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cause of the wide range in the amount of information available on the HAPs. For example, only 70 of the 189 HAPS are included in EPA's National Ambient Volatile Organic Compound (VOC) Data Base (5-71, a compilation of data on more than 300 commonly measured volatile a i r contami-
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w h i c h to conduct health risk assessments for many of t h e HAPS may be lacking. Similarly, the atmospheric reactions and products of some HAPS have been widely studied, but information o n the transformations of most HAPS is scarce or nonexistent. This paper presents an overview of the ambient measurement and transformation surveys. illustrating what information is available and what is lacking for assessing the public health risks from the 189 HAPs. A survey of proven and potential ambient air sampling/analysis methods for HAPS has recently been completed ( 4 ) : publication of all three EPA-sponsored surveys with full tabulations of data is planned (8).
CAAA's 189 Hazardous Air Pollutants
Transformations of Hazardous Air Pollutants T
THOMAS J. KELLY, R. MUKUND, CHESTER W. SPICER, A L B E R T J. P O L L A C K Battelle. Columbus. OH 43201-2693
he Clean Air Act Amendments of 1990 (CAAA) made a pronounced change in the regulatory approach to toxic air contaminants. Title 111, Section 112 of the CAAA accelerated the pace at which air contaminants are designated and regulated by defining a list of 189 hazardous air pollutants (HAPs) ( I ) . Title 111 is aimed at reducing the public health risk from exposure to HAPs in ambient air and requires regulation of routine and accidental emissions of each HAP from large industrial sources and from small commercial [Le., "area") sources. Maximum achievable control technology emission standards are to be
developed by EPA for each of the HAPs, with the reduction of human health risks as the mandated goal of HAPS emission control. The stated risk reduction goals of Title 111 include a 75% reduction in cancer incidence caused by HAPs from area sources. Section 112 also specifically calls for ambient monitoring of HAPS in urban areas and for consideration of atmospheric transformations and other factors that can increase the health risks from HAPS [ I ) . As initial steps toward Title I11 goals, EPA bas sponsored surveys of the reported ambient measurements ( 2 ) . atmospheric transformations and fate (31, and ambient sampling/ analysis methods ( 4 ) for the 189 HAPs. These surveys are needed be-
378A Environ. Sci. Technol.. Vol. 28, No. 8. 1994
The HAPS list The 189 chemicals designated as HAPS are remarkably diverse, consisting of industrial chemicals and intermediates, pesticides, chlorinated and hydrocarbon solvents, metals, combustion byproducts, chemical groups such as polychlorinated biphenyls (PCBs),and mixed chemicals such as coke oven emissions. Some of the HAPS are ubiquitous ambient air contaminants, primarily VOCs. Many other HAPs were assigned to the list because of their recognized toxicity in workplace environments, but have not previously received attention as ambient air contaminants. About onethird of the HAPS are semivolatile organic compounds: that is, they may exist in both vapor and particulate phases in the atmosphere. To facilitate collection of information in the surveys described here, the 189 HAPS were organized into 10 categories of chemically similar substances. The categories include 2 to as many as 49 HAPs. The nitrogenated and oxygenated hydrocarbons categories contain the most HAPs: 49 and 40 compounds,
0013-936X/94/0927-378A$O4.50/0 @ 1994 American Chemical Society
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respectively. The other categories are halogenated hydrocarbons with 27 HAPs, inorganics ( 2 3 ) , aromatics (181,pesticides (151, halogenated aromatics (81, phthalates (4), hydrocarbons (3), and sulfates ( 2 ) . This classification is somewhat subjective, a n d other categorizations could be made. Several of the HAPs could reasonably be assigned to more than one chemical category.
The ambient measurements survey The survey of ambient measurements of the HAPs (2) was conducted by computerized and manual searches to locate ambient data. T h e computerized searches included the Chemical Abstracts files from 1967 through November 1992, Chemical Abstracts Previews current files, and National Technical Information Service files from 1964 through November 1 9 9 2 . Other sources of information included review articles, reference books, technical journals, proceedings of air quality conferences, unpublished data sets from recent urban air monitoring studies, and the National Ambient VOC Data Base ( 5 , 6). The recent revision of the National VOC Data Base (7) was in progress at the time of this survey; as a result, this survey included data from the previous version of that database ( 5 , 6), that is, through about 1988. However, many of the additional data now included in the updated National VOC Data Base were also independently included in the present survey. In keeping with the aim of providing data for health risk assessment, the focus of this survey was on ambient data in populated areas of the United States. Data from remote sites and data indicating direct source impacts were excluded. For frequently measured HAPs, the intent of this review was not to catalog every data point or sample, as attempted in the National VOC Data Base (5-7), but to characterize the typical range of concentrations of HAPs and the locations and relative numbers of measurements made of the various HAPs. For HAPs that have rarely been measured in ambient air, an attempt was made to locate all reported ambient measurements. All individual ambient measurements were weighted equally as samples in this survey; that is, no distinction was made on the basis of the duration of each sample. The 189 HAPs include some redundant compounds, in the form of chemical groups ( e g , xylenes and
cresols) and their respective individual isomers. Searches were performed for both the individual isomers and the groups, but ambient data were found only for individual isomers. The HAP denoted as polycyclic organic matter (POM) is composed of numerous individual compounds, and the POM compounds measured are not always clearly defined i n reports of ambient measurements. To focus on potential health risks from POM, this survey addressed eight POM compounds identified as possible or probable human carcinogens (9, 10). Ambient data were compiled for the sum of those eight compounds. The product of this survey is an extensive table that lists for each HAP the locations, dates, number of samples, means (and/or medians), and ranges of ambient measurements, along with citations to the pertinent literature ( 2 ) .Comments on the sampling and the number of nondetects reported in ambient samples are also included in the table ( 2 ) .
Atmospheric transformations survey The survey of atmospheric transformations of the HAPs ( 3 )was conducted in much the same manner as the survey of ambient concentrations, by a combination of computer and manual searches. The computer databases u s e d i n c l u d e d those noted above for the concentration survey. Two further valuable resources were the ABIOTIK, database ( 1 1 ) and a recently published Handbook of Environmental Degradation Rates ( 1 2 ) . ABIOTIK, provides the measured rate constants for the abiotic degradation of organic compounds in the atmosphere and includes the published rate constants for several possible atmospheric reactions for the hundreds of chemicals listed, along with the pertinent literature citations. The handbook reports biotic and abiotic degradation rates for 331 chemicals in air, water, and soil. In this study the rate data were used to estimate atmospheric lifetimes and identify significant transformation processes for the HAPs, a n d the literature cited was reviewed for product information. T h e atmospheric lifetimes of HAPs are important as indicators of the rates at which those compounds are removed from the atmosphere and at which hazardous products may be formed. For some HAPs, estimates of the atmospheric lifetime have been reported in the literature.
380 A Environ. Sci. Technol., Vol. 28,No. 8,1994
In other cases we used reported rate constants, or estimated rate constants based on structure-reactivity relationships, along with assumed values of atmospheric reactants to estimate an atmospheric lifetime. The following reactant concentrations were used to represent the urb a n a t m o s p h e r e : ozone (Or31, 1.5 x lo*’ molecules/cm3 (i.e., 60 ppbv); OH radical, 3.0 x l o e molecules/cm3; hydroperoxy radical (HO,.), 1.0 x l o 9 molecules/cm:’; and nitrogen trioxide radical (NO:,,), 2.5 x l o 9 mo1ecules/cm3. Transformation of HAPs by photolysis, by reactions with liquid water, and removal from the atmosphere by dry and wet deposition were also included as possible pathways. Lifetimes were classified into broad ranges of 5 days, representing (in the context of an urban atmosphere) rapidly transformed or removed, moderately persistent, and long-lived species, respectively. The lifetime estimates are exponential values, that is, indicating the time required for the HAP concentration to decrease to l / e (i.e., 37%) of its original value by chemical transformation or removal from the atmosphere. The available information on atmospheric transformations of HAPs was restricted almost entirely to reactions in the gas phase. Chemical reactions of semivolatile HAPs in the particulate phase were not addressed i n this survey. Consequently, the atmospheric lifetimes determined for such compounds are dominated by the gas-phase data. However, the actual atmospheric lifetimes of semivolatile HAPs will depend on the extent of their vaporparticle partitioning. HAPs for which vapor-particle partitioning may be important are appropriately indicated, and deposition of particle-phase material was considered as a factor in the atmospheric lifetimes of particulate-phase HAPs, including semivolatile compounds. The product of the survey of HAP transformations is a lengthy table with an associated list of more than 140 literature citations ( 3 ) .For each HAP, the table includes the chemical formula or structure, the major removal process(es), the atmospheric lifetime, the reported transformation products, comments on the data, and references to the pertinent literature. Survey results A summary of the surveys on ambient measurements and atmos-
BLE 1 immaries of the atmospheric concentrations and transformations surveys mpound
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140, and the number of samples ranges from zero to >10,000. For 74 of the HAPs, the number of samples is zero; that is, no ambient measurements were found. This is an important observation, because it is difficult to assess the potential health risk of a HAP whose ambient concentration is unknown. Furthermore, for 116 HAPs (61% of the list) fewer than 100 measurements were found. In contrast, for 42 chemicals (22% of the list) >IO00 measurements were found. This illustrates the primary characteristic of the 189 HAPs: they are a unique set of chemicals, some frequently measured in ambient air and others rarely or never measured. Figure 1 explores what types of chemicals predominate among those 74 HAPs for which no amhient data were found, by showing the total number of HAPs and the numher with no ambient data for each of the 10 HAPs categories indicated in Table 1. Sixty of the 74 HAPs with no ambient data fall in just two categories, the nitrogenated and oxy-
Ambient Concentration measurements Median orb Locations Samples ranwas (rglm’)
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genated organics, which together make up almost half of the HAPs. The aromatics, pesticides, and inorganics categories, among others, also contain a few HAPs for which no ambient measurements were found, but in general the great majority of HAPs in these categories have been measured at least occasionally. There are several reasons for the scarcity of ambient data for some HAPs. For the nitrogenated and oxygenated organics, the most likely reason is the lack of sampling and analysis methods for these compounds at ambient levels in air. Because of their water solubility and reactivity, measurement of these chemicals is more difficult than measurement of VOCs, and consequently methods for such compounds are still in development ( 4 , 13). Method development is necessary before these HAPs can be adequately measured and regulated. For other chemical categories, the lack of ambient data may have other causes. Most measurements of pesticides have been made in agricultural areas and so are not directly relevant to the exposure of the ur-
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ban U.S. population and are not included in this survey. Many other chemicals have been measured in the workplace but not in ambient air. For example, titanium tetrachloride, elemental phosphorus, and dye intermediates such as 3,3'dimethoxybenzidine are listed as HAPs (Table 1).Although the potential toxicity of these chemicals has been established, their ambient concentrations have not been measured because they have been considered unlikely to be present in ambient air. For such compounds, ambient measurements focused in areas of known sources may be the best approach to assessing human health risks. A final reason for the lack of ambient data for some HAPs is the ambiguous nature of the identification on the CAAA list. A good example is "coke oven emissions." The emission of a variety of toxic substances from coke ovens, including benzene, other aromatics, polycyclic organic matter, and sulfur compounds, is well documented. However, it is impossible to quantify those substances in ambient air originating from coke oven emis-
sions in the face of other sources of the same compounds without, for example, detailed source apportionment modeling. As a result, measurements of coke oven emissions as a chemical group in urban air simply do not exist. The representativeness of the HAPs data for use in health risk assessments is an important issue. Some compounds, such as the chlorinated and aromatic hydrocarbons, have been measured thousands of times in dozens of locations, and the geographic spread of the data is wide. Thus sufficient data may exist to estimate typical and elevated human exposures to these chemicals. However, nearly two-thirds of the 189 HAPs have been measured fewer than 100 times, and a similar number in fewer than five locations. For most of the HAPs, therefore, the representativeness of the existing data is doubtful. Atmospheric transformations Table 1 shows that the availahility of information on the atmospheric transformations of HAPs varies widely. For 81 of the HAPs, information on reaction rates. atmos-
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624-83-9 80-62-6 1634-04-4 101-14-4 75-09-2 101-68-8 101-77-9 91-20-3 98-95-3 92-93-3 100-02-7 79-48-9 684-93-5 62-75-9 59-89-2 56-38-2 82-68-8 87-86-5 108-95-2 106-50-3 75-44-5 7803-51-2 7723-14-0
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