Apparent threshold

A dose-related elevation of EP or ZPP in lead workers has been documented extensively (Herber 1980 Matte et al. 1989). Correlations between PbB levels and log EP or ZPP indicate an apparent threshold for EP elevation in male workers at 25-35 pg/dL (Grandjean and Lintrup 1978 Roels et al. 1975) for FEP and a threshold of 30-40 pg/dL for EP (Roels and Lauwerys 1987 Roels et al. 1979). The threshold for EP elevation appears to be somewhat lower (20-30 pg/dL) in women than in men (Roels and Lauwerys 1987 Roels etal. 1975, 1976, 1979 Stuik 1974), regardless of whether exposure is primarily by inhalation (occupational) or oral (nonoccupational). These studies were controlled for possible confounding factors such as iron deficiency or age, both of which increase erythrocyte ZPP. 

It is possible that lead s interference with heme synthesis may underlie the effects on vitamin D metabolism. Evidence that lead affects heme synthesis in the kidney was presented in the section on hematological effects. In addition, apparent thresholds for the effects of lead on renal vitamin D metabolism and for erythrocyte protoporphyrin accumulation are similar. 

Hearing thresholds in children may be affected adversely by lead exposure at low blood lead levels (Robinson et al. 1985 Schwartz and Otto 1987, 1991). Robinson et al. (1985) reported that hearing thresholds increased linearly with maximum historical PbB levels of 6.2-56.0 pg/dL. In the analyses by Schwartz and Otto (1987, 1991), the probability of lead levels studied (NHANES II and HHANES data, respectively), from <4 to >50 pg/dL, with no apparent threshold. There is also some evidence suggesting that lead exposure may cause postural disequilibrium in children (Bhattacharya et al. 1993). The children evaluated in that study had a geometric mean PbB for the first 5 years of life of 11.9 pg/dL, the range was 5.1 to 28.2 pg/dL. 

When environmental stress cracking tests are carried out to detect the time to onset of crazing, results are generally obtained as a function of applied strain, and possibly also of temperature. There is then an apparent threshold strain below which crazing does not occur for a very long time. A discussion of how this data can be treated has been given by Wright [18]. 

Given these restrictions, oat, barley and alfalfa canopies required treatments with more than 1 pphm HF, about 5 pphm O3 or CI2, 20 pphm SO2, and 40-60 pphm NO2 or NO before apparent photosynthesis rates were measurably depressed by the end of 2 hr of exposure. Above these apparent threshold values the 2-hr depressions induced were linearly related to pollutant concentrations applied up to those that caused visible foliar injury to the tissues. Foliar necrosis occurred to some plant tissues within the canopies exposed to approximately 15 pphm HF,... 

An example of this modeling procedure is shown in Fig. 4 for the total dissociation cross section of the Cr+(CO)6 complex [9]. The energy at which the Cr+(CO)5 product signal first deviates from zero clearly depends on the sensitivity of the experiment, but certainly rises above the background at about 0.5 eV (Fig. 1). This apparent threshold energy differs appreciably from the E0 value obtained from modeling with Eq. (3), 1.59 0.09 eV, as indicated by where the dashed line deviates from zero in Fig. 4. The convolution of Eq. (3) over the... 

The quantum yield for the photosensitized reduction of PVS (using Ru(blpy)2+ as sensitizer) has been correlated with the Interfaclal surface potential of the SIO2 colloid (controlled by varying the Ionic strength of the medium) (22). The correlation curve (Figure 11) shows that up to an Interfaclal potential of ca. -40 mV the quantum yield Is not affected. Increasing the potential above this apparent threshold value results In a sharp Increase In the quantum yield. A similar correlation curve was obtained when Zn-TMPyP was used as sensitizer Instead of Ru(bipy). ... 

The conductivity of Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) at 30 °C was measured by Zawodzinski et al. [58-60] for membranes immersed in liquid water and for partially hydrated membranes in contact with water vapor. The conductivity of Nafion 117 decreases roughly linearly with decreasing water content, as shown in Fig. 32. The value of the conductivity measured at A = 14 (saturated vapor), 0.06 0.01 S/cm, agrees with the value previously reported by Rieke and Vanderborgh [62]. The dependence of conductivity on water content for Membrane C and Dow membranes is somewhat more complicated. In both cases, the conductivity decreases roughly linearly with decreasing water content until an apparent threshold is reached, at which point the conductivity drops substantially (Fig. 32). Above the water content where the steep drop occurs, the specific conductivity of both membranes is substantially higher than... 

The first photochemical study of this reaction was carried out in 1969 by Oldershaw and Porter [104], who photolyzed static N2O/HI samples at different wavelengths, and used final product analyses to deduce reaction probability versus photolysis wavelength. This provided clear evidence of a substantial entrance channel barrier (i.e., 4400cm ) for the highly exoergic reaction (4a), which was later confirmed and quantified by Marshall et al. [40,41], who carried out experimental rate constant versus temperature measurements as well as ab initio calculations of the stationary points on the potential surface. Oldershaw and Porter were also able to discern the appearance of reaction (4c) with an apparent threshold of 13,500 1400cm, in accord with the thermochemistry, as well as our observations, as discussed below. 

Apparent threshold An immeasurable concentration of a chemical below which no damage is produced. 

Emission from Xe(7p) at 4680 A was studied from about 70 to 200 eV.64 The cross section Q4680 normalized to that for emission of the resonance lines Q7680 is shown in Fig. 5. Again the apparent threshold coincides with... 

The apparent threshold for emission of the potassium D-lines in K + Hg collisions is 4 0 (+0-4) eV the absolute size of the emission cross section at 10 eV has been estimated69 to 3 A2. Again no emission from excited potassium states other than 42P has been observed up to 100 eV collision energy upper limits for emission at 4050 and 6930 A are 1 % of that at 7680 A. No emission from excited mercury states at 5440 and 5780 A was observed either. 

Emission from Ne(2p53p ) at 6266 A was studied in collisions between ground state Ne-atoms from the apparent threshold up to about 2keV.91 The spectral analysis was made by interference filters no values of the spectral resolution were given. At 60 eV the emission cross section rises sharply a weak tail is observed below 60eV, the apparent threshold is estimated to be about 40 eV by the present author. The cross section has a similar shape as found in89 for the u.v.-emission in Ar 4- Ar. No estimation of the absolute cross section has been made. 

Emission from excited He, Ne and Ar-atoms has been studied for several combinations of rare gases.92 He + He, He + Ne, He + Ar, Ne 4- Ne, Ne + Ar and Ar + Ar were studied from the apparent threshold up to about 500 eV. The spectroscopic analysis was made with interference filters of 70 A bandwidth (for more details about the studied wavelengths see Table IV(a) and IV(b). Except for He 4- Ar the apparent threshold energies are between 25 and 60 eV. The smallest cross sections which could be detected were of the order of 10 21 cm2. The shape of the emission cross sections is similar to what was found in Refs. 89 and 91. 

System Studied wave- length Contributing excited states Apparent threshold [eV] Emission cross sections Estimated Collision absolute energy size [cm2] [eV] Reference... 

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