Workplace Exposure Monitoring: an Overview

Dr. Richard Griffiths, PhD, MSc (Env Man), MEd, Cert Ed, Grad R.S.C, FRSH, FRIPH, CMIOSH, MIIRSM

Throughout the UK, employers and employees come into contact with workplace hazards every day, which is why the Health and Safety Executive (HSE) publish a wide variety of literature and information detailing those hazards and how to monitor them. They publish a series of Guidance Notes on Environmental Hygiene and Medicinal topics, as well as referencing various websites that describe their effects. In addition to environmental hazards, there may be physico-chemical hazards caused by the physical or chemical properties of the substance, or toxicological hazards, which arise from a chemical causing harmful effects to living organisms. Toxic effects may be acute or chronic, local or systemic, and reversible or irreversible.

Why do we Need Workplace Exposure Monitoring?

Some hazards are more common to specific industries such as lead and radiation, or specific occupations such as farm workers and coal miners. Irrespective of this, each hazard can enter the body by a number of routes, such as inhalation, absorption and ingestion, where they target specific organs, for example the lungs and liver, or systems (lymphatic) within the human body. These can give rise to either acute effects: short-term response induced by a single dose or limited exposure to the agent, or chronic effects: long-term response, usually after repeated exposures to a sublethal concentration of the agent on the human body.

The most important factor is that everyone is different and these hazards may affect individuals in different ways according to their age, medical condition, and gender, and give rise to acute and/or chronic health effects. Within the groups of persons at risk from exposure to such hazards, there are certain sections of the population that are particularly vulnerable, for example the elderly, the debilitated or chronically sick, pregnant and breast-feeding women and their children, and/or people with suppressed immune systems.

Controlling Workplace Hazards

There are also many preventative and protective measures used in the workplace to control such hazards, including the following:

• Elimination.

• Substitution.

• Engineering control measures.

• Immunisation.

• Permits to work (PTW).

• Safe systems of work (SSOW).

Other controls include:

• Legislation, for example the Asbestos, Lead, Ionising Radiation and COSHH Regulations.

• International and British Standards, such as Workplace Exposure Limits and “suitable and sufficient” Risk Assessments.

Monitoring

Despite all of these measures, problems can arise in the workplace due to various hazardous agents, and those agents will require some form of measuring or monitoring. The analysis and measurement of chemical agents must be carried out in a consistent and reproducible manner, such as using the Methods for the Determination of Hazardous Substances (MDHS) Guidance, which is approved by the HSE.

The main purposes of monitoring these hazards are a qualitative analysis of the sample to determine its exact constituents, or a quantitative analysis to determine the exact concentrations of a particular contaminant. Monitoring procedures should establish when, where and how the monitoring is to be done; how sampling is to be carried out and the results interpreted; and how frequently samples should be taken. Monitoring consists of sampling and testing the air quality in respect of the general atmosphere in the workplace, or in respect of the immediate environment around individual employees who may be at particular risk.

Sampling Techniques

However, there are certain limitations on the use and reliability of each method used. Sampling techniques involve a number of factors that need to be considered:

• Positioning:

• Within the general working atmosphere.

• In the operator’s breathing zone.

• Close to the source of the contaminant.

• Method of analysis used.

• Duration:

• Spot sampling.

• Grab sampling.

• Continuously-monitored sampling.

• Frequency.

Static sampling is the preferred method used to assess effectiveness of workplace controls as the sample remains in a fixed position for an accurate result. Personal sampling of the operator's breathing zone is used to assess personal exposure on a chart recorder, data logger or warning device, and so does not need constant attention.

Active and Passive Sampling

There are two basic methods of sampling, based on the way in which the sample is collected. Diffusion or passive sampling is where the contaminant passes over the sampling system in natural air currents and diffuses into a chamber containing an absorbent material, which can be removed for analysis. Mechanical or active sampling is where a continuous stream of air is pumped through a tube containing activated charcoal or silica gel, absorbing any gases or vapours; the collected pollutant can then be determined in a laboratory.

Passive devices employ absorbent material to sample concentrations of airborne pollutants without using a pump to draw air through the collector. These systems are used for continuous sampling over a period of time, and can only produce cumulative results for that period as a whole. Active devices use a pump to provide airflow through the analysing instrument, and can be used for both spot and continuous sampling.

Solid materials like silica gel, activated charcoal and various types of porous resin readily absorb certain gases and vapours, which can then be chemically analysed. The amount of pollutant collected can be determined back in the laboratory. Vapours collected on absorbents within samplers can be chemically analysed using a variety of techniques, from gas chromatography, (GC) to infrared spectrophotometry. Thus it is possible to identify the actual chemical collected and calculate its concentration.

Stain Tube Detectors

Stain Tube Detectors are perhaps the most convenient method of analysing gaseous contamination of the workplace air. A predetermined volume of air is drawn over a chemical reagent supported in a graduated glass tube. The contaminant reacts with the reagent and a coloured product or stain is produced of varying length, dependant on concentration.

Other methods used are Draeger multigas detectors, which have a bellows pump and a Draeger tube, selected to suit the particular measurement to be carried out; automatic multigas detectors, which have an electrically-operated bellows pump model that can be set to switch off when the selected number of strokes for each particular tube is complete; and toxicator gas detectors incorporate a system that draws air across a chemically-impregnated tape which changes colour in reaction to the level of contaminant.

Direct Measuring Instruments

Direct measuring instruments involve a number of analytical mechanisms including chemical reactions, which are designed to produce a colour change that enables a qualitative analysis to be made; and electrical detection, in conjunction with chemical or electrochemical processes. Physical methods are based on the absorption of ultraviolet or infrared radiation in proportion to the concentration of the contaminant. These direct measuring instruments indicate concentration on a simple dial or digital readout. The instrument can be used for static monitoring or portable monitoring and if pre-set to a given concentration, an audible or visual alarm is activated when that concentration is exceeded.

Ultraviolet and Infrared Detection

Photo ionisation detectors involve the contaminant being drawn into a cell and ionised by ultraviolet radiation, which generates a current proportional to the concentration of contaminant present. The technique can be used for continuous monitoring or general screening of a workplace.

Infrared (IR) analysers rely on the specific absorption characteristics of certain chemicals. The absorption of radiation at specific wavelengths is proportional to the concentration and provides an accurate measurement of concentration for a wide range of gases that exhibit absorption in the IR range.

Dust Monitoring

Most dust particles are too small to be seen with the naked eye, particularly those that are inhalable or respirable. Dust monitoring involves using a Tyndall Beam, which is a light beam that reflects the dust particles and makes them readily visible. The illuminated dust cloud can be observed directly during the work activity in question, or a camera can be set up to photograph the dust emission. Although no numerical measurements are made, the equipment enables us to observe exactly how and where the dust is.

Gravimetric and chemical analysis of dusts, such as asbestos, silica, cement and wood can also be carried out. The dust collected using a simple particulate filter is determined gravimetrically, i.e. by weighing. Generally, the filter medium is weighed before and after the sampling, and the mass of dust collected is given by weight difference. For some dust samples, such as silica-containing dusts, chemical analysis will be carried out to assess toxic potential.

Measuring Airborne Contaminants

Airborne Contaminants can be measured using personal or static sampling techniques. Variations that are considered within these techniques include within shifts, between shifts, or between processes or individuals. Typically, a measured volume of air is drawn through a membrane filter mounted in a sampler, and the mass of dust collected is determined by weighing the filter before and after sampling. The method can be used for total inhalable dust, which is the fraction of airborne material that enters the nose and mouth during breathing, or for total respirable dust, which is the fraction of airborne material that approximates to that penetrating to the gas exchange region of the lung.

Similarly, microscopy can be used to count fibres collected on the filter of a sampling device and the length can be measured with an eyepiece fitted with a calibrated scale. By calculating the number of fibres in a known proportion of the sample collected, the number in the whole sample, and the airborne concentration can be calculated.

Biological Monitoring

Most biological agents are naturally occurring materials that are encountered in the workplace, for example:

• Through healthcare work and associated laboratory activities.

• During the handling or processing of animal- or plant-derived materials.

• Through exposure to biological agents, which are incidental to the work being carried out, such as sewer workers coming into contact with rats' urine.

Biological monitoring may be defined as a regular measuring activity where selected, validated indicators of the uptake of toxic substances are determined to prevent damage to health. Its purpose is to assess the extent of exposure, uptake and metabolism of chemicals in the workplace. This involves the measurement and assessment of workplace agents (or metabolites) in tissues, secretions, excretions or even expired air, to evaluate exposure and health risk compared to an appropriate standard. An example is the detection of significant levels of lead in blood, which indicates the presence of potentially harmful levels of absorbed lead. The concentration of bromide in blood is an indicator of methyl bromide exposure, and the concentration of mandelic acid in urine is an indicator of styrene exposure.

The HSE have derived Biological Monitoring Guidance Values (BMGVs) for interpreting biological monitoring measurements. There are two types of BMGV; Health Guidance Values are set at a level at which there is no scientific evidence that the substance is likely to be injurious to health; and Benchmark Guidance Values are practicable, achievable levels.

When considering biological hazards, it includes the considerable range of commonly encountered micro-organisms, as well as those genetically engineered. The three main categories of micro-organism that we are concerned with are:

• Fungi - mushrooms, mould and yeasts.

• Bacteria - Legionella and zoonoses: animal bacteria infections, such as Leptospirosis, E. coli, tetanus, anthrax and Q fever (recent outbreak at Dunblane).

• Viruses - hepatitis or HIV/AIDS.

Radiation Detection

The principal instrument used to detect radiation is the ionisation chamber, or the Geiger-Muller tube (Geiger counter). Scintillation counters employ materials that produce light (fluorescence) on interaction with ionising radiation. The degree of fluorescence can be used to measure the level of radiation. Film Badges and Thermoluminescent Dosimeters (TLD) detect personal radiation exposure. The former by means of a film badge that contains a photographic film, which is affected by radiation in the same way as such film is affected by light exceeded, and the dose received can be measured. The TLD is often worn on the fingers to estimate dose to the hands.

Measuring Hearing

The measurement of hearing performance is necessary in order to detect actual noise-induced hearing loss. A common use of audiometric testing at the pre-employment stages is to establish a baseline against which any deterioration due to poor noise control arrangement can be measured, and to detect any existing noise-induced hearing loss to safeguard the employer against false accusations that hearing loss is due to this employment. Instruments used to measure sound, including fluctuating sound levels, must be able to measure the intensity of the noise of specific frequencies or a weighted sound intensity, and the duration of the exposure if a dose measurement is required.

Appropriate instruments that are used are simple sound level meters, integrating sound level meters and personal sound exposure meters (dosimeters). Care is required in the interpretation of results as they are affected by the different situations being measured, such as:

• Exposure to a continuous noise level.

• Exposure to one significant level of noise during the working day.

• Exposure to more than one significant level of noise during the working day.

Vibration (an oscillatory motion involving an object moving back and forth) may be measured using an accelerometer.

Conclusions

Clearly, there is a wide and diverse range of methods and instrumentation that can be used to measure workplace exposure of a wide range of hazards and the most appropriate should be used to gain an accurate representation of the actual hazard involved.

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