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Octachlorostyrene(OCS) is a persistent, highly bioaccumulative, and toxic halogenated aromatic compound. It is not commercially manufactured, but is reportedly an inadvertent by-product of processes that combine carbon and chlorine under elevated temperatures. Magnesium production, chloride solvent production, aluminum plasma etching, aluminum degassing with hexachloroethane, chlorination of titanium, waste incineration, and chloro-alkali production with graphite anodes process are considered to be candidate occupations that produce OCS. However, recent advances have been made in process technology and pollution prevention practices in some of these industries, such as largely eliminating the electrolytic manufacture of chlorine and aluminum degassing with hexachloroethane, both of which have likely resulted in reductions in known sources of OCS.

OCS is used in pesticide products to increase the effectiveness of the active ingredients, to make the product easier to apply, or to allow several active ingredients to mix in one solution.

Perchlorostyrene is a component used in pesticide formulations.

ChEBI: Octachlorostyrene is an organochlorine compound.

Moderately toxic by ingestion.When heated to decomposition it emits toxic vapors ofClí.

OCS is bioaccumulative in aquatic food webs. Due to its low water solubility (water solubility = 1.74E-03 mg l^-1; log P (octanol–water) = 7.460), OCS tends to rapidly partition from water and binds to sediments and suspended solids. Bioconcentration through direct uptake may be an important mechanism in aquatic species.
In aquatic systems, OCS is expected to adsorb to suspended solids and sediments based on its Koc value ranging from 200 000 to 10 000 000. OCS has been detected in water at concentrations as high as 7.2 ng l^-1, but levels typically are well below 1 ng l^-1. While there is the potential for volatilization from aquatic systems based on an estimated Henry’s law constant of 2.3 ×10^-4 atm m³ mol^-1, volatilization (vapor pressure= 1.32E-05mmHg) is likely attenuated by adsorption to particles. Bioaccumulation by aquatic organisms is likely based on a bioconcentration factor that is estimated to range from 8100 to 33 000. Field estimates of bioaccumulation factors range up to 1 400 000 (from water to rainbow trout in Lake Ontario). Mean concentrations in Lake Ontario sediments and rainbow trout were 13.6 ng g^-1 dry weight (ppb) and 2.6 ng g^-1 wet weight (ppb), respectively. The highest concentrations found in fish as part of the National Study of Chemical Residues in Fish (conducted by the US Environmental Protection Agency (EPA)) were from Bayou D’Inde, Louisiana (138 ng g^-1 (ppb)), Freeport, Texas (65.3 ng g^-1 (ppb)), River Rouge, Michigan (50.7 ng g^-1 (ppb)), and Olcott, New York (49.6 ng g^-1 (ppb)). Temporal studies, while limited, have indicated a substantial decline in concentrations of OCS since the 1970s. In contrast, relatively low OCS levels in freshwater mussels and fish from Belgium and Romania ranged from 0.01 to 0.18 ng g^-1 wet weight (ppb), and those in marine fish (bib, sole, and whiting) ranged from 0.01 to 0.02 ng g^-1 wet weight (ppb).
In terrestrial systems, OCS is expected to bind to soil particles. In the atmosphere, OCS (in the vapor phase) is degraded by reactions with photochemically produced hydroxyl radicals. OCS weakly absorbs ultraviolet light between 295 and 310 nm with slow photolysis. Major transformation products of photolysis include heptachlorostyrene and two isomers of hexachlorostyrene, while minor transformation products of photolysis include pentachlorostyrene and tetrachlorostyrene.
Monitoring of OCS in western Hudson polar bears showed no change during 1991–2007. This suggests the persistency of OCS in the environment, though it did not further accumulate.

The mechanisms of toxicity and the human toxicological properties of OCS have not been well characterized. Exposure to OCS decreased GSH, increased reactive oxygen species and cytosolic caspase-3 activation in human Chang liver cells, and led to cell death. These results suggest that the toxicity in cells may be via apoptotic processes.