Table of Contents 2

Abbreviations 3

Executive Summary 6

Introduction 21

Approach 23

Background and Analysis 25

Listing Criteria 54

Federal Approaches 54

State Approaches 55

Recommendations 74

References 81

Appendix A: Comparison of SC MACs with Other Exposure

and Dose Limits 85

Appendix B: Application of Proposed Listing Criteria 92

Appendix C: Examples of Protocol Use for Deriving ACLs 96

Appendix D: Curriculum Vitae of Reviewers 101

On January 1, 1999, the University of South Carolina, through its Institute of Public Affairs’ Center for Environmental Policy, entered into a contract with the South Carolina Department of Health and Environmental Control (DHEC) to review South Carolina Air Pollution Regulation 62.5, Standard No. 8, "Toxic Air Pollutants". Because of the nature of the research and its congruency with the stated objectives of the Hazardous Waste Management Research Fund, the Fund also provided some support.

The goal of this project was to provide DHEC with science-based methods for determining which air pollutants should be regulated under Standard No. 8, and for specifying appropriate ambient levels for the protection of public health. Specifically, the project team was asked to develop criteria to guide additions to and deletions from the list of air pollutants regulated by DHEC as air toxics. Also, protocols for establishing maximum ambient concentrations (MACs) under the Standard were to be developed. Finally, the use of these criteria and protocols were to be demonstrated by applying the criteria to the 58 compounds now regulated by DHEC as air toxics but not recognized by the US Environmental Protection Agency (EPA) as hazardous air pollutants (HAPs), and employing the protocols for setting ACLs for several compounds. An important constraint was that the criteria and protocols developed must make efficient use of existing information, recognizing that agency time and resources are finite.

To accomplish project objectives, a Workgroup was formed with representation of disciplines essential for developing air pollutant standards for protection of public health. The group consisted of:

As a background for this study, a survey of state HAP regulations throughout the United States was performed. Information obtained from the survey of state regulations was analyzed graphically and statistically to identify factors associated with the policy decisions of the various states in regard to the number of HAPs regulated and whether the state regulates HAPs by setting ambient concentration limits (ACLs). Next, policies and approaches for controlling HAPs, including selection of pollutants to be regulated and development of ACLs, were investigated and analyzed. Also, the relationship between the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs) and the EPA's Reference Concentrations (RfC) was explored for HAPs regulated in South Carolina.

Based on these findings, suggested criteria for pollutant selection and ACL-setting protocols were developed. Then, the use of these criteria and protocols was demonstrated by application to various pollutants. Finally, a draft report was reviewed by three recognized authorities in relevant fields: Dr. Elizabeth L. Anderson, Dr. Donald E. Gardner and Dr. Gary P. Carlson. (See Appendix D for their curricula vitae.) This report reflects the Workgroup's responses to the reviewers' comments.

Numerous examples of listing and delisting criteria are found in laws, regulations, exposure guidelines, and policy statements. For HAPs, such criteria generally consist of a definition of HAPs, and the procedures for establishing that a pollutant is or is not consistent with that definition. HAPs definitions generally exclude pollutants as regulated under other laws or regulations (e.g., "criteria" pollutants regulated by EPA, or airborne contaminants in workplaces regulated by the OSHA), and from pollutants not emitted during routine operations (e.g., accidental leaks). The definitions also differentiate HAPs from other ambient air pollutants based on their known or potential effects on human health or the environment. HAPs are usually chemicals that may cause "adverse" effects on human health and the environment. Also, definitions may distinguish HAPs from other pollutants by specifying the types of effects to be avoided and the severity of these effects. Finally, some states have established numerical toxicity rating systems to decide which pollutants to regulate. Often the initial list of HAPs consists of pollutants recognized as human toxicants by an organization that has performed a detailed evaluation of many contaminants.

The second component of current listing criteria, the procedures for listing and delisting, are as diverse as the definitions of HAPs. In all states with their own HAP regulations, citizens and other organizations may petition for listing or delisting of a pollutant. In some states, such as California, revision of the regulated HAP list requires a formal risk assessment and approval by peer reviewers. In Michigan, all pollutants are considered to be HAPs unless proven otherwise by formal risk assessment.

Approaches for establishing safe levels for human exposures have been developed over the last century. Three distinct methods have been employed for ACL development (Calabrese and Kenyon, 1991). The most scientifically rigorous, but resource intensive, approach is to evaluate risk as a function of exposure levels for each regulated pollutant. ACLs are then set at levels unlikely to cause adverse health or environmental effects. Different approaches have developed for setting acceptable levels for carcinogenic pollutants and for non-carcinogenic toxic pollutants. Both approaches are often applied to a single pollutant with multiple potential health effects. An expedient alternative to these methods has been the derivation of ACLs from occupational exposure limits (OELs). The following is a brief overview of these methods.

For noncancer endpoints, acceptable levels of pollutant exposures generally are based on the concept that a specific adverse effect will not occur below a threshold exposure level of the pollutant. An example of this method is that first used by EPA to develop the Reference Dose (RfDs) for ingestion (US EPA, 1993) and Reference Concentration (RfCs) for inhalation. First, the no-observed-adverse-effect-level (NOAEL) is determined, i.e. the highest experimental exposure level at which no statistically significant differences in the rates of adverse effects were observed between an exposed group and a control group. For determining the safe levels for airborne contaminants, the NOAEL is most often determined from animal inhalation experiments. The RfC is then obtained by dividing the NOAEL by various uncertainty and modifying factors. For setting noncancer ACLs, the advantages of this approach are: potency, duration and exposure route are accounted for; it relies on primary sources; and ACLs may be directly developed for general population exposure (Patrick, 1994c).

For convenience and to obtain conservative risk estimates, cancer endpoints often are assumed not to have a threshold. Lower doses lead to lower excess risk of cancer (i.e., cancer risk above the background level). The regulatory treatment of environmental carcinogens has been to define a level of acceptable risk contributed by the chemical exposure and to determine the corresponding dose. This generally requires extrapolation to the doses experienced in the environment from the higher doses used in animal assays or encountered in occupational exposures. Usually, a linear low-level dose-response is assumed (Crump, et al., 1976; US EPA, 1996). For air pollutants, the concentration corresponding to the extrapolated dose is then calculated. In 1996, EPA proposed a more flexible approach for relating effects and exposures that considered a carcinogen's mode of action (US EPA, 1996).

Recently, efforts have been made to unify the approaches used for noncarcinogens and carcinogens (Crump, et al., 1996; Gaylor, et al., 1998). One such method, termed the benchmark dose (BMD) approach, has been applied for setting acceptable exposure levels for non-cancer endpoints.

In addition to the careful use of the well-established procedures discussed above, independent peer review of the process is recognized as essential for obtaining standards and guidelines of the highest scientific quality. The purpose of peer review is to develop recommendations with a solid scientific foundation. Bias and potential conflict of interest can be minimized by careful selection of reviewers.

Another approach for developing ACLs is to apply safety factors to OELs. The OELs most frequently used as a basis of ambient air limits are the ACGIH TLVs. Although OELs and their documentation represent the most extensive database of airborne contaminant concentrations considered safe for human exposure, they have several drawbacks as the basis of ACLs. OELs are developed to protect persons often less susceptible to a chemical’s toxic effects than the general population. They assume an intermittent, not continuous, exposure. They do not account for environmental fate. Although OELs are divided by uncertainty factors to account for these to differences, the appropriate factors for each pollutant are at best order-of-magnitude estimates. Finally, standards based on OELs cannot be easily modified to reflect new scientific studies because the detailed methods and data used to derive an OEL are often not available.

Calabrese and Kenyon (1991) recommended that OELs not be used as the basis of ACLs unless their documentation suggests that the OEL is a "reasonable surrogate" for a NOAEL or lowest-observed-adverse-effect-level (LOAEL). The ACGIH TLVs in particular have been sharply criticized for use as the basis of OSHA PELs and in developing state and local ACLs for HAPs. Nevertheless, TLVs may be the only secondary data source available for some compounds. The following regression equation (R² of 0.70) was developed to relate TLVs to RfCs:

log₁₀(RfC) = -3.5 + 1.1 log₁₀(TLV)

where both RfC and TLV are expressed in m g/m³.

In addition, to the methods described above, hybrid approaches have been used or recommended. For instance, Caldwell, et al. (1998) described a method that makes extensive use of secondary databases and establishes a hierarchy of secondary data. Tier I values are those from EPA sources "with the lowest uncertainties, most comprehensive peer review, and greatest consistency in derivation." Tier II values are taken from other EPA data sources, as well as from California EPA (CAL/EPA) and the Agency for Toxic Substances Disease Registry (ATSDR). Tier III values are "comprised mostly of qualitative indicators of potential HAP hazards with the addition of default values" and have the highest degree of uncertainty of health hazard information of the three tiers.

This section presents for DHEC's consideration the Workgroup's conclusions regarding air toxics regulation in South Carolina, including its findings and concerns.

1. Need for SC MAC Standards

EPA MACT standards were aimed at establishing a modicum of control over HAPs after two decades of ineffective federal activity. They focus on only certain source categories and pollutants. The SC MACs complement federal HAPs emission standards. They provide public health and environmental protection in areas near sources not regulated by EPA and for some pollutants not regulated by EPA. Thus, the Workgroup concluded that the SC Standard is an essential tool for controlling air toxics.

2. Definition of TAPs

The Workgroup noted that Standard No. 8 does not define TAPs. Many other air pollution regulations incorporate a definition to distinguish regulated pollutants from other materials and other air pollutants. Such definitions often address the severity of effects to be avoided.

If DHEC revises Standard No. 8, the Workgroup suggests that inclusion of a definition of TAPs be considered. The Workgroup here provides a model that may serve as a starting point should DHEC decide to develop such a definition:

This definition devotes fewer words to environmental effects than to human health effects. This is not a reflection of the relative importance of these effect categories, but of the relative lack of methods and data for setting standard to avoid environmental effects.

3. Acceptable Risk

Development of ambient concentration limits and evaluation of residual risk of federal MACT standards both require that a level of acceptable risk be established. EPA's goal is to limit cancer risk to 10^-4 for the most-exposed persons, and to 10^-6 for the majority of exposed persons. "Most-exposed" refers to individuals who might be living close to a pollutant source and are possibly at greater risk. "Majority of exposed people" refers to the general population. Although no such levels have been established in South Carolina, an acceptable risk goal is essential for developing MACs to limit excess cancer risk in exposed populations. The acceptable risk goal selected should be reasonable, taking into account the rate of cancer mortality in the general population.

4. Background Levels

Eight federal HAPs (bis(2-ethylhexyl) phthalate, benzene, carbon tetrachloride, chloroform, ethylene dibromide, ethylene dichloride, formaldehyde, and methyl chloride) were estimated to have continental and regional scale background levels exceeding the benchmark screening concentrations developed for the EPA Cumulative Exposure Project (CEP) (Woodruff, et al., 1998). The MACs do not consider that single source emissions of a pollutant are superimposed on the background levels of that pollutant. Thus, compliance with MAC standards does not assure acceptable risk levels for pollutants with significant background levels. The ubiquitous nature of these eight pollutants suggests that the federal government should regulate their ambient concentrations. However, the 1990 emissions database used in the CEP was subject to considerable uncertainty. EPA is scheduled to release a more accurate assessment this year. Until the original CEP findings are confirmed, enforcement of MAC standards can assure, at least, that the contribution of individual sources does not elevate risk in excess of the acceptable risk level. If concentrations cannot be reduced by regulation as HAPs, the EPA should consider regulating these compounds as criteria pollutants.

5. Multiple Sources

Even when background levels are low and all single sources are in compliance with Standard 8, emissions from several nearby sources may cause the ambient concentration to exceed the MAC standards. This can result from the mixing of plumes from different sources near these sources. Although this probably occurs in relatively few areas of the State and only under specific meteorological conditions, it is important to be aware of this possibility. Such situations are best handled on a case-by-case basis.

6. Multiple Media and Multiple Pathway Exposures

Environmental exposure to some contaminants may result from exposure to more than one medium (e.g., air, water, soil) or by more than one transport pathway. For example, persons may be exposed to lead from a regulated stationary source, from soil near their homes contaminated with lead from historical motor vehicle emissions, and from deteriorating lead-based paints indoors. Like exposure from multiple sources, compliance with Standard 8 does not assure acceptable risk levels when such complex exposures occur. But also like exposure from multiple sources, the areas of the state and pollutants that present multiple media/multiple pathway hazards are limited. Such problems require more complex interventions than simply setting ACLs, and, like multiple sources, are best addressed on a case-by-case basis.

7. Protection of Susceptible Persons

The general population is known to have a range of sensitivity to individual pollutants. The very young and the chronically ill are often more susceptible. The aim of air pollution regulation should be to protect the normal range of sensitivity within the community including those known, or reasonably believed, to have increased susceptibility. Uncertainty factors are often used to extrapolate to levels protective of susceptible subpopulations. The alternative BMD approach accounts statistically for normal variation of susceptibility. ACLs set by EPA and some other agencies have taken susceptible persons into account. It is important to note, however, that due to differences in genetic predisposition, some people's response to chemical exposure may be outside the range of those expected of the general population. This group is said to be hypersusceptible. Sufficient information is rarely available to determine safe levels for hypersusceptible persons. Thus, it must be recognized that the MACs may not be protective of this extremely susceptible group.

This section presents the Workgroup's recommendations to DHEC regarding Standard 8. These recommendations are based on the Workgroup's experience in environmental public health and, more specifically, in developing guideline levels, and its analysis of various approaches to regulating and controlling airborne contaminants. The primary aim of these recommendations is to place the regulation of TAPs in South Carolina on a firmer scientific footing. A secondary consideration is to develop an approach that makes optimal use of pollutant toxicity information available from secondary sources, thereby accommodating the agency’s limited resources for setting MACs.

1. Identification of candidate pollutants for regulation as TAPs

The Workgroup recommends consideration of pollutants for listing identified through: (1) formal petition by persons or organizations in South Carolina; (2) review of emissions information; (3) a periodic review of available databases and regulations promulgated by other agencies; and (4) ambient air data and information. The Workgroup recommends that DHEC set up mechanisms for funneling relevant information from permitting and air monitoring groups to those responsible for TAPs regulation development if these mechanisms do not already exist. The Workgroup also recommends that DHEC perform a periodic review of the secondary sources of toxicity information listed below.

2. Multiple Pollutant Exposures

MAC standards make no provision for protecting public health and the environment from the effects of multiple air toxics. If the effects of pollutants at ambient levels are different, standards for individual pollutants should adequately protect health

and the environment. If the effects are additive, the following formula is often used:

where C_i = the concentration of pollutant i,

MAC_i = the MAC of pollutant i, and

n = equals the number of additive pollutants in the mixture.

In each group, the sum of the ratios of concentration to the MAC value of each member of the group must not exceed unity. It is possible that, although each individual exposure does not exceed its MAC value, the permissible total may be exceeded and exposures must be reduced. If DHEC revised Standard No. 8, the Workgroup recommends incorporating the above formula to address exposure to pollutants with additive effects.

Some pollutant combinations are antagonistic or synergistic; i.e. one pollutant nullifies or exacerbates the effects of another. No simple equation can be applied to evaluate risk of all such combined exposures; they must be considered on a case-by-case basis.

3. Acute Effects

The MACs were developed to control chronic effects of air toxics. Nevertheless, compliance is based on the maximum one-hour emission potential of a source. Thus, acute effects during normal source operations should also be prevented by compliance with Standard 8. Accidental release rates during emergencies may exceed the maximum one-hour emission rate used in Standard 8 compliance calculations. However, such releases are regulated under other state and federal programs. Thus, the Workgroup recommends that Standard 8 remain focused on controlling chronic health and environmental effects and that the use of the source maximum one-hour emission potential for compliance determination be continued.

4. Procedures for listing new pollutants to be regulated as TAPS

The procedure for listing and delisting of TAPs for regulation in South Carolina should be flexible and take advantage of the pollutant toxicity evaluations performed by scientifically reputable agencies and organizations. The Workgroup recommends adding a pollutant to the list of regulated pollutants if emission to the ambient air from a source regulated under SC Standard 8 is likely and evidence exists of the pollutant's potential to cause adverse public health and environmental effects, as described in the definition of TAPs. Sufficient evidence of potential toxicity may include, but need not be limited to, any one of the following conditions:

Carcinogenic Effects:

1. EPA classifies pollutant as a known, probable, or possible human carcinogen (Class A, B1, B2 or C), or, under EPA's proposed guidelines for carcinogen classification, as a "known/likely" human carcinogen (http://www.epa.gov/iris/);
2. International Agency for Research on Cancer (IARC) classifies pollutant as a human carcinogen, a probable, or a possible human carcinogen (Group 1, 2A, or 2B);
3. National Toxicology Program in its Report on Carcinogens classifies the pollutant as a known human carcinogen or as an agent that may be reasonably anticipated to be a carcinogen.
   (http://ntp-server.niehs.nih.gov/NewHomeRoc/AboutRoC.html)
4. The American Conference of Governmental Industrial Hygienists (ACGIH) classifies the pollutant as a confirmed human carcinogen, a suspected human carcinogen, or a confirmed animal carcinogen with unknown relevance to humans (Classification A1, A2, or A3) as listed in the most recent Threshold Limit Values.

1. EPA has developed a RfD or RfC for the pollutant (http://www.epa.gov/iris/).
2. California EPA has developed a chronic inhalation Reference Exposure Level for the pollutant (http://www.oehha.ca.gov/air/hot_spots/index.html).
3. The ACGIH has a TLV for the pollutant in its most recent listing.
4. The Agency for Toxic Substances and Disease Registry (ATSDR) has developed a Minimal Risk Level (MRL) for the pollutant.

Classification as a known, likely, or possible human carcinogen, or the equivalent, by one of the national and international organizations listed above based on a weight-of-evidence approach should be adequate evidence for listing a pollutant and establishing a MAC under SC Standard 8. The Workgroup recommends that currently unlisted pollutants not be listed on the basis of carcinogenicity if classified as "cannot be determined", or the equivalent, by the organizations listed above.

Alternatively, the existence of exposure or dose limits set by one of the organizations listed above, also should be adequate evidence for listing and regulation. Although some of these limits are intended for use in the workplace, they are nevertheless an indication of known human toxicity. The organizations listed above utilize rigorous, peer-reviewed protocols for classifying compounds carcinogenicity and setting exposure and dose limits.

5. Procedures for delisting pollutants regulated as TAPS

Removing a pollutant from the list of regulated TAPs may be expected to be more difficult than adding a pollutant to the list because the previous evidence suggesting that the pollutant is potentially harmful to human health or the environment must be conclusively refuted. The most scientifically sound evidence would consist of studies, superior in scope or methods to prior research, demonstrating that the pollutant does not cause the adverse effects shown or implied in previous studies. The following steps are recommended: (1) The petitioner for the delisting of a pollutant is required to supply evidence that the pollutant does not conform to the definition of a TAP. This includes copies of recent scientific studies used to make this argument. (2) Upon receipt of the petition, DHEC informs stakeholders of their intent to consider delisting, and requests stakeholder comments. (3) DHEC then reviews the scientific literature regarding the adverse impact of this pollutant, including the information supplied by those outside the agency. (4) If it finds that the pollutant does not conform to the definition of a TAP, DHEC then will recommend delisting to the SC legislature. As appropriate, DHEC may initiate the procedure for delisting using external peer review as needed.

For a pollutant listed based solely on its carcinogenic potential, evidence in support of delisting may include the following:

The Workgroup recommends that a listed pollutant remain listed if all the above organizations classify the pollutant’s carcinogenicity as "cannot be determined" or the equivalent. The Workgroup also recommends that the classifications, regulations and guidelines from the organizations mentioned above be reviewed on a regular basis to identify delisting candidates.

6. Procedure for Developing MAC Standards

The procedure recommended below is based on the Workgroup's belief that ACLs may be developed by use of secondary data sources, such as the EPA RfCs or the CAL/EPA Reference Exposure Levels. As a basis of an ACL, the secondary source value must have been derived for the same exposure scenario as the ACL, or must be adjusted to compensate for differences in exposure scenarios. Also, it must be recognized that the methods and data used to develop a secondary source value limit its accuracy. For instance, EPA has limited ability to update RfC values. Thus, many RfCs were not developed using the latest methods and data. Nevertheless, the secondary sources recommended for use in deriving MACs utilized the best methods and databases available at the time a guideline level was developed. Also, the cost of developing and updating ACLs from primary data sources exceeds DHEC’s current resources. Thus, the approach recommended by the Workgroup conserves agency resources by not duplicating risk assessments performed by other organizations.

The procedure suggested below applies a hierarchical approach that ranks secondary sources according to: (1) the quality of the data used; (2) the uncertainties in establishing safe levels; (3) the consistency of methods used in their derivation; and (4) their use of peer review. The recommended protocol assumes that the MACs will continue to focus on the chronic effects of community exposure. The recommended approach is very similar to that used by Caldwell, et al. (1998) for developing benchmark concentrations for cancer and chronic non-cancer effects.

The recommended hierarchy of data sources is shown in Table I. If a pollutant is classified as a human carcinogen by EPA, IARC, NTP, or ACGIH as described in Recommendation 4, the highest priority health effect value available for the pollutant is used to obtain the inhalation unit risk (IUR). Then, the concentration corresponding to the acceptable risk value is calculated by dividing the risk by the IUR.

The third priority source is the EPA oral unit risk or ED₁₀ values. Although they are established using scientifically rigorous protocols, they should be used as the basis of ACLs only if portal-of-entry effects are negligible or reliable adjustments can be made to account for the differences. Factors that may be affected by portal-of-entry include absorption efficiency, metabolism, and target organ/tissue.

Caldwell, et al. (1998) developed benchmark concentrations that were used to identify potentially hazardous conditions. Thus, they selected the lowest cancer potency that had been determined by EPA as a default potency, that of methylene chloride. Here we recommend that the mean IUR in the EPA IRIS database be used as the default because this is the best point estimate for an unknown IUR. As of April 2000, the mean was 6x10^-3 (m g/m³)^-1.

In recognition of the continual changes in the toxicity databases, the Workgroup recommends that a periodic survey of the data sources and update of MAC values be performed.

Table I. Hierarchy of Data Sources Recommended for Developing South Carolina MACs.

Endpoint	Priority	Data source*	Health Effect Value
Cancer	1	US EPA	Inhalation Unit Risk
	2	US EPA	Oral Unit Risk values or ED10s expressed in terms of inhalation unit risk values
	3	CAL/EPA	Inhalation Unit Risk values
	4	Default	Default potency: average of EPA IURs
Chronic Non-cancer	1	US EPA	Inhalation Reference Concentrations
	2	US EPA	Provisional Inhalation Reference Concentration
	3	CAL/EPA***	Reference Exposure Levels developed for the California Hot Spots Program for chronic toxicity
	4	ATSDR	Minimal Risk Levels developed for chronic toxicity
	5	ACGIH	TLVs

For chronic non-cancer effects, the MAC is set equal to the highest priority health effect value (Table I). If a TLV must be used as the basis for a MAC, the Workgroup recommends that the following equation, derived from regression of TLVs against the RfC, be used to obtain the MAC:

log₁₀(MAC) = -3.5 + 1.1 log₁₀(TLV)

where both MAC and TLV are expressed in m g/m³.

In 1995, the South Carolina Department of Health and Environmental Control (DHEC) formed a committee to discuss concerns expressed by South Carolina industry representatives that the South Carolina Air Pollution Regulation 62.5, Standard No. 8, "Toxic Air Pollutants", should be withdrawn. Their rationale was that the federal Hazardous Air Pollutants (HAPs) emissions standards would provide sufficient protection to the public from exposure to hazardous air pollutants. They further requested that, in the event that Standard No. 8 was not withdrawn, the list of regulated pollutants be reduced to include only the 188 hazardous air pollutants (HAPs) regulated by the United States Environmental Protection Agency (EPA). The committee met over a two-year period and, as a result of these meetings, revisions were proposed to this Standard, which were approved by the DHEC Board and submitted to the Legislature in December 1997. During discussions at the Board meeting, the staff was requested to review the chemicals that were not on the EPA list and report back to the Board at a later date. The DHEC staff plans to review the entire list periodically to ensure that the compounds listed, along with their ambient standards, are still appropriate to provide adequate protection to the citizens and environment of the state.

On January 1, 1999, the University of South Carolina, through its Institute of Public Affairs’ Center for Environmental Policy, entered into a contract with DHEC to review South Carolina Standard No. 8. Because of the nature of the research and its congruency with the stated objectives of the Hazardous Waste Management Research Fund, the Fund also provided some support for this project.

The goal of this project was to provide DHEC with science-based methods for determining which air pollutants should be regulated under Standard No. 8, and for specifying appropriate ambient levels for the protection of public health. Specifically, the project team was asked to develop criteria to guide additions to and deletions from the list of air pollutants regulated by DHEC as air toxics. Also, protocols for establishing maximum ambient concentrations (MACs) under the Standard were to be developed. Finally, the use of these criteria and protocols were to be demonstrated by applying the criteria to the 58 compounds now regulated by DHEC as air toxics but not recognized by EPA as hazardous air pollutants, and employing the protocols for setting MACs for several critically important compounds. An important constraint was that the criteria and protocols developed must make efficient use of existing information, recognizing that agency time and resources are finite.

Terms such as hazardous air pollutants (HAPs), air toxics (ATs), toxic air contaminants (TACs) and toxic air pollutants (TAPs) are used by various regulatory agencies in the United States to designate the same class of pollutants, although the specific pollutants regulated vary from agency to agency. These terms refer to air pollutants that may cause adverse effects on human health and the environment, but are not regulated by EPA as "criteria pollutants." They are used interchangeably throughout this report except when discussing the regulations of a specific agency.

To accomplish project objectives, a Workgroup was formed with representation of the major disciplines essential for developing air pollutant standards for protection of public health. The group consisted of:

As a background for this study, a survey of state HAPs regulations throughout the United States was performed. Graduate assistants searched the World Wide Web for information relevant to HAPs regulation and made contact by telephone with state government officials responsible for HAPs control. Particular attention was paid to states that have developed HAPs standards, beyond simply enforcing EPA regulations. Information on these standards was obtained and summarized.

Next, policies and approaches for controlling HAPs, including selection of pollutants to be regulated and development of ambient concentration limits (ACLs), were investigated and analyzed. Based on these findings, recommended criteria for pollutant selection and ACL-setting protocols were developed. Then, the use of recommended criteria and protocols was demonstrated by application to various pollutants. Finally, three recognized authorities in relevant fields, Dr. Elizabeth L. Anderson, Dr. Donald E. Gardner and Dr. Gary P. Carlson, reviewed a draft report. (See Appendix D for their curricula vitae.) After responding to the reviewers' comments, this report was submitted to DHEC.

The recommendations developed here are based on the premise that the goals of a state hazardous air pollutants standard should be congruent with the mission of the state health and environment regulatory agency. The criteria for adding and deleting pollutants to be regulated and the protocols for setting ACLs should follow from the standard's goals. This report begins with a summary of the background for regulation of HAPs, including federal and state regulations. More detailed discussions of federal efforts to control HAPs may be found in many of the references, particularly Patrick (1994) and Calabrese and Kenyon (1991). This is followed by specific analyses and recommendations related to listing criteria, and setting ambient concentration limits (ACLs) for air pollutants.

Information obtained from the survey of state regulations was analyzed graphically and statistically to identify factors associated with the policy decisions of the various states in regard to the number of HAPs regulated and whether the state regulates HAPs by setting ACLs. Potential associations were assessed for the following state characteristics: EPA region; population; population rank; percent of population less than 5 years old; percent of population greater than 70 years old; percent of population white, black, Asian/American, Indian/Pacific islander, and Hispanic; gross state product; per capita personal income; sum of modeled HAPs concentrations from the EPA Cumulative Exposure Project (CEP); area; and major industries. All characteristics were obtained from The World Almanac and Book of Facts 1999 (1998) except the CEP data taken from Woodruff, et al. (1998).

The means of possibly associated variables for states with ACLs were compared with the means for states without ACLs using Student t-tests. The means were considered to be different for the two groups if the "t" statistic was significant at p<0.10 for tests with either unequal or pooled variances. Also, determinants of the number of ACLs set by a state were investigated using stepwise regression. The independent variables consisted of the state's physical, demographic, and economic characteristics listed above. Variables entered into the regression with a p<0.10 were considered to be significantly associated with the number of regulated pollutants. All calculations were performed using SAS (Version 6.12, SAS Institute, Inc., Cary, NC).

The relationship between the American Conference of Governmental Industrial Hygienists Threshold Limit Values (TLVs) and the US Environmental Protection Agency's Reference Concentrations (RfC) for HAPs regulated in South Carolina was explored. The base 10 logarithm of the RfC was plotted against the logarithm of the TLV. Also, regression was used to develop an equation for predicting RfCs from TLVs. Both graphical and statistical analyses were performed using Sigmaplot 5.0 (SPSS, Inc., Richmond, CA).

The foundation for HAPs regulation by the EPA was established in the 1970 Clean Air Act (CAA) Amendments. In Section 112 of the Act, HAPs were defined as air pollutants "which may reasonably be anticipated to result in an increase in mortality, or an increase in serious irreversible, or incapacitating reversible, illness."(42 USC §7412) EPA was given a mandate to identify HAPs to be regulated and to establish standards for these pollutants. Under the Act, the Agency had broad authority to decide which pollutants to regulate, but less latitude in determining the type and bases for the standards. EPA was required to set emission limits that "provide an adequate margin of safety to protect the public health," but was not authorized to consider the technical feasibility or the economic consequences of these standards.

The 1970 CAA Amendments also required EPA to regulate another class of pollutants, the "criteria pollutants". In response, EPA developed and promulgated the National Ambient Air Quality Standards (NAAQS) for criteria pollutants, i.e. ambient levels that are protective of human health with an adequate margin for safety, and of other adverse effects not related to human health. Unlike the approach for controlling criteria pollutants, the 1970 CAA Amendments required EPA to establish emission standards, not ambient standards, for HAPs. Subsequently, minor changes in federal HAPs policy were made in the 1977 CAA Amendments, for instance allowing HAPs standards to regulate work practices. Nevertheless, EPA's basic regulatory approach remained unchanged until 1990.

Conflict regarding the pollutants to be regulated and the relative importance of public health protection, economic impact, and technical feasibility marked the first 20 years of federal HAPs regulation. The discord played out in political and judicial arenas, as summarized by Robinson and Pease (1991) and Patrick (1994a). In 1987, he U.S. Court of Appeals ruling in National Resources Defense Council, Inc. v. EPA clarified the basis for setting HAPs emission limits. In response to the court's decision, EPA (US EPA, 1989) defined the risk levels deemed to be acceptable in setting emission limits. The goals were to "provide maximum feasible protection against risks to health from hazardous air pollutants by (1) protecting the greatest number of persons possible to an individual lifetime risk level no higher than approximately 1x10^-6 [one in one million] and (2) limiting to no higher than approximately 1x10^-4 [one in ten thousand] the estimated risk that a person living near a plant would have if he or she were exposed to the maximum pollutant concentrations for 70 years."

Calabrese and Kenyon (1991) characterized the second decade of HAPs regulation as follows:

Prior to 1990, federal standards were set for only 8 HAPs -- asbestos, beryllium, mercury, vinyl chloride, benzene, radionuclides, inorganic arsenic, and coke oven emissions. The task set before EPA was enormous. Specific decisions included which pollutants to regulate as HAPs and the problem of balancing risks versus cost and feasibility. Over 65,000 chemicals were in use in industry and many more pollutants were recognized to be formed by chemical reaction in the air post emission. Most of these potentially hazardous agents lacked relevant toxicological data. To expedite control of HAP's health hazards, Congress, in its 1990 Amendments to the CAA, mandated EPA to set emission standards for 189 HAPs (172 single compounds and 17 pollutant categories). This selection of HAPs was developed from a list of 224 pollutants: (1) regulated under Section 313 of the 1986 Emergency Planning and Community Right-to-Know Act, (2) reported to EPA as exceeding the reportable quantities in the Comprehensive Emergency Response and Compensation Liability Act, and (3) regulated by states and listed in EPA's National Air Toxics Clearinghouse. Prior to the Amendments, EPA recommended additions and deletions based on toxicity and air pollution potential (Patrick, 1994b). The 1990 Amendments required completion of standards for all 189 HAPs by the year 2000. One HAP, caprolactam, has been delisted, lowering the number of HAPs to be federally regulated to 188.

As EPA began developing standards, they often found it more efficient to develop standards for specific source categories, instead of individual pollutants, because many source categories emit more than one HAP. From 1970 through 2000, 109 standards were to be developed for 174 source categories. By 1999, EPA had promulgated 43 standards for 78 sources. Federal standards for HAPs are found in Title 40 of the Code of Federal Regulations, Parts 61 and 63.

The federal emission limits for existing sources are based on "maximum achievable control technology" (MACT), defined as the technology used by the best performing 12% of sources within a source category or subcategory, or the best performing five sources in a category or subcategory with fewer than thirty sources. The emission limits for new sources are required to be no less stringent than the best performing similar sources. For area sources, which usually consist of many small sources, the emission standards are based on generally available control technology (GACT). In addition to setting MACT and GACT standards required in Section 112, the 1990 CAA Amendments required EPA to address: HAPs emissions from utilities; the problem of mercury emitted to the atmosphere accumulating in fish; deposition of air pollutants in the Great Lakes; deposition of HAPs in the Chesapeake Bay and coastal waters; radionuclides; incinerator emissions; a strategy for air toxics in urban areas; and a national assessment of air toxics ambient concentrations and emissions. Brief summaries of these provisions may be found in Waxman (1994) and Bailey (1994).

The 1990 CAA Amendments recognized that MACT standards might not be fully protective of public health, because MACT emissions are set at levels achievable with existing technology, irrespective of health and environmental impacts. Thus, EPA, after setting MACT standards, is required to assess the "residual risk", the risk to health and the environment remaining after compliance with MACT emission standards. If the MACT standards do not protect public health with an "ample margin of safety", then EPA must set more stringent emission limits. By 2001, some allowable emission rates may be reduced from MACT emission levels to those required to limit cancer risk to 10^-4 for the most-exposed persons, and to 10^-6 for the majority of exposed persons. Standards protecting against adverse environmental effects are required to consider issues of cost, energy, and safety, as well as impact on environmental quality.

EPA has numerous on-going efforts to estimate HAPs emission and exposures, and to evaluate their impact on public health and the environment, including the National Toxics Inventory (NTI) and the Cumulative Exposure Project (CEP). The NTI compiles HAP emission estimates based on the Toxic Release Inventory (TRI) and numerous other sources of emissions data, such as information accumulated in developing MACT standards. EPA analyzed the 1993 NTI emission estimates in its 1996 and 1997 National Air Quality and Emission Trends Reports (US EPA, 1998; US EPA, 1998b). In the 1996 report, total 1993 HAPs emissions were estimated to be 3.7 million tons, with 41.5%, 34.6% and 23.9% from mobile, area, and point sources, respectively. These figures changed dramatically in the 1997 report, even though the same year's emission data were considered. The total HAPs emissions for 1993 were estimated to be 8.1 million tons, with 21.0%, 18.0% and 61.0% from mobile, area, and point sources, respectively. Although these estimates have significant bearing on the effectiveness of controlling HAPs emissions from point sources, no explanation of the differences was offered. South Carolina was ranked in the moderate HAPs emissions category, 77,000 to 167,000 tons per year (US EPA, 1997b).

The Cumulative Exposure Project (CEP) estimated air toxic exposure levels for 1990 in each of 60,803 census tracts in the contiguous United States by air quality modeling (Woodruff, et al., 1998; Caldwell, et al., 1998). These estimated exposures were then compared with benchmark concentrations (BCs), the potential regulatory thresholds of concern derived from available standards and guideline levels. Several of the findings based upon the assessment of 1990 data are of special significance to regulators and relevant to this investigation, including:

• the estimated background concentrations exceeded BCs for 8 HAPs (bis(2-ethylhexyl) phthalate, benzene, carbon tetrachloride, chloroform, ethylene dibromide, ethylene dichloride, formaldehyde, and methyl chloride) in 100% of the census tracks;
• neglecting their already high background concentrations, benzene and formaldehyde exceeded the BCs in 90% of the census tracts, and ethylene dichloride exceeded its BC in 21% of the census tracts;
• of the 40 Tier I HAPs (i.e., those with the most certain BC levels), 35 exceeded their cancer BC, and 13 exceeded their cancer BC by a factor of 100 or more in 10% of the census tracts, an area of the US containing a population of 20 million people;
• measured concentrations, where available, generally exceeded modeled concentration estimates
• for 20% of modeled HAPs, insufficient data are available to develop BCs, although weight-of-evidence indicates potential carcinogenicity;
• noncancer BCs could not be developed for 50% of HAPs;
• insufficient information is available to consider reproductive and developmental effects in BC determination, even for compounds such as benzene and 1,3-butadiene that are know to have these effects.

A further analysis of the 1990 CEP data estimated that exposure to HAPs in the US causes a median cancer risk of 18 lifetime cases per 100,000 people (Woodruff, et al., 2000). It must be recognized that the 1990 emission estimates upon which the CEP results are based have a high degree of uncertainty. Thus, more refined concentration estimates are needed to make thoughtful recommendations for improving federal HAPS regulations. However, most measured HAPs concentrations are higher than the CEPS estimates. Thus, the CEP preliminary findings engender valid concern.

EPA was able to promulgate standards for only 8 HAPS prior to 1984 and none between 1984 and 1990. Unable to regulate HAPs expeditiously, the federal government encouraged states to develop their own air toxics programs (US EPA, 1985). The states were confronted with growing public concern about air pollution's role in high urban cancer rates, and the prevalence of toxic contaminants, especially carcinogens, in the nation's air (Calabrese and Kenyon, 1991). Thus, state agencies took up the task of developing HAPs standards. In 1984, only nineteen states were regulating HAPs; by 1989, all states had some means of regulating HAPs (STAPPA/ALAPCO, 1989).

States were found to employ a wide variety of approaches, including agency policy, formal regulations, and informal case-by-case activities (STAPPA/ALAPCO, 1989). Specific regulatory efforts may be categorized as ambient concentration limits (ACLs), lowest achievable emission rates (LAER), risk assessment, and various control technology requirements. The types of state regulations in use in 1989 are summarized in Table II. Obviously, states placed greater emphasis on controlling new sources than existing sources, although many regulations require that modified sources meet new source standards. This approach is similar to automotive emission control in that control of existing stationary sources is phased in as aging facilities and processes are updated.

Table II. Number of States with Their Own HAPs Regulations in 1989, by Type of Regulation.¹