Document ba6EX37QX5Yw2wqM5pBanYqKD

ENGINEERING 4 A Risk assessments have become a necessary ingredient in complying with air toxics policies and regulations by Richard V. Crume 92 Pollution Engineering July 1990 A number of state and local agencies have developed air toxics policies and regulations for the control of toxic com pound emissions from industrial proc esses. These policies and regulations commonly require that air inhalation risk assessments be performed to dem onstrate that public health risks associ ated with the industrial processes are below established de minimis values. Often the risk assessment must include an evaluation of the potential for both carcinogenic and non-carcinogenic health effects in the surrounding com munity, including various sensitive re ceptors (e.g., school students and nurs ing home residents). A consideration of health effects from multipathway expo sures (e.g., the deposition of toxic air contaminants on surface waters, crops, and soils where subsequent human ex posure may occur) may also be required. This article does not attempt to de scribe risk assessment methodologies and procedures. Instead, various steps are described that can streamline the execution of air toxics risk assessments, increase the likelihood of first-time ap proval of an assessment, and reduce risks when calculated values exceed de minimis levels. Air toxics and risk assessments Air toxics are hazardous air pollut ants discharged into the atmosphere by various sources including industrial fa cilities. The degree of hazard associated with air toxics exposure varies widely, depending on the air contaminant's tox icity; the route, level and duration of exposure; and the size of the exposed population. The principal route of ex posure is inhalation, although skin ab sorption and the ingestion of aircontaminated surface waters, crops, and soils also are possible. The consequences of exposure to air toxics include both carcinogenic and non-carcinogenic human health effects. These effects are often characterized us- A ing risk assessment practices, as speci fied in various state and local air toxics policies and regulations. Risk assess ment is used to express the likelihood that individuals within an exposed population will suffer adverse health ef fects as a result of exposure. The adverse effect is death from can cer when the contaminant is a carcino gen, while exposure to non-carcinogens may lead to a variety of fatal or nonfatal health effects. Risk is expressed as a probability. For example, in an exposed population of one million, if one person would be expected to die from lifetime exposure to a toxic air contaminant, the risk of death associated with the contaminant is one in one million, or l O'*. Risk cal culations often are based on worst-case exposure scenarios, such that the risk is more likely overestimated or conser vatively calculated and resulting health protection measures are more certainly sufficient. Regardless of whether worst-case or typical-case assumptions are used, envi ronmental toxicologists consider risk assessment results to be upper bound estimates of risk, where the true risk lies somewhere between the calculated value and zero. Air toxics risk assessments are ap plied in a variety of situations. For ex ample, as part of a state's permitting procedures, new industrial facilities may be required to use risk assessment techniques to demonstrate that risks are below specified de minimis values. Additionally, both new and existing facilities may be required to perform risk assessments in situations where catastrophic releases of toxic air con taminants are possible. Finally, risk as sessments may be performed for a vari- Air toxics are hazardous air pollutants discharged into the atmosphere by various sources, including industrial facuities. VAB.0001125548 July 1990 Pollution Engineering 93 i I POLLUTION ENGINEERING A The level of the de minimis risk value may have a significant impact on the required complexity of the assessment M \< K III M.ll! W IMUMIM \l *il^k Maximum individual risk Figure 1. Mi individual's maximum risk can be reduced by increasing stack height. ety of other purposes, including compli ance with the Resource Conservation and Recovery Act (RCRA), the Com prehensive Environmental Response, Compensation, and Liability Act (CERCLA), and the Superfund Amendments and Reauthorization Act (SARA). Examples of air toxics policies and regulations for California are presented in Table 1. Although California leads the nation in air toxics policies and regulations, several other states also have active air toxics programs. Steps can be taken in planning for and conducting an air toxics risk assess ment that will streamline the assess ment process, increase the likelihood of first-time approval of the assessment to avoid repeated revisions, and ulti mately reduce risk to an acceptable level. These steps include: Preliminary contacts with air regula tory agencies; Preparation of a risk assessment pro tocol document; Use of screening techniques; and Use of model refinements, process technology and emission controls to re duce risk. Preliminary contacts An important first step in conducting an air toxics risk assessment is to be come familiar with the risk assessment requirements of the air regulatory agency with jurisdiction over the emis sion source in question. Although usu ally not required, a preliminary meeting with the load air regulatory agency can provide valuable insight regarding how the risk assessment should be per formed and documented, and what in formation should be provided to ensure future acceptance of the final assess ment without major revision. Most agencies make available risk as sessment guidelines and reference docu ments. However, these materials sel dom provide the detailed information needed to perform the entire assess ment. An added problem is that air tox ics risk assessment policies are becom ing more sophisticated and policies and reference documents may not be up to date. Thus, initial contacts with the regulatory agency take on critical im portance in determining how to per form the assessment. When regulatory agencies require as sessments to be repeated or extensively revised, a common reason is that the risk assessment was not designed with appropriate objectives in mind. These objectives may include the calculation of one or more of the following meas ures of risk: Maximum individual risk -- the risk of death from cancer for a hypothetical adult exposed at the point of maximum predicted ambient air concentration for 94 Pollution Engineering July 1990 a 70-year period, 24 hours per day. Risk for the maximum exposed indi vidual -- same as the maximum indi vidual risk, but for the point of actual exposure for the adult living closest to the emission source. Excess cancer burden -- the increased incidence of cancer death in the community surrounding the facility, usually based on either a 70-year expo sure period or on the actual expected operating lifetime of the facility. Sensitive receptor evaluation -- cal culation of the maximum individual risk for high-risk individuals such as children, asthmatics and the elderly. Non-carcinogenic effects -- potential health problems associated with com pounds causing non-carcinogenic acute or chronic health effects. Multipathway risks -- calculation ot risk, taking into account non-inhalation exposure routes such as the ingestion of air toxics including contaminated soil, fish, water, and crops or the ab sorption of air contaminants through the skin. Worker effects -- adverse health ef fects in on-site employees as well as in workers in nearby facilities. Target populations and points of ex posure for these measures of risk are summarized in Table 2. Sometimes, one or more of the above measures of risk are erroneously ex cluded from a risk assessment due tc miscommunication with the regulator} agency or incorrect assumptions about what logically should and should not be included in an air toxics risk assess ment for a given facility. Because differ ent agencies have varying philosophies about the purposes and objectives o\ risk assessments, it is important to dis cuss the objectives of the risk assess ment with the appropriate agency as a first step in designing a risk assessment study. Other important topics to raise with the agency include: What de minimis values have beer established by the agency in determin ing the levels at which calculated risks are considered significant? Is the use of emissions factors satis factory, or will emission testing will be required? What are the requirements for the use of specific dispersion models or risk as sessment methodologies? Should the worst-case, worst-plausi ble case, average or typical values be used for emission rates and other criti cal parameters? What reference sources should be VAB.000112554 f Ti *a r r ll A used to determine compounds of inter est to the agency and to determine ap propriate unit risk factors? Should risks be calculated over a 70year lifetime period or over the ex pected operating lifetime of the facility? Should 24-hour community exposure be assumed, even when many residents may spend 8 to 10 hours per day work ing outside the community? At what distance from the facility site should excess cancer burdens be calcu lated? Often these distances are ex pressed in terms of 10`7 or 10"6 risk lev els. What are acceptable risk-screening approaches? Are future revisions or amendments expected to the agency's risk assessment policies? Although the question of de minimis values may not seem important from the viewpoint of conducting an air tox ics risk assessment, knowledge of the de minimis value may be very useful in determining the degree of refinement required by the assessment. For example, if the de minimis value is relatively stringent -- a maximum individual risk of 10 6, or one cancer death in an exposed population of one million -- the use of precise dispersion models and assumptions may be neces sary to determine whether risk is below the de minimis value. On the other hand, if the de minimis value is less stringent -- a maximum individual risk of 1 O'5 or higher -- more conservative, less costly dispersion models and as sumptions may be adequate to demon strate compliance with the de minimis value. The level of the de minimis risk value may have a significant impact on the required complexity of the assess ment, affecting costs as well as time for completion. Protocol document Before an air toxics risk assessment is performed, a protocol document that outlines all procedures and assump tions to be used in the study should be prepared. Preparation of a protocol document not only provides an oppor tunity for all parties to consider and agree upon the risk assessment method ology, it also gives the regulatory agency staff who review the protocol the oppor tunity to sign off on the proposed risk assessment plan. A well-written protocol document ' that has the approval of all interested parties can minimize the revisions, cor rections and associated delays required idiiah l-". -I | * i S, M ^ a Is tf } Nil H 1 later in the study, more than justifying the extra time and expense required to prepare the document. The protocol document should spec ify all models, procedures, parameters and assumptions to be used in the risk assessment. The document also may be an appropriate place to remind inter ested parties that risk assessment is not an exact science, and the many inherent uncertainties in the process are accom modated through a series of conserva tive, health-protective assumptions. Screening techniques Considerable time and cost savings can be realized through the use of screening techniques. A good example of a screening technique is the use of the PTPLU dispersion model in place of the standard Industrial Source Com plex (ISC) model. Although the ISC model provides more precise results than PTPLU, ISC is more expensive and time-consuming to run. If the regu latory agency will allow use of screening models such as the PTPLU, and if the screening model will provide results that are sufficient to demonstrate com pliance with the de minimis value, its use should be considered. Another screening technique involves the use of worst-case values for a variety of risk assessment parameters such as emissions factors. Worst-case values usually are easier to select and justify than more realistic values and many regulatory agencies encourage or re quire their use. The hazard associated with worst-case values is that their use may result in an artificially high esti mate of risk. In fact, using worst-case values throughout a risk assessment can lead to calculated risks that are three or more orders of magnitude higher than a best estimate. Risk reduction In situations where calculated risk values exceed de minimis values, steps often can be taken to refine the calcula tions such that the calculated risk val ues are reduced. For example, where screening techniques have been used, portions of the risk assessment can be repeated using more refined and realis tic models and assumptions. If the risk values are still unaccept able, risk can be reduced further by im plementing various process modifica tions. These include: Reducing hours of operation or oper ating capacity; reducing feed rate or modifying feed composition; and increasing stack height. In the case of stack height, calcula tions can be performed to determine what height would result in sufficient dilution of the plume sue July 1990 Pollution Engineering 95 I "H fm 1I