Environmental Risk Assessment Reports - Chapter 24

CHAPTER 24 Risk Assessment of Airborne Chemicals Jeanne C. Willson CONTENTS I. Introduction.................................................................................................466 II. Conceptual Site Models..............................................................................466 A. Indirect Exposure Pathways.........................................................469 B. Project Manager Role in Conceptual Site Model Development .....................................................................469 C. Developing Data Quality Objectives............................................470 1. State the Problem.........................................................471 2. Identify (Define) the Decision.....................................471 3. Identify Inputs to Decision..........................................471 4. Define Study Boundaries.............................................471 5. Develop a Decision Rule.............................................471 6. Specify Limits on Decision Errors..............................471 7. Optimize Design for Obtaining Data..........................472 D. DQO Process: Final Check ..........................................................472 III. Estimating Chemical Concentrations at Exposure Points: Transport Models........................................................................................473 IV. Occupational Exposure and Risk Assessment...........................................473 A. Describing Toxicity: Reference Concentrations and Unit Risk................................................................................474 V. Conclusion: Risk Characterization and Informing the Risk Manager........................................................................................476 References...................................................................................................477 465 © 2001 by CRC Press LLC 466 A PRACTICAL GUIDE TO ENVIRONMENTAL RISK ASSESSMENT REPORTS I. INTRODUCTION Risk assessment professionals argue endlessly about how much soil people eat, if any, or whether certain groundwater sources will be used as sole sources of residen-tial drinking water, and a host of other risk assessment exposure questions. But nobody argues about whether people breathe air. When chemicals are in the air, people are exposed. Discussion of airborne chemical risk assessment centers around modeled predictions, the toxic effects of the chemicals (especially at low doses), probabilities of accidental releases, the hazards of inhaling small particulate matter, and indirect pathways. Project managers have many opportunities to inject rationality into the air toxics risk process, regardless of their level of technical involvement. In this chapter, we will discuss the typical issues that arise in evaluating air toxics, with special emphasis on what managers should watch for, and we will discuss the general approach to risk assessment* as it applies to air toxics, including: · Developing a conceptual site model · Applying the DQO process · Using appropriate exposure and toxicity information to develop a risk characterization II. CONCEPTUAL SITE MODELS Evaluating risk from chemicals in air is not quite as simple as detecting its presence somewhere and plugging detected values into a model. While a consulting risk assessor will probably do this evaluation, direct input and oversight from project managers at this point is significant and critical. Project managers know the site (or situation) and know what happens. That knowledge, plus common sense, provides 90% of what is needed to develop a conceptual site model, which describes all of the significant ways in which people may contact site-related chemicals and which will be the foundation of the risk assessment. Fortunately for all of us, the mere presence of a chemical anywhere is not enough to cause a risk. Enough of it must (1) move to and (2) contact someone (a receptor) before there is a risk. Actually we can be more specific than that about the require-ments for significant exposure that might indicate complete exposure pathways from a source to a receptor, via air. 1. A source must exist, such as an incinerator or ventilation stack, an evaporation pond, fugitive (nonpoint) emissions from an industrial facility, or any other signif-icant source of chemical that is open to the air. A secondary source might be water in a home that releases aerosols when used for showering, cooking, flushing toilets, watering the lawn, watering a vegetable garden, and so forth. 2. A release mechanism is required. For air, look for (1) volatilization, (2) wind release of particulates from contaminated soils, (3) emission through ventilation * For additional information on this general approach, the EPA’s 1999 risk assessment guidance is still the best single summary available at this writing (see References). © 2001 by CRC Press LLC RISK ASSESSMENT OF AIRBORNE CHEMICALS 467 of stacks, etc., and (4) negative pressure that develops inside basements that sucks in volatile chemicals, radon, etc. from soils or groundwater-derived vapors sur-rounding building foundations. 3. A transport mechanism may be required if potentially exposed people are located away from the release point. For air, transport mechanism means “wind.” 4. The obvious exposure medium is air, but consider also (1) deposition of particulate matter on outdoor soils that may be eaten directly, tracked inside, and eaten as “incidentally ingested” house dust, or absorbed by garden vegetables, which may be eaten; (2) deposition in indoor house dust; (3) attachment to dust particles, which are then readily deposited in the lungs (this is the mechanism for radon exposure). Other potential routes to exposure media are possible. 5. An exposure point is required. The amount of chemical that actually reaches a person or an ecological receptor is the amount that is significant, not the amount emitted from the source. The selection of transport models and placement of monitoring stations should account for this distinction. Air measurements should be made in the breathing zone — 3 to 6 feet off the ground — not at the ground from a flux chamber or far above head on a telephone pole. Also note that direct measurements made away from a source are likely to measure other sources as well. We found, for example, that measured cadmium and other metals may originate from domestic wood burning, not from metal mines. 6. Receptors must be present, now or in the future. Is that downwind cabin a year-round residence or just a summer home? Did the transport model predict concen-trations at the housing development or in the middle of a fallow field? If the only possible receptors are maintenance people or occasional visitors, what is their expected exposure frequency? At this point in the analysis, risk assessors generally note that there must also be a route of exposure: oral, inhalation, or dermal absorption. Whether there is a route of exposure can be debated for certain con-taminated media; for example, not everyone has to pump and drink the ground-water. The debate is a minor issue for air; since everyone breathes, inhalation is an obvious route of exposure. Another exposure route that may be important for airborne chemicals is eye exposure that may result in significant irritation and tearing. Potentially complete pathways are compiled into descriptive lists and graphic presentations to provide guidance to the risk assessment (see Figure 1). Conceptual site models can be elaborate, with molecules of a chemical being chased all over the countryside. Perhaps this tendency is an ill-guided response to public pressure and concern. The author even heard of a serious proposal to evaluate the risk to humans posed by being bitten by wild animals exposed to windblown (radioactive) particulate matter deposited on the soil. The best way to argue against such foolish-ness is to identify a limited number of exposure pathways that will cause the greatest potential exposure. If those pathways are managed so that risk is negligible, then other pathways derived from those are also almost guaranteed* to be negligible. A factor that is not always considered in risk assessments is degradation of chemicals. Chemicals in air may be photodegraded or oxidized, and this may result in greatly reduced risk. On the other hand, it also results in smog formation in cities. * Note that it is part of the job description of a risk assessor to never be virtually 100% certain of anything. © 2001 by CRC Press LLC Figure 1 Elements of site conceptual model. © 2001 by CRC Press LLC RISK ASSESSMENT OF AIRBORNE CHEMICALS 469 Since this may be overlooked, project managers should suggest it to risk assessors if some exposure pathways are found to pose a potential risk of health effects based on hypothetical modeling. A. Indirect Exposure Pathways In the early 1980s, the infancy of environmental risk assessment, it was deemed sufficient to make simple assumptions about daily intake of water, soil, and air by adults living full time with chemicals in those media. Simple calculations of chemical intake were made and risk was calculated. As risk assessment grew up, it became apparent that these simple assumptions were inadequate in some important ways: children are not tiny adults, exposure rates can vary widely, and chemicals move from one environmental medium to another. These things need to be accounted for in risk assessments. In the field of air toxics, additional pathways became known as “Indirect Pathways.” Guidance for evaluating indirect pathways has been formalized under some programs (see, e.g., guidance for hazardous waste and other combustion facilities: U.S. EPA; 1990, 1993a, and 1994. Such federal and state guidance doc-uments show how a full and complex set of exposure pathways can be evaluated. Pathways include deposition on plant leaves and deposition on soils, root uptake and translocation, uptake by cattle and accumulation in beef and milk, and so forth. The approach is conservative and protective, but the resulting risk models have not been validated. By combining many conservative decisions together, there is a real danger of producing an unrealistically high estimate of risk. Some people argue that in the absence of data such estimates are appropriate to fully protect people. The models provide a good starting point for understanding the fate of air toxics. In many cases, it will be worthwhile to gather supplementary information to refine the risk estimates. It may also be worth using quantitative uncertainty analysis methods such as probabilistic or Monte Carlo analysis, or fuzzy logic analysis (for further discussion, see Burmaster and Appling, 1995). B. Project Manager Role in Conceptual Site Model Development As the risk assessor develops the conceptual site model, the project manager should gather and provide as much information as possible about the site, historic conditions and occurrences, known or potential exposures, worker behavior and job duties, recreational visitation rates, current land use and likely future land development plans, and so forth. It may be important at this stage to collect additional data. While mangers with bottom-line accountability are naturally reluctant to spend project resources for additional data collection, however, it can pay off in lower remedial costs or improved public confidence. A cost-benefit analysis may help a project manager decide whether additional data collection might be cost-effective. For example, assume that the choice is either to collect air monitoring data in a nearby housing development or to use conservative air modeling in the risk assessment. On one hand, data collection can be expensive. If air monitoring data in the nearby © 2001 by CRC Press LLC ... - tailieumienphi.vn