Threshold Zone Research Articles

Variations in hematologic responses to increased lead absorption in young children.

In the study of human populations, much emphasis is placed on the concentration of lead in whole peripheral blood. There is a considerable body of evidence which indicates that this measurement reflects recent and current assimilation of lead. While broad ranges in blood lead concentration have been associated with differing risks of toxicity for groups, it is not a precise index of adverse effect per se, even at elevated levels. Within the red blood cell itself there is not a close association between the concentration of lead and such adverse metabolic effects as the increased loss of potassium caused by lead. Above the apparent "threshold zone" of approximately 30-50 mug Pb/100 ml whole blood, equivalent metabolic effects on heme synthesis may be seen over an interval of at least 20 mug Pb/100 ml whole blood. This variation will be examined with particular reference to the interrelationship between the concentrations of lead and protoporphyrin in peripheral blood. The data indicate that limitations in both precision and accuracy of measurement account for a relatively small fraction of the observed variations. Together with other experimental and clinical information, they suggest that concurrent dietary deficiency of iron may be one of the important modifying factors in the responses of subjects with increased lead absorption. It is suggested that suspected adverse effects upon the various organ systems associated with increased lead absorption be measured directly and that the CaEDTA mobilization test for lead should be more fully explored as a measure of the "metabolically active" fraction of the total body lead burden.

Variations in Hematologic Responses to Increased Lead Absorption in Young Children

In the study of human populations, much emphasis is placed on the concentration of lead in whole peripheral blood. There is a considerable body of evidence which indicates that this measurement reflects recent and current assimilation of lead. While broad ranges in blood lead concentration have been associated with differing risks of toxicity for groups, it is not a precise index of adverse effect per se, even at elevated levels. Within the red blood cell itself there is not a close association between the concentration of lead and such adverse metabolic effects as the increased loss of potassium caused by lead. Above the apparent “threshold zone” of approximately 30–50 μg Pb/100 ml whole blood, equivalent metabolic effects on heme synthesis may be seen over an interval of at least 20 μg Pb/100 ml whole blood. This variation will be examined with particular reference to the interrelationship between the concentrations of lead and protoporphyrin in peripheral blood. The data indicate that limitations in both precision and accuracy of measurement account for a relatively small fraction of the observed variations. Together with other experimental and clinical information, they suggest that concurrent dietary deficiency of iron may be one of the important modifying factors in the responses of subjects with increased lead absorption. It is suggested that suspected adverse effects upon the various organ systems associated with increased lead absorption be measured directly and that the CaEDTA mobilization test for lead should be more fully explored as a measure of the “metabolically active” fraction of the total body lead burden.

Thresholds and hysteresis of surface ionization under nonuniform conditions

The influence of nonuniformity of the atom arrival rate, of the emitter temperature and of the adsorption properties of its surface on characteristics of thresholds and hysteresis of the surface ionization is analyzed. It is shown that, unlike the uniform conditions, in the threshold zone the ion current changes with the emitter temperature monotonically and not in a form of a jump ; the width of hysteresis is substantially diminished ; the slope of the first threshold curve is not obligatory equal to the heat of ion adsorption; for a polycrystalline surface the critical temperature for hysteresis to vanish is related to the minimum work function and not to the effective work function.

A Unified Theory of Cavitation Damage

The Cavitation Damage Number proposed by the author has been modified to incorporate the concept of strain-energy instead of the dynamic yield stress of the material. The correlation between the Cavitation Damage Number and the Number of Cavities has been extended to different materials and different sizes. A third nondimensional number known as the Inception of Damage Number has been derived and correlated with the critical cavitation parameter. Detailed investigations in the threshold zone indicate the validity of the functional relationship between the critical cavitation parameter and the Inception of Damage Number. In this paper, the part played by each parameter is analyzed and some of the experimental results arrived at by earlier investigators are explained in the light of the present theory.