Document 8VXd0pjyDg0mpwYqNMK4Lgb4K

GRAVIMETRIC DUST SAMPLING GRAVIMETRIC DUST SAMPLING -- FUNDAMENTAL CONCEPTS AND PRINCIPLES* L.R. GARDINER Industrial Hygiene Laboratory, Chamber of Mines of South Africa Research Organization One of llie fundamcnial problems of mining, from the health poim of xicxx. is that it produces airborne dust and. as ex cry miner knows, excessive dust exposure max lead to lung disease, commonly termed pneumoconiosis Cdustv lung'). The solution is obvious: reduce the dust exposure of mineworkers to lex els which are considered acceptable. A considerable amount of time, effort and expense is directed towards dust sampling on South African mines. For such efforts to be fruitful, however, they must be based on sound principles of dust measurement and dust control, and their effectiveness verified by medical stu dies. Alter all. the success of a dust control programme is ultimately measured in terms of an overall reduction in the incidence ol pneumoconiosis. The introduction of grax imelric dust sampling throughout the South African mining industry as an alternative to present methods is currently the subject of considerable attention. Discussion on this subject ob viously requires a basic understanding of the method and tamilinrily with terminology used to describe it. This paper seis out the fundamental principles and concepts underlying gravimetric sampling for respirable dust and examines its role in providing an improved measure of the health hazard from dust exposure. Principles of dust sampling for hazard evaluation Nearly thirty years ago, world dust experts gathered in Johannesburg to discuss the need to improve dust sam pling and control techniques in order to mitigate the ef fects of dust exposure. That historic International Pneumoconiosis Conference in 1959 recommended the principles under which dust sampling should be conducted in order to assess the health hazard from dust exposure. The main principles have remained virtually unchanged to the present day and have gained wide spread international acceptance. They are as follows measurements of the respirable dust fraction gravimetric assessment of the collected dust lung-period sampling to give average dust levels over a shift. Each of these principles is now examined in more detail. Respirable dust Dust particles are conveniently sized by reference to their diameter, generally expressed in microns (pm). The diameter of an irregular particle estimated using a microscope and graticule by the projected area method is the `geometric' diameter. It has physical meaning and is most commonly used to denote actual particle size. The aerodynamic equivalent diameter (AED) is a hypo thetical but valuable concept used to denote the dia meter of a spherical particle of unit density having the same falling velocity as the particle in question. For example, an irregular quartz particle (density 2 600 kg/m3) with a diameter of 3,1 pm will have an equivalent (AED) diameter of about 5 pm; a coal par ticle (density 1 100 kg/m3) but 3,8 pm in size will also have an AED of 5 pm. Both particles will behave in exactly the same manner aerodynamically when breathed into the lungs, despite being physically different in size. The equivalent diameter concept therefore classifies par ticles aerodynamically, regardless of their shape, size or density. Respirable dust was originally defined in qualitative terms by the British Medical Research Council (BMRC) in 1952 as `that fraction of dust reaching the alveoli'. (Alveoli are air sacs deep in the lung where gas exchange takes place; damage in this region, caused by deposited dust particles, may lead to pneumoconiosis). The BMRC selected the horizontal elutriator (MRE 113A) as the practical device for measuring respirable dust with the fraction of dust penetrating this sampler constituting the respirable fraction. The rationale behind this was that the dust collection efficiency of the horizon tal elutriator could be made to simulate fairly closely the dust deposition characteristics of the alveolar portion of the lung. Here, therefore, was a practical method of measuring that fraction of a dust cloud which could pose long-term health risks. *Paper presented at the Mine Ventilation Society of South Africa Symposium "Airborne Pollutants: Their Effects, Measurement and Control", 25-26 March 1987 Journal ofthe Mine Ventilation Society ofSouth Africa, March, 1989 53 PAPER From experimental studies of dust deposition, it was de cided to set a value of 50% penetration at 5 pm AED. Respirable dust was therefore defined numerically by a penetration curve (Figure 1) whose maximum particle size is 7,1 pm AED. an exact simulation of the deposition curve for dust in the lung particle size is expressed in aerodynamic equivalent diameters e.g. a 5 pm (AED) quartz particle will ap pear under the microscope to be only 3,1 pm in size other definitions of respirable dust have also been recognised e.g. U.S. ACGIH definition the BMRC definition was adopted at the Interna tional Conference on Pneumoconiosis held in Johan nesburg in 1959. It is therefore frequently referred to as the `Johannesburg' curve. the Government Mining Engineer in South Africa has recognised the Johannesburg definition of respirable dust for local use. Gravimetric assessment `Gravimetric' means `measurement by mass' and refers to the mass of dust collected after a period of sampling dusty air. The mass of dust is typically determined by measuring the weight increase of a filter using a micro balance. Strictly speaking, `gravimetric' refers to the ana lytical rather than the sampling process, but the term `gravimetric dust sampling' has, through common usage, been taken to mean `dust sampling followed by gravi metric assessment'. Figure 1 BMRC/JOHANNESBURG RESPIRABLE DUST CURVE A formula for the penetration curve defining the respi rable fraction* is given below:- where P = penetration of dust to the lung (%) d = size of particle (pm, AED) D = maximum size of particle (7,1 pm AED) (* equivalent to the dust collected by the MRE 113A filter) Values of P for varying dust size are set out in Table 1. Gravimetric means measurement by mass and is the mass of dust collected after a period of sampling dusty air Dust levels measured gravimetrically are expressed in terms of concentration by dividing the mass of dust (mg) by the volume of air sampled (m3) giving a mass concen tration in mg/m3. (This is quite distinct from number con centration e.g. konimeter dust `counts' expressed in par ticles per millilitre of air sampled, or p/m). The principle of measuring the mass concentration of respirable dust was founded on the results of epidemio logical research, particularly on coal mines. Epidemio logy is the science of relating medical manifestations of disease with the agent or process presumed to cause the Table 1 QUANTITATIVE DEFINITION OF RESPIRABLE DUST (BMRC/JOHANNESBURG DEFINITION) 1> CD d (urn) 9(1 Ml 70 til) 3.2 3.9 4,5 50 40 5.0 5 5 50 20 10 5.9 ft-.T b.9 0 7,1 Respirable dust as defined in Table 1 is therefore not simply all particles less than a stated size (as is so often quoted) but an increasing proportion of dust particles smaller than ~ 7 pm (AED). This description approxi mates to the fraction of dust penetration to the lung, in the course of which larger particles are selectively re moved. Several points should be emphasized the penetration function is a theoretical curve and not disease. It typically requires detailed study of a select group of workers over a long time, during which two aspects are monitored: average dust exposures of workers in occupational groups medical and occupational histories of these workers. From such studies, the relationship between dust exposure (obtained from dust measurements and occupa tional histories), and clinical evidence of the disease is 54 Journal ofthe Mine Ventilation Society ofSouth Africa, March, 1989 GRAVIMETRIC DUST SAMPLING hopefully clarified and the results are expressed in a family of exposure (or dose) -- response curves. An example is given in Figure 2. Figure 2 HYPOTHETICAL EXPOSURERESPONSE CURVES SHOWING PROBABILITY OF CONTRACTING PNEUMOCONIOSIS AFTER 25, 35 OR 45 YEARS EXPOSURE TO DIF FERENT DUST CONCENTRATIONS The classic epidemiological study was the so-called `25 pit scheme' conducted in the U.K. over 15 years. During this period, detailed chest x-rays and dust exposure levels of all miners were accumulated, the object being to provide a quantitative link between dust exposure and coal-workers' pneumoconiosis. Poor correlation was initially obtained when particle number was the parameter used to measure dust concen trations, despite this being of respirable size. It was only after gravimetric sampling was undertaken that good cor relation was found between mass concentration of respi rable dust (measured by the MRE 113A instrument) and progression of the disease. This was a major breakthrough because for the first time a quantitative relationship for the health hazard of respirable coal dust was available in terms of a controlla ble parameter. It was, furthermore, developed not from theory or animal studies but from clinical and environ mental evidence gleaned from a group of coal mineworkers in the course of normal work. The link also provided a logical basis for dust control establish a link between dust exposure and dust disease control that dust parameter which correlates with the disease to an acceptable level observe a reduction in the prevalence of the disease with time. The relationship between silica dust exposure and silicosis has, unfortunately, not received the same depth of study as coal dust. However, there is general consen sus that mass is a better indicator than surface area for estimating the harmful effect of respirable silica. Long-period sampling Epidemiological studies reveal two factors which are paramount in determining whether or not a mineworker contracts pneumoconiosis :- his cumulative exposure to respirable dust the time period over which this occurs. These points are illustrated graphically in Figure 2 which comprises a set of hypothetical exposure -- response curves relating the probability of contracting pneumoconiosis after 25, 35 or 45 years exposure to dif ferent mean respirable dust concentrations. For a given exposure period (say 35 years), risk increases rapidly with increase in mean dust concentration (curve AB). However, a miner would suffer the same risk of 4% (point B) whether it was after 25 years at 5 mg/m3 (point C), 35 years at 4 mg/m3 (point B) or after 45 years at 3 mg/m3 (point D). Dust levels during a shift vary tremendously, so in or der to obtain a reliable mean concentration, measure ments require the use of a long-period sampler. A full shift is usually considered the most practical period of measurement and this has the advantage of monitoring a mineworker's total exposure. Gravimetric dust standards Having discussed the principles of gravimetric dust sam pling, it is important to understand how gravimetric dust levels are interpreted, and what degree of dust exposure is acceptable. The answers, of course, depend on what dust standard is permitted. Without benchmark stan dards, it is difficult to quantify whether or not a given Dust levels measured gravimetrically are expressed in terms of concentration by dividing the mass of the dust (mg) by the volume of the air sampled m3), giving a mass concentration in mg/m3 working environment is potentially unhealthy. Conver sely, a healthy environment can only be effectively de fined in terms of specified levels of pollutants. Pneumoconiosis is a long-term disease in that it may not appear before 15 to 20 years of cumulative dust ex posure. It is daily exposure to respirable dust over many years rather than the occasional peak level in a shift that is most likely to lead to the disease. Respirable dust standards must obviously be formulated on this basis. Journal ofthe Mine Ventilation Society ofSouth Africa, March, 1989 55 PAPER However, what are harmful dust levels? Epidemiological studies again provide the answer in the form of the expo sure -- response curve (Figure 3). There is no evidence that a lower level exists below which respirable dust causes no injury. (The so-called `zero risk' standard of 2 mg/m3 legislated for the U.S. coal mining industry does in fact represent a finite but negligible risk). Gravimetric standards therefore represent a balance between what is considered an acceptable risk (expressed in per cent probability of contracting the disease after a specified time) and the likely impact on the mining in dustry of adopting that value. Toxicity Mine dusts have different health effects on the body depending on their mineralogical or chemical make-up e.g. asbestos, coal, quartz, diesel soot, metal fumes. Dif ferent standards therefore apply depending on the type of airborne dust. For pneumoconiosis type diseases, the respirable size fraction is also specified. Concentration The amount of dust inhaled in a given time increases di rectly with concentration. Gravimetric standards are generally expressed in terms of an average concentration over the work shift e.g. 3 mg/m3. Figure 3: HYPOTHETICAL DUST PROFILE OVER A FULL SHIFT ILLUSTRATING THE CONCEPTS OF FULL-SHIFT MEAN CONCENTRATION AND TIME-WEIGHTED AVERAGE (TWA) CONCENTRATION Although standards should be set at realistic levels, bearing in mind the degree of dust control attainable with current technology, they should also require a measure of improvement on existing levels i.e. the stan dard should be seen as a target. Dust standards have his torically been tightened as improvements in control tech nology appear and as social pressures come to bear on mining industries. Gravimetric dust standards are usually specified in terms of three criteria:- toxicity -- how harmful? concentration -- how much present? exposure -- how much inhaled? Exposure The amount of dust inhaled at a given concentration in creases with time of exposure to the dust. Gravimetric standards normally refer to an 8-hour shift (40 hour week). Dust concentration and time together give the dust exposure or dose:- Dust exposure = concentration x time For example, two coal mineworkers whose full-shift mean respirable dust concentrations are 3 mg/m3 and 4 mg/m3, but whose shift times (bank-to-bank) are 8 hours and 6 hours respectively, will experience the same dust exposure (24 mg.hrs) 56 Journal ofthe Mine Ventilation Society ofSouth Africa, March, 1989 GRAVIMETRIC DUST SAMPLING Both workers consequently suffer the same health risk from respirable dust, despite the fact that the mean dust concentrations differ. Clearly, dust exposure can be decreased by:- reducing the average dust concentration reducing the period of exposure a combination of the above two. The three criteria for gravimetric dust standards are illustrated in Figure 3 which shows a hypothetical profile of dust concentrations experienced by a miner during his shift. Assume that the miner collected his personal gravime tric sampler from the lamphouse at 05h30 and returned it at 15h30, still running at a flow rate of 2,0 /min. After gravimetric analysis, the mass of respirable dust on the filter amounted to 3,0 mg. The following then applies:- Sampling period = 10 hours Air volume sampled = 1,2 m3 Full-shift mean dust concentration =3 1,2 = 2,5 mg/m3 Dust exposure (concentration x time) = 2,5 x 10 = 25 mg.hr ~mr Assume, also, that the Government Mining Engineer (GME) has set a respirable dust standard of 3,0 mg/m3 over an 8 hour period. Are dust conditions within the law? Maximum dust exposure allowed by GME = 3,0 x 8 = 24 mg.hr m3 This particular shift is therefore fractionally above what is allowed. The same conclusion could be reached by comparing the permissable dust concentration, 3,0 mg/m3 (over 8 hours), with the calculated figure of 2,5 mg/m3 (over 10 hours), after weighting this to 8 hours (2,5 mg/m3 for 10 hours is equivalent to 3,1 mg/m3 for 8 hours). Again, the time-weighted average concentration for the shift is marginally higher than the allowable level, based on the stipulated 8 hour period. An important point to note is that although the fullshift mean concentration (2,5 mg/m3) is apparently within the dust standard (3,0 mg/m3), after weighting for the shift period (10 hours) it actually exceeds the stan dard. This emphasises the need to assess dust exposure and not just dust concentration. Several other points are evident from Figure 3:- dust levels experienced by miners may vary by orders of magnitude during a shift (and even between shifts). A statistical approach is therefore required to obtain reliable estimates of dust exposure any attempt to measure dust exposure (or mean dust concentration) with one reading from a short-term sampler (e.g. modified thermal precipitator over 15 minutes, or a konimeter over < 1 second) will lead to totally erroneous results any attempt to measure meaningful improvements re sulting from dust control measures will require com parisons based on average dust concentrations meas ured over a good proportion of the shift travelling and/or waiting time may be a significant fraction of the total shift period during which the miner is in `fresh' air. In the example, the working face mean dust concentration of 5,2 mg/m3 reduces to 2,5 mg/m3 when averaged over a full shift personal gravimetric dust sampling can be used as a cost-effective screening tool to identify dusty sections. In the above example, extra dust control measures are probably not required, although spot dust meas urements during the middle of the shift may have created the impression that dust conditions are hazar dous. Summary The average mass concentration of respirable dust, measured over as long a period as possible, gives the most reliable measure of hazard dust exposure. These three principles of gravimetric sampling form the most rational foundation for programmes designed to measure and control the hazard from long-term exposure to respi rable dust. Gravimetric dust standards, selected on the basis of epidemiological studies that quantify the relationship be tween dust exposure and dust disease, provide the key to reduce the prevalence of pneumoconiosis in the mining industry. Acknowledgements This paper is published with the permission of the Chamber of Mines of South Africa Research Organiza tion. THE AUTHOR Dr Laurie Gardiner graduated from Bristol Univer sity in 1974 with a B.S.c.(lions.) in Chemistry and Geology and was awarded his Fh.I). in Analytical C hcmislry in 1978. He is currently chief of the Air Pollution Division in the Industrial Hygiene Labora tory of the Research Organization of The Chamber of Mines of South Africa. Journal ofthe Mine Ventilation Society ofSouth Africa, March, 1989 57