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The underdefined nature of the blood lead-air lead relationship from biological and statistical grounds
1068
Discussions
Thus the final correlations obtained may have some rough degree of emph'kal utility, but they do not establish that this two-phase zone may be modelled as a "rapid bubbling fluidized bed", as this terminology is generally used.
Universlty of Cincinnati, Cincinnati, OH 45221-0171, U.S.A.
WILLIAMLICHT
REFIglIENCE Azhel D. and Llapis A. I. (1983) Hydrodynamics of gas-llquid flows. In Handbook of Fluids in Minion (edited by Cheremisinoff N. P. and Gupta R.), Chapter 16, pp. 427-452. Ann Arbor Science, MI.
AUTHORS' REPLY We see that Dr Lk:ht appreciates our efforts to characterize the zone above the catenary grids. If one is to follow the conventional definition, we agree that it is not truly a "fluidized bed". However, it certainly looks and acts like one. Contrary to the contention that we do not form bubbles, we indeed do produce them and wish we could show him our pilot unit so he could verify this observation for himself. Since the work in this paper was completed, we have discovered that much of the liquid in this zone appears as very fine vertical sheets plus a dispersion of a few droplets. It is indeed difficult to characterize this zone, especially when the turbulence is included. The Azbel correlation is applicable to the zone above the grid where the gas bubble has not yet burst. Thus the "rapid bubbling fiuidized bed" model is currently the best defined model available to describe our system. The model, with modification, enables us to predict pressure drops to within 0.1 inch water gauge and to more accurately predict mass transfer in larger systems. Addressing the comment of reproducibility, there are now over 50 catenary grid units in operation with some as large as 6 feet in diameter. All data are very consistent to the mean of these data as reported in this paper. It can also be noted that in a unit where the grid discharges into a very low velocity section ( < 3 m s- t), the top of the fiuidized zone is very evident.
Southern Illinois Universily, Carbondahe, IL 62901, U.S.A.
H. E. HESKETH K. C. SCHIFFTNER
THE UNDERDEFINED NATURE OF THE BLOOD LEAD-AIR LEAD RELATIONSHIP FROM BIOLOGICAL AND STATISTICAL GROUNDS* I have read with interest Dr Colombo's account of his efforts to find a 'Universal' dose-response relationship for blood lead in relation to air lead. He has shown a prudent command both of the data and of analysis by admitting that discrimination among a number of possible models is not potable on the basis of the data available to him. I take no exception. Of course for any curvifincar relationship, it is possible to define a short segment which appears to be linear. Although our original paper (Goldsmith and Hexter, 1967) dealt with *Colombo A. 1485-1493.
(1985) Atmospheric Environment 19,
PbA levels above 30 pg m - ~ and we therefore had to include data over about two orders of magnitude for PbA, much of the more recent debate has dealt with a smaller range, as indeed is done in Colombo's paper. As is true in some other dose--response situations, there must be a range at the lower end where the relationship is so poorly defined as to be undeterminable. For purposes of considering air quality standards originally at the California Department of Health in 1969, we did not feel that below 2 #g m - 3 of PbA we could define a dose-response relationship. Colombo suggests that the situation has not changed much in the intervening 16 years. I have trouble conceptualizing the "Generic Compartment." If it refers to bone, where most of the body burden of lead in adults is stored, then there should be no transfer coefficient to the environment. Figures 3 a-e are eaeeHent examples of what I teach should never be published in what purports to be a scientific article. Only one panel, 3e, looks like it has any evidence of an association, and that certainly does not justify the extension of the purported regression lines beyond the cluster of observations. The Williams data (1969) with a correlation coefficient of R = 0.90 (linear) and R = 0.92 (lead exposure) also do not meet the criteria I teach, because the points are in two clusters, neither of which alone shows much of an association. Neither can the 'on-duty PbA' leveLs meet the criteria for steady state exposure levels. It may be more fruitful to follow carefully the decrease in blood lead to U.S. populations, as lead in air has diminished over the past 10 years, than it is to keep reanalyzing data that are well known, if not fully understood.
Rehov Clil Hachoresh 4, POB 473, Omer 84965, Israel
JOHN R. GOLDSMITH, M.D., M.P.H.
REFERENCES Goldsmith J. R. and Hexter A. C. (1967) Respiratory exposure to lead: epidemiological and experimental dose-response relationships. Science, Wash. 1~;8, 132-134. Williams M. K.. King E. and Walford J. (1969) An investigation of lead absorption in an electric accumulator factory with the use of personal samplers. Br. J. ind. Med. 26, 202-216.
AUTHOR'S REPLY I thank Dr Goldsmith for his comments on my paper. They offer me the opportunity to be more specific on some points of my work. The exact nature of the "Generic Compartment" need not be defined. Lead pathways from and to it are only given as example of pouible routes. As outlined in Section 3, Equations (1) and (2) hold irrespective of the number of and interactions among compartments. Figures such as 3a-e showing ciusterlike data scarcely associated at a v/sual inspection are certainly not uncommon if we sample, as in those cases, a number of individuals from a population, measuring two uncontrolled parameters on each individual in the sample. Data such as those of Griffin (Table 3 and Fig. 6) where more than one individual PbB concent~tion is available, each at a given, controlled, well-spacedapart PbA level, are known to comply better with a visual appreciation of association between variables and, indeed, with a statistical analysis. I would incidentally add that, merely on the basis of Griffin's data, the algebraic form of the relationship PbB vs PbA does not appear so poorly defined at its lower end as Dr Goldsmith claims. I believe indeed that it is the lack of additional data such as those of Griffin (also covering about two orders of magnitude for PbA, i.e. from
Discussions
Thus the final correlations obtained may have some rough degree of emph'kal utility, but they do not establish that this two-phase zone may be modelled as a "rapid bubbling fluidized bed", as this terminology is generally used.
Universlty of Cincinnati, Cincinnati, OH 45221-0171, U.S.A.
WILLIAMLICHT
REFIglIENCE Azhel D. and Llapis A. I. (1983) Hydrodynamics of gas-llquid flows. In Handbook of Fluids in Minion (edited by Cheremisinoff N. P. and Gupta R.), Chapter 16, pp. 427-452. Ann Arbor Science, MI.
AUTHORS' REPLY We see that Dr Lk:ht appreciates our efforts to characterize the zone above the catenary grids. If one is to follow the conventional definition, we agree that it is not truly a "fluidized bed". However, it certainly looks and acts like one. Contrary to the contention that we do not form bubbles, we indeed do produce them and wish we could show him our pilot unit so he could verify this observation for himself. Since the work in this paper was completed, we have discovered that much of the liquid in this zone appears as very fine vertical sheets plus a dispersion of a few droplets. It is indeed difficult to characterize this zone, especially when the turbulence is included. The Azbel correlation is applicable to the zone above the grid where the gas bubble has not yet burst. Thus the "rapid bubbling fiuidized bed" model is currently the best defined model available to describe our system. The model, with modification, enables us to predict pressure drops to within 0.1 inch water gauge and to more accurately predict mass transfer in larger systems. Addressing the comment of reproducibility, there are now over 50 catenary grid units in operation with some as large as 6 feet in diameter. All data are very consistent to the mean of these data as reported in this paper. It can also be noted that in a unit where the grid discharges into a very low velocity section ( < 3 m s- t), the top of the fiuidized zone is very evident.
Southern Illinois Universily, Carbondahe, IL 62901, U.S.A.
H. E. HESKETH K. C. SCHIFFTNER
THE UNDERDEFINED NATURE OF THE BLOOD LEAD-AIR LEAD RELATIONSHIP FROM BIOLOGICAL AND STATISTICAL GROUNDS* I have read with interest Dr Colombo's account of his efforts to find a 'Universal' dose-response relationship for blood lead in relation to air lead. He has shown a prudent command both of the data and of analysis by admitting that discrimination among a number of possible models is not potable on the basis of the data available to him. I take no exception. Of course for any curvifincar relationship, it is possible to define a short segment which appears to be linear. Although our original paper (Goldsmith and Hexter, 1967) dealt with *Colombo A. 1485-1493.
(1985) Atmospheric Environment 19,
PbA levels above 30 pg m - ~ and we therefore had to include data over about two orders of magnitude for PbA, much of the more recent debate has dealt with a smaller range, as indeed is done in Colombo's paper. As is true in some other dose--response situations, there must be a range at the lower end where the relationship is so poorly defined as to be undeterminable. For purposes of considering air quality standards originally at the California Department of Health in 1969, we did not feel that below 2 #g m - 3 of PbA we could define a dose-response relationship. Colombo suggests that the situation has not changed much in the intervening 16 years. I have trouble conceptualizing the "Generic Compartment." If it refers to bone, where most of the body burden of lead in adults is stored, then there should be no transfer coefficient to the environment. Figures 3 a-e are eaeeHent examples of what I teach should never be published in what purports to be a scientific article. Only one panel, 3e, looks like it has any evidence of an association, and that certainly does not justify the extension of the purported regression lines beyond the cluster of observations. The Williams data (1969) with a correlation coefficient of R = 0.90 (linear) and R = 0.92 (lead exposure) also do not meet the criteria I teach, because the points are in two clusters, neither of which alone shows much of an association. Neither can the 'on-duty PbA' leveLs meet the criteria for steady state exposure levels. It may be more fruitful to follow carefully the decrease in blood lead to U.S. populations, as lead in air has diminished over the past 10 years, than it is to keep reanalyzing data that are well known, if not fully understood.
Rehov Clil Hachoresh 4, POB 473, Omer 84965, Israel
JOHN R. GOLDSMITH, M.D., M.P.H.
REFERENCES Goldsmith J. R. and Hexter A. C. (1967) Respiratory exposure to lead: epidemiological and experimental dose-response relationships. Science, Wash. 1~;8, 132-134. Williams M. K.. King E. and Walford J. (1969) An investigation of lead absorption in an electric accumulator factory with the use of personal samplers. Br. J. ind. Med. 26, 202-216.
AUTHOR'S REPLY I thank Dr Goldsmith for his comments on my paper. They offer me the opportunity to be more specific on some points of my work. The exact nature of the "Generic Compartment" need not be defined. Lead pathways from and to it are only given as example of pouible routes. As outlined in Section 3, Equations (1) and (2) hold irrespective of the number of and interactions among compartments. Figures such as 3a-e showing ciusterlike data scarcely associated at a v/sual inspection are certainly not uncommon if we sample, as in those cases, a number of individuals from a population, measuring two uncontrolled parameters on each individual in the sample. Data such as those of Griffin (Table 3 and Fig. 6) where more than one individual PbB concent~tion is available, each at a given, controlled, well-spacedapart PbA level, are known to comply better with a visual appreciation of association between variables and, indeed, with a statistical analysis. I would incidentally add that, merely on the basis of Griffin's data, the algebraic form of the relationship PbB vs PbA does not appear so poorly defined at its lower end as Dr Goldsmith claims. I believe indeed that it is the lack of additional data such as those of Griffin (also covering about two orders of magnitude for PbA, i.e. from