Estimated half-lives

Figure 2 Typical log-transformed decays of DFR for estimating half-lives of methio-carb (o) and thiophanate-methyl (+).

Dislodgeable foliar residue as a function of time and estimating half-lives of the pesticides after high-volume application... 

Estimated half-lives for chlorinated hydrocarbon pesticides vary widely [16, 22] Aldrin, 1-9 Dieldrin, 3-7 Chlordane, 1-8 Heptachlor, 1-4 and DDT, 3-10 years. Half-lives for PCBs range from one year to 16 years [23]. Other types of pesticides, e.g. organophosphates, triazines, carbamates and ureas, are generally less persistent [16, 24],... 

Biological. Based on aerobic soil die away test data at 10 to 30 °C, the estimated half-lives ranged from 590 to 650 d (Coover and Sims, 1987). 

Surface Water. Estimated half-lives of chlorobenzene (1.0 tig/L) from an experimental marine mesocosm during the spring (8-16 °C), summer (20-22 °C), and winter (3-7 °C) were 21, 4.6, and 13 d, respectively (Wakeham et al, 1983). 

Soil. The estimated half-lives of dibenzofuran in soil under aerobic and anaerobic conditions were 7 to 28 and 28 to 112 d, respectively (Lee et al., 1984). 

Soil. The major soil metabolite is 2,6-dichlorobenzamide which degrades to 2,6-dichloro-benzoic acid. The estimated half-lives ranged from 1 to 12 months (Hartley and Kidd, 1987). Under field conditions, dichlobenil persists from 2 to 12 months (Ashton and Monaco, 1991). The disappearance of dichlobenil from a hydrosol and pond water was primarily due to volatilization and biodegradation. The times required for 50 and 90% dissipation of the herbicide from a hydrosol were approximately 20 and 50 d, respectively (Rice et al., 1974). Dichlobenil has a high vapor pressure and volatilization should be an important process. Williams and Eagle (1979) found... 

In addition to ARSAC approval, the protocol must also be approved by ethics committees in the normal manner for studies in man. The study should be conducted in between four and eight consenting subjects, in facilities where any spills of radiolabelled materials can be contained and monitored. Normally, subjects will be required to provide blood samples and to collect all excreta for a period determined by the known or estimated half-lives of the parent compound and metabolite. With cooperative subjects, recoveries of radioactivity should be close to 100%. Samples will be assayed for radioactivity and by cold chromatographic methods, and every attempt should be made to identify major metabolites... 

Figure 3.6 Estimated half-lives of triclosan in various environmental matrices. (Source Halden and Pauli, 2005.)...

Polybrominated Biphenyls. In air, the two processes that may result in significant degradation or transformation of PBBs are photooxidation by hydroxyl radicals and direct photolysis. The estimated half-life of pentachlorobiphenyl in air due to reaction with hydroxyl radicals is 41.6 83.2 days (Atkinson 1987a). Based on a structure-activity relationship for the estimation of half-lives for the gas-phase reactions of hydroxyl radicals with organic compounds (Atkinson 1987b), the estimated half-lives of hexabromobiphenyl and decabromobiphenyl due to reaction with OH radicals are 182 and 2,448 days, respectively. These half-lives are consistent with the half-life of pentachlorobiphenyl due to reaction with OH radicals. However, the half-lives of brominated biphenyls expected to be present in the particulate phase in the air may be even longer than the estimated half-lives due to gas phase reaction. Therefore, the Iransfonnation of the hexa- and other higher brominated PBBs in the atmosphere due to reaction with OH radicals may not be irrportant. 

The experiments discussed above determined the rate of disappearance and the half-life of elemental phosphorus in water in open systems. The phosphorus in these experiments disappeared due to hydrolysis/oxidation and evaporation. Spanggord et al. (1985) studied the loss of elemental phosphorus in sealed reaction flasks. In a closed reaction flask with argon-saturated water, the loss of white phosphorus can only be due to hydrolysis. The estimated half-life for hydrolysis at ambient temperatures was 84 hours (Spanggord et al. 1985). The estimated half-lives of white phosphorus at ambient temperatures due to a... 

There are limited data available on the elimination of other CDD congeners in humans. In the Flesch-Janys et al. (1996) study of 48 workers at a German pesticide facility, elimination half times were estimated for several CDD congeners. The estimated half-lives were 15.7 years for 1,2,3,7,8-PeCDD, 8.4 years for 1,2,3,4,7,8-HxCDD, 13.1 years for 1,2,3,6,7,8-HxCDD, 4.9 years for 1,2,3,7,8,9-HxCDD, 3.7 years for 1,2,3,4,6,7,8-HpCDD, and 6.7 years for OCDD. In a study of six German workers with high... 

Hydrolysis not expected to be significant based on estimated half-lives for hydrolysis of acetamide of 261, 3950, and 46 yr at pH 5,7,9, respectively, which were calculated using experimental acid and base hydrolysis rate constants for acetamide (Mabey Mill 1978 selected, Howard et al. 1991). 

Biodegradation estimated half-lives of 14 h in river, eutrophic pond, eutrophic lake and > 103 h in oligotrophic lake by the one compartment model (Smith et al. 1978). 

Volatilization estimated half-lives, L, = 51.6 h in streams, ty, = 65.37 h in rivers and ty, = 3104.3 h in lakes were estimated by using Henry s law constant and assuming the wind velocity to be 3 m/s, the current velocities of the streams, rivers, and lakes 2,1, and 0.01 m/s with depths of lakes 50 m and that of the streams and lakes 1 m deep (Lyman et al. 1982 quoted, Howard 1990). 

Atmospheric fate Cresols are not expected to persist in the atmosphere because (1) cresols have low estimated half-lives (less than 1 day) (2) they are sensitive to photolysis and (3) the water solubility of cresols may cause transport of cresols from the atmosphere to the soil or aqueous environment. The photodegradation half-life of cresol isomers during the daytime is 8-10 h while at night it is 2 min. Daytime half-lives would be reduced under smog conditions. Cresols are highly soluble compounds, and gas scavenging will be an efficient removal process as is reflected by high concentrations in rain. 

Indole will mainly exist in the vapor phase and is degraded in the atmosphere by reaction with photo-chemically produced hydroxyl radicals, nitrate radicals, and ozone with estimated half-lives of 2-3 h, < 1 min, and 6 h, respectively. Indole is expected to readily biodegrade under both aerobic and anaerobic conditions in soil and water. Bioconcentration in aquatic organisms should be low, given an estimated bioconcentration factor value of 25. 

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