1. INTRODUCTION

Potential health effects of air toxics are a major concern to the public. In recent years, air toxic emissions have become the focus of several regulations in California. Routine releases of air toxics are regulated by the Air Toxics "Hot Spots" Information and Assessment Act of 1987, commonly known as AB 2588, which established in California a statewide program for the inventory of air toxics emissions from individual facilities as well as requirements for risk assessment and public notification of potential health risks. Facilities that handle acutely hazardous chemicals are required to prepare a risk management and prevention plan (RMPP) which must include a consequence analysis of offsite risk due to their accidental releases.

This paper summarizes the air toxics regulations of California. For a facility subject to these regulations, extensive information on facility emissions and potential health risks are required. The paper also describes a PC-based integrated modeling system designed to facilitate the compliance with all requirements of emissions inventory, dispersion modeling, risk assessment and reporting set forth in these regulations.

2. CALIFORNIA AIR TOXICS REGULATIONS

To gather information on air toxics from facilities that may pose localized adverse health effects, the Air Toxics "Hot Spots" Information and Assessment Act, commonly known as AB 2588, was passed in 1987. It established a statewide program for the inventory of air toxics emissions from individual facilities as well as requirements for risk assessment and public notification of potential health risks. A facility subject to AB 2588 must complete, and update every four years, a detailed inventory of its emissions of substances on the ARB list which is known as Appendix A-I of the Emission Inventory Criteria and Guidelines Regulations (ARB, 1993). These ARB guidelines specify the types of emission that each class of facility must inventory; requirements for facility diagrams, including emission locations; requirements for source testing, measurement and estimation techniques; and standardized report formats. An air toxics inventory plan (ATIP) is first submitted to the local air district. Once the plan is approved, the facility has 180 days to complete an air toxic inventory report (ATIR). Local air pollution control districts are to determine which facilities will be required to prepare risk assessments based on a prioritization process which involves consideration of potency, toxicity, quantity of emissions, proximity to sensitive receptors (hospitals, daycare centers, worksites and residences) and any other factors determined by local districts to cause potential significant risk. As part of this process, districts are to categorize facilities as high, intermediate or low priority. Facilities designated as "high priority" are required to submit a health risk assessment (HRA) within 150 days following the District's notification. Risk assessments are to be reviewed by the districts and the Office of Environmental Health Hazard Assessment (OEHHA). Upon approval of a risk assessment, a facility is required to give notice to all exposed persons if the district determines that there is a significant health risk associated with emissions from the facility.

The "Hot Spots" Act was amended in 1992 by the Senate Bill 1731 (the Calderon bill). Under SB 1731, a facility with significant risk must prepare and implement a risk reduction audit and plan (RRAP). The implementation of the RRAP must reduce emissions below the significant risk level within five years.

In addition to the above regulations dealing with routine releases, a facility handling acutely hazardous materials must submit registration forms and/or provide risk management and prevention programs (RMPP). The registration form must include a general description of the processes and principal equipment involved in the handling of acutely hazardous materials. A RMPP is required if the local administering agency (e.g., the fire department), determines that the facility may pose an accident risk. Guidance for preparing a RMPP has been developed by the California Office of Emergency Services (OES, 1989). It must include a report and analysis of accidents at the facility involving acutely hazardous materials during the past three years; information about equipment used to handle these materials; controls and procedures to minimize risks; and a schedule for implementing additional safety equipment or procedures. The RMPP also must consider the results of a hazard and operability (HazOp) study identifying the hazards associated with the handling of an acutely hazardous material due to operating error, equipment failure or external events which may present an accident risk. The RMPP further must consider an offsite consequence analysis which examines the hazard identified in the HazOp study in conjunction with adverse air dispersion and other environmental conditions. The offsite consequence analysis must include a clearly drawn map that shows potentially affected areas, other zones of vulnerability, and the levels of expected exposure in each zone.

3. ATIRAS - AIR TOXICS INVENTORY REPORT AND ANALYSIS SYSTEM

As described above, California air toxics regulations require facilities to prepare and submit extensive information under strict guidelines. The Emission Inventory Criteria and Guidelines Regulations establish stringent requirements for the preparation of a facility emissions inventory, including emission quantification methods and reporting forms. Requirements for health risk assessments are established in the AB 2588 Risk Assessment Guidelines published by the California Air Pollution Control Officers Association (CAPCOA, 1993). As defined under AB 2588, a health risk assessment includes a comprehensive analysis of the dispersion of hazardous substances in the environment, the potential for human exposure, and a quantitative assessment of both individual and population-wide health risks associated with those levels of exposure. Dispersion modeling also plays a major role in the RMPP offsite consequence analysis of accidental releases.

All the information requirements by air toxics regulations are rather intensive and are best handled by a computerized information management system. Thus, Applied Modeling Inc. (AMI) has developed an integrated modeling system named ATIRAS (Air Toxics Inventory and Risk Assessment System). ATIRAS is an user-friendly PC software system which runs on IBM PC and compatibles with 386/387 or better processors and a minimum of 4 MB of RAM. As described below, the ATIRAS system is composed of five modules for emissions inventory, facility prioritization, air dispersion modeling, health risk assessment and computer graphics.

3.1 Emissions Inventory Processor

Emissions data calculations and reporting are handled by the ATIR PRO module which is the Air Toxics Inventory Report (ATIR) PROcessor. This module is completely menu-driven and has several user friendly features such as searches for chemical names, their Chemical Abstract Service (CAS) numbers, the facility's standard industrial classification (SIC) and source classification codes (SCC). It accepts site-specific emission factors such as those derived from source test results or those recommended by regulatory agencies. At the user's request, it can search default emission factors from built-in data bases such as the EPA-developed AP-42 and FIRE systems. Processing of facility information and emissions data is based on the use of the reporting forms developed by the ARB: Form FAC contains facility's identification and description; stack data are reported on Form STK; devices are shown on Form DEV and Form PRO is used to report emissions from each process within each device. In addition, there are supplementary forms such as Form S-UP which is used to report substances below the degree of accuracy or those not required to be quantified, and Form SUM which summarizes the facility-wide emissions totals for all emitted pollutants. For each of these forms, data entry screens are used to facilitate the data entry. Default values, menus and built-in error traps are used extensively to minimize data entry errors. The facility emissions inventory can be printed on these reporting forms and/or submitted electronically in ASCII format. From the base case inventory, the system allows easy updates and evaluation of "what-if" scenarios for permitting and planning purposes.

3.2 Facility Risk Prioritization

With the emissions developed by ATIR PRO, the user can perform the facility prioritization to assess whether it is categorized as a "high priority" facility. The ATIRAS system uses the ranking procedures recommended by CAPCOA (CAPCOA, 1990). One of these procedures is the Emissions and Potency Procedure which primarily relies on three parameters to prioritize facilities: emissions, potency or toxicity, and the proximity to potential receptors. This procedure has also been modified to take into consideration other factors such as exposure periods and non-inhalation exposure for multipathway pollutants. Basically, a facility receives two scores: one for carcinogenic effects and the other for non-carcinogenic effects. The facility is then ranked based on the highest of these two scores. Normally, a facility is designated "high priority" or Category A if its highest score is above 10. A high priority facility is required to prepare and submit a health risk assessment.

3.3 Air Dispersion Modeling

Air concentrations from routine releases of air toxics are predicted by the Short-Term version of the Industrial Source Complex (ISCST3) model. This Gaussian dispersion model is recommended by the U.S. EPA for modeling sources (point, area and line) located in urban or rural areas. The complex terrain algorithm of the COMPLEX I model has been implemented in ISCST3. Hence, this model is applicable to both flat and complex terrain. Input file to the ISCST3 model is set up by a menu-driven data entry program which extensively uses default values, menus and built-in error traps to minimize data entry errors. ATIRAS can also generate the building downwash inputs using the EPA-developed BPIP program. Since air toxics risks are predicted for several receptors, ATIRAS has implemented a program to automatically generate a grid of receptors surrounding the modeled facility.

For acutely hazardous materials, dispersion modeling required for an offsite consequence analysis is performed with the toxic screening TSCREEN model. This model incorporates screening techniques recommended by the U.S. EPA for estimating ambient impacts of accidental releases (U.S. EPA, 1992). The model is quite versatile since it can handle both instantaneous and continuous releases of gases, particles and liquids from various sources (e.g., storage tanks, pipes ...). The model handles instantaneous and continuous releases of liquids from pressurized storage, and the evaporation from surface impoundments such as lagoons. TSCREEN can also perform a screening dispersion of denser-than-air gases from vertical jet releases or area sources. For refined modeling of dense gas releases, the ATIRAS system can also interface with advanced dispersion models such as DEGADIS or SLAB.

3.4 Health Risk Assessment

Potential health risks are predicted by the ACE2588 (Assessment of Chemical Exposure for AB 2588) risk assessment model (AMI, 1991; Tran and Sears, 1991). Developed by Applied Modeling Inc. (AMI), this model incorporates the algorithms and recommendations of the CAPCOA AB 2588 Risk Assessment Guidelines (CAPCOA, 1993). The ACE2588 model is designed for multipathway risk assessment and is capable of evaluating potential health risks from multiple pollutants (cancer and non-cancer) emitted from multiple sources located in a wide range of settings (flat or complex terrain, rural or urban, onshore or offshore). ACE2588 assumes that all emission sources from the facility are modeled by a guideline dispersion model (e.g., ISCST3) with appropriate meteorological data, stack parameters and receptor locations.

Air toxics and associated health effect data are contained in an input file to the ACE2588 model. This database contains over 150 carcinogens and noncancer pollutants. Unit risk factors for computing cancer risk through inhalation and potency slopes for estimating risk from non-inhalation pathways are specified for carcinogens. Reference exposure levels and oral doses (for multipathway pollutants) are required for non-cancer pollutants. In addition, toxicological endpoints (e.g., kidney, immune system, liver etc.) affected by the pollutants are specified through indicator flags.

For pollutants with non-inhalation exposure (i.e., multipathway pollutants such as arsenic, chromium and TCDD), concentrations in other media are required for risk assessments. For these and other multipathway pollutants, the ACE2588 model computes pollutant deposition and concentrations in soil, drinking water, vegetation (root and non-root crops), farm animals (cattle, pigs, poultry, goats and sheep) and fish. Chemical-specific factors (such as half-life in soil, root uptake, diet transfer coefficients and bioconcentration factors) and site-specific factors (such as water surface area, average annual rainfall and percentage of animal diet provided by grazing) are considered in the calculations.

Potential carcinogenic and noncancer health risks are quantified with the calculated exposure doses. Carcinogenic effects are predicted by the individual lifetime cancer risk at each receptor and the population excess cancer burden. For noncancer pollutants, hazard indices are calculated for all applicable toxicological endpoints (e.g., kidney, cardiovascular system, liver etc.) due to acute and chronic exposure.

3.5 Computer Graphics

The ATIRAS system extensively uses computer-generated graphics for both data checking, visualization and reporting. The system can generate plots of facility boundaries, location of emission sources, nearby buildings and receptors. These plots provide a rapid visual mean of checking the accuracy and validity of modeling inputs. A contour mapping program is also built into ATIRAS. Combined with appropriate site-specific street or topographical maps, it can generate isopleths of predicted risks. Zones of significant impact are clearly depicted on these risk isopleth maps. Graphics generated by ATIRAS can be viewed on-screen or printed.

4. SUMMARY

Facilities emitting air toxics in California are regulated by several regulations which require extensive information on facility operation, emissions and risk assessment. Preparing and submitting these information are very time consuming and costly. A PC-based user-friendly integrated modeling system named ATIRAS has been developed to fully meet the requirements of these regulations. The system incorporates methodologies and mathematical models for dispersion and risk analyses approved by regulatory agencies. The system is an efficient tool in the preparation of facility emissions inventory and risk assessment, and is applicable to a wide range of air toxics impact analyses, from short-term accidental releases to long-term routine releases.
 

5. REFERENCES

Applied Modeling Inc., 1991. User's Guide to the Assessment of Chemical Exposure for AB 2588 (ACE2588) Model. Report prepared for Santa Barbara County APCD, February 1991.

Air Resources Board, 1993. Proposed Amendments to the Emission Inventory Criteria and Guidelines Regulation for the Administration of the Air Toxics "Hot Spots" Information and Assessment Act of 1987. Prepared by the Technical Support Division, California Air Resources Board, April 1993.

CAPCOA, 1993. Air Toxics "Hot Spots" Program Revised 1992 Risk Assessment Guidelines. Prepared by the Toxics Committee of the California Air Pollution Control Officers Association, October 1993.

CAPCOA, 1990. Air Toxics "Hot Spots" Program Facility Prioritization Guidelines. Prepared by the AB 2588 Risk Assessment Committee of the California Air Pollution Control Officers Association, July 1990.

Office of Emergency Services, 1989. Guidance for the Preparation of a Risk Management and Prevention Program. Prepared by the Hazardous Material Division, California OES, November 1989.

Tran, K.T. and R. Sears, 1991. Development of A Comprehensive Risk Assessment Model for Air Toxics. Paper No. 91-113.1. Presented at the 84th Annual Meeting & Exhibition, Vancouver, Canada, Air & Waste Management Association, June 1991.

U.S. Environmental Protection Agency, 1992. Workbook of Screening Techniques for Assessing Impacts of Toxic Air Pollutants (Revised). EPA-454/R-92-024, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, December 1992.