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Gas Detection

Published: 01st Jan 2004

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When using or being exposed to hazardous substances at work people's health may be put at risk as a result of over exposure to these materials

In the UK we have a well developed, goal orientated approach to ensure that the risk experienced by our chemically exposed workers is properly managed. Using good occupational hygiene practice as a guide, the system is embodied within the COSHH Regulations. This article focuses on the key principles contained within COSHH that could be beneficial worldwide.

The regulations require employers to assess the risk of chemicals hazardous to health (these could include exposure to dusts, solvent vapours, gases or a synergistic or additive effect caused by exposure to one or more substances). After the risk to health has been assessed, the COSHH regulations require the employer to implement control measures and to establish good working practices.

COSHH Regulation 10 requires employers to measure / monitor the concentration of hazardous substances in the air which are inhaled by workers where it is considered that there is a serious risk to health if an employee is overexposed to this material (i.e. if control measures fail or deteriorate). In addition, air monitoring must be carried out when employees are exposed to certain specified substances and processes. However, the monitoring does not need to take place if you can show that you are preventing or adequately controlling employees' exposure to hazardous substances, for example if you have in place a system which automatically sounds an alarm upon detection of the hazardous substance in question.

Gas monitoring

A volume of any gas at the same temperature and pressure contains the same number of molecules irrespective of what the gas is. This means that measuring gas by volume is very convenient. Gas measurements at higher levels are in % (volume) and at lower levels parts per million, ppm or milligram's per cubic metre (mg/m3).

Whilst different gases have different densities, they do not totally separate into layers according to their density. Heavy gases tend to sink and light gases tend to rise, but their constant motion means that there is continuous mixing (i.e. they do not behave like liquids).

So in a room where there is a natural gas (such as methane) leak, the gas will tend to rise because it is lighter than air but the constant motion means that there will be a considerable concentration at floor level. This will happen in perfectly still conditions but if there are any air currents, the mixing will be increased.

In order to determine the risk to employees' health from exposure to hazardous gases, the employer should complete a risk assessment. The best way of controlling a risk is to prevent exposure but if this is not possible, a process may have to be enclosed or ventilation and extraction equipment used or special handling procedures employed. In order to ensure that employees are working within a safe environment, the HSE publishes Guidance Note EH40 each year to help employers to compare employees' exposure to hazardous substances to a recognised standard.

EH40 sets out the Occupational Exposure Limits (OEL's) currently used in the UK.

The list gives long term (8 hour) exposure limits (LTELs) applicable to exposure during a normal working day and short term (15 minute) exposure limits (STELs) which are applicable to occasional exposure to higher levels (Where there is no 15- minute limit the rule of thumb is applied that the 15 minute limit is equivalent to a level of three times the 8-hour limit used).

When mixtures of toxic gases are encountered the effects on health are often additive and this needs to be taken into account (exposure to two gases with similar effects, each at 50% of their OELs may be equivalent to working at an OEL or the two gases together may have an enhanced effect).

The following data has been extracted from EH40/2002:

LTEL (mg/m 3 ) STEL (mg/m 3 )
1 GASES WITH MELs (8HR TWA) (15 MIN TWA)
Ethylene oxide C 2 H 4 O 9.2
Formaldehyde HCHO 2.5 2.5
Hydrogen Cyanide HCN 11
2 GASES WITH OESs
Ammonia NH 3 18 25
Carbon dioxide CO 2 9150 27400
Carbon monoxide CO 35 232
Chlorine Cl 2 1.5 2.9
Chlorine dioxide ClO 2 0.28 0.84
Fluorine F 2 1 ppm 1 ppm
Hydrogen bromide HBr - 10
Hydrogen chloride HCl 2 8
Hydrogen fluoride HF 1.5 2.5
Hydrogen sulphide H2S 7 14
Nitric acid HNO 3 5.2 10
Nitrogen dioxide NO 2 5.7 9.6
Ozone O 3 0.4
Phosphine PH 3 0.42
Sulphur dioxide SO 2 5.3 13
Sulphur hexafluoride SF 6 6070 7590

When monitoring of exposure is called for records of the monitoring must be kept and employees must be told about their risks, the precautions and the results of any monitoring and health surveillance.

Gaseous toxic substances are especially dangerous because they are usually invisible and often odourless. Also their physical behaviour is not always predictable since their temperature and pressure affect their properties dramatically as do the effects of draughts.

Static or personal sampling

Monitoring of hazardous substances including gases is undertaken using one of two methods: Static (background) sampling or personal sampling. Static sampling involves the positioning of the sampling equipment in one location during the entire sampling period. This was commonly used before small, portable sampling equipment was available. The main problem associated with static sampling is that it is very rare that a worker spends time in one location. A static or background sample gives only information on the contaminant concentration at one position in the workplace and may over or underestimate the workers' actual exposure. Therefore personal sampling (which can be compared; the limits of which are listed in EH40/2002) is considered to be today's preferred sampling approach.

Personal sampling involves attaching a personal sampler within the breathing zone of the operator. Attaching the equipment within the breathing zone (i.e. within inches of the nose / mouth region of the individual to be monitored) is a non-intrusive approach to sampling the air and gases which the individual is breathing. Ideally, the dose should be measured directly at the organ of interest (i.e. the lungs), but due to obvious reasons (and lack of volunteers for sampling) the air which is being inhaled by the body is sampled.

Static monitoring can be undertaken within the workplace to obtain information on the likely sources of a chemical which contributes to personal exposure of individuals. As static monitoring does not generally reflect the amount of contaminant which employees are breathing in, the levels obtained during the sampling period should not be compared to the standards which are given in EH40/2002. However, static sampling could be undertaken for a variety of reasons including:

  • Checking the effectiveness of control measures
  • Identifying emission sources o Determining background workplace contaminant concentrations
  • If no suitable personal monitoring methods are available / practical
  • When continuous monitoring alarm systems are installed

An approach to air sampling

When conducting an air sampling survey, care needs to be taken to ensure that the sampling is undertaken in a manner which will reflect the normal working conditions within the area to be sampled. The UK's Health & Safety Executive has produced HS(G) 173 "Monitoring Strategies for Toxic Substances". This gives details of a structured approach to sampling which involves the elements of an initial appraisal, a basic survey, a detailed survey and a third level strategy.

An initial appraisal should look at the workplace and work practices employed. Information relating to the substances used, number of people exposed, shift patterns, etc. The information gathered should be sufficient to determine whether further work is needed, and whether air monitoring should be undertaken. If it is decided that air sampling is required, a basic survey should be carried out.

A basic survey normally involves:

  • Personal sampling for the identified "high risk" groups and "worst case" exposures. Judgement will be needed here to classify workers into groups who are likely to have similar exposure patterns and to decide which of these groups are thought to have the highest personal exposure during their working day. These are known as SEGs.
  • Sampling should ideally concentrate on determining and assessing peak exposure levels. However, if reasonable assumptions regarding exposures during the entire shift cannot be assessed, then a more detailed survey may need to be carried out
  • Background or static sampling should be undertaken where necessary to evaluate the risks to ancillary workers to identify the main sources of exposure
  • Examining local extraction systems and other controls where present to evaluate their effectiveness

If the results of the survey suggest that employee exposure is well below relevant OEL's, then so long as care has been taken when designing the survey, it can usually be assumed that personal exposure to the material sampled for is acceptable. However, it should be considered that the survey should take into account all "reasonably foreseeable circumstances" which could result in higher operator exposures.

Following the basic survey, if the results have indicated that exposure is likely to be around or approaching the relevant exposure limit, or if there is a wide variation in results, a more detailed survey will be required.

A detailed survey could look at the following aspects:

  • The collection of personal samples covering the whole shift
  • Focussing the sampling programme on several consecutive samples covering the working period
  • The sampling programme should take into account variations in working patterns and conditions which could influence the exposure

In some circumstances, for example where exposure patterns are more complex, a further "third level" sampling survey may need to be undertaken. In cases such as this, more data should be collected and sufficient samples taken to allow the results to be analysed using statistical techniques.

Routine surveys

Some companies may choose to adopt a routine sampling programme, where the exposure to hazardous substances is regularly assessed. Routine samples may be carried out for a number of reasons: to check that control measures are operating effectively, to ensure that OEL's are not exceeded, to provide data for long term studies and to comply with legislation (for example COSHH, and the Control of Lead at Work Regulations).

Sampling techniques

Sampling techniques used to assess the level of gases present within the workplace are undertaken primarily using one of two methods: active or passive (diffusive) sampling.

Active sampling involves the use of a pump which pulls air through the sampling media. The amount of air drawn through the media by the pump can be calculated and the concentration of contaminants present in the air can be assessed. Depending upon the material to be sampled, the pump will have to draw air through the media at a set rate. Sampling for gases and vapours should normally involve the use of a low flow pump (which pulls air from between 10 - 500ml/min).

For active sampling, irrespective of contaminant, an air sample will always involve:

  • A pre-determined flow rate
  • A flow time
  • A quantity

From the flow rate and flow time a volume can be derived, and by dividing this into the quantity a concentration is calculated.

Passive (diffusive) sampling is a fairly recent air sampling technique which involves the passage of molecules of the contaminant through a membrane followed by adsorption or absorption. Depending upon the type of media used analysis by a laboratory may be required.

Alternatively, direct reading detector tubes can be used to give instantaneous results. The type of tube to be used is dependent upon the sampling protocol and the material to be sampled. This use of direct reading detector tubes and passive sampling techniques do not involve the use of an air monitoring pump.

Active sampling techniques for gases and vapours

1.Adsorption Adsorption of gaseous contaminants onto a suitable solid medium is one of the most widely used personal sampling techniques. Air is drawn through a glass or stainless tube packed with the sorbent (charcoal, silica gel, porous polymers, coated sorbents or molecular sieves dependant upon the material to be sampled) at a low flow rate. There are usually two sections in the adsorption tube, the sorbent layer which is separated by foam from a back up section which allows "break through" to be detected. Following sampling, the adsorbed contaminant is desorbed, using thermal or chemical techniques.

Factors which can affect the adsorption efficiency include:

  • Temperature - adsorption is an exothermic process (i.e. it evolves heat) and is, therefore, reduced at higher temperatures. In addition, if there is a reaction between the adsorbed material and the surface, or between two adsorbed materials, the rate of reaction may increase with temperature. If tubes have to be stored after sampling, before analysis, they should be kept cool in an icebox or refrigerator
  • Humidity - water vapour is adsorbed by polar solvents and therefore reduces their ability to adsorb the contaminants
  • Sampling Flow Rate - a too high flow rate can reduce the adsorption efficiency of the sampling media
  • Channelling - if too high a flow rate is used, and / or the tube is located in a horizontal position, the particles of the adsorbent may settle into layers, so channels are produced, along which the sampled air will pass. This results in the surface area of the adsorbent in contact with the air being reduced and saturation occurs more rapidly. This means that the "useful" life of the tube is reduced

2. Absorption This active sampling technique involves the bubbling of contaminated air through a liquid solution contained within a glass or plastic vessel (referred to as a bubbler). Analysis of the liquid solution takes place in a laboratory and commonly uses calorimetric analysis. The main disadvantage of this technique is that if the bubbler is not kept upright, spillage of the sorbent can occur.

Various types of bubblers are available in which to hold the liquid solution. However, the most common bubbler used today is referred to as the midget impinger. It was originally designed for particulate sampling, but is now mainly used as a bubbler for gas absorption. It is generally used with 10 - 20mls of liquid at a flow rate of between 0.5 and 1 litre / minute. Too much liquid, or an excessive flow rate can result in sample loss. Factors which can affect the efficiency of gas collection by a pure solution include:

  • The volume of air sampled
  • The volume of the sorbent
  • The volatility of the contaminant

As the collection efficiency is largely dependent on the concentration of gas and the sampling rate, collection efficiency can be improved by increasing the sorbent volume or cooling the collector (this reduces the volatility of the contaminant).

Because of the inherent hazards associated with some absorption techniques (employees "wearing" a hazardous substance in a bubbler for a shift), new methods are now being developed which involve the impregnation of a filter with the absorption medium (isocyanate sampling methods for example). These are preferable because they are safer and easy to manage.

Diffusive (passive) sampling techniques

Diffusive samplers provide a simple, reliable, and economical method for air sampling. During the early use of diffusive samplers, there was considerable scepticism with regards to their effectiveness, but experience has shown that the technique is as good as established "active" methods for many applications. Although not as sensitive as active techniques (they therefore require a longer sampling time) they are a good, relatively inexpensive means of obtaining a large number of personal exposure samples. They are also often more "worker acceptable" than active sampling equipment as the equipment does not involve the worker wearing a pump.

There are a number of disadvantages associated with the use of diffusive samplers; the main ones are as follows:

o Most diffusive samplers do not have a back up layer to detect breakthrough

  • Overloading the collector decreases the uptake rate
  • Low air speeds can result in insufficient flow to replace the substance adsorbed by the sampler
  • Excessive air movement can induce turbulence inside the sampler
  • Worker movement can also induce turbulence
  • Temperature and pressure can affect the diffusion co-efficient If a passive method is used then it is important to ensure correct validation of the technique and meticulous adherence to manufacturers instructions.

Indicator tubes

Indicator tubes, also referred to as colorimetric indicator tubes, are commonly used to determine the concentration of a particular gas. The glass tubes are generally packed with coloured crystals which react with the gas to be monitored to give a colour change. A known volume of air is drawn through the tube (occasionally using a hand pump) and the concentration of gas can be calculated from the colour change present in the tube.

The reading from this type of sampling equipment is dependent upon a number of factors:

  • Temperature - the rate of the reaction with the gas will be increased by the temperature of the air, and ambient conditions leading to an overestimate of the concentration may occur if the temperature is above 20ºC or an underestimate if the temperature is substantially below 20ºC
  • Interfering reactions - often more than one gaseous contaminant may be present within the air sampled. In some cases, this can lead to one or more of these gases reacting with the chemicals inside the tube. This can lead to an over- or underestimation of the gaseous concentration within the air (depending upon the particular reaction)
  • Pump accuracy - the reading obtained from the indicator tube depends on the correct volume of air being drawn through the tube. If this is reduced due to leakages, or if it is not operated properly, the gas concentration will be underestimated
  • Shelf life - the chemicals within the tube can deteriorate over a time. Most tubes have a shelf life of approximately 2 years if stored at room temperature

The results obtained from this sampling technique are not considered to be very accurate as they are not personal samplers and they should not be compared to OEL's. However they can give a good indication as to whether a problem exists, and, if so, the extent of the problem.

Technical advances have meant that the process of gas detection in a working environment is simpler today than it has ever been. For this reason it is now more important than ever to ensure that these techniques are employed in a planned and systematic way so that results are representative of actual exposures. After all, it is people's health we are looking to protect when we follow these methods.

Catherine Rosie is an Occupational Hygienist with Sypol Ltd, health, safety and environmental consultancy and training experts.

Tel: +44 (0) 1296 415715, Fax: +44 (0) 1296 397106

Email: sales@sypol.com . www.sypol.com

Published: 01st Jan 2004 in Health and Safety International

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