Threshold similarity value

The dendrogram illustrates the cluster merging sequence and the corresponding values of the similarity measure employed. A threshold similarity value, selected by the user, splits the data into the perceived correct number of clusters. Generally, the threshold level should be chosen so that intercluster distances are considerably greater than the intracluster distances. 

By definition, any target structure that is a substructure (or superstructure) of the database molecule will have a subsimilarity (or supersimilarity) of 1.0, and hence a subsimilarity (supersimilarity) search can be viewed as a form of fuzzy substructure (superstructure) search. A further difference from the systems described thus far is in the nature of the output. The description of similarity searching that has been given previously assumes that a user browses down a ranked output until sufficient database structures have been identified. Here, conversely, the user specifies a threshold similarity value, and the output consists of an unordered list of all database structures (or database reactions) that have a similarity to the target that exceeds the chosen threshold value. 

Fig. 1.15 Cumulative distribution curve showing the fraction of possible edges formed as a function of similarity threshold value. The light grey dashed line corresponds to a threshold similarity value of 0.75...

The user specifies threshold similarity values for each of the two components of reaction similarity. For the candidate reaction to be considered a hit, both molecule and reaction-centre similarity values must exceed their respective user-specified thresholds ... 

Care ought to be taken in handling acetic acid to avoid spillage or otherwise breathing vapors. Wash any exposed areas with large amounts of water. Once the odor of acetic acid vapors is noticeable, the area should be abandoned immediately. The U.S. threshold limit value for acetic acid is 10 ppm (25 mg/m ). Similar values prevail in Germany (75). 

A commercially interesting low calorie fat has been produced from sucrose. Proctor Gamble has patented a mixture of penta- to octafatty acid ester derivatives of sucrose under the brand name Olestra. It was approved by the FDA in January 1996 for use as up to 100% replacement for the oil used in preparing savory snacks and biscuits. Olestra, a viscous, bland-tasting Hquid insoluble in water, has an appearance and color similar to refined edible vegetable oils. It is basically inert from a toxicity point of view as it is not metabolized or absorbed. It absorbs cholesterol (low density Hpoprotein) and removes certain fat-soluble vitamins (A, D, E, and K). Hence, Olestra has to be supplemented with these vitamins. No standard LD q tests have been performed on Olestra however, several chronic and subchronic studies were performed at levels of 15% in the diet, and no evidence of toxicity was found. No threshold limit value (TLV), expressed as a maximum exposure per m of air, has been estabhshed, but it is estimated to be similar to that of an inert hpid material at 5 mg/m. ... 

The threshold limit value—time integrated average, TLV—TWA, of chlorine dioxide is 0.1 ppm, and the threshold limit value—short-term exposure limit, STEL, is 0.3 ppm or 0.9 mg /m of air concentration (87,88). Chlorine dioxide is a severe respiratory and eye irritant. Symptoms of exposure by inhalation include eye and throat irritation, headache, nausea, nasal discharge, coughing, wheezing, bronchitis, and delayed onset of pulmonary edema. Delayed deaths occurred in animals after exposure to 150—200 ppm for less than one hour. Rats repeatedly exposed to 10 ppm died after 10 to 13 days of exposure. Exposure of a worker to 19 ppm for an unspecified time was fatal. The ingested systemic effects of low concentration chlorine dioxide solutions are similar to that of chlorite. 

Similar values have been obtained for AHffMesSi ) from two independent studies. The bond dissociation enthalpy DHfMeaSi-SiMea) = 332 +12 kJ moC was obtained from a kinetic study on the very low pressure pyrolysis of hexamethyldisilane and the enthalpy of formation of trimethylsilyl ion, AHf (MeaSi ) = 617.3 + 2.3kJmor, was determined using threshold photoelectron-photoion coincidence spectroscopy (TPEPICO). Both data are related to AHf°(Me3Si ). 

Application of a 0.1M solution of yttrium chloride to the eyes of rabbits caused no injury similar exposure of eyes from which the corneal epithelium had been removed resulted in immediate slight haziness of the cornea, which subsequently became opaque and vascularized. The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) for yttrium and compounds is 1 mg/m as Y. 

If spilled on the skin or in the eyes liquid N2IL, can cause severe local damage or burns and can cause dermatitis. In addition it can penetrate skin to cause systemic effects similar to those produced when the compound is swallowed or inhaled. Inhalation of the vapor causes local irritation of the respiratory tract and eyes. On short exposure systemic effects involve the central nervous system. Resultant symptoms include tremors, on exposure to higher concentrations, convulsions and possibly death follow. Repeated exposures may cause toxic damage to the liver (fatty liver) and kidney (interstitial nephritis), as well as anemia. The threshold limit value of hydrazine is 1 ppm (1.3 mg/m3)(Ref 20) (See also Refs 27 c and 33a)... 

For occupational exposure to chemicals as opposed to environmental exposure, other parameters such as threshold limit values (or maximum exposure limits) are determined in a similar way and are based on exposure for an eight-hour working day. 

The American Conference of Governmental Industrial Hygienists (ACGIH) (1997) has recommended 5 iiig/m as the 8-h time-weighted average threshold limit value for occupational exposures to benzoyl peroxide in workplace air. Similar values have been used as standards or guidelines in many countries (International Labour Office, 1991). 

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