Evaluation of the public health risks associated with semivolatile metal and dioxin emissions from hazardous waste incinerators.

The public health impacts associated with stack emissions from hazardous waste incinerators have become a major concern in recent years. Most evaluations of incinerator stack emissions have focused on three classes of compounds: metals, semivolatile, and volatile compounds. These investigations have been complicated by the difficulty and expense of analyzing the emissions and the limited amount of toxicity information for many of the compounds that have been detected. The results of over 20 trial burns at hazardous waste incinerators were assembled in an attempt to determine which compounds may pose a significant threat to the public health. The risks associated with semivolatile emissions were found to be inconsequential, although further study of dioxins and dibenzofurans emissions appears to be warranted. The risk associated with the emission of cadmium and perhaps chromium (VI) may pose a significant risk to public health at certain facilities. Controls on waste feed or air pollution control devices should be employed to reduce the emission of these metals. Any monitoring of metal emissions from hazardous waste incinerators should focus on cadmium and chromium (VI).


Introduction
Hazardous waste disposal practices have undergone major changes during the past decade. As new Federal and State statutes have become effective, the disposal of hazardous wastes into unsecured landfills has been curtailed (1,2). These new statutes have mandated many changes in hazardous waste disposal practices (3,4). Within a few years, the disposal of most types of hazardous wastes to any landfill will be virtually prohibited unless the wastes have been suitably treated. Only the residuals from an accepted waste treatment process will be permitted to be disposed of into a hazardous waste landfill.
The new Federal and State hazardous waste treatment standards provide for what constitutes an acceptable waste treatment practice (5,6). These stndards designate either a minimum level of treatment or a specific technology that must be employed to treat each class ofhazardous waste. Both Federal and California treatment standards are based primarily on the best demonsted available technology that can be employed to treat the waste.
Incineration is a very effective technology for destroying organic wastes (7)(8)(9). Federal regulations require that hazardous waste incinerators achieve a minimum of 99.99 % destruction and removal efficiency (10). Facilities that process certain wastes containing polychlorinated dibenzodioxins are required to achieve a 99.9999% destruction efficiency (10).
Even efficient hazardous waste incinerators emit small quantities of organic compounds from the stack (7,(11)(12)(13)(14). Organic compounds in stack emissions are usually attributed to three sources: uncombtisted wastes and fuel, waste or fuel constituents that are only partially combusted, and compounds formed due to chemical reactions occuring during the combustion process. While only a small portion of thermally stable organic compounds survive the combustion process, the metallic components ofhazardous wastes are not susceptible to thermal destruction. Metals can be oxidized and transformed.
Compounds emitted from the stack that have resulted from fiagments or the recombination of fragments produced from waste or auxiliary fuel constituents are known as products ofincomplete combustion (PICs). They include the polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs). The PCDDs and PCDFs are two classes of compounds that have received considerable public attention in recent years. The notoriety associated with these substances stems from animal bioassays that demonstrate that these compounds are potent carcinogens and reproductive toxicants (15)(16)(17). PCDDs and PCDFs have been detected in the emissions from certain types of incinerators (7).

Trial Burns
Federal, State, and local statutes and regulations govern permitting hazardous waste incinerators in California (18)(19)(20). A number ofrequirements are stipulated before a permit is issued to operate a hazardous waste incinerator. Trial burns are usually conducted at facilities to determine the incinerator's efficiency in destroying certain designated constituents, the principal organic hazardous constituents, in a waste stream.
During a trial burn, a representative waste, which is often augmented with thermally stable compounds, is processed under normal operating conditions. Samples of the waste, emissions from the stack, and perhaps ash and air pollution control equipment blowdown strams are collected and analyzed to determine the levels of the designated constituents of concern. Using the results ofthe trial burn, the efficiency ofthe incinerator is determined by comparing the amount ofa principal organic hazardous constituent that is emitted from the stack with the amount processed during the trial burn. Of late, the results of trial burns have also been employed to evaluate the impact ofincinerators on public health (21,22). The emission rates of various constituents ofconcern are monitored during a trial burn. Air dispersion modeling is employed to estimate the impact ofthe incinerator's emissions on ambient air adjacent to the facility. Dose-response relationships are then employed to evaluate the public health impacts associated with the stack emissions.

Semivolatile Compounds and Metals
As the number oftrial burns at different facilities has increased, more information has become available that can be employed to delineate which toxic constituents emitted from an incinerator's stack pose a substantial threat to the public health. This report will evaluate the potential impact on public health associated with the emissions of metals and semivolatile compounds from the stacks of hazardous waste incinerators.

Facilities
Trial burns representing a wide range of incineration technology were included in this study. The emissions ofmetals and semivolatile compounds from five liquid injection incinerators, five liquid injection/rotary kilns, two rotary kilns, one fixed hearth, two fluidized bed incinerators, and six industrial boilers, a cement kiln, and an aggregate kiln were evaluated for potential impact on the public health (Table 1). Volatile emissions from these facilities were evaluated separately in a companion study (23). Descriptions of the waste feeds used and the air pollution control equipment associated with the incinerators that were the subject of a trial burn are found in Table 1.

Stack Sampling and Analytical Procedures
Since the results of many independent studies were used, the analytical procedures employed varied at the different facilities. However, stack gases were sampled for metals, dioxins, and semivolatile compounds using a similar procedure, a modified method 5 sampling train (MM5). Using a modification of EPA method 5 sampling train, stack gases are drawn through a series offilters and XAD-2 resin traps. Semivolatile constituents were determined in the various studies using gas chromatography/ mass spectrometry methods. The levels ofmetals inparticulates captured by filters were determined using inductively coupled argon plasma, atomic absorption, or visible spectroscopy. The emission rates from the stack were then determined, as described previously, from the level ofeach constituent captured by the MM5 (23).

Air Dispersion Modeling
Using the stackemission rates determined for the various constituents, airdispersion modeling was employedto determine the impactofthe facility had itbeen located atanurban coastal setting oran inland rural location in California. Meteorological data from Los Angeles International Airport (urban) and Edwards Air Force Base (rural) were employed to model the air emissions. The approachemployedtomodelthestackemissionshasbeenpreviously described (23).

Health-Based Criteria
CriteriadevelopedbytheCaliformiaDepartmentofHealthServices or the EPA were employed to evaluate the potential public health impacts associated with the maximumannual average concentration oftoxicants inairadjacentto each facility. Thecompilation and use of the criteria to estimate the cumulative risk associated with the emission ofindividual compounds has been previously described (23). When available, ambient air criteria wereemployedtoevaluatetheriskassociated with exposureby the inhalation pathway. Otherwise, amodifieddrinking watercriterion was employed to evaluate the exposure. Ifmore than one criterionwas available, themostconservativecriterion was employed.

Statistical Analysis
Regressionanalyses wereemployed to identify significant correlations between the emission rates of Cd, As, or Cr and the cumulative risk associated with the emission rates of these metals. Statistical analyses were preformed using the SYSTAT statistics software package on a IBM PS/2 model 60 (38).

Semivolatile Compounds
Although many ofthe waste strams processed by the facilities contained substantial quantities of semivolatile compounds, only low levels ofthis class ofcompounds were detected in the stack emissions. Eight carcinogenic semivolatile compounds were identified in the stack emissions from these incinerators ( Table   2). Only three carcinogenic semivolatile compounds, benz[a] anthracene, 2,4,6-trichlorophenol, and bis(2-ethylhexyl)phthalate were detected at more than one facility. The risk associated with the emission ofthese carcinogenic semivolatile compounds appears to be inconsequential (Table 3).

Dioxins and Dibenzofurans
Relatively few studies were located where dioxin emissions were monitored at a trial burn. Both the California Department ofHealth Services dioxin equivalence approach (39) (24) Chevron (25) Mobay (26) Liquid injection/rotary kiln incinerator SCA Chemical Services 1983 (27) SCA Chemical Services 1988 (28) Rollins (29) Du Pont (11 ) Energy Systems Company (30) Fixed hearth incinerator Trade Waste Incinerator (11) Aggregate or lime kiln Florida Solite (31) Rockwell ( with dioxin emissions. Unless information was provided concerning the dioxin congeners, all tetra through hepta forms were assumed to be chlorinated at the 2, 3, 7, and 8 positions. Based on this assumption, a highly conservative estimate of risk was ascertained. At only two of nine incinerators did the risk associated with dioxin emissions exceed the benchmark of 10-6 risk (Table 3).

Metals
Nine facilities were identified where metal emissions were hRisk(x 10-6). monitored during the course of a trial burn. Four carcinogenic metals were detected in the emissions from these facilities. The maximum risk associated with the stack emissions ranged from 400 x 10-6 to 0.2 x 10-6 ( Table 4). Cadmium and chromium were responsible for virtually all ofthe carcinogen risk associated with metal emissions at these facilities (Table 4).
Only the hexavalent form ofchromium is considered to exhibit carcinogenic activity. Since only total chromium was measured in the stack emissions, the evaluation employed the highly conservative assumption that all the chromium emitted from the stack is in the hexavalent form. The actual risk associated with exposure to this metal is probably considerably lower than that displayed in Table 4.
The emissions of cadmium, chromium, and arsenic were evaluated to identify correlations with the cumulative risk associated with metal emissions. Cadmium emissions strongly correlated with metal emissions, with a coefficient of deter-  mination (r2) of 0.98 (Fig. 1). Arsenic and chromium did not significantly correlate with the cumulative risk associated with metal emissions (data not shown).
Other noncarcinogenic metals were also evaluated to determine if they pose a significant threat to the public health. Exposure to lead and mercury occurred only at a fraction ofa percent ofthe applicable ambient air criteria. However, mercury, a volatile metal, was not expected to be effectively sampled by the methods used in these trial burns (41). 'No analysis for this metal. bResults expressed as risk (x 109) in a rurl/urban area. ND, not detected.

Discussion
The results of20 trial burns were evaluated to determine ifthe emission of semivolatile compounds or metals could be a source ofsignificant risk to the public health. Dioxins and dibenzofuans were evaluated separately from other semivolatile compounds.

Dioxin and Dibenzofurans
Dioxins and dibenzofurans were detected in the stack emissions offive hazardous waste incinerators in this study. Only after employing the highly conservative assumptions that all forms of the tetra through hepta dioxins and dibenzofurans that were detected are chlorinated in the 2, 3, 7, and 8 positions (if not otherwise specified), using the maximum average annual concentration projected by the air dispersion modeling and using the California Department ofHealth Services equivalency procedure was it demonstrated that the risk associated with these compounds substantially exceeded 10-6 at two of the facilities. Estimates ofrisk based on the EPA equivalency approach did not markedly exceed 10-6 at any facility. Had information been available concerning the actual distribution of the dioxin and dibenzofuran congeners, it is likely that the estimates of risk at all of the facilities would have been much lower.
Much higher levels of dioxin emission have been detected at other types of incinerators (42). However, lower combustion temperatures, less uniform mixing ofthe wastes, and lower oxygen levels in the combustion chamber are thought to be responsible for the higher level ofdioxins detected at these types of incinerators. Although dioxins have been monitored at relatively few facilities, it appears that the monitoring ofthis class ofco ¶pounds may be unnecessary at hazardous waste incinerators. Clearly, more studies are needed to determine ifthe emission of this class ofcompounds from hazardous waste incinerators can pose a significant threat to public health.

Semivolatile Compounds
Although semivolatile compounds composed a significant portion ofthe wastes processed during the trial bums, only very low levels ofthese compounds were detected in stack emissions. Several ofthese trial bums were conducted specifically to evaluate the combustion ofcertain semivolatile compounds such as PCBs and pentachlorophenol.
The risk associated with stack emissions ofthis class ofcompounds appears to be inconsequential. Semivolatile compounds tend to be less thermally stable than volatile compounds (43). Based on this study, there does not appear to be a need to monitor the emission of semivolatile compounds from hazardous waste incinerators.

Metals
The results ofthis study demonstrate that the emission ofcadmium and perhaps chromium may pose a significant risk to public health near certain hazardous waste incinerators. Based on the extremely conservative assumption that all chromium is in the 6+ oxidation state, the cumulative risk associated with the emission ofcadmium and chromium substantially ded 10-6 for five of seven facilities. However, only cadmium emissions were found to correlate with the cumulative risk associated with metal emissions.
The highest risk was associated with the American Cyanamid facility, an incinerator with no air pollution equipment. The Trade Waste Incinerator facility produced the next highest risk. The significant amount ofsand and gravel processed during the trial burn at this facility probably contributed to the high metal emissions.
The Du Pont, Chevron, and SCA Chemical Services facilities were equipped with air pollution control equipment. Although this equipment did not appear to be a type that is highly efficient in removing Cd, Cr, or As, the lowest risks due to metal emissions were associated with the Du Pont and Chevron incinerators. The air pollution control equipm^ent operating during the SCA Chemical Services 1983 trial burn was not functioning optimally; it was reported that much lower emissions of metals would be anticipated during future operations.
An aggregate and a lime kiln were included this study. While the risk associated with metal emissions from the Rockwell lime facility was relatively low, a relatively higher risk was associated with metal emissions from the Florida Solite incinerator. The operation of a baghouse during the trial burn, an efficient air pollution control device for removing Cd, Cr, and As, probably accounts forthelowerriskassociated with metal emissions from the Rockwell incinerator. Florida Solitewas notequipped with effectiveairpollutioncontrolequipmentforparticulateemissions.
The emission of metals are governed by the level of metals in the waste feed, the nature ofthe combustion process, and the type ofair pollution equipment operating at the facility. Many ofthe trial burns processed solid wastes or wastes containing sizable amounts ofmetls. Only one ofthese facilities was equipped with an efficient air pollution control device for reducing the emissions of cadmium, chromium, and arsenic (44).
Most of the trial burns were conducted at a time when the emissions ofmetals were not ofparticular concern. The level of metal emissions observed in the study may not be representative of facilities equipped with air pollution control devices designed to minimize the emission of particulates. This study is valuable for delineating which metals should be targeted in managing and monitoring the operation ofhazardous waste incinerators. The emissions of cadmium and perhaps chromium (VI) should be controlled by managing the types of waste processed by the facility or by installing adequate air pollution control equipment. Any monitoring ofmetal emissions from these facility should be focused on these metals.
While mercury emissions did not appear to pose a significant threat to the public health, the analytical procedures employed inthe trial burns did not appear to be apable ofdetecting volatile mercury emission. Volatile emissions may compose the major portion the mercury emissions from hazardous waste incinerators. Further studies are necessary to better quantify mercury stack emissions and determine ifthe emissions can pose a threat to public health.
Efforts to determine which constituents in stack emissions may pose a significant risk to public health prompted a study aimed at identifying which operating conditions are associated with changes in the enmission ofthese compounds (45). The results of over 20 trial burns were included in this study. Few significant correlations between emissions rates and most operating parameters were identified. This finding is probably due to the inherent differences in facility design, operating conditions, sampling locations, waste feeds, and air pollution control devices, which resulted in high variability in the emissions from the various facilities.
For the incinerators included in this study, the risk associated with volatile emissions correlated with the number of volatile carcinogens evaluated and the heat input of the facility. These results suggest that the size ofthe facility (as indicated by its heat input) can be correlated with the risk associated with stack emissions. These findings also suggest that the determination ofrisk associated with volatile emissions is dependent on the number of carcinogenic volatile compounds employed in the assessment.
The only operating parameter that significantly correlated with the risk associated with metal emissions was excess oxygen. This finding could be due to the entrainment ofparticulates resulting from increases in air flow through the combustion chamber or decreases in the residence time in the air pollution control equipment. Further studies are needed to determine if either mechanism is responsible for the apparent correlation between the risk associated with metal emissions and excess oxygen.
Efforts to construct hazardous waste incinerators are often hampered by public concerns that these facilities pose a substantial risk to public health. The ability to effectively monitor these facilities to insure that they do not pose a significant risk to public health should aid in addressing these concerns. The routine monitoring oftoxic emissions from hazardous waste incinerators is not currently practical due to technical difficulty and the expense ofsampling and analysis. Focusing the monitoring on total hydrocarbon emissions and selected metal emissions should facilitate routine stack monitoring.