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Environmental fate & pathways

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Chloromethane enters the environment from natural and industrial sources but the main entrance of chloromethane to the environment is natural. The contribution from natural sources has been estimated to be well over 90%, and may reach 99%, of the total release (CICAD, 2000), the anthropogenic entrance is only 10% or less.Therefore, the Industrial activities add only a small, insignificant (less than 1%) amount of chloromethane to the ambient air levels relative to all of the non-industrial sources. Natural production is estimated to be around 4000 ktonne/yr (Harper, 2000). This is about a factor of eight times higher than the total global anthropogenic production, which was estimated to be 158 ktonne for Europe and 497.7 ktonne/yr globally (EuroChlor, 2006). It is well known that bacteria, fungi, plants (esp. potatoes (Varns, 1982) and conifers (Gribble, 2006)) and marine organisms produce chloromethane constantly in high quantities from natural processes due to the widespread availability of salt and organic matter. Also forest fires or agricultural biomass, and volcano eruptions lead to enormous natural emissions of chloromethane (CICAD, 2000).

According to the ICCA HPV-SIDS from 2004, hydrolysis of chloromethane in water is relatively slow with a half-life ranging from 62 days to about 1.1 year, but chloromethane is a gas, most industrial releases would be expected to be to the air and the hydroxyl radical atmospheric half-life is estimated to be approximately one year. Any releases to surface water or surface soil would be expected to evaporate immediately because the values of its Henry’s law constant and vapour pressure are high, which suggests that volatilization of chloromethane, will be significant in surface waters. The results of the Mackay Level III (SIDS, 2004) also indicate that the environmental compartment of concern is air (> 99%).

Therefore, the principal sink for chloromethane in the troposphere is chemical reaction with hydroxyl radicals. Surface deposition and washout are unimportant sinks for chloromethane (Graedel & Keene, 1995; CICAD, 2000). The measured octanol/water partition coefficient (log Kow of 0.91) is low, indicating a low potential for bioaccumulation and low tendency of adsorption to soil and sediment. The substance is readily biodegradable.