Absorption different ages

In contrast to the estimated dietary requirement of approximately 35 ug/kg/day in adults, the requirement of infants has been estimated to be 0.2-0.6 ug/kg/day (35). This difference is likely explained by the differences in degree of absorption or in excretion rates and susceptibility to Mn at different ages. Because of the vulnerability of the newborn to deficiency and excess of Mn, we have investigated Mn in various infant diets and the extent to which it is absorbed. The concentration of Mn in human milk is very low, 4-8 ug/L, as compared to 20-50 ug/L in cow s milk and 50-1300 ug/L in U.S. infant formula (6,36,37). Thus, Mn intake of breast-fed infants will be 0.5-0.9 ug/kg/day, while the intake of formula-fed infants will be considerably higher and highly variable depending on the formula used. 

Mraz FR and Eisele GR (1977a) Gastrointestinal absorption of Nb by rats of different ages. Radiat Res 69 591-593. 

In surface renewal models the liquid surface is assumed to consist of a mosaic of elements with different age at the surface. The rate of absorption at the surface is then an average of the rates of absorption in each element, weighted with respect to a distribution function (t)—see Eq. 6.2-5. Under this heading of surface renewal theory we will also occasionally mention results of Higbie s [23] so-called penetration-theory, which can be considered as a special case in which every element is exposed to the gas for the same length of time before being replaced. The main emphasis of this section is on the Danckwerts [24] approach using the distribution function for completely random replacement of surface elements ... 

From these results it was concluded that neonatal age is a period of greatly increased metal absorption different organ distribution different metal-metal interaction and higher acute oral metal toxicity. It therefore seems reasonable to consider the early neonatal age as a critical period for metal accumulation and therefore of metal toxicity. 

Figure 10 Spectral diffusion broadening as a function of waiting time fwfor different ageing times. The sample is protoporphyrin IX in a dimethylformamide/glycerol glass temperature is 100 mK. Insert Broad-band absorption spectrum. The arrow marks the wavenumber where hole burning was performed.

Another family of feedbacks arises because the radical differences in the albedo (reflectivity) of ice, snow, and clouds compared to the rest of the planetary surface, which causes a loss of the absorption of solar radiation and thereby cools the planet. Indeed, the high albedo of snow and ice cover may be a factor that hastens the transition into ice ages once they have been initiated. Of course, the opposite holds due to decreasing albedo at the end of an ice age. As simple as this concept may appear to be, the cloud-albedo feedback is not easy to quantify because clouds reflect solar radiation (albedo effect) but absorb... 

Often, absorption occurs by multiple routes in humans. Dean et al. (1984) reported deaths and toxic effects as well as lowered blood cholinesterase levels and excretion of urinary 4-nitrophenol in several children who were exposed by inhalation, oral, and possibly dermal routes after the spraying of methyl parathion in a house. In the same incident (Dean et al. 1984), absorption was indicated in adults who also excreted 4-nitrophenol in the urine, though at lower levels than some of the children, and in the absence of other evidence of methyl parathion exposure. In this study, the potential for age-related differences in absorption rates could not be assessed because exposure levels were not known and the children may have been more highly exposed than the adults. Health effects from multiple routes are discussed in detail in Section 3.2. 

Extraction studies have also been carried out by grinding the ageing cements and extracting the soluble ions with water (Wilson Kent, 1970 Crisp Wilson, 1974). Ion content was determined using atomic absorption spectroscopy. The experiments give different, but complementary, results to those of Cook (1983), since what is extracted are those ions that have been released from the glass powder but not yet insolubilized by reaction with the polyacid. 

The question now arises how the various maxima that are sometimes observed can be explained. Two explanations seem to be possible. One could ascribe the maxima to transitions to different electronic states of the small particles In this case, the structured absorption spectrum would only be seen if the sample had a narrow size distribution and one would expect a maximum to change position upon particle growth by aging. 

Children s Susceptibility. No studies were located in which comparisons were made between the sensitivity of children and adults to the toxicity of americium. Animal studies indicate that juvenile dogs are less susceptible than adults to americium-induced bone cancer (Lloyd et al. 1999). No direct evidence was located to indicate that the pharmacokinetics of americium in children may be different from that in adults. Based on dosimetric considerations related to differences in the parameters of available models, as well as studies in animals, it seems likely that children may be more susceptible to americium toxicity than are adults by virtue of age-related differences in pharmacokinetics. Absorption of ingested americium may be as much as 200 times greater in neonatal animals than in adults. (Bomford and Harrison 1986 David and Harrison 1984 Sullivan et al. 1985). 

---