N-Hexane: Sources, Environmental Impact, and Human Exposure, Lecture notes of Literature

Download N-Hexane: Sources, Environmental Impact, and Human Exposure and more Lecture notes Literature in PDF only on Docsity! n-HEXANE 157 5. POTENTIAL FOR HUMAN EXPOSURE 5.1 OVERVIEW n-Hexane is a highly volatile component of the paraffin (also the alkane or aliphatic) fraction of crude oil and natural gas, and it is a constituent of heating and motor fuels refined from petroleum. Exposure from contact with vapors or emissions from these refined petroleum products is the most widespread form of low-level exposure for the general population. Most n-hexane in these fuels is oxidized (and therefore destroyed) as part of the combustion process to provide heat or drive internal combustion engines. Small amounts of n-hexane, along with other petroleum compounds, volatilize to the atmosphere during handling, storage in fuel tanks, or through incomplete combustion. Recent research (Ahearn et al.1996) suggests that certain fungi may be able to produce n-hexane. These fungi may be common in older buildings, and in some parts of the country may provide exposures from previously unsuspected indoor sources. n-Hexane is also produced as a relatively pure product for a number of specialized end uses, primarily as a solvent or as a component of certain glues and adhesives. Especially in urban areas, n-hexane may be a typical component of nonpoint source runoff when rainfall washes hydrocarbons deposited on roads and other surfaces into surface waters. Spills of refined petroleum products or of commercial n-hexane products may introduce n-hexane into soils or surface waters. Around urbanized areas, spill sites, refineries, tank storage facilities, underground storage tanks (e.g., at gas stations), or waste sites, can be sources of n-hexane subsequently transported into sediments or groundwater. Once introduced into deeper sediments or groundwater, n-hexane may be fairly persistent since its degradation by chemical hydrolysis is slow and opportunities for biodegradation may be limited under anoxic conditions or where nutrients such as nitrogen or phosphorus are in limited supply. In the atmosphere, the main degradation pathways involve destruction through the action of free radicals such as hydroxyl radicals. n-Hexane has been identified in at least 60 of the 1,467 current or former EPA National Priorities List (NPL) hazardous waste sites (HazDat 1998). However, the number of sites evaluated for n-hexane is not known. The number of these sites within the United States can be seen in Figure 5-1. 5.2 RELEASES TO THE ENVIRONMENT According to the Toxics Release Inventory (TRI), in 1996, a total of 70,685,942 pounds (32,062,933 kg) of n-hexane was released to the environment from 534 reporting facilities (TRI96 1998). Table 5-1 lists amounts released from these facilities. An estimated 77,303 pounds (35,064 kg) was released to publicly owned treatment works (POTWs), and an estimated 11,625,623 pounds (5,273,348 kg) were transferred THHEXANE Figure §-1. Frequency of NPL Sites with n-Hexane Cortamination 5. AOTENTIAL FOR BUMAN EXPOSLBE oe OE OS LD CUeER Derived from Wazbat 1908 1S n-HEXANE 161 5. POTENTIAL FOR HUMAN EXPOSURE offsite (TRI96 1998). The TRI data should be used with caution because only certain types of facilities are required to report. This is not an exhaustive list. Since n-hexane is a component of refined petroleum products, there is considerable potential for releases to environmental media through the use of heating and motor fuels. Table 5-2 summarizes the uses of petroleum products according to major demand categories (e.g., “transportation”) and displays estimated use levels in barrels (and liter equivalents) and by percentages for the various end-use demands for specific fuel types (e.g., kerosene or fuel oil). While n-hexane can be a minor constituent (less than 1% by weight) of several of these petroleum products, its physical properties as a light alkane make it most suitable for use in gasoline. Approximately 98% of the demand for gasoline involves transportation, mainly cars and trucks. The composition of gasolines has changed over the years, mainly in an effort to maintain the so called octane ratings of the fuels. Since the 1980s the growing use of nonleaded gasolines has led to a growing percentage of high-octane benzene and toluene in gasoline blends. For modern gasoline mixtures, the total percentage by weight of the n-hexane component is approximately 3% (Brugnone et al.1991; Heath et al.1993; Stelljes and Watkin 1993). Of the 2,608 million barrels of motor gasoline consumed for transportation in 1992 (designated “transportation” in Table 5-2) about 27,300 million pounds (12,409 million kg) are from the n-hexane fraction (PennWell 1994; Stevens 1988). This figure is about 76 times the 358 million pounds (143 million kg) of commercial n-hexane produced annually in the 1970s (Marks et al.1980). Most gasoline, along with its n-hexane fraction, is consumed during its combustion in motor cars and other engines. However, gasoline use results in a variety of emission losses from refueling, evaporation while gasoline is stored in fuel tanks or ignition systems, and exhaust releases when there is incomplete combustion of fuels (EPA 1994h). EPA only tracks trends in total hydrocarbon or total volatile organic compound (VOC) emissions, so that quantitative estimates for the n-hexane released from automobiles and trucks are not available. Assuming that only 1% of the n-hexane of motor fuels is released to environmental media, such releases could be on the same order of magnitude as the total amount of relatively pure n-hexane associated with the major end-uses described in Chapter 4. In addition to emissions to the atmosphere, releases from heating and motor fuel uses to other environmental media are possible as a result of leaks and spills at refineries, pipelines, large tank batteries (or tank “farms”), above- and below-ground storage tanks, tanker trucks and railroad tanker cars, or from minor releases at garages or around homes and workplaces. Crude oil spills also result in the release of n-hexane to the air or other environmental media. mHEXANE 5. POTENTIAL FOR HUMAN EXPOSURE Table 5-2. Demand Patterns for Major Petroleum Products (1992) 162 Product Residential Commercial industrial Transportation Electric utilities Total Motor gasoline? 0 (0.0) 15 (<1.0) 37 (1.4) 2608 (98.0) 0(0.0) 2,660 Oo 2,385 5,883 414,672 0 Kerosene 14 (73.3) 2 (13.3) 2 (13.3) 0 (0.0) 0 (0.0) 15 1,749 318 318 0 0 Distillate fuel oil 148 (13.6) 80 (7.3) 196 (18.0) 654 (60.0) 12(1.1) 1,090 23,532 12,720 31,164 103,986 1,908 Residual fuel oil 0 (0,0) 30 (7.5) 62 (15.5) 172 (42.9) 136 (33.9) 400 0 4,770 9,858 27,348 21,942 Liquid 106 (16.5) 19 (3.0) 513 (79.9) 5 (0.8) 0 (0.0) 643 petroleum gas 16,858 3,021 81,567 795 0 and ethanes * Top line: millions of barrels (% of total sectoral demand} Bottom line: millions of liters © Typically contains >1% n-hexane Note: 1 barrel = 42 U.S. gallons = 159 liters Source: PennWell 1994 n-HEXANE 163 5. POTENTIAL FOR HUMAN EXPOSURE In addition to releases associated with the ordinary use of refined petroleum products as a fuel, ongoing research (Ahearn et al.1996) suggests that a variety of fungi found in ducts and insulation materials in homes or office buildings are capable of releasing gases that include n-hexane. Where the buildings have poor ventilation properties, commonly referred to as “sick-building syndrome” (Sundell 1996), the indoor air releases of n-hexane may sometimes be sufficient to pose public health concerns. n-Hexane is also among the various off-gassing constituents encountered at sanitary landfills (Brosseau and Heitz 1994; O’Leary and Walsh 1995). There is also evidence (McKay et al.1996) that marine phytoplankton produce a variety of non-methane hydrocarbons, including small amounts of n-hexane, from the metabolism of polyunsaturated lipids in dissolved organic materials. Very small amounts of n-hexane may also be among the biogenic emissions from different types of terrestrial vegetation (Isidorov et al.1985; Winer et al.1992). 5.2.1 Air According to the Toxics Release Inventory (TRI), in 1996, the estimated releases of n-hexane of 58,649,487 pounds (26,603,233 kg) to air from 534 reporting facilities accounted for about 82.9% of total environmental releases (TRI96 1998). Table 5-1 lists amounts released from these facilities. The TRI data should be used with caution because only certain types of facilities are required to report. This is not an exhaustive list. Most releases of n-hexane to environmental media are to air. Based on its Henry’s law constant, n-hexane discharged to water will volatilize rapidly; however, the amount volatilized will vary depending on a number of factors including the temperature, turbulence, and depth of the receiving water. n-Hexane spilled onto surface soils will also volatilize to the air. Data sources were not identified allowing comprehensive quantitative estimates of the amount of n-hexane released on an annual basis to the air. In addition to releases from such commercial applications as edible oil extraction, the other major sources of atmospheric releases would be from emissions related to the n-hexane contained in heating and motor fuels. n-Hexane was identified in air samples collected at 17 of the 60 NPL hazardous waste sites where it had been detected (HazDat 1998). n-HEXANE 166 5. POTENTIAL FOR HUMAN EXPOSURE 5.3.2 Transformation and Degradation 5.3.2.1 Air n-Hexane does not absorb ultraviolet (UV) light at 290 nm and is thus not expected to undergo direct photolysis reactions. The dominant tropospheric removal mechanism for n-hexane is generally regarded to be decomposition by hydroxyl radicals (Atkinson and Carter 1984; Atkinson et al.1982). Calculations assuming typical hydroxyl radical concentrations suggest a half-life of approximately 2.9 days (SRC 1994b). While n-hexane can react with nitrogen oxides to produce ozone precursors under controlled laboratory conditions (Montgomery 1991), the smog-producing potential of n-hexane is very low compared to that of other alkanes or chlorinated VOCs (Kopczynski et al.1972). Hydroxyl ion reactions in the upper troposphere, therefore, are probably the primary mechanisms for n-hexane degradation in the atmosphere. As with most alkanes, n-hexane is resistant to hydrolysis (ASTER 1995; Lyman et al.1982). The proposed decomposition of n-hexane in air is shown in Figure 5-2. 5.3.2.2 Water Although few data are available dealing explicitly with the biodegradation of n-hexane in water, neither hydrolysis nor biodegradation in surface waters appears to be rapid compared with volatilization. In surface waters, as in the atmosphere, alkanes such as n-hexane would be resistant to hydrolysis (ASTER 1995; Lyman et al.1982). Biodegradation is probably the most significant degradation mechanism in groundwater. One study was identified (McClay et al.1995) that documented the ability of Pseudomonas mendocina bacteria to metabolize n-hexane in laboratory microcosms simulating groundwater conditions. Mixed bacterial cultures as well as pure cultures are documented as capable of metabolizing n-hexane under aerobic conditions (Heringa et al.1961; Rosenberg et al.1992). A study of a biofiltration system to remove VOCs from air used a sludge-like composting biofiltering system that was effective in causing the biodegradation of n-hexane (Morgenroth et al.1996); this study involved a special composting system to allow the introduction of nitrogen fertilizers to overcome a nutrient limitation. Most of the available literature deals with petroleum mixtures containing several types of alkanes. In general, linear alkanes (such as n-hexane) are viewed as the most readily biodegradable fractions in petroleum (Leahy and Colwell 1990), particularly when oxygen is present in solution. Figure 5-2. Degradation of n-Hexane in Air by Free Radicals CH3gCH2CH,CHzCH2CHg n-hexane Source: Atkinson 1985 Oz OH | ——* CH3CH,CH,CH,CHCH, ———> radicals . peroxy radical ae NO, Cap 0 CH,CHgCH,CH,CHCH + Noe Oo CH3CH,CH,CHCH,CH, aldehydes ketones nitrates SYNSOdX]a NYANH HOd WILNS1Od “S SNVXSHU 4£9L n-HEXANE 168 5. POTENTIAL FOR HUMAN EXPOSURE Since n-hexane is highly volatile, it is often excluded from the list of constituents included in studies on biodegradation or bioremediation of petroleum wastes or in studies of surface waters receiving pollutant loads from runoff or discharges. Attention is generally focused on complex mixtures of hydrocarbons, starting with fractions heavier or less volatile (usually C10 or longer chain alkanes, aromatics such as benzene or toluene, and PAHs (polyaromatic hydrocarbons)) than the lighter constituents of gasoline (Crawford et al.1995; Latimer et al.1990; Rosenberg et al.1992; Sauer et al.1993; Shaw et al.1986). Once introduced into groundwater, n-hexane may be fairly persistent since its degradation by chemical hydrolysis is slow and opportunities for biodegradation may be limited under anoxic conditions or where nutrients such as nitrogen or phosphorus are in limited supply. 5.3.2.3 Sediment and Soil The findings presented above in Section 5.3.2.2 on bioremediation in groundwater are relevant for many soil and sediment systems. Figure 5-3 outlines the probable biodegradation of n-hexane based on metabolites isolated from a pure culture of Pseudomonas (Heringa et al.1961). The most important biodegradation processes involve the conversion of the n-hexane to primary alcohols, aldehydes and, ultimately, into fatty acids. Similar processes are encountered with other light hydrocarbons such as heptane. In general, unless the n-hexane is buried at some depth within a soil or sediment, volatilization is generally assumed to occur at a much more rapid rate than chemical or biochemical degradation processes. Once introduced into deeper sediments, n-hexane may be fairly persistent since its degradation by chemical hydrolysis is slow and opportunities for biodegradation rnay be limited under anoxic conditions or where nutrients such as nitrogen or phosphorus are in limited supply. 5. 4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT The widespread use of n-hexane as an extractant in the laboratory creates problems in interpreting concentration readings at low levels. Even with good quality control, it may often be impossible to determine whether to attribute a measured value to the actual levels in a sample or to contamination from n-hexane in the laboratory environment (Otson et al.1994). For the most part, n-hexane is not a common target analyte from water or soil samples. While data based on ambient air samples or sampling in the air of various workplace or residential environments are more numerous, most EPA regulatory programs rely on bulk measurements of total hydrocarbons or total volatile compounds rather than on measurements of specific compounds such as n-hexane (Bishop et al.1994; DeLuchi 1993). n-HEXANE 171 5. POTENTIAL FOR HUMAN EXPOSURE 1993). A complication in such testing is that hydrocarbons in the smoke may have been introduced from sources such as polluted urban air or n-hexane from cigarette lighters. The air in well ventilated office buildings in urban areas of California contained n-hexane levels of approximately 0.55 µg/m3 (1.5 ppbv) (Daisey et al.1994). Other studies of heavily polluted urban areas have suggested that the air in offices will have n-hexane levels at least an order of magnitude lower than the peak levels in rush-hour traffic in cars or other vehicles (Chan et al.1994); the same studies showing that the median concentrations averaged over an entire commuting trip are about the same as for the time-averaged median concentrations of n-hexane in office buildings (<9 µg/m3 or <3.2 ppbv). Recent research suggests that gases released by various fungi in ductwork, inner walls, and crawl spaces contain a variety of VOCs, including n-hexane (Ahearn et al.1996). Of the total levels of VOCs measured in situ from fungal-colonized insulation materials in a 2-year-old office building, n-hexane comprised about 2.69% of the total measured VOCs; in air samples collected under laboratory conditions using cultures prepared from fungal isolates, the n-hexane contribution to the total measured VOCs was 4.85% (Ahearn et al.1996). Measurements of the interior air of an office building located in the Houston area showed total VOC concentrations (expressed as carbon) in excess of 100 ppbv. The VOCs levels included detectable amounts of n-hexane as well as toluene and benzene (Ahearn et al.1996). Concentrations in some workplace settings may be higher than typical ambient air levels. Samples collected in tire factories during the 1970s showed median n-hexane levels of 25.9 ppmv around the work area where the rubber curing took place (Van Ert et al.1980). When workers have assembled items in poorly ventilated rooms, n-hexane levels ranging from 500 to 2,500 ppmv have been documented (Iida 1982). Similar workplace findings, usually in countries other than the United States, have been documented, with n-hexane concentrations in workspace air in excess of 500 ppmv (Graham et al.1995). Elevated levels in air are also found in substance abuse cases, where pure n-hexane or mixtures containing significant amounts of n-hexane are used to produce a “high” (Altenkirch et al.1977; Graham et al.1995). n-Hexane is a common trace component in landfill gases at many waste sites (Brosseau and Heitz 1994; O’Leary and Walsh 1995). The n-hexane concentrations of these emissions have been documented to range from 3 to 10 mg/m3 (1.1-3.6 or 1,100-3,600 ppbv). While these levels would be expected to decrease rapidly as the landfill gases were dispersed into the ambient air, areas near the ground or pockets of air in trenches or excavations could reach levels significantly above the concentrations normally encountered in ambient air. For instance, data averaged over 15-minute intervals during site remediation n-HEXANE 172 5. POTENTIAL FOR HUMAN EXPOSURE work at a reclaimed oil refinery site showed levels as high as 121.51 mg/m3 (43.70 ppmv) in the air around a backhoe digging trench in the petroleum-contaminated soils (Verma et al.1992). Samples from the same study averaged over a typical 8-hour workshift for the area around the backhoe showed average levels of 3.06 mg/m3 (1.10 ppmv). Even higher levels (perhaps in excess of 10,000 ppmv) are possible around large spills of n-hexane; at such elevated concentrations, as with many components of gasoline-type hydrocarbons, there could be considerable danger from explosions, which are possible when the n-hexane levels exceed approximately 1.2% of the volume of air (Merck 1989). Since 0.1% by volume is equivalent to 1,000 ppmv, this flash-point level for n-hexane would be at a level of 12,000 ppmv or higher. 5.4.2 Water In general, data on levels in water or groundwater are very limited, with no information being identified in the literature. Information on levels in public water supplies was not identified. Since n-hexane is highly volatile, typical treatment techniques for drinking water supplies in larger towns and cities would be expected to volatilize the n-hexane before it could enter the distribution system. It is likely that some n-hexane would be found in groundwater contaminated by gasoline leaks from underground storage tanks (UST). This could be a matter of concern for some domestic groundwater wells used for drinking water supplies. Since the emphasis in UST programs is usually on the more soluble aromatic fractions (e.g., benzene) or on bulk measurements of total petroleum hydrocarbons (TPH) (Potter 1993), no information could be identified in the literature dealing explicitly with n-hexane. 5.4.3 Sediment and Soil Very little information could be identified dealing with n-hexane levels in sediments and soils. n-Hexane has been identified among the contaminants in an offsite oilfield-disposal pit in New Mexico (Eiceman et al. 1986). Since n-hexane is a trace constituent of crude oil and natural gas, as well as a component of refined petroleum products, soil or sediment contamination with n-hexane can be expected near oilfield production sites, large soil spills, slush pits and other areas around refineries, and in waste sites where petroleum products or other n-hexane-containing wastes had been disposed. Detections would also be likely near many tank storage facilities, pipelines, truck or rail transfer sites, car repair facilities, automobile assembly or storage facilities, and auto and truck fueling facilities (DeLuchi 1993). n-HEXANE 173 5. POTENTIAL FOR HUMAN EXPOSURE At many waste sites, n-hexane has been detected in the landfill gases vented from the soils at the disposal sites (Brosseau and Heitz 1994; O’Leary and Walsh 1995). While information in the literature is extremely limited, trace levels of n-hexane are probably found in the soils or the soil gases at many waste disposal sites. n-Hexane has been identified in the soil at 14 sites and in sediments at two sites among the 60 NPL hazardous waste sites where it was detected in some environmental medium (HazDat 1998). 5.4.4 Other Environmental Media n-Hexane is exempted from analysis of most foodstuffs (the exceptions are spice oleoresins and corn endosperm oil) or as an inert ingredient in pesticide formulations (see Chapter 4; Firestone 1997). Testing for alkanes is often directed at compounds less volatile (e.g., Cl0 or higher) than n-hexane (Hernandez et al.1995). There is, therefore, limited information in the literature on the levels of n-hexane encountered in foodstuffs. Analyses carried out in the 1960s and 1970s would have sometimes involved analytical methods not considered accurate by contemporary standards. Caution is also needed in interpreting published results to make sure the testing did not involve materials that had not yet gone through the complete cycle of solvent recovery, heating, and final vacuum treatment to recover the n-hexane solvent and remove as much as possible of this hydrocarbon from the final product intended for human consumption. Before these recovery processes, the crude oil or meal products can be expected to show appreciably high levels of n-hexane. In studies of fully processed edible oil products carried out in the 1960s it was determined that n-hexane residues were generally at levels below 10 ppm (Watts and Holswade 1967). Recent investigations using more precise modern analysis techniques (Hautfenne et al. 1987) concluded that residual n-hexane residues for refined food products would be less than 2 ppm. If the standard assumption of 80 g of fat consumed per 70-kg person per day is made, such residual levels would be the equivalent of no more than 2.29 µg/kg/day of n-hexane, which is a toxicologically insignificant amount. No recent studies could be identified that were performed in the United States on levels in expired air. A study of hydrocarbon contents in expired air in the Chicago area carried out in the late 1970s found average n-hexane concentrations from 54 human volunteers breathing urban air to be approximately 4.7 ng/L (Krotoszynski et al.1979). Most studies of n-hexane have involved occupational exposures, especially in shoe or sandal factories, in Japan or Italy. Since n-hexane is rapidly metabolized by humans to such compounds as 2,5-hexanedione, investigations of toxic substances in blood or urine typically focus on these metabolites (Mutti et al.1993; van Engelen et al.1995). In one study of shoe assembly workers in n-HEXANE 176 5. POTENTIAL FOR HUMAN EXPOSURE inhalation exposure risks. Dermal exposures are also possible from hexane-containing household products. For very small children, accidental ingestion of hexane-containing materials is also a potential exposure risk. Other potential exposures are possible from hazardous waste sites. There have been no documented secondary or take-home exposures for children from materials transferred from the parents’ workplace on clothes, skin, hair, tools, or other objects (NIOSH 1995). Such exposure risks are not expected to be a concern with n-hexane because it is highly volatile. Although concentration levels were not reported, studies have shown detections of n-hexane in human breast milk (Pellizzari et al.1982). In chapter 2, the discussion of PBPK modeling suggests the likelihood for breast milk transfers to nursing infants. No studies were identified dealing with levels of n-hexane in amniotic fluid, meconium, cord blood, or neonatal blood that would document prenatal exposures. There are no studies dealing with exposure or body burden measurements on children. Given the absence of such studies targeted at children, it is unknown whether children are different in their weight-adjusted intake responses to n-hexane. 5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES In addition to individuals who are occupationally exposed to n-hexane (see Section 5.5), there are several groups within the general population that have potentially high exposures (higher than background levels) to n-hexane. These populations include individuals living in proximity to sites where n-hexane is produced or sites where n-hexane is disposed, and individuals living near the 60 NPL hazardous waste sites where n-hexane has been detected in some environmental media (HazDat 1998). Work situations where n-hexane is used as a solvent or in adhesives and where there are very poor ventilation conditions could also involve elevated exposure risks. Workers in poorly ventilated confined areas (e.g., warehouses, garages, tunnels) or trenches where n-hexane levels could build up from engine exhaust or from off-gassing, as in some landfill sites, might also experience higher exposures. Workers in tire-manufacturing facilities may have a heightened potential for health hazards since the rubber vulcanization process can involve exposures to n-hexane (Graham et al.1995). Individuals who subject themselves to substance abuse by inhaling n-hexane or vapors from products containing significant levels of n-hexane would also experience potentially high exposure levels (Altenkirch et al.1982; Graham et al.1995). n-HEXANE 177 5. POTENTIAL FOR HUMAN EXPOSURE 5.8 ADEQUACY OF THE DATABASE Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the Administrator of EPA and agencies and programs of the Public Health Service) to assess whether adequate information on the health effects of n-hexane is available. Where adequate information is not available, ATSDR, in conjunction with the NTP, is required to assure the initiation of a program of research designed to determine the health effects (and techniques for developing methods to determine such health effects) of n-hexane. The following categories of possible data needs have been identified by a joint team of scientists from ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would reduce the uncertainties of human health assessment. This definition should not be interpreted to mean that all data needs discussed in this section must be filled. In the future, the identified data needs will be evaluated and prioritized, and a substance-specific research agenda will be proposed. 5.8.1 Identification of Data Needs Physical and Chemical Properties. Data on physical and chemical properties are essential for estimating the partitioning of a chemical in the environment. The data on known physical and chemical properties form the basis of many of the input requirements for environmental models that predict the behavior of a chemical under specific conditions including those in hazardous waste landfills. Most of the necessary data on physical and chemical properties are available for n-hexane. Production, Import/Export, Use, Release, and Disposal. Production methods for n-hexane are described in the literature, and there does not appear to be a need for further information. Uses of n-hexane are documented, although a detailed description of all uses is not available. Quantitative estimates of production levels for the more highly purified forms of n-hexane are available. The amounts of n-hexane associated with many types of motor and heating fuels can only be roughly estimated. Information on import and export levels is lacking. This information would be useful for estimating the potential for environmental releases from manufacturing and use industries as well as the potential environmental burden. However, it is difficult to obtain this information in the detail desired since it is generally considered to be confidential business information for those industries that manufacture n-hexane. Information on disposal practices is limited. n-HEXANE 178 5. POTENTIAL FOR HUMAN EXPOSURE According to the Emergency Planning and Community Right-to-Know Act of 1986,42 U.S.C. Section 11023, industries are required to submit chemical release and off-site transfer information to the EPA. The Toxics Release Inventory (TRI) contains this information. This database will be updated yearly and provides a list of industrial production facilities and emissions. n-Hexane was added to the TRI process, with data available for an inventory baseline of 1996 (TRI96 1998). Environmental Fate. n-Hexane is a highly volatile hydrocarbon and will partition to the atmosphere if released into surface waters or onto land surfaces. The fate of n-hexane in air is reasonably well described, with free radical degradation from hydroxyl radicals being of major importance. In water, biodegradation studies in surface water and groundwater are very limited, with most studies involving various petroleum fractions. Few studies were identified dealing explicitly with the fate of n-hexane in soils. Available studies (Heringa et al.1961, Leahy and Colwell 1990, Rosenberg et al.1992) indicate that n-hexane, along with other linear alkanes, is readily biodegraded under aerobic conditions. In soils near the surface, n-hexane’s high volatility will usually result in it rapid transfer to the atmosphere. Given the volatility of n-hexane and it ready biodegradation under aerobic conditions, the most important data need would involve degradation processes in groundwater, especially under anoxic conditions. Further research is needed to identify the rates of any relevant abiotic decay and transformation mechanisms (e.g., hydrolysis). These kinds of studies are important because they provide information about the movement or fundamental mechanisms of destruction of n-hexane in the environment and aid in understanding the behavior of n-hexane at hazardous waste sites. Bioavailability from Environmental Media. Inhalation studies of humans indicate that n-hexane is bioavailable from the atmosphere. Although n-hexane in water or soil is likely to undergo transport to the air because of its volatility (although this would not necessarily be the case with n-hexane in groundwater), pharmacokinetic absorption studies using the oral and dermal routes of exposure would help clarify the bioavailability of n-hexane from water, soil, plant material, and other environmental media. Food Chain Bioaccumulation. The physical constants for n-hexane (high volatility) and a low estimated BCF value for a typical fathead minnow forage fish (ASTER 1995) suggest that n-hexane will not concentrate significantly in aquatic organisms. No empirical information is available concerning BCFs for particular species or concerning the bioaccumulation or biomagnification of n-hexane in environmental media other than water. Information concerning the accumulation of n-hexane in several trophic levels would be useful in estimating human dietary intake; however, little intake is expected.