be hidden for years. All companies have a legal requirement to deal with this under the Health and Safety at Work Act, and they also have to protect both employees and others by complying with the Control of Substances Hazardous to Health Regulations 2002 (COSHH).

There are eight steps to COSHH: assess the risks; decide precautions required; prevent and control exposure; ensure control measures are maintained; monitor the exposures; carry out health surveillance; prepare emergency plans; ensure employees are trained and informed. It is also a fundamental principle of a company's requirements under their risk assessment strategies and policies to address this area. Where any potential hazards or processes are identified, a strategy of air sampling regimes and methodologies needs to be set in motion and constantly revisited.

Examining the methodology
There are many methods of taking air samples, but the one most commonly used is to connect a battery-operated sampling pump to a filter medium. The pump has to be capable of drawing air through the media at a constant rate for more than eight hours, over a wide range of conditions. The methodology is a simple process of drawing a known volume of air onto a sampling media, and analysing the results. Where it is appropriate to carry out personal air monitoring, the air to be sampled is the space around the worker's face from where the breath is taken - the breathing zone.

For dusts and fumes, this process is carried out by the person being monitored wearing a personal air sampling pump. To this, an air sampling head is attached containing either a pre-weighed or treated filter media. Media include glass fibre, PVC, PTFE, and mixed-cellulose ester (MCE) among others. The sampling pump is then set to a required flow rate, calibrated against a flowmeter or rotameter, then worn for eight hours to monitor exposure during a normal working shift. At the end of this time, the filter is removed and sent off for post weighing. In the case of heavy metals, it is sent off for further chemical analysis by atomic adsorption, X-ray fluorescence or other process.

For gases and vapours, different absorption media are used. These usually consist of glass tubes containing either charcoal, silica gel or other polymer materials, which act as sponges, absorbing the substances as they are drawn through the tube. The air sampling pump is again deployed. And, as air is sampled through these tubes at a pre-set rate, the material absorbs the substances.

These are then sent off for analysis, usually at an accredited laboratory. Analytical techniques are usually gas chromatography-based, which give a chemical composition breakdown of the sample. Other sampling media include chemical liquid bubblers and impingers, gas bags and passive badges.

Within the legal limit
All results have to be made and referred to the applicable exposure limits, or workplace exposure limits (WEL). These are legally binding and are usually expressed and specified as a time-weighted average of long-term (eight hours) or short-term (15 minutes). Although primarily British limits, in some cases WELs also reflect some European limits, and are known as indicative occupational exposure limit values. These limits are set under the Chemical Agents Directive, and member states are obliged to implement national limits for the substances listed.

All sampling has to be representative of an operator's exposure, and should be done during worst-case scenarios, or when the potential for exposures is at its highest. Asbestos and lead have their own regulations due to their toxicity and morphological nature - these are the Control of Asbestos at Work Regulations, and Control of Lead at Work Regulations 2002.

Real-time monitoring can aid results
The methodologies discussed only give results retrospectively, and a total or average exposure level is obtained. Many hygienists also prefer to supplement this data by using real-time direct-reading instruments when they are monitoring. For gases and vapours, many direct-reading photo-ionisation detectors and portable gas detectors can be used, which will display data to parts-per-million levels.

For dusts and fumes, devices such as Casella's Microdust Pro exist. This is a light-scattering device that allows the user to obtain a real-time numerical and graphical indication of the dust and fume levels in an area without having to wait for analytical data. This information is valuable and allows walkthrough surveys to be carried out and hot spots identified, where further in-depth personal monitoring can then be undertaken. It can also be used to identify which machines or processes may be generating the most dust into the environment, allowing these to be isolated, enclosed or replaced.

Gary Noakes is product manager at Casella Measurement

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