Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants

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Process Engineering Guide: Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants

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Process Engineering Guide: Hygiene: Estimating and Understanding Personal Exposure to Inhalation of Vapors on Chemical Plants CONTENTS 0 INTRODUCTION

1 SCOPE 2 FIELD OF APPLICATION 3 DEFINITIONS 4 KEY CONCEPTS 4.1 Occupational Exposure Limits

4.2 Exposures to Vapor - Background, Task and Incident 4.3 Variability and Target Exposure Levels 4.4 Units

5 OVERVIEW OF METHOD 6 APPLICATION OF METHOD 6.1 Chemical Substances – List and Rank 6.2 Personnel - List and Rank 6.3 Plant – Define Zones 6.4 Zone Information 6.5 Integrate Exposures Over All Zones



7 UNDERSTANDING 8 INCIDENTS APPENDICES A VENTILATION B ESTIMATING CONCENTRATION AND EXPOSURE C VARIABILITY OF EXPOSURE LEVELS D CALCULATION OF INCIDENT QUANTITIES E SUBSTANCE INDEX F GLOSSARY G REFERENCES



TABLES 1 STEPS IN A STUDY 2 TYPICAL EMISSION RATES FROM EQUIPMENT

3 TYPICAL EMISSION RATES FROM EQUIPMENT 4 EFFECTIVE WIND SPEED 5 NATURAL VENTILATION RATES OF BUILDINGS 6 SUBSTANCE INDICES FOR ACUTE INCIDENTS FIGURES 1 FLOWCHART 2 RESULTS OF MONITORING

3 (LOG) NORMAL DISTRIBUTIONS DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE



INTRODUCTION It is a vital aspect of chemical plant design and operation to minimize any contact of the operating and maintenance personnel with the chemicals on the plant. Often the most significant route of exposure to chemicals is by inhalation, and in this case it is relatively easy to measure exposure. This has led to the development of standards which indicate levels of exposure which should minimize any risk to health. These will be called Occupational Exposure Limits (OELs) in this Guide as a generic name. Given an OEL as a design criterion it might seem essential to be able to estimate personal exposures. In practice the complexity of the problem has prevented any systematic approach being developed. Consequently plant design has tended to be evolutionary, based on experience of earlier plant, and levels of exposure found thereon. There are some major problems with this evolutionary approach. Firstly there may be very little previous experience of a particular plant design, or standards may have tightened dramatically. Secondly there is very little appreciation of which control measures are really effective, and which are more trouble than they are worth. This Guide is intended to provide a means to estimate personal exposures on chemical plants to aid in such design problems. It also provides a means to rank emission sources in terms of their importance in health terms. Control measures can then be applied to problems of real significance, with an increased likelihood of achieving genuinely cost-effective solutions. The problem is found to be well suited to analysis on a computer using a spreadsheet program, and this approach is recommended in the text. The whole topic of monitoring exposure levels and designing appropriate controls and procedures is central to Occupational Hygiene. It is anticipated that this approach will normally be carried out by a combination of Occupational Hygienists and Process Engineers, with other disciplines being consulted as required.



The recommended method for estimating exposures considers the materials handled, the personnel at risk, and the potential sources of exposure. This Guide considers primarily chronic exposure to toxics by vapor inhalation. The method should however highlight acute problems and is potentially useful in assessing risks due to other exposure routes e.g. dusts. This method has been used as part of hygiene studies on plants in Europe. The total exposure of an individual is considered to be a combination of background and task exposures. Methods of estimating such exposures, by dividing the plant into zones, determining release sources and local dispersion, are presented. To make progress on the problem of estimating exposures which may be random both in time and space, an 'averaging' approach has been adopted. Variations around the 'average' and short term exposures due to incidents are also considered. Potential acute problems due to unplanned incidents are also examined. This Guide does not attempt to define or recommend good practice in plant design or operation. Its purpose is to help assess alternative designs or operating procedures. The inherent uncertainty in the information used to estimate exposures can be considerable; errors in estimates can be large. However sensible application of this Guide should improve understanding of potential exposures on a plant, rank exposure hazards and indicate areas for improvement or of over-design.



1 SCOPE This Engineering Guide covers the estimation of the likely exposures of personnel to toxic vapors on chemical plants. A methodology is given which should be used as a complement to the standard design and Hazard Study procedures. 2 FIELD OF APPLICATION This Guide applies to the process engineering community worldwide. 3 DEFINITIONS

For the purposes of this Guide no specific definitions apply.

4 KEY CONCEPTS 4.1 Occupational Exposure Limits In this Guide, the term Occupational Exposure Limit indicates the criterion set for plant design and operation. Different countries have different terms for such standards. Well known examples include Threshold Limit Values (TLVs) and OSHA Permissible Exposure Limits (PELs), both US, and MAKs and TRKs from Germany. The UK system of Occupational Exposure Limits has changed from Control Limits and Recommended Limits to Maximum Exposure Limits (MELs) and Occupational Exposure Standards (OESs). These will be more strictly enforceable under the new Control of Substances Hazardous to Health Regulations. The legal status and rules for deciding compliance with such standards vary from country to country. This Clause can only lay out some general guidance, based mainly on our current understanding of the UK system. Specific guidance may need to be obtained from the relevant Occupational Health Department to ensure that the correct criterion, either national or in-house, has been selected, and is being applied correctly.



OELs are based either on animal experiments (toxicology) or human experience. It should be understood that the data underlying such standards are often not very extensive, or reliable. The safety margin provided by the standards for different substances can vary also. For this reason it is often stated that OELs should not be regarded as fine lines dividing safe and dangerous conditions. They can be thought of as limits which should be met with as much leeway as is reasonably practicable. In the UK system (see Guidance Note EH40) it is stated that OELs should 'not normally be exceeded'. This phrase has never been defined. In GBHE it has been taken to mean that 90% or 95% of all measured exposures for a work-group should be below the OEL. It is important to understand that this means that the average exposure of a work-group meeting the criterion will be significantly less than the OEL - typically between 25% and 50% of the OEL in fact. Consequently the target criterion for average personal exposure is not the OEL itself but a fraction of it. Most OELs are set for measurements over 8-hour shifts, known as 8-hour time-weighted averages (TWAs). For some substances it is more important to measure short-term peaks, as the effects occur rapidly - an obvious example would be carbon monoxide. Such OELs are called Short-Term Exposure Limits (STELs), and in the UK are 10-minute TWAs. The calculations in this Guide are based on 8-hour TWAs, but the procedure can handle other time bases as appropriate. More complex questions e.g. on the effects of exposure to mixtures of substances, or the effect of unusual work-patterns, should be referred to the relevant Occupational Health Department. A clear understanding of the criterion to be applied is essential. 4.2 Exposures to Vapor - Background, Task and Incident It is helpful to classify some different mechanisms by which people might be exposed to vapors on a plant. Just walking around a plant causes some exposure, which we label as 'BACKGROUND' exposure. The source of this can be from vapor or dust clouds as they dissipate or from pools of liquid as they evaporate. The vapor or dust clouds can be the fugitive emissions from plant equipment (i.e. minor leaks from seals, glands, flanges etc.), or the consequence of earlier emissions associated with a task (see below).



The second class is the exposure directly associated with a person carrying out some work, i.e. a 'TASK' exposure. The average exposure of people in a work-group is thus the sum of their average 'TASK' and 'BACKGROUND' exposures. For both 'BACKGROUND' and 'TASK' exposures a medium-term view should be taken. In general any source of emission likely to occur more frequently than say once a month should be included in the daily averages. Rarer events should be included only if they are sufficiently sizeable to affect the daily average. A further class of exposure is that due to 'INCIDENTS'. Incidents by definition are infrequent but may involve significant releases of material. Although such events are unlikely to make a noticeable contribution to the time weighted average exposure, incidents may have an acute impact on the health of the exposed person. The identification and quantification of potential incidents should occur during Hazard Studies. This Guide should be used to complement the Hazard Studies by contributing to quantifying the consequences of significant releases. 'INCIDENTS' involve a different definition of toxic hazard from longer-term exposures. For instance lead compounds are well known toxins, but are most unlikely to cause an acute injury by inhalation. Thus the Occupational Exposure Limit - 8 hour TWA - is not relevant for 'INCIDENTS'. It is suggested that the IDLH (Immediately Dangerous to Life or Health) criterion be used for this purpose. Further consideration of incidents is given in Clause 8. 4.3 Variability and Target Exposure Levels There are many reasons why exposure levels measured in the field do vary considerably (see Guidance Note EH42). In the process outlined above, only the mean exposure is estimated at each stage. This is clearly the simplest procedure rather than considering ranges. The variability of the exposure levels is compensated for by setting a target at a fraction of the Occupational Exposure Limit (OEL). In general we recommend the value of one-quarter of the OEL for plant design purposes (see 4.1). Specific cases can be considered in more depth by comparisons with similar types of plant which have been closely monitored. This topic is developed in more detail in Appendix C.



4.4 Units The units used in this Guide are normally milligrams, seconds and meters. Thus emission rates of contaminants are expressed in mg/s, air speeds in m/s, flow rates of air through the plant in m3/s and concentrations in mg/m3. Concentrations are often expressed in ppm (vol/vol). The conversion factor involves the Molecular Weight of the substance MW. The formula is:

(24.45 is the volume in m3 occupied by 1 kmol of gas at 25 °C and 1 atmosphere). Occasionally, different time bases are used e.g. the units of exposure are taken as mg.min/m3. 5 OVERVIEW OF METHOD On a plant a variety of people will be exposed to a variety of chemicals escaping from a variety of process equipment. This method for estimating exposure offers a way of handling these different variables. It considers the chemicals handled the personnel on the plant, and the layout and types of equipment. Particular emphasis is given to the distribution of equipment on the plant, and the plant is considered as a series of 'zones'. The method considers background and task exposure. The key steps in the method are to: (a) Define the extent of the study. (b) Divide the plant into zones. (c) Gather the information. (d) Compute the exposures.



Figure 1 and Table 1 describe the steps. Each step in the process is examined in more detail in Clause 6. The use of spreadsheet techniques to collate data and calculate the various exposures is highly recommended. Spreadsheets simplify the repeat calculations for different chemicals or personnel, or for examining design changes. The cumulative exposure by vapor inhalation is estimated by considering the various exposures that personnel may experience on a plant. Each particular exposure is calculated from the exposure time and the concentration of the target material in the local atmosphere i.e.:

The local concentration is estimated from the local vapour emission rate and the effective local ventilation rate, where:

To provide a basis for operating the (existing) process outlined in Figure 1, in the most efficient manner.



FIGURE 1 FLOWCHART





6 APPLICATION OF METHOD 6.1 Chemical Substances - List and Rank The first stage is to consider the flowsheet of the process. From this, it is possible to develop a list of substances which might present a significant inhalation hazard. These substances can then be ranked in order of importance by taking account of the following: (a) The quantity. (b) The conditions of containment (quality, temperature, pressure). (c) The Occupational Exposure Limit (OEL). (d) The concentration Immediately Dangerous to Life or Health (IDLH). (e) The saturated vapor pressure (svp) at ambient temperature. The IDLH and svp are both intrinsic properties of the substance and can be combined into a single Substance Index for ranking purposes (see Appendix E). The Substance Index can be compared to a number of other substances to give an understanding of the significance of the inhalation hazard. This should then be combined with data on the quantities being handled, and the quality of containment to decide for which substances, if any, a detailed analysis is required. For relevant substances the following physical properties should be obtained: (1) The molecular weight. (2) The solubility in water, and in main components of relevant process

streams. (3) The saturated vapor pressure across the range of process temperatures

and process streams.



6.2 Personnel - List And Rank Personnel who might be exposed then need to be identified. The most obvious categories generally are process operators, and regular maintenance tradesmen, especially fitters. Other tradesmen and supervisors should be considered. The aim is not to produce a complete listing of those who might be exposed to any extent, but a practical short list of those who might be the most significantly exposed. Given this selection of the more relevant personnel, a list of tasks, which might cause significant exposure, should be produced for each chosen work-group. 6.3 Plant - Define Zones Normally a plant is not homogeneous in terms of exposure. All aspects of ventilation, emission sources, and occupacity by personnel will cause the exposure to vary significantly. The recommended method is to divide the plant into zones (say 10 - 20) which are at least reasonably homogeneous. The principal division should be in terms of the quality of ventilation provided. For example outside areas should be separated from internal areas. Normally, different floors should also be separated. Further sub-division of zones can be carried out if there are sub-areas where a significantly different exposure pattern is likely. This will mainly be due to the items of equipment, or operations in these sub-areas. This method also uses the concept of Zone 0, where personnel spend time, but there is no exposure. This can include time off-plant, and time in amenities areas and control rooms. Careful thought should be given as to whether these areas are indeed zero-exposure. Control rooms for example can easily be contaminated, especially by low-volatility materials brought in on people’s clothing. The relatively long time spent in the control room can then cause significant exposure.



6.4 Zone Information

6.4.1 Process Equipment

Having defined the extent of the plant zones, information is now required on the equipment which could be a source of exposure. This is achieved by listing in each zone:

(a) all equipment which could contain significant quantities of the

material under consideration,

(b) and for that equipment, the process stream data (pressure, temperature, state, composition and in particular concentration of target material).

The list of equipment should include plant items and any other potentially significant sources such as drains, sumps or pipelines passing through the zone. Note should also be made of any equipment in other zones which releases material into the zone. For example, if an item has a remote vent or drain pipe in another zone, then the location of such outlets should also be noted.

Process stream data should be available from the process flowsheet.

6.4.2 Fugitive Sources

The equipment identified above should now be examined to determine the possible escape of material to atmosphere. There are several routes for these fugitive emissions; these include:

(a) Seals on moving shafts. Such shafts include pumps, compressors,

agitators, control valves etc. Release rates will depend upon the complexity of the design and the wear and maintenance performance. Tables 2 and 3 give an indication of typical rates.



(b) Vents. These include process vents, where allowance has been made for a recognized gas flow to atmosphere, breathing vents, where flow is occasional and due to displacement of gas (often contaminated air) during filling or temperature changes, and emergency relief vents. Estimates can be made of the quantities of vapor vented either by calculating vapor flows in vents or by calculating the volumes and compositions of displaced contaminated air.

(c) Open vessels. This category covers atmospheric-pressure

uncovered tanks, pits, drain systems etc. often containing potentially contaminated water.

(d) 'Leaks'. Material can leak through joints, drain valves, corrosion holes etc. In a well designed and maintained system, leaks from joints may be negligible. However when corrosion is a problem, that is when materials of construction have a short life, serious leaks can occur.



For possible sources not covered above (e.g. fume cupboards and laminar flow booths) specialist advice may be required.

For each fugitive source, an estimate is required of the quantity, properties, and frequency of chemicals which can be released into each zone. It is recognized that the accuracy of these estimates will be limited. Particular problems occur with sources where conditions vary with time. Examples include mechanical equipment which wears (pump seals), where corrosion is significant, and where material is released occasionally, either due to an infrequent process operation or say tank breathing. It may be necessary in these cases to estimate both an average and worst release.

These figures, which are based on CMA guidance (ref 8) should be used only as preliminary estimates. Actual leak rates will depend upon the suitability of the equipment for the duty, the severity of the duty, the standard of maintenance etc. Advice should be sought from Machinery or Piping Sections where necessary.



6.4.3 Time Spent In Zones

In order to estimate people’s exposure to contaminants it is necessary to know where they spend their time and what they do. For an existing plant, the plant manager should be able to provide the information. For a new plant the information may have to be inferred from the operating instructions. The information needed is the average number of minutes per shift spent in each zone and on each task. It is often necessary to consider what is done over a week or a month to arrive at an average figure.

6.4.4 Tasks

Material can also be released into the atmosphere as a result of carrying out tasks. Such tasks are usually those where material is transferred into open vessels or where process joints have to be broken. Examples include:

(a) Filling/emptying containers. Containers include tankers, cylinders, drums,

bottles, sacks etc. Typical tasks are packing product, charging, sampling, draining, purging etc. Material escapes either due to displacement of contaminated air, spillage, or evaporation from wetted equipment (e.g. dip pipes).

(b) Maintenance of equipment and/or contents. Work on process equipment

can lead to release of material either during decontamination (purging/washing) or maintenance (replacement or repair of damaged parts, catalyst, packing etc.). Particular problems occur when material cannot be removed effectively due to blockages.

A list of all tasks performed in each zone with potentially significant exposure should be prepared. These tasks should include both process and maintenance operations. The frequency and extent should be noted. Estimates of the quantities of material associated with each task are required. For example in a sampling operation, how much material has to be purged before the sample is taken? How big is the sample? What form is it in? How much vapor is displaced?



6.4.5 Estimate Effective Local Ventilation Rates

Ventilation is as yet one of the least certain areas in the estimation of personal exposure. To ease calculation it is assumed that contaminated air leaves a zone at a steady rate to be replaced by air from another zone or the outside environment. An estimate of the volumetric rate is required for the estimation of zone concentrations given later.

For an indoor area three approaches are possible-calculation, measurement and simple estimation. There are methods available (Goodfellow, ref 3, provides an introduction and further references) for the prediction of the ventilation in a building when full details are known of its construction. However these are involved and unlikely to be worth using for the type of study envisaged here.

There are also methods for measuring the air changes per hour for existing buildings. When designing a new structure, it may be possible to take measurements on an analogous existing system. Where the ventilation is forced, the air flow rate should be known.

Simple estimation involves predicting a 'normal' number of air changes per hour and from this a volumetric flow rate. A simple table is given for guidance in Appendix A. Local ventilation can be applied to major sources, such as a sample booth or a hooded plating bath. A way of dealing with these is to ascribe an efficiency factor. For example, a well designed fume cupboard allows only 0.1% of the material released in it to escape from the front. So effective source strength can be used in the study, which is one thousandth of the actual source strength. For outdoor areas the problem should be simpler. At its simplest the air can be assumed to travel through the plant at a steady speed, equal to the wind speed, under plug flow. However there are a number of complicating factors. Choosing the appropriate wind speed is only one (see Appendix A). A chemical plant represents a permeable obstruction to the wind. Air flow accelerates through the gaps between plant items and eddies behind them. Overall the wind speed is attenuated. However some estimate can be made of the volumetric flow rate through a zone (see Appendix A).



6.4.6 Estimate Fugitive Vapor Emission Rates

The CMA (ref 8) give guidance on how to measure vapor emission rates from items of equipment, and the Appendices refer to the use of existing methods, particularly as developed by the Environmental Protection Agency (EPA).

For material to be a hazard by the vapor inhalation route, the released material needs to be present as a vapor. For direct vapor/gas releases, the vapor rate is the same as the release rate. For liquid releases, the fate of the liquid and mass transfer into the vapor phase should be considered. The simplest assumption is to assume all released material evaporates into the zone; this may be true for a small leak of hot liquid onto a poorly drained surface. For other liquid releases, particularly where material is drained away or treated, only some fraction of the released material evaporates locally. Removal of liquid may cause problems in other zones e.g. where liquid in drains meets hot water.

Mass transfer may occur by flashing if a superheated liquid is released, or more generally, by evaporation from a wetted surface. Although it is relatively straightforward to calculate the rate of vapor release from a flashing stream if the process stream conditions are known, calculations of evaporation rates are less precise. The evaporation rate is a function of the wetted area, pool temperature and composition (defining vapor pressure) and local wind speed.

A useful correlation, due to Clancey, (ref 2) for evaporation from a well mixed pool is:



For evaporation of a component in a dilute solution, mass transfer of the component within the solution may be limiting. In this case the vapor rates will be overestimated using the above equation.

6.4.7 Estimate Task Vapor Emission Rates

From the information obtained in 6.4.4, estimates of the material released may be made. The approach adopted will be similar to that for estimating vapor rates for fugitive sources. The quantity and nature of material escaping from the equipment, and the fraction of that material evaporating locally should be calculated. As task releases usually only last for a limited time, an average vapor emission rate should be calculated.

6.4.8 Estimate Preliminary Background Concentrations

When making a preliminary estimate of background concentrations it is assumed that the air entering a zone is free of the contaminant. Allowance for the air being already contaminated is made later. The preliminary background concentration is the sum of the fugitive emission rate and the task emission rate in the zone divided by the volumetric air flow rate and multiplied by an inefficiency factor, see Appendix B.

6.4.9 Estimate Task Exposures

When carrying out a task, personnel may be exposed to concentrations in excess of the background, as the task may be the cause of material release, and the personnel may be working close to the source. The task exposure can be calculated as:

The effective task ventilation rate is estimated by assuming the person works in a 'task zone', a nominal 3m cube around the equipment or source. Further details are given in Appendix B.



Knowing the task concentration and the duration of the task or exposure time, the task exposure can be calculated as:

6.5 Integrate Exposures Over All Zones

6.5.1 Revise Background Concentrations

In the preliminary estimation (6.4.8) of background concentrations it was assumed that the air entering a zone was uncontaminated. Frequently this is not the case and an allowance has to be made for the contamination of the incoming air. The easiest way to do this is to add the quantity of contaminant brought into the zone to the emission rate in the zone before calculating the concentration. Where a number of zones interact iteration may be required but that should not be difficult using a spreadsheet.

6.5.2 Estimate Exposure Due To Background

The average background exposure in mg.min/m3 for a shift is estimated by multiplying the average number of minutes per shift spent in each zone by the background concentration in the zone and summing over all zones. If the pattern of work is very variable it may be necessary to investigate the various patterns to check that none gives rise to an unacceptably high exposure over a shift.

6.5.3 Estimate Average Personal Exposure

Since incidents are by definition relatively rare events, they are ignored in estimating average personal exposure. The average personal exposure in mg.min/m3 per shift is estimated by adding the average background exposure to the average exposure due to tasks. This per shift figure may need to be converted to the correct time basis (usually 8 hr Time Weighted Average, TWA) for comparison with the relevant exposure criterion.



7 UNDERSTANDING The procedure described so far enables an estimate to be made of personal exposure which may be compared with the OEL. If this is fully satisfactory no more may need to be done. However, much information is contained in the way the estimate is built up which may be used to understand the main sources of exposure. This understanding may then be used to reduce personal exposure. This reduction may be necessary to meet the OEL, although in general it is not enough simply to comply with OELs; exposures should be reduced as far as reasonably practicable. The following understanding is likely to be obtained (a) Which chemicals are particularly troublesome? (b) Who is most at risk and whether they are likely to be overexposed; (c) The distribution of background concentrations and the location of any 'hot-

spots'; (d) A ranking of emission sources; (e) A ranking of tasks. The understanding may demonstrate the need for changes such as those listed below: (1) The process may need to be altered to avoid the use of a chemical if no

satisfactory way can be found of containing it. (2) Items of equipment may need to be changed or modified to improve

containment if they are shown to be major emission sources. (3) The method of operation can often be changed to separate people from

high concentrations once the location of the latter has been identified. (4) The ventilation may need to be improved, either generally or by the

provision of local extraction. (5) Additional personal protection may be needed, but this should be seen as

a last resort.



If the calculations have been computerized it is relatively simple to evaluate the expected effect of each change. This should enable the provision of a satisfactory working environment in a cost effective manner. The estimates for background concentrations and task exposures should be checked to ensure that STELs are not exceeded and that acute problems do not occur. 8 INCIDENTS As discussed in 4.2, INCIDENTS are single exposures of such a magnitude as to possibly cause direct damage to health. Not all toxic substances fall into this category; for instance lead compounds are most unlikely to cause an acute injury by inhalation. So the first step in defining possible incidents is to determine what level of exposure, if any, could cause immediate health effects. The criterion proposed here is IDLH (Immediately Dangerous to Life or Health). This was developed by NIOSH in the USA to assist respirator selection. The IDLH concentration represents the maximum level from which one could escape in 30 minutes without any escape impairing symptoms, or any irreversible health effects. Values for a wide variety of substances are listed in Reference NIOSH/OSHA. A new system of Emergency Response Planning Guides (ERPG) is currently being developed by the American Industrial Hygiene Association (AIHA). These standards will probably be more thoroughly researched than the IDLH ones but are, as yet, only available for a few substances. Using the approach of this Guide, it is possible to estimate the scale of events which will lead to concentrations above the IDLH value in occupied areas of the plant. This scale will depend on whether the emission is inside or outside. Such estimates give quantitative criteria for Hazards and Operability Studies. The acute hazard associated with any substance can be estimated using the Substance Index approach - see Appendix E. This can usefully be refined in early stages of Hazard studies using rough estimates of typical indoor and outdoor ventilation. An example is given in Appendix D.



There is no agreement as yet as to what frequency events causing exposures to IDLH concentrations might be allowed. It is strongly recommended that the relevant Occupational Health Department is involved in such decisions, both to confirm that the IDLH value is reasonable, and also to agree any choice of frequency.



APPENDIX A VENTILATION A.1 OUTDOOR VENTILATION As stated in the main text, the ventilation rate for outdoor zones is based on wind speed. The problem then remains of choosing an appropriate wind speed. Wind speed is approximately log-normally distributed and the geometric mean wind speed frequently lies between 3 and 4!m/s. Places such as Baton Rouge, Lousiana is at the bottom of the range; Newark, New Jersey is in the middle with Cardiff Wales at the top end. Lower values are found for Gladstone, Australia (1.8 m/s) and Tokyo, Japan (2.8!m/s) and higher ones for Wattisham, Suffolk, England (4.6 m/s) and Anglesey, Wales (5.6!m/s). Users should satisfy themselves that they are using a value appropriate to the area concerned. Wind speed is normally measured at a height of 10m. The wind speed varies with height, e.g. at 2m in average conditions and surroundings it is 70% of its value at 10m. The effect of plant will be to slow the wind further and reduce the local effective ventilation. Estimation of ventilation is particularly difficult for outside zones which contain walls. A tentative set of effective wind speeds is suggested in Table 4. This suggestion assumes that the wind speed is reduced uniformly, from an 'exposed' value to an 'indoor' value, as the plant zones become more confined.

The air flow rate for an outdoor zone is found by multiplying the effective wind speed by a suitable cross-sectional area. If the wind direction is assumed to be uniformly distributed, the mean cross-sectional area of a rectangular zone is the height of the zone multiplied by the sum of the length and width of the zone multiplied by 0.64.



A.1.1 Downwash from Vents If vents are set close to the roof, vented material is likely to be entrained in the building wake. This is discussed clearly by Bryant, (ref 1) 'When the actual stack height is equal to or less than 1.5 building heights i.e. the effective stack height is appreciably less than one building height, effluent entrained in the wake could be carried to ground level in the immediate vicinity of the building on which the stack stands.' This was considered by Scorer and Barrett (ref 6) with respect to the short-term concentration likely to be found near a building with a low chimney. They assumed the pollution to be spread over a cone of semi-angle 12° bent round the building, in a moderate wind of about 5 m/s, this representing the worst weather conditions. In winds of higher speeds the pollution would be less, and in lower speeds the wake would not be well developed. From the formula given by Scorer and Barrett:

where Ps is the typical maximum short term concentration per unit release rate (unit/m3 per unit/sec) and is the length of the trajectory to the place where the pollution is measured (m) In general, on the assumption that the place of interest is at ground level outside the building, the length of trajectory is given by the height of the stack above ground level plus the distance from the stack to the outside edge of the building. To convert Ps into the long-term concentration PL it is suggested by Bryant that, corresponding to the assumed semi-vertical angle of 12°, a swing of the wind factor of 360/(2 x 12) = 15 may be applied.

This is tantamount to assuming a uniform wind rose and spreading released material uniformly in all directions. In light winds the pollutant may not be brought down to ground level. The above assumes a vent on the roof of a closed building. If one side of the building is open or has an open structure built against it, eddying may cause contaminant to enter the building or open structure.



A.2 NATURAL VENTILATION OF INDOOR AREAS The following table (Table 5) has been taken from the 1970 Institute of Heating and Ventilation Engineers (IHVE) Guide. It does not apply if there are large doorways or forced ventilation.



APPENDIX B ESTIMATING CONCENTRATION AND EXPOSURE B.1 ESTIMATING ZONE CONCENTRATIONS

The choice of K factor is something of an art. The factor allows for three items: (a) How uniformly the air is distributed through the zone. (b) How uniformly the sources are distributed through the zone. (c) How uniformly the worker 'samples' the air throughout the zone. For most zones K=1 should be used. If there is doubt about the uniformity of any of the three items, a value of K greater than one should be used. The plug flow value should be used only when all three items are favorable. Air distribution is favorable in uncluttered outdoor zones or where the ventilation has been specially designed to achieve plug flow. Source distribution is favorable where it is uniform or sources are near the air exits (and flow reversal does not happen). Worker distribution is favorable when it is uniform or the worker stays deliberately upwind of sources. Some users may wish to make the inefficiency factor a product of three individual terms, each representing one of the contributing items.



B.2 ESTIMATING TASK EXPOSURES There are two major routes for exposure when carrying out a task: (a) Evaporation from a wetted surface

This is the most frequent source of exposure with liquids. Tasks usually relate to transferring materials. Examples include tanker discharge where material can be spilled during hose handling, draining equipment, and sampling where excess liquid wets bottles and gloves etc.

(b) Release of gas or vapor

There are several tasks when contaminant vapor is released directly into the local environment where people are working. Examples include maintenance work on equipment which has not been completely purged, filling containers where contaminated air is displaced, dipping tanks etc. The gas release may be of limited duration (i.e. less than the time someone is present) or may occur throughout the task.

To estimate the task exposure, estimates are needed of the average vapor emission rate, the local ventilation, and the time spent on the task.



B.2.1 Worked Examples

(a) Liquid spillages Consider an operation where a small quantity of material is spilled. This could be a filling or maintenance operation. Assume 500 g is spilled and this covers a floor area of say1m2. Assume operation is outside and task lasts 15 min. Assume material is spilled at ambient say 10 °C; vapor pressure is 0.01 bara; mol wt 100.

Assume wind speed 2 m/s, then evaporation rate from equation (4) is

Assume task takes place in 3 x 3 x 3 m cube.

Assume K = 1 , no obvious maldistribution of worker or ventilation, then

As pool persists for longer than the 15 min of the task, the worker will be exposed to an average 7 mg/m3 concentration throughout the task.



(b) Release of gas

Consider operation where non-toxic solid is added to an atmospheric vessel containing a volatile toxic liquid. Exposure occurs as operator is exposed to the displaced contaminated air.

Assume 0.2 m3 of solid added. Assume vapor pressure of liquid is 0.1 bara; mol. Wt. = 120; temperature of vessel 50 °C. Assume operation takes place inside a building with 2 air changes/hour.



APPENDIX C VARIABILITY OF EXPOSURE LEVELS When sufficient personal monitoring is carried out there are two obvious limitations on the type of distribution. Firstly no result can be less than zero, and secondly occasional high results will occur, with the frequency tailing off at higher values. The simplest distribution with these characteristics is the log-normal distribution (lnd), and this often fits experimental data very well. In practice the fit of a set of results to a lnd is very easily demonstrated by plotting the cumulative percentage of results below a certain value, against that value on Log-probability paper. Figure 2 shows plots of two distributions, with a median of 10. NIOSH (ref 11) discusses deviations from lnd in more depth. The distribution of a log-normal distribution is defined by the geometric mean and the geometric standard deviation (gsd). The gsd has been found to vary in practice from about 1.3 to 10, although more commonly between 1.5 and 2.5. Figure 3 demonstrates the difference between a normal distribution, and lnd’s with gsd’s of 1.5 and 3. It will be seen that a gsd of 1.5 is not dissimilar to the normal distribution. The gsd of 3 however gives a much more skewed distribution, with the arithmetic mean largely due to occasional high level excursions. This is more characteristic of plants where background levels are very low, relative to the occasional excursions. Compliance with a standard is best considered as the percentage of results below the standard. This should be at least 90%, and preferably 95%, to show good compliance. The results of the method developed in this Guide generate arithmetic means, so the relationship between this mean and 90 and 95 percentiles is vital. Fortunately, the ratio of the arithmetic mean to the 90 and 95 percentiles does not vary much with gsd. This means that if the arithmetic mean of a set of results, which are log-normally distributed is less than one-quarter the OEL, then 95% compliance is guaranteed, whatever the gsd. With favorable gsd’s of 1.6 or less the arithmetic mean can be one-half the OEL for 95% compliance. As stated earlier, we generally recommend the value of one-quarter of the OEL for plant design purposes. This errs slightly to caution, but care has been taken in earlier stages not to accumulate safety factors.







APPENDIX D CALCULATION OF INCIDENT QUANTITIES The quantity to cause an incident, as discussed in Clause 8, depends on: (a) The IDLH concentration (b) The effective ventilation of the zone. In the earlier stages of plant design

rough estimates of outdoor and indoor ventilation rates can be made following Appendix A.

As an example consider a substance with an IDLH concentration of 100 mg/m3, over 30 minutes. The zone will be given dimensions of 10m x 5m x 5m (250 m3). In Appendix A.1 it was suggested that a value of 1 m/s be used as an average wind speed, together with a formula for Mean flow area. Assuming a K of 1 this gives an effective ventilation rate (outdoors),



APPENDIX E SUBSTANCE INDEX E.1 INTRODUCTION

There is a need for a simple way of ranking substances for their difficulty of control for hygiene purposes. The Substance Index as described below, is relevant to short-term exposures and incidents, rather than chronic exposures.

Two principal characteristics of a substance that indicate its relative difficulty of control for hygiene purposes are its 'volatility' and its 'toxicity'. As will be seen both can be expressed as atmospheric vapor concentrations, so the ratio of the two is a dimensionless index of toxic hazard.

Here 'volatility' is an indication of the ease with which a substance may pass into the vapor phase. A simple measure of this is the partial pressure exerted by the specified substance. For pure substances, rather than mixtures, the saturated vapor pressure (svp) can be used. For the examples below, svp at ambient conditions is used - care should be taken to use relevant data for any real-life examples.

This comparison applies strictly to vapors arising from liquids and solids, e.g. evaporation from a pool. Emissions of gases can be treated in the same way, as they produce a gas cloud of the pure substance, i.e. at 1 bar, rather than the svp of a liquid. This underestimates the immediate impact of a gaseous emission, but has the merit of simplicity. Strictly, Substance Indices for gases should be compared only to other gases.

Liquefied gases behave like a mixture of a gas and a liquid. Gas flashes off until the pool cools to the boiling point of the substance, then it behaves as a liquid with svp of 1 bar. The simplified formula for gases is appropriate in this case.



E.2 INCIDENTS When considering 'INCIDENTS' a suitable measure of the 'toxicity' is the IDLH concentration. The simple index is thus:

where P is the relevant pressure in bars, converted in this formula to ppm by assuming ideality. As discussed this pressure is 1 bar for a gas, the saturated vapor pressure for a pure liquid, and the partial pressure for liquid mixtures. For liquids the pressures should be at the liquid temperature. Table 5 assumes 25°C. E.3 SHORT TERM HAZARDS The same logic can be used when considering short term emissions, against the Short Term Exposure Limit (STEL), averaged over 10 minutes. In this case the STEL value replaces IDLH. This is recommended only when the STEL presents the primary standard for compliance, i.e. short-term exposures are more relevant than longer term ones. Of the compounds in Table 5 this applies only to chlorine, and possibly trichlorofluoromethane. Advice should be sought from the relevant Occupational Health Department for specific substances. E.4 LONGER TERM When considering long term (CHRONIC) effects the effect of volatility is more complex. A highly volatile material will reach higher concentrations more quickly, but over longer time periods a lower volatility substance can exert the same average concentration. High volatility can even aid control in that it may be possible to leave a known emission to disperse. On the other hand low volatility substances can drain away, or be treated with absorbent. The impact of volatility can be discounted, provided it is realized that the emission rates used in calculations are the effective emissions, i.e. the quantity left to evaporate after alternative outlets have been considered, such as losses to drains, or conversion to different compounds over time.



Then the best index to the hazard presented by a substance is simply the value of the OEL. Comparisons may be better in mg/m3 rather than ppm because emissions are normally considered in weight rather than molar terms.

The usefulness of Table 6 can be judged by considering two well-known toxic hazards, chlorine and mercury. Chlorine ranks easily the worst INCIDENT substance. Mercury on the other hand presents essentially no INCIDENT hazard, but is very difficult to contain on a long term basis.







