|【Organo-fluorine Compounds: PFOS・PFOA】|
Hyogo Prefectural Institute of Public Health and Environmental Sciences
– physicality, environmental pollution and toxicity –
Organo-fluorine Compounds, such as PFOS (Perfluorooctanesulfonic acid) and PFOA (Perfluorooctanoic acid) and their chlorides belong to fluorine-containing surfactants. PFOS is a sulfone acid type of surfactant, while PFOA is a carboxylic acid type. Surfactants are composed of the hydrophilic group, the hydrophobic group and the lipophilic group within an identical molecule. It is one type of amphiphatic organic compound, which can change surface physical-chemical properties, including the decrease in surface tension, by adsorbing at the boundaries of the liquid and gas phases with a different polarity and liquid phase. Surfactants demonstrate effective activities, such as detergency, emulsification, dispersion, solubilization and moistening with this physicality. Many types of surfactants are present by combining the hydrophilic group and the hydrophobic group. Using the dissociation state of the hydrophilic group in water, surfactants are classified into the following four groups: anionics, cationics, amphoterics and nonionics.
PFOS and PFOA are anionics and have the structure consisting of a fluorine atom with which all hydrogen atoms from the linear-alkyl chain, which is a hydrophobic group, are replaced. Since PFOS and PFOA are surfactants, they are soluble in water and oil (aqueous solubility: PFOS 570mg/L，PFOA 3,400mg/L). The van der Waals radius of a fluorine atom is higher than that of a hydrogen atom, so that the carbon atoms of the hydrophobic alkylic framework are coated and the chemical stability of the module is increased. Also, the free energy of the fluorocarbon hydrophobic group (Rf group) is generally smaller than that of the hydrocarbon hydrophobic group (R group). For example, if they cover the water surface as a surfactant, the surface free energy becomes smaller and the surface tension is significantly decreased. Therefore, critical micelle concentration, which is a rough indication of the limitation of detergent action, becomes much lower than hydrocarbon-containing surfactants, which have the same carbon numbers and can demonstrate effective surface activities even with a low concentration.
Since PFOS and PFOA have properties such as strong water repellency, oil repellency, chemical stability, thermal stability, chemical resistance and non-adherence, they are used for home and industry as fiber textiles, paper, water proofing repellents for leather product, oil repellence processing, stain-resistance processing, coating agents for nonstick cookware, mold-releasing agents, coating agents for semiconductor materials and aircraft hydraulic liquids. Particularly, PFOA is used as an emulsifier when polytetrafluoroethylene (Teflon) is produced, which is an organofluorine polymer.
There are two methods to synthesize the fluorocarbon framework of PFOS and PFOA. One method is the electrochemical fluorination method of octyl (actually, fluorination of n-octanesulfonic acid and n-octanoic acid), which is an alkyl group. The other is the telomerization (a polymerization reaction that retains a degree of polymerization much lower than standard polymerization) of tetrafluoroethylene monomer (C2F4). The low-molecular polymer generated is called telomere. These synthetic methods, especially in the electrochemical fluorination method, possess a branched-chain or produce a variety of similar impurities which have different carbon numbers. Furthermore, Organo-fluorine compounds, which have carbon numbers different from PFOS and PFOA, are produced. Consequently, many derivatives and similar chemical compounds have been detected in the environment. The similar chemical compounds, which possess the perfluoroalkyl group including PFOS and PFOA, have been generically named PFCs.
PFOS and PFOA have been produced by the DuPont and 3M U.S. companies since about 1950. However, regarding the environmental pollution problems caused by PFOS and PFOA, highly concentrated PFOS was found in the human sera of those who work for 3M (range: ND-13 mg/L，N=327) as publicized in a paper in 1999 by 3M, which is known for its waterproof spray “Scotchgard”. In 2000, “Cease PFOS production by 2002” was decided, which has drawn attention. Afterwards, environmental surveys and human blood tests were implemented in Canada, the UK, Sweden, Germany and Australia. It was clear that high concentrations in the environment including animals in polar areas and humans (PFOS: roughly 10μg/L，PFOA: roughly μg/L) are widespread. PFOS and PFOA are persistent. Persistence and ecological magnification were found. By accumulating toxicity data, PFOS and PFOA have been examined if they will be added to the list of POPs (Persistent Organic Pollutants) of the Stockholm Convention.
In Japan, low concentrations (2-20μg/L, N=26) of PFOS were detected from a few residents in 2002 (PFOA: under the determination limit). Additionally, surveys conducted at more than a hundred domestic rivers and coastal areas demonstrated the detection of PFOS: 0.2-530ng/L，PFOA: 0.2-67,000ng/L in 2003, so that the existence of areas of high concentration and widespread pollution could found. These high concentration areas were focused in the Kansai region. Afterwards, human blood and the concentration in running water were examined. In Kyoto, Osaka and Hyogo, especially high PFOA levels (approx. 10μg/L) were confirmed. These results and those of new river surveys were published in the newspapers in May, 2007. Subsequently, Hyogo and Osaka prefectures conducted a large scale survey with respect to rivers, coastal areas, underground water and discharge within Hyogo and Osaka. The results indicated that high PFOA (maximum level: 670ng/L) was partly detected in the Kanzaki-Ai River System, which runs through Osaka and Hyogo. This kind of partial pollution was also found in the Kanto region.
With respect to the pollutant sources of PFOS and PFOA, the environmental research in Japan and in each country reported that emissions from sewage-treatment and manufacturing plants are significant. In addition, it is presumed that generation of substances related to fluorotelomer alcohol by environmental microbial metabolism is also associated.
There are many reports on the toxicity of PFOA. Acute toxicity is not very strong (LD50: approx. 500mg/kg). Tests using rats demonstrated that carcinogenicity, such as hepatic tumors and pancreatic tumors, as well as hepatic disorder, lipid metabolic disorder and developmental disorder were observed. Mechanisms associated with fatty acid metabolism by linking to the peroxysome proliferator activated receptors likely to cause hepatotoxicity. Immunotoxicity was also reported. DuPont and 3M stated that no significant influence to hazardous property on humans was found, based on a long-term epidemiologic survey concerning many workers who work at affected factories. However, recently, knowledge is being accumulated, such as a report on the significantly negative correlation between PFOS and PFOA concentrations in cord serum, and nursing infants' weight and head circumference. The Risk Assessment Committee of the U.S. EPA described in their report that “Human carcinogenicity is suggested”. The U.S. EPA established concentration levels against PFOA in drinking water at 500ng/L and ordered DuPont to address levels higher than that.
With respect to the PFOS and PFOA issue, a large number of fines and a string of lawsuits against DuPont and 3M by citizens owing to a violation of reports on health risks set under the US Toxic Substances Control Act took place. Consequently, the 2015 Complete Control Program of PFOA and 8 major companies which produce PFOA related substances, including DuPont, 3M, Asahi Glass, Daikin was started in the US. Controlling PFOS and PFOA has been examined in the EU, as well as in many countries. In Japan, at the present time, PFOS and PFOA are only specified as class 2 chemical substances based on the Law Concerning the Examination and Regulation of Manufacture of Chemical Substances.