Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) refers to chemical compounds that contain the element chromium in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately 136,000 tonnes (300,000,000 lb) of hexavalent chromium were produced in 1985. Additional hexavalent chromium compounds are chromium trioxide and various salts of chromate and dichromate, among others. Hexavalent chromium is used in textile dyes, wood preservation, anti-corrosion products, chromate conversion coatings, and a variety of niche uses. Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating. Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state. Hexavalent chromium can also be found in drinking water and public water systems.
Inhaled hexavalent chromium is recognized as a human carcinogen. Workers in many occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products and those who grind and/ or weld stainless steel. Workers who are exposed to hexavalent chromium are at increased risk of developing lung cancer, asthma, or damage to the nasal epithelia and skin. Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive.
Video Hexavalent chromium
Toxicity
Hexavalent chromium compounds are genotoxic carcinogens. Due to its structural similarity to sulfate, chromate (a typical form of chromium(VI) at neutral pH) is transported into cells via sulfate channels. Inside the cell, hexavalent chromium (chromium(VI)) is reduced first to pentavalent chromium (chromium(V)) then to trivalent chromium (chromium(III)) without the aid of any enzymes. The reduction occurs via direct electron transfer primarily from ascorbate and some nonprotein thiols. Vitamin C and other reducing agents combine with chromate to give chromium(III) products inside the cell. The resultant chromium(III) forms stable complexes with nucleic acids and proteins. This causes strand breaks and Cr-DNA adducts which are responsible for mutagenic damage. According to Shi et al., the DNA can also be damaged by hydroxyl radicals produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell, which can cause double-strand breakage.
Both insoluble salts of lead and barium chromates as well as soluble chromates were negative in the implantation model of lung carcinogenesis. Yet, soluble chromates are a confirmed carcinogen so it would be prudent to consider all chromates carcinogenic.
Chronic inhalation from occupational exposures increases the risk of respiratory cancers. The most common form of lung malignancies in chromate workers is squamous cell carcinoma. Ingestion of chromium(VI) through drinking water has been found to cause cancer in the oral cavity and small intestine. It can also cause irritation or ulcers in the stomach and intestines, and toxicity in the liver. Liver toxicity shows the body's apparent inability to detoxify chromium(VI) in the GI tract where it can then enter the circulatory system.
Of 2,345 unsafe products in 2015 listed by the EU Commission for Justice, Consumers and Gender Equality some 64% came from China, and 23% were clothing articles, including leather goods (and shoes) contaminated with hexavalent chromium. Chromate-dyed textiles or chromate-tanned leather shoes can cause skin sensitivity.
In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3). The U.S. National Institute for Occupational Safety and Health proposed a REL of 0.2 µg/m3 for airborne exposures to hexavalent chromium.
For drinking water the United States Environmental Protection Agency (EPA) does not have a Maximum Contaminant Level (MCL) for hexavalent chromium. California has finalized a Public Health Goal of 0.02 parts per billion (ppb or micrograms per liter) and established a MCL of 10 ppb.
Maps Hexavalent chromium
Remediation of hexavalent chromium in groundwater and drinking water
There are mainly three types of methods to remediate hexavalent chromium in ground water and drinking water: 1) reduction of toxicity, 2) removal technologies and 3) containment technologies. Reduction of toxicity of hexavalent chromium involves methods using chemicals, microbes and plants. Some removal technologies include transporting contaminated soil offsite to a landfill, using ion exchange resins to reduce chromium(VI) concentrations to less than detectable limit and granular activated carbon (GAC) filter. Containment technologies can be employed with the use of physical barriers such as grouts, slurries or sheet piling. Attempts have been made to test the removal or reduction of hexavalent chromium from aqueous solutions and environment. For example, a research study conducted by the School of Industrial Technology, University Sains Malaysia in 2010 found that chitosan coated with poly 3-methyl thiophene can be effectively employed for removal of hexavalent chromium ions from aqueous solutions. Chitosan is a very cheap, economical, and environmentally friendly substrate for coating of this polymer. Adsorption of chromium(VI) is found to be effective in the lower pH range and at higher temperatures and subsequent desorption is readily achieved upon alkaline treatment of the adsorbent. Another study done by the American Industrial Hygiene Association indicates that the airborne hexavalent chromium in acidic mists of an electroplating tank collected on PVC filters was reduced over time after mist generation.
Exposure and safety issues
Hexavalent chromium is a constituent of tobacco smoke.
Australia
Kooragang Island, New South Wales
Hexavalent chromium was released from the Newcastle Orica Kooragang Island ammonium nitrate plant on August 8, 2011. The incident occurred when the plant entered the 'start up' phase after the completion of a five-yearly maintenance overhaul. The "High Temperature Shift catalyst began the process of 'reduction'" where steam passes through the catalyst bed and out the SP8 vent stack. At this time lower temperatures in parts of the plant caused some of the steam to condense lower which caused chromium(VI) from the catalyst bed to dissolve into the liquid present. The amount of condensate overwhelmed the drainage arrangements resulting in the emission of condensate through the SP8 vent stack. The leak went undetected for 30-minutes releasing 200 kg of chromium(VI) into the atmosphere exposing up to 20 workers at the plant and 70 nearby homes in Stockton.
The town was not notified of the exposure until three days later on Wednesday morning. This accident sparked a major public controversy, with Orica criticized for playing down the extent and possible risks of the leak.
The office of Environment and Heritage in Stockton collected 71 samples. Low levels of chromium were detected in 11 of them. These 11 samples were taken within six residential blocks close to the Orica plant, two of which were from water samples collected immediately south of the six block area.
The Select Committee on the Kooragnang Island Orica Chemical Leak released their report on the incident in February 2012. They found that Orica's approach for addressing the leak's potential impact grossly inadequate. Orica failed to realize the potential impact that prevailing winds would have on an emission 60 meters high. Orica failed to inspect the area immediately downwind and notify the Office of Environment and Heritage until August 9, 2011. In Orica's initial report to the Office of Environment and Heritage they failed to disclose that the emissions had escaped off-site. In the initial report to WorkCover Orica did not disclose potential impacts on workers as well as that the substance emitted was chromium(VI). Orica's Emergency Response plan was not well understood by employees particularly about notification procedures. The original notification of residents in Stockton was only to households immediately downwind of the emission which failed to realize the potential for contamination of the surrounding area as well. The information presented at the original notification downplayed potential health risks and subsequently provided incomplete information and has led to a lack of trust between Stockton residents and Orica officials.
In 2014, Orica plead guilty to nine charges before the Land and Environment court and was fined $768,000. NSW Health findings ruled that it is very unlikely that anyone in Stockton will develop cancer as a result of the incident.
Bangladesh
Toxic poultry feed contaminated by chromium-based leather tanning (as opposed to traditional slower vegetable tanning) is entering the food supply in Bangladesh through chicken meat, which is the most common source of protein in the country. Tanneries in Hazaribagh Thana, an industrial neighborhood of Dhaka, emit around 21,600 cubic meters of toxic waste each day. The tanneries also generate as much as 100 tonnes per day of scraps, trimmed raw hide, flesh and fat, which are processed into feed by neighborhood recycling plants and used in chicken and fish farms across the country. Chromium levels ranging from 350 to 4,520 micrograms (0.35 to 4.52 mg) per kilogram were found in different organs of chickens which had been fed the tannery-scraps feed for two months, according to Abul Hossain, a chemistry professor at the University of Dhaka. The study estimated up to 25% of the chickens in Bangladesh contained harmful levels of chromium(VI).
Greece
Eastern Central Greece
The chemistry of the groundwater in eastern Central Greece (central Euboea and the Asopos valley) revealed high concentrations of hexavalent chromium in groundwater systems sometimes exceeding the Greek and the EU drinking water maximum acceptable level for total chromium. Hexavalent chromium pollution here is associated with industrial waste.
By using the GFAAS for total chromium, diphenylcarbazide-Cr(VI) complex colorimetric method for hexavalent chromium, and flame-AAS and ICP-MS for other toxic elements, their concentrations were investigated in several groundwater samples. The contamination of water by hexavalent chromium in central Euboea is mainly linked to natural processes, but there are anthropogenic cases.
Thebes - Tanagra - Malakasa (Asopos) Basin
In the Thebes - Tanagra - Malakasa basin, Eastern Central Greece, which supports many industrial activities, concentrations of chromium (up to 80 ?g/L Cr(VI)) and Inofyta (up to 53 ?g/L Cr(VI) were found in the urban water supply of Oropos). Chromium(VI) concentrations ranging from 5 to 33 ?g/L Cr(VI) were found in groundwater that is used for Thiva's water supply. Arsenic concentrations up to 34 ?g/L along with chromium(VI) levels up to 40 ?g/L were detected in Schimatari's water supply. In the Asopos River, total chromium values were up to 13 ?g/L, hexavalent chromium was less than 5 ?g/L and other toxic elements were relatively low.
Iraq
In 2008, defense contractor KBR was alleged to have exposed 16 members of the Indiana National Guard, as well as its own workers, to hexavalent chromium at the Qarmat Ali water treatment facility in Iraq in 2003. Later, 433 members of the Oregon National Guard's 162nd Infantry Battalion were informed of possible exposure to hexavalent chromium while escorting KBR contractors. One of the National Guard soldiers, David Moore, died in February 2008. The cause was lung disease at age 42. His death was ruled service-related. His brother believes it was hexavalent chromium. On November 2, 2012, a Portland, Oregon jury found KBR negligent in knowingly exposing twelve National Guard soldiers to hexavalent chromium while working at the Qarmat Ali water treatment facility and awarded damages of $85 million to the plaintiffs.
United States
History of the EPA's chromium policies in the United States
Prior to 1970, the federal government had limited reach in monitoring and enforcing environmental regulations. Local governments were tasked with environmental monitoring and regulations, such as the monitoring of heavy metals in wastewater. Examples of this can be seen in larger municipalities such Chicago, Los Angeles, and New York. A specific example was in 1969, the Chicago Metropolitan Sanitary District imposed regulations on factories that were identified as having large amounts of heavy metal discharge.
On December 2, 1970 the Environmental Protection Agency (EPA) was formed. With the formation of the EPA, the federal government had the funds and the oversight to influence major environmental changes. Following the formation of the EPA, the United States saw groundbreaking legislation such as the Clean Water Act (1972) and the Safe Drinking Water Act (1974).
The Federal Water Pollution Control Act (FWPCA) of 1948 was amended in 1972 to what is more commonly known as the Clean Water Act (CWA). The subsequent amendments provided a basis for the federal government to begin regulating pollutants, implement wastewater standards, and increase funding for water treatment facilities among other things. Two years later, in 1974, the Safe Drinking Water Act (SDWA) was passed by congress. The SDWA aimed to monitor and protect the United States drinking water, and the water sources it is drawn from.
As part of the SDWA, in 1991, the EPA placed chromium under its list of maximum contaminant level goals (MCLG), to have a maximum contaminant level (MCL) of 100 ppb. In 1996, the SDWA was amended to include a provision known as the Unregulated Contaminant Monitoring Rule (UCMR). Under this rule, the EPA issues a list of 30 or less contaminants that aren't normally regulated under the SDWA. Chromium was monitored under the third UCMR, from January 2013 through December 2015. The EPA uses data from these reports to assist in making regulatory decisions.
Current policies in the United States
The current EPA standard in measuring chromium, is in reference to total chromium, both trivalent and hexavalent. However, preliminary findings from water tests conducted by the Environmental Working Group (EWG), suggest that chromium is most often found in the hexavalent form; the more toxic of the two. Often trivalent and hexavalent chromium are mentioned together, when in fact they are very different from each other. This is an issue because hexavalent chromium can cause cancer, whereas trivalent chromium does not.
In 1991, the MCL for chromium exposure was set based on potential of "adverse dermatological effects" related to long-term chromium exposure. Chromium's MCL of 100 ppb has not changed since its 1991 recommendation. In 1998 the EPA released a toxicological review of hexavalent chromium. This report examined current literature, at the time, and came to the conclusion that chromium was associated with various health issues.
Before the EPA can adjust the policy on chromium levels in drinking water, they have to release a final human health assessment. The EPA mentions two specific documents that are currently under review to determine whether or not to adjust the current drinking water standard for chromium. The first study the EPA mentioned that is under review is a 2008 study conducted by the Department of Health and Human Services National Toxicology Program. This study looks at chronic oral exposure of hexavalent chromium in rats, and its association with cancer. The other study mentioned is a human health assessment of chromium, titled Toxicological Review of Hexavalent Chromium. The final human health assessment is currently in the stage of draft development. This stage is the first of seven. The EPA gives no forecast to when the review will be finalized and if a decision will be made.
Pending regulations in the United States
In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California's proposed limits. The EPA currently limits total chromium in drinking water to 100 parts per billion, but there is no established specifically for chromium(VI). In the same year, the California Environmental Protection Agency had proposed a goal of 0.2 parts per billion, despite a 2001 state law requiring a standard be set by 2005. A final Public Health Goal of 0.02 ppb was established in July 2011.
As of August 2016, the EWG alongside activist Erin Brockovich is pushing for the EPA to change the nationwide MCL to be more reflective of the California standards. This response comes as result of the resignation of a top state health official in North Carolina. Governor Pat McCrory had been attempting to remove warnings against drinking water potentially contaminated by hexavalent chromium from Duke Energy coal burning facilities. The confusion on the regulating guidelines and health safety of hexavalent chromium within drinking water has led to multiple standards. The EWG's push for stricter national guidelines is done in hopes to eliminate the confusion among citizens and state officials as well as to protect the health of the population from this carcinogen.
The EWG released an analysis in September 2016 of federal data from nationwide drinking water tests which showed that seventy-five percent of samples contained hexavalent chromium at levels higher than the California public health goal of 0.02 ppb. The EWG estimates that the US will see 12,000 excess cases in cancer by the end of the century if hexavalent chromium exposure in drinking water is not reduced.
California
Davenport
Monterey Bay Unified Air Pollution Control District monitored airborne levels of hexavalent chromium at an elementary school and fire department, as well as the point-source. They concluded that there were high levels of hexavalent chromium in the air, originating from a local cement plant, called Cemex. The levels of hexavalent chromium were 8 to 10 times higher than the air district's acceptable level at Pacific Elementary School and the Davenport Fire Department. The County of Santa Cruz sought help of the Health Services Agency (HSA) to investigate the findings of the Air District's report. Cemex voluntarily ceased operations due to the growing concern within the community, while additional air samples were analyzed. The HSA worked with Cemex to implement engineering controls, such as dust scavenging systems and other dust mitigation procedures. Cemex also made a change in the materials they used, trying to replace current materials with materials lower in chromium. The HSA also monitored the surrounding schools to determine if there were any health risks. Most schools came back with low levels, but in the case of higher levels a contractor was hired to clean up the chromium deposits. This case highlights the previously unrecognized possibility that hexavalent chromium can be released from cement-making.
Paramount
Air quality officials are taking enforcement action against two metal-processing plants they believe are contributing to hexavalent chromium emissions in Paramount, California.
Hinkley
Hexavalent chromium was found in drinking water in the southern California town of Hinkley and was brought to popular attention by the involvement of Erin Brockovich and Attorney Edward Masry. The source of contamination was from the evaporating ponds of a PG&E (Pacific Gas and Electric) natural gas pipeline Compressor Station located approximately 2 miles southeast of Hinkley. Between 1952 and 1966, chromium(VI) was used to prevent corrosion in the cooling stacks. The wastewater was then dumped into the unlined evaporating ponds. As a result, the chromium(VI) leaked into the groundwater source. The 580 ppb chromium(VI) in the groundwater in Hinkley exceeded the Maximum Contaminant Level (MCL) of 100 ppb for total chromium currently set by the United States Environmental Protection Agency (EPA). It also exceeded the California MCL of 50 ppb (as of November 2008) for all types of chromium. California first established an MCL specifically for hexavalent chromium in 2014, set at 10 ppb. Prior to that only total chromium standards applied.
A more recent study found that from 1996 to 2008, 196 cancers were identified among residents of the census tract that included Hinkley -- a slightly lower number than the 224 cancers that would have been expected given its demographic characteristics. In June 2013 Mother Jones published an article regarding work by the Center for Public Integrity that was critical of the study, and some others by the same researcher, John Morgan. This comes in contrast with the conclusions reached by the EPA and California's Department of Public Health that chromium(VI) does in fact cause cancer.
At the time that a PG&E background study of chromium(VI) was conducted, average chromium(VI) levels in Hinkley were recorded as 1.19 ppb with a peak of 3.09 ppb. The PG&E Topock Compressor Station averaged 7.8ppb and peaked at 31.8ppb. The California MCL standard was still at 50 ppb at the completion of this background study. In comparison, the Office of Environmental Health Hazard Assessment (OEHHA) of the California EPA, proposed a health goal of 0.06 ppb of chromium(VI) in drinking water in 2009. In 2010, Erin Brockovich returned to Hinkley in the midst of claims that the plume was spreading, despite PG&E cleanup efforts. PG&E continues to provide bottled water for the residents of Hinkley as well as offer to buy their homes. All other ongoing cleanup documentation is maintained at California EPA's page.
Illinois
In Chicago's first ever testing for the toxic metal contaminant, results show that the city's local drinking water contains levels of hexavalent chromium more than 11 times higher than the health standard set in California in July 2011. The results of the test showed that the water which is sent to over 7 million residents had average levels of 0.23 ppb of the toxic metal. California's Office of Environmental Health Hazard Assessment designated the nation's new "public health goal" limit as 0.02 ppb. Echoing their counterparts in other cities where the metal has been detected, Chicago officials stress that local tap water is safe and suggest that if a national limit is adopted, it likely would be less stringent than California's goal. The Illinois Environmental Protection Agency (Illinois EPA) has developed a chromium(VI) strategic plan which outlines tasks in order to reduce the levels of chromium(VI) in Illinois' drinking water. One of which is to work with the U.S. EPA to provide significant technical assistance to the City of Chicago to ensure they quickly develop an effective chromium(VI) specific monitoring program that makes use of the U.S. EPA-approved methods.
Massachusetts
Cambridge Plating Company, now known as Purecoat North, was an electroplating business in Belmont, Massachusetts. A report was conducted by the Agency for Toxic Substances and Disease Registry (ATSDR), to evaluate the association between environmental exposures from the Cambridge Plating Company and health effects on the surrounding community. The report indicated that residents of Belmont were exposed to chromium via air emissions, as well as groundwater and soil. However, six types of cancer were evaluated, and the incidence was actually found to be average, in most cases, across all types, if not a little bit lower than average. For example, in kidney cancer the number of observed cases was 7 versus an expected of 16. While that was the case for most diseases, it was not for all. The incidence of leukemia among females was elevated in Belmont, MA during 1982-1999 (32 diagnoses observed vs. 23.2 expected). Elevations in females were due to four excess cases in each time period (11 diagnoses observed vs. 6.9 expected during 1988-1993; 13 diagnoses observed vs. 8.7 expected during 1994-1999) while elevations among males were based on one to three excess cases. ATSDR deemed Cambridge Plating as an Indeterminate Public Health Hazard in the past, but No Apparent Public Health Hazard in the present or future.
Missouri
In 2009, a lawsuit was filed against Prime Tanning Corporation of St. Joseph, Missouri, over alleged hexavalent chromium contamination in Cameron, Missouri. A cluster of brain tumors had developed in the town that was above average for the population size of the town. The lawsuit alleges that the tumors were caused by waste hexavalent chromium that had been distributed to local farmers as free fertilizer. In 2010 a government study found hexavalent chromium within the soil but not at levels that were hazardous to human health. In 2012, the case ruled that $10 million would be distributed to over a dozen farmers affected in the northwest Missouri area. The Tanning Corporation still denies that their fertilizer caused any harm. Some residents claim that the tumors were a direct cause from the chromium exposure, but it is difficult to determine what other future impacts might arise from exposure in the specific Missouri counties.
Texas
On April 8, 2009, the Texas Commission on Environmental Quality (TCEQ) collected ground water samples from a domestic well on West County Road 112 in Midland, Texas (U.S.), in response to a resident complaint of yellow water. The well was found to be contaminated with chromium(VI). The Midland groundwater reached higher levels of contamination than the EPA mandated maximum contaminant level (MCL) of 100 parts per billion. The current groundwater plume of chromium lies under approximately 260 acres of land at the West County Road 112 Groundwater Site. In response, the TCEQ installed filtration systems on water-well sites that showed contamination of chromium.
To date, TCEQ has sampled water from 235 wells and has installed over 45 anion-exchange filtration systems from this site determined to be centered at 2601 West County Road 112, Midland, Texas. The TCEQ continues to sample wells surrounding the area to monitor the movement of the plume. In addition, they continue to monitor the effectiveness of the anion-exchange filtration systems by sampling on a year-quarterly and the filters are maintained at no cost to the residents.
As of March 2011, the West County Road 112 Ground Water site was added to the National Priorities List (NPL) also known as the Superfund List by the U.S. Environmental Protection Agency (EPA). From 2011 to 2013, TCEQ installed groundwater monitors and conducted groundwater sampling. In 2013, TCEQ began sampling residential soil and confirmed that it was contaminated from use of the contaminated groundwater for garden and lawn care.
According to the EPA, ongoing investigations have not concluded the source of the contamination and cleanup solutions are still being developed. Until such investigations are complete and remediation established, residents will continue to be at risk for health effects from exposure to the groundwater contamination.
Wisconsin
On January 7, 2011 it was announced that Milwaukee, Wisconsin had tested its water and hexavalent chromium was found to be present. Officials stated that it was in such small quantities that it was nothing to worry about, although this contaminant is a carcinogen. In Wisconsin, Milwaukee's average chromium(VI) level is 0.194 parts per billion (the EPA recommended maximum contaminant level (MCL) is 100 ppb). All 13 water systems tested positive for chromium(VI). Four out of seven systems detected the chemical in Waukesha County, and both Racine and Kenosha Counties had the highest levels averaging more than 0.2 parts per billion Further testing is being conducted. There is no further information available as of October 2016.
See also
- California Proposition 65 (1986)
References
External links
- hinkleygroundwater.com
- Hexavalent Chromium, also known as Chromium 6 was a major carcinogenic in the Hinkley case, of which was the subject of the poignant movie, "Erin Brockovich."
- "I never thought I'd see another Hinkley, California, but I'm afraid I might be wrong." Hexavalent chromium was first discovered in April of 2009 at more than 50 times the acceptable federal levels in a water well near West County Road 112.
- ATSDR Case Studies in Environmental Medicine: Chromium Toxicity U.S. Department of Health and Human Services
- Hexavalent Chromium National Emphasis Program
- Factors controlling the heterogeneous distribution of Cr(VI) in soil, plants and groundwater: evidence from the Assopos basin, Greece by M. Economou-Eliopoulos, I. Megremi and Ch. Vasilatos. in the Chemie der Erde/Geochemistry, 71 (1), 39-52, 2011 Elsevier
- 3M US: OSHA Hexavalent Chromium Standard--An overview of the Chromium Six (CrVI) standard
- Environmental Health: "Selected science: an industry campaign to undermine an OSHA hexavalent chromium standard"
- Australian National Pollutant Inventory Chromium VI fact sheet
- US OSHA Health and Safety Topics: Hexavalent Chromium"
- National Institute for Occupational Safety and Health - Hexavalent Chromium
- M. Economou-Eliopoulos, D. Antivachi, Ch. Vasilatos and Megremi, I. "Evaluation of the CR(VI) and other toxic element contamination and their potential sources: The case of the Thiva basin (Greece)". Geoscience Frontiers. 3: 523-539. doi:10.1016/j.gsf.2011.11.010. CS1 maint: Multiple names: authors list (link)
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