Threshold effects

Organophosphonates are similar to polyphosphates in chelation properties, but they are stable to hydrolysis and replace the phosphates where persistence in aqueous solution is necessary. They are used as scale and corrosion inhibitors (52) where they function via the threshold effect, a mechanism requiring far less than the stoichiometric amounts for chelation of the detrimental ions present. Threshold inhibition in cooling water treatment is the largest market for organophosphonates, but there is a wide variety of other uses (50). 

Polysilicates are effective in potable HW systems and provide threshold effect protection against hardness destabilization and red water. They also function by laying down a protective calcium silicate film. Polysilicates used for this type of application typically require a dose rate of 8 to 15 ppm Si02 above that of the natural orthosilicate found in the MU water. Where polysilicates are employed for dualtemperature and LPHW systems, however, the silicate reserve required is higher and ranges from 50 to 150 ppm. 

Hardness stabilization, functioning at substoichiometric levels by crystal growth retardation, (threshold effect). This attribute is useful in low heat-flux HW applications. 

Sludge conditioners are required. Originally these were based on starches and lignins, but modem carbonate cycle treatments use carbonate-polymer programs, where the polymer (such as a phos-phinocarboxylic acid) provides a combination of threshold effect, crystal distortion, and sludge dispersion to minimize scaling and prevent sludge deposition. 

Phosphonates exhibit all the properties of polyphosphates, such as threshold effect, crystal distortion, and sequestration, but are superior in their effectiveness. They provide good chelates for calcium, magnesium, iron, and copper and are commonly used where iron fouling is a problem. Their sequestering properties are generally superior to other common chelants, such as EDTA and NTA. 

A temperature measuring device that reports data back to a recorder. Threshold effect ... 

Far less than the stoichiometric amount of sequestrants precipitation of insoluble salts from water hardness can be prevented by slowing down the formation of crystals and crystal growth. This process is called the threshold effect. It has long been used in the preparation of boiler feeding water, e.g., steam vessels of railroad engines. Originally sodium pyrophosphate was used for this task, but alkylphosphonic acids and derivatives thereof are superior in their effect. 

Indeed our scale makes it possible to make a more subtle comparison of risk levels, which allows us to classify substances in ascending risk order and can be directly interpreted (an ii value of 4.62% for instance means that with a vapour equilibrium concentration at 21 C in air this concentration cannot exceed 4.62% of LEL value, which is equivalent to the reading of the dial of an explosimeter). Note in particular that it is clearly seen by examining the tables for these substances that the same code 11 set by the regulations or 3 by NFPA hide very different risk situations, which is well shown by II code. Vice versa, the threshold effects of these codes tend to overestimate insignificant risk differences (for instance between ethylbenzene and styrene). 

Reference Dose (RfD)—An estimate (with uncertainty spanning perhaps an order of magnitude) of the daily exposure of the human population to a potential hazard that is likely to be without risk of deleterious effects during a lifetime. The RfD is operationally derived from the NOAEL (from animal and human studies) by a consistent application of uncertainty factors that reflect various types of data used to estimate RfDs and an additional modifying factor, which is based on a professional judgment of the entire database on the chemical. The RfDs are not applicable to non-threshold effects such as cancer. 

H2O, D2O and HOD with D detection at the Lyman-a wavelength excitation. Based on theoretical analysis, this single rotational state product propensity is attributed to a dynamically constrained threshold effect in the HOD photodissociation process.41... 

This model does not allow for a threshold effect, and thus may overstate the risk at low doses. It tends to yield higher estimates of human risk than the other models discussed below. 

Accommodation of competing hypotheses about threshold effects. 

Schwartz J, Landrigan PJ, Feldman RG, et al. 1988. Threshold effect in lead-induced peripheral neuropathy. J Pediatr 112 12-17. 

The number of affected individuals in subsequent generations should be higher than in autosomal dominant disease, again were it not for the threshold effect (see below). 

Because there are hundreds or thousands of copies of mtDNA in each cell, the phenotypic expression of a mitochondrially encoded gene depends on the relative proportions of mutant and wild-type mtDNAs within a cell this is termed the threshold effect . 

Threshold effects on lung function or morphology in several species of laboratory mammals occur at airbome nickel concentrations of 0.1 to 0.2 mg/m3, depending on nickel compound and duration of exposure. 

Uncertainty factor 3 no unusually susceptible populations were identified and the end point was a threshold effect. More severe headaches are known to occur in some patients medicated with other nitrate esters and the threshold for vasodilatation in the key study did not vary greatly among individuals. 

Uncertainty factor 3 severe headaches are known to occur in angina patients medicated with nitroglycerin and the threshold for vasodilatation does not vary greatly among individuals. The effect was also a threshold effect for central nervous systems depression (no change in cognitive abilities slight imbalance in one of several sensitive motor tests). Individual variation in susceptibility to central nervous system depressants such as anesthetics varies no more than 2-fold. 

Time scaling Not applied. Cardiac sensitization is an exposure and blood concentration related threshold effect. Several studies have shown that blood concentrations of halocarbons do not increase greatly with time after 15-55 min of exposure, and exposure duration did not influence the concentration at which the effect occurred. 

Time scaling Not applied inadequate data. Based on the rapidity with which blood concentrations approached equilibrium in human subjects, the similarity of lethality values in rats exposed for 4 or 6 h, and the fact that cardiac sensitization, the most sensitive end point in studies with halocarbons, is a concentration-dependent threshold effect, the 6-h value was used for all exposure durations. ... 

Mutagenicity and carcinogenicity are generally considered to be non-threshold effects, unless a non-genotoxic mechanism can be established with a NOEL (or NOAEL or LOAEL). Risk assessment is based on establishing whether exposure is prevented. A similar process of preventing exposure also applies for skin and respiratory sensitisers, since there is no means of identifying a dose or concentration below which adverse effects will not occur in someone already sensitised to a particular substance. 

Although no threshold effect was specified by Wayne et al., a later analysis by Hasselblad et al. suggested a threshold estimate of 235 Mg/m (0.012 ppm). A similar study was carried out over a shorter period by Koontz in Seattle, where oxidant concentrations were approx-... 

The two most critical factors in terms of air quality standards are duration of exposure and concentration. These two factors determine the exposure dose for a plant. In determining the response of vegetation, concentration is more important than time. A given dose presented to a plant in a short period has a greater effect than the same dose applied over a longer period. This suggests a threshold effect for plant populations and is probably related to the repair mechanisms inherent in bio-... 

The following are the major subjects of the book the various institutions, agencies, and programs involved in chemicals regulation (Chapter 2). The data for hazard assessment (Chapter 3) and the hazard assessment process, i.e., identihcation and characterization of the various toxicological effects and the associated test methods (Chapter 4). Standard setting for threshold effects (Chapter 5) and non-threshold effects (Chapter 6). Exposure assessment (Chapter 7) and risk characterization (Chapter 8). Regulatory standards set by various bodies (Chapter 9) and combined actions of chemicals in mixture (Chapter 10). 

For threshold effects, traditionally, a level of exposure below which it is believed that there are no adverse effects estimated, based on an approximation of the threshold termed the No-Observed-(Adverse)-Effect Level (NO(A)EL) and assessment factors this is addressed in detail in Chapter 5. This estimated level of exposure will in this book be termed tolerable exposure level. Examples, where this approach is used, include establishment of the Acceptable/Tolerable... 

---