Arsenic in Drinking WaterâA Global ... - ACS Publications

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Arsenic in Drinking Water—A Global Environmental Problem Joanna Shaofen Wang and Chien M. Wai* Department of Chemistry, University of Idaho, Moscow, ID 83844-2343; *[email protected]

Arsenic Toxicity and Environmental Problems Arsenic (As, Z = 33) is a naturally occurring and ubiquitous element found in the earth’s crust. It is classically considered as a soft metal and shares many of its toxicity effects with the other heavy metals such as lead and mercury. It generally occurs in nature as sulfides and oxides. Contamination of groundwater with arsenic is a global environmental problem because arsenic can enter groundwater systems from weathering and leaching of arsenic minerals in rocks and soil. The affinity of arsenic for sulfur is revealed in many natural arsenic-containing sulfide minerals, for example, As4S6 (orpiment). This affinity also accounts for the toxicity of arsenite As(III) compounds through its interactions with protein thiols in the human body (1). Trivalent arsenic is extremely poisonous because it can bind strongly to sulfur groups in amino acids, the building blocks of protein as illustrated for cystine in Scheme I. Sulfur containing amino acids include methionine, cysteine, and cystine. Proteins with sulfur containing groups can react with As(III) to form products that will cause biological body malfunction. Throughout human history arsenic has been seen as a mysterious element. Ancient Chinese used arsenic trioxide (pee-song) to kill rats and fungi in rice fields to protect and increase crop yield (2). According to a description in Tian Gong Kai Wu (Exploitation of the Works of Nature), a Ming Dynasty technical book published in 1637, pee-song production workers usually could not work more than two years before losing their hair and becoming ill. Occasionally, peesong was also used to kill people in some well-known murder cases described in the Chinese classical literature. There are also many arsenic-related stories in the history of the western world. For example, Napoleon Bonaparte’s death was suspected to be caused by arsenic poisoning (3). These are just a few examples of arsenic poisoning cases described in the literature. Today, arsenic in the environment has become a global concern because of widespread, chronic arsenic poisoning found in a number of countries affecting a large number of people. This paper summarizes recent literature information regarding global groundwater contamination by arsenic and related health problems, and the controversies around government regulations of arsenic in drinking water. Analytical determination of arsenic and techniques for removing arsenic from water will also be discussed.

in bottled water according to the U.S. Code of Federal Regulations (CFR) (5). However, analyses (6, 7) suggest that the standard of 50 ppb has a substantial increased risk of cancer and is not sufficiently protective of public health. The U.S. Environmental Protection Agency (EPA) was required by the Safe Drinking Water Act Amendments of 1996 to propose a new standard of arsenic by January 2000 and to finalize that regulation by January 2001 (8). In 2000, Congress extended this deadline to June 22, 2001. Based on the accumulating scientific information and data on the health effects of arsenic, in January 2001 EPA issued regulations that set a MCL level of 10 ppb-arsenic standard for drinking water (9). The Bush administration withdrew the 10 ppb standard in March 2001, three days before it was to take effect. On October 31, 2001, the EPA affirmed the appropriateness of the MCL and reinstated 10 ppb as the new MCL for arsenic in drinking water. The new arsenic MCL effective date was February 22, 2002 and water systems must meet this standard by January 2006 (10). Of the 74,000 systems regulated by arsenic MCL, about 4000 systems will have to install treatment devices or take other means to comply with this new MCL regulation. The new standard is estimated to affect about 13 million people, mainly in the West, Midwest, and New England, where arsenic levels in many well waters are greater than 10 ppb. According to the EPA’s estimate, this new standard will reduce cases of bladder cancer by 19–31 and prevent 5–8 deaths annually from this cancer nationwide. It will also reduce lung cancer cases by 19–25 annually and prevent 16–22 deaths per year from lung cancer. This new standard will also prevent numerous cases of other non-cancerous diseases, such as diabetes and heart disease (11). EPA estimated that the average annual household water bill would increase by $32 and the cost would be higher ($327) for systems treating fewer than 3300 households. The U.S. EPA will provide $30 million over the next two years for research and development of more costeffective technologies to help small systems meet the 10 ppb standard. In addition, EPA will provide technical assistance and training to operators of small systems, which will reduce their compliance costs; EPA will also provide funding to states for their drinking water programs through the Public Water Systems Supervision Grants Program (11). SH

+

Protein

Arsenic in Drinking Water—Regulatory Controversy The arsenic standard for drinking water established by the World Health Organization (WHO) in 1963 was 50 ppb; the guideline set in 1993 was 10 ppb. In the USA, the maximum contaminant level (MCL) of arsenic in drinking water is 50 ppb, a level established in 1942 by the U.S. Public Health Service (4). This is also the permissible level of arsenic www.JCE.DivCHED.org

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Cl2AsR

SH S

Protein

AsR

+

2HCl

S

Scheme I. Proteins with sulfur containing groups can react with As(III) to form products that will cause biological body malfunction.

Vol. 81 No. 2 February 2004

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Arsenic Contamination of Groundwater and Related Health Problems The first well-documented, large-scale arsenic contamination of deep well waters and its correlation to “Blackfoot Disease” (BFD) occurred nearly half a century ago in Taiwan (12–17). The symptoms of BFD start with spotted discoloration on the skin of extremities, especially on the feet. The spots change from white to brown and eventually to black, hence the name. The affected skin gradually thickens, cracks and ulcerates. Amputation of the affected extremities is often the final resort to save the BFD victims. The clinical symptoms in arsenic poisoning are described in more detail in a recent ACS Symposium Series Book (18). Outbreaks of BFD increased rapidly around 1950 when the number of deep artesian wells drilled by local villagers for drinking reached a maximum. Analysis of the well water showed arsenic concentration ranged from 100 to 1810 ppb (15). The number of BFD victims has been decreasing since 1956 after purified drinking water was made available to the local inhabitants. In a 1977 survey of more than 40,000 local inhabitants in a BFD-affected area in Taiwan (14), a positive dose– response relationship between the concentration of arsenic in well water and the prevalence rate for skin cancer was established. Medical examinations, with special attention to skin lesions and peripheral vascular disorders, were performed on 40,421 people in the endemic area of Taiwan. A total of 428 cases of skin cancer and 370 of BFD were recorded. The overall prevalence rates for skin cancer, keratosis, and hyperpigmentation were 10.6, 71.0, and 183.5 per 1000, respectively. The male to female ratio in skin cancer was 2.9 (14). A systematic study of arsenic species and other trace elements in related well waters of a representative village (Putai) in the BFD area in southwest Taiwan was conducted in 1994 (17). Averaging over the three wells studied, the total dissolved arsenic concentration was 671 ± 149 ppb, with a range of 470–897 ppb for all the well waters with 54 samples collected from the Putai area. The average value of the dissolved arsenic in all the well waters analyzed by this study was thus about 13 times greater than the 50 ppb MCL. The main arsenic species found in the well waters of the BFD area were inorganic arsenic species As(III) and As(V), with an average ratio of As(III)/As(V) of about 2.6. The individual wells showed a variation of As(III)/As(V) ratio from 1.1 to 5.2. Monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) were below the detection limit; insoluble suspended arsenic accounted for about 3% of the total arsenic in the well waters. Arsenic-related diseases were also reported later in other areas of Asia, including Inner Mongolia of China, Bangladesh, India, and Vietnam. These diseases are all related to drinking waters containing high levels of arsenic. Most arsenic-affected areas in Inner Mongolia of China are located in the arid region of the Hetao Plain between the Yellow River on the south and the Inshan Mountains on the north. The first patient diagnosed as suffering from arsenic poisoning in Inner Mongolia was recorded in 1990 (19). In 1996, a survey team was organized by Asia Arsenic Network experts and members of the Institute for Control and Treatment of Endemic Disease in Inner Mongolia, and an extensive field investigation was carried out among 15 villages in 208


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3 counties (19). In this survey, arsenic concentrations in 96% of the tested well waters in this area exceeded 50 ppb, with the highest concentration of 1080 ppb. A sample of 1602 people and a control group of 126 people were examined for skin lesions. In the sample group, 612 people (35.4%) showed skin lesions related to arsenicosis (such as hyperpigmentation, depigmentation, keratosis), or skin cancer, on their abdomen, palm, or sole. Males were found to be more affected by arsenic-contaminated well waters than females. No one in the control group was found to have skin lesions (19). Bangladesh, which has historically struggled to supply clean water, has the largest arsenic-affected population in the world. In the mid-1980s, with the assistance of the World Bank, many shallow tube wells were dug to meet the daily water requirements of the local people. No one had thought to check for arsenic contamination of groundwater when the wells were being dug. The catastrophe of arsenic contamination in well water that occurred in the 1950s in Taiwan apparently was unheeded in Bangladesh. In 1992, it was discovered that the well water, which had provided a solution to the country’s water supply problems, came with a hidden poison (20–24). According to the latest report (22), perhaps as many as 2 million wells drilled were contaminated with arsenic with concentrations exceeding 50 ppb. Approximately 44% of the total area of Bangladesh (34 districts) is affected; 53 million rural people may be suffering from arsenic poisoning. The highest concentration (14 ppm) in a shallow tube water was found in Pabna, Bangladesh, which is far above the allowed level—50 ppb—for drinking water in Bangladesh (25). The duration of arsenic exposure in West Bengal, India is uncertain, but it is thought that the problem began in the late 1960s when digging of tube wells commenced as part of a statewide irrigation plan (26). The first group of patients identified to be suffering from arsenic poisoning was in 1983 (27). Areas affected by arsenic contamination were all located in the upper delta plain of the Ganges River. More than 800,000 people from 312 villages were drinking arsenic contaminated water. More than 175,000 people exhibited arsenical skin lesions (28, 29). Hair, nails, scales, urine, and liver tissue analyses showed that the residents had drunk this arsenic contaminated drinking water for years. Nickson et al. (21) suggested that the arsenic in the alluvial sediments was derived from sulphide deposits in the Ganges basin. However, the copper belt of Bihar, which contains small amounts of arsenopyrite, and coal basins of Damodar Valley, which contain moderate concentrations of arsenic, are drained by rivers that flow far to the south of the Ganges tributary system. Alternatively, arsenic may occur as contaminants from past and present mining and smelting activity. A feature article in the journal Environmental Science and Technology in 2001 showed a serious arsenic contamination problem in the Red River Valley of Vietnam, the city of Hanoi and its surrounding rural district (30, 31). Arsenic levels in some Vietnam groundwater wells exceeded 3000 ppb. In a contaminated rural area, the groundwater used directly as drinking water had an average concentration of 430 ppb. Analysis of groundwater from the lower aquifer for the Hanoi water supply showed arsenic levels around 280 ppb in three of eight water treatment plants and 37–82 ppb in another five plants. Table 1 shows arsenic contaminations in groundwater of selected locations in Asia.


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Chemistry for Everyone Table 1. Arsenic Concentrations in Groundwater in Selected Areas of Asia 0Location

Conc. As (ppb)

0Taiwan: Southwest Coast a

0Ref.

100–1810

015

0Taiwan: Putai

470–897

017

0Chinese Inner Mongolia (Hetao Plain)

50–1080

019

0Bangladesh: Ganges Delta 0Bangladesh: Pabna (North District) 0India: West Bengal 0Vietnam: Hanoi and Red River Valley a

10–2040

023

50–14,000

025

50–3400

027

1–3050

030

This is the area where “Blackfoot Disease” occurred.

Table 2. Chemical Forms of Common Arsenic Speciesa Arsenic Species

Chemical Form

Arsine

AsH3

Methylarsine

(CH3)AsH2

Dimethylarsine

(CH3)2AsH

Trimethylarsine

(CH3)3As

Monomethylarsonic Acid (MMA)

CH3As(O)(OH)2

Dimethylarsinic Acid (DMA)

(CH3)2As(O)(OH)

Arsenite

H3AsO3

Arsenate

H3AsO4, H2AsO4

a

Information derived from references 46 and 47.

In the United States, some states reported arsenic in drinking water higher than 50 ppb several decades ago, including Minnesota (32), Oregon (33), California (34), Alaska (35), and Utah (36, 37). New Hampshire (38) and Wisconsin (39) reported contaminated areas in 1999. In randomly selected household water samples in New Hampshire, concentrations of arsenic ranged from not detectable to 180 ppb, with water from domestic wells containing higher concentrations of arsenic than the ones from municipal sources. Water samples from drilled bedrock wells had the highest arsenic concentrations. According to one estimate, over 2.5 million people in the U.S might be supplied with drinking water containing more than 25 ppb arsenic and 350,000 people supplied with water of 50 ppb arsenic or more (40). Other countries such as Canada (41), Chile (42), Mexico (43), Argentina (44), and Finland (45) all have a potential threat from arsenic contamination of groundwater. Figure 1 shows areas of the world with arsenic contamination problems. Methods for Arsenic Determination In natural waters, arsenic can exist in different oxidation states depending on the redox environment. The toxicity, bioaccumulation, and transport of arsenic are often www.JCE.DivCHED.org

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Figure 1. Groundwater arsenic contaminated areas in the U.S. (82), Taiwan (83), Inner Mongolia of China (19, 84), Bangladesh (22), and India (28, 85).

dependent upon its species in the system under investigation. The trivalent inorganic species arsenite is more toxic to biological systems than the pentavalent species arsenate because it can bind to thio groups. Organoarsenicals such as MMA and DMA also exist in the natural environments, but their toxicities are lower than the inorganic arsenic species. MMA, DMA, arsenite, and arsenate are the common species present in natural waters. Arsenite usually exists as arsenous acid (H3AsO3, pKa = 9.22) and arsenate as arsenic acid H3AsO4 (pKa = 2.20) and its deprotonated species such as H2AsO4 (pKa = 6.97) and HAsO42 in aqueous environments (46). Information on the distribution of arsenic species is therefore important to assess its toxicity in drinking water. Penrose (47) compiled the approximate toxicity order of various arsenic compounds, which, in decreasing order, is: arsines > arsenite > arsenoxides > arsenate > pentavalent arsenicals > arsonium compounds > metallic arsenic. The chemical formulas of some common arsenic species are shown in Table 2. Total arsenic in water can be measured directly by many techniques, including flame atomic absorption spectrometry (AAS) or graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma–atomic emission spectrometry (ICP–AES), inductively coupled plasma–mass spec-


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trometry (ICP–MS), atomic fluorescence spectrometry (AFS), and neutron activation analysis (NAA). Preconcentration may be required for water samples with very low arsenic contents. Determination of arsenic species generally relies on separations based upon the principles of solvent extraction, chromatography, and selective hydride generation. Preconcentration and separation of arsenic species by solvent extraction is one method for arsenic speciation in environmental waters (48). The dithiocarbamate extraction method is widely used for preconcentration of arsenic and other trace metals. Dithiocarbamate reagents such as sodium dithiocarbamate or ammonium pyrrolidinedithiocarbamate form waterinsoluble metal complexes with As(III) that can be extracted into an organic solvent such as chloroform. Back extraction of the As(III)–dithiocarbamate complex from the organic phase can be achieved using HNO3. In a second aliquot, a reducing agent is used to convert arsenate As(V) to arsenite As(III) followed by solvent extraction with the dithiocarbamate reagent. The difference between the arsenic concentrations in the two aliquots allows determination of As(V) and As(III) in the water sample (49, 50). The organic compounds, MMA and DMA in aqueous solutions, can be converted to arsenites such as CH3AsI2 and (CH3)2AsI using a mixture of potassium iodide, sodium thiosulfate, and sulfuric acid and then analyzed by chromatographic methods (49, 50). Gas chromatography (GC) (51), liquid chromatography (LC) (52), ion chromatography (IC) (53), and supercritical fluid chromatography (SFC) (54, 55) have been successfully employed to separate various arsenic species. Both capillary and packed columns have been utilized with a variety of detection methods. The choice of detector depends on the specific derivatization procedure and degree of selectivity required. Detection methods that have been coupled to gas– liquid chromatography (GLC) for arsenic determination include ICP, AAS, AFS, microwave emission spectroscopy (MES), electron capture detection (ECD), and flame ionization detection (FID) (56, 57). In addition to the volatile species that can be determined by GC, ions and non-volatile organometallic species can be separated with LC. When sensitive and selective detectors are used for measurement, the determination of nanogram quantities of eluants can be obtained with high performance liquid chromatography (HPLC). Speciation of As(III) and As(V) in sediment extracts by HPLC–HG (hydride generation) atomic absorption spectrophotometry was discussed recently (52). Several detectors have been coupled to HPLC including ultraviolet (UV), AAS, GFAAS, ICP, and conductivity (CD). Hydride generation techniques for the speciation of arsenic involve selective reduction of the hydride-forming arsenic species to the corresponding arsines (58). At pH 5–7, arsenite can be reduced to arsine and at pH less than 1, both arsenite and arsenate are reduced to arsines. MMA and DMA are reduced to methylarsine and dimethylarsine at pH less than 1. After generation, the arsines may be either passed directly into the detection device or accumulated prior to detection. Separation is achieved by controlling and varying the pH and reduction system between sample aliquots. When a hydride collection step is incorporated prior to detection, it is usually accomplished by freezing the arsines out in a liquid nitrogen-cooled trap (59). After collection, the volatile 210


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arsines can then be separated in the order of their boiling points by slow warming of the trap, and subsequently transferred to the detection device (60). Methods of Removing Arsenic from Water A general procedure for treating drinking water in a municipal water treatment plant often includes the following steps: (1) addition of chemicals, (2) interaction of the chemicals with sediments and organic particles, (3) sedimentation of large particles, (4) filtration of water, (5) disinfection of water with chlorine, and (6) distributing the treated water. However, this general procedure may not be effective for removing low levels of arsenic from water. The concentration of arsenic and the ratio of As(III)/As(V) in drinking water must be known before water treatment can effectively remove arsenic. The treatment steps may include oxidation, adsorption, membrane processing, precipitation, coagulation, chlorination, acidification, and so forth. pH correction and ion exchange demineralization may also be required. Some of these steps are described below.

Oxidation One method to reduce the toxicity of arsenic is to oxidize arsenite to arsenate. Arsenate is more adsorbable in adsorption processes and easier to be co-precipitated. This step is often necessary for removal of arsenic. Chemical reagents are usually required for oxidation of As(III) to As(V). Simple mechanical addition of pure oxygen is not effective because of the slow kinetics of As(III) oxidation by dissolved oxygen. Immediate oxidation of As(III) to As(V) occurs in the presence of Cl2 at 1 mg/mL level (61). Ozone rapidly oxidizes As(III) with a dose at 0.1 mg/L, but it has undesirable reactions with natural organic matter (62). Free chlorine or hypochlorite is effective, but it may also react with natural organic matter to produce undesirable chlorinated by-products (63). MnO2 loaded onto sand filters has been shown to be an effective oxidizer for As(III) (62). The most effective initial molar ratio is MnO2:As(III) = 14:1. At this ratio the reaction rate was not affected by pH changes between 5–10. Other chemicals that may be used for oxidation of As(III) include FeCl 3, H2O2/Fe 2 (Fenton’s reagent), KMnO 4, MnO2, and Fe(VI) (64–69). Adsorption After oxidation, arsenate can be removed by adsorption. Activated alumina, typically a mixture of amorphous and γAl2O3, is a common, commercially available adsorbent material for water treatment (70, 71). Because OH groups are exchanged for As(V) anions, this process could also be considered as ion exchange. At pH 5.5–8.5 the anion order of preference for activated alumina from most to least is (72): OH > [HAsO42, Si(OH)3O] > F > H2AsO4 > [SeO32, HSeO3] > [SeO4 2, SO42] > [Cl, NO3 , HCO3] > H3AsO3

For As(V) removal, activated alumina has been shown to perform best around pH 5. For As(III) removal, pH 8 is most effective, because the H3AsO3 species exhibits neutral charge at lower pH values (73). The maximum adsorptive capacity of activated alumina is 5–24 mg As adsorbed per g


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of media at As concentrations of 0.05–0.2 ppm (63, 73). Operation of the activated alumina system may produce hazardous chemicals such as HCl and NaOH. MnO2-coated sand (MDCS) is another adsorbent material on which metal ions can be oxidized from soluble to insoluble forms that can be held by the absorber (74). MDCS can oxidize As(III) to As(V) before adsorption. MDCS can adsorb As(V), and can also oxidize Fe2+ ions to Fe3+, which in turn adsorbs As(V) (75). Therefore, MDCS can remove excess iron and manganese from water. The As(V) can also be adsorbed onto or co-precipitated with iron and manganese. MDCS can remove more than 80% of arsenic, but this efficiency requires iron to be present (76, 77). Ferrous with oxygen incorporated with MDCS has been shown to be the best combination since the products of these reactions were more filterable (64).

Membrane Processing In membrane processing particles are separated on the basis of their molecular size and shape with the use of pressure and specially designed semi-permeable membranes. Reverse osmosis is one of the membrane processing methods in which a semi-permeable membrane is used to transport water from a dilute solution to a concentrated one. When the external pressure is greater than the osmotic pressure, the membrane allows water from a salt solution to diffuse into a diluted solution. The advantages of this technique include high rejections of neutral and charged particles, no chemical compounds involved, simplicity, and automatic operation. Ion Exchange Strong-base anion exchange (SBA) is the best ion exchange method for arsenic removal. The preferred pH for the operation is 8–9. The order of preference of SBA resins for anions from most to least is as follows (78): HCrO4 > CrO42 > ClO4 > SeO42 > SO42 > NO3 > Br > HAsO42 > CN > NO2 > Cl > H2AsO4 > OH >CH3COO > F

Chemical Precipitation Chemical precipitation is a simple way of removing dissolved ions in water. After precipitation, filtration or gravity settling can be used to remove the precipitate and unwanted solute. Arsenic removal may be accomplished by altering the process, such as changing the coagulant type or increasing the dosage (79). Iron or alumina salt precipitating agents react with natural bicarbonate alkalinity to form ferric or alumina hydroxides as illustrated by this reaction: Fe2(SO4)3 3Ca(HCO3)2 → 2Fe(OH)3 ↓ 3CaSO4 ↓ 6CO2 ↑

In this reaction, Fe(OH)3 and CaSO4 precipitates are formed that can be easily removed from water. Arsenic may be removed by a simple precipitation method, co-precipitation onto a hydroxide phase such as Fe(OH)3, or adsorption onto a solid oxyhydroxide surface site (63). The anion order of preference for iron hydroxide adsorption follows this order (80): H2AsO4 > H2PO4 > H2AsO3 > Si(OH)3O > HCO3

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Chemical Coagulation Coagulation and flocculation are used to remove nonsettleable colloidal solids and slow-settling suspended solids. To reduce surface charge on particles and allow them to conglomerate, coagulants such as alumina and iron salts or some polyelectrolytes are added to water and rapidly mixed. This will destabilize particles, allowing them to aggregate. The coagulants Fe2(SO4)3 and FeCl3 exhibit the highest possible arsenic removal when conditions are optimal. Fe2(SO4)3 has several operational advantages over FeCl3 including more gradual oxidation and slower flocculation formation, easy filtration resulting in a high ability to treat a larger volume of water, and reduced backwashing frequency. These features may result in a more economical operational procedure (81). The methods described above can all be used to remove arsenic in drinking water. A major problem of applying sophisticated arsenic removal techniques is the cost. In some developing countries, very cheap materials such as coconut shells, rice husks, wood charcoal, and sand are often used to remove arsenic in drinking water. These primitive techniques may not result in efficient removal of arsenic from contaminated waters. Conclusions Groundwater contamination of arsenic is one of the most serious global environmental problems facing human beings today. Arsenic contamination in drinking water has created serious health problems in Taiwan, Inner Mongolia of China, Bangladesh, and India. The groundwater arsenic contamination problem has also been found in other countries, including Canada, Mexico, Argentina, Finland, and the U.S. Water treatment techniques are available to reach the new 10-ppb arsenic MCL standard by 2006, although the cost may be high for small communities. Because the toxicity of arsenic depends on its oxidation states and chemical forms, measuring all arsenic species in natural water systems is important for environmental monitoring and future regulatory considerations. Literature Cited 1. Subcommittee on Arsenic in Drinking Water, Committee on Toxicology, National Research Council. Chemistry and Analysis of Arsenic Species in Water, Food, Urine, Blood, Hair, and Nails. In Arsenic in Drinking Water; National Academy Press: Washington, DC, 1999; Chapter 3, p 27. 2. Song, Ying-Hsing. Tian Gong Kai Wu (Exploitation of the Works of Nature); Song, Y. X. Ed.; China, 1637, (Ming Dynasty). 3. http://more.abcnews.go.com/sections/science/dailynews/ napoleon000504.html. See also http://www.didyouknow.cd/ napoleon.htm (accessed Nov 2003). 4. Code of Federal Regulations, 40 CFR 141.11, revised July 1992, U.S. GPO: Washington, DC, 1992. 5. Code of Federal Regulations, 21 CFR 103.35, revised April 1992, U.S. GPO: Washington, DC, 1992. 6. Morales, K. H.; Ryan, L.; Kuo, T. L.; Wu, M. M.; Chen, C. J. Environ. Health Perspectives 2000, 108 (7), 655. 7. Karagas, M. R.; Stukel, T. A.; Morris, J. S.; Tosteson, T. D.; Weiss, J. E.; Spencer, S. K.; Greenberg, E. R. American J. Epidemiology 2001, 153 (6), 559.


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