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Isla de Vieques (Vieques) is an island in the Commonwealth of Puerto Rico and is located roughly 7 miles east of the main island of Puerto Rico. Until May 1, 2003, the United States Navy (Navy) owned about half of the land on Vieques and conducted military training exercises on the east side of the island. These exercises included various types of bombing and shelling, which took place at the Live Impact Area (LIA). The residential areas of Vieques are located more than 7.9 miles west of the center of the LIA. On May 1, 2003, the Navy ceased all military training exercises at Vieques and turned its land on the eastern portion of Vieques over to the U.S. Department of Interior.
In 1999, a resident of Vieques asked ATSDR to determine whether the Navy's operations on Vieques caused residents to be exposed to levels of environmental contaminants that could present a public health hazard. For the last 3 years, ATSDR has studied this issue extensively and is publishing its findings in a series of public health assessments (PHAs). This PHA addresses the public health implications of exposure to air contaminants potentially released from Navy property.
To characterize air quality at Vieques, ATSDR identified and obtained a wide range of relevant data. Specifically, ATSDR initiated an air sampling study during a recent military training exercise and reviewed relevant studies prepared by the following parties: the Puerto Rico Environmental Quality Board (PREQB), several academic and independent researchers from universities and private organizations in Puerto Rico, the U.S. Environmental Protection Agency (EPA), and the Navy and its contractors.
ATSDR's findings are summarized below. Later sections of this report describe how ATSDR reached these conclusions.
ATSDR's modeling considered nearly 100 different contaminants believed to be released to the air during live bombing exercises and simulated how these contaminants moved through the air. The modeling analysis predicted that chemicals emitted from bombing exercises dispersed to extremely low levels over the 7.9 miles that separate the emissions source (the LIA) and the receptor (the residential area of Vieques). For a majority of the contaminants released, the estimated concentrations in the residential areas are so low that even highly sensitive air sampling devices would likely not be able to measure them. In the case of particulate matter, for example, emissions from live bombing exercises were predicted to account for less than 1% of the concentrations of particulate matter that were recently measured in the residential areas of Vieques. This comparison suggests that emissions sources located in the residential area of Vieques–and not emissions from the past live bombing exercises–accounted for nearly all of the particulate matter that residents breathed in the past.
In summary, whether considering acute or chronic exposure scenarios, ATSDR's modeling estimates indicate that emissions from live bombing activities did not cause ambient air concentrations of explosion byproducts, including metals released from soil, to reach levels known to be associated with adverse health effects. ATSDR concludes, therefore, that chemicals released to the air during the past live bombing exercises did not pose a health hazard.
ATSDR acknowledges that this finding is based entirely on a modeling analysis, which has inherent uncertainties and limitations. However, as Section V.C describes, ATSDR has reason to believe that the modeling analysis has not understated exposures and public health implications. Of particular note, the approaches ATSDR used to estimate emissions of contaminants are based on, and consistent with, EPA modeling guidance and several assumptions ATSDR made likely overstate the actual emissions. These observations, combined with the fact that estimated ambient air concentrations for most contaminants considered were several orders of magnitude lower than concentrations of health concern, lead ATSDR to believe that the modeling analysis presents a reasonable account of exposures that occurred on Vieques and does not understate the exposures that residents might have experienced.
In May 1999, a resident of Isla de Vieques (Vieques), Puerto Rico, requested that the Agency for Toxic Substances and Disease Registry (ATSDR) determine whether contaminants released from the United States Navy's (Navy's) bombing range pose a public health hazard. This request was submitted as a petition to the agency. ATSDR accepted this petition and has since been investigating public health concerns related to operations at the Navy's former bombing range.
ATSDR is responding to this petition in a series of public health assessments (PHAs) that examine what contaminants entered the environment, how these contaminants moved through the environment, and what levels of contamination residents might have contacted. ATSDR then uses this information to determine whether residents were exposed to levels of contamination that might cause health problems.
To be most responsive to the petitioner and the people of Vieques, ATSDR is publishing a series of PHAs that address very specific questions. This PHA focuses on the public health implications of exposure to air contaminants, by responding to four key questions that the petitioner and residents of Vieques have asked ATSDR. ATSDR's responses to these questions are found throughout Section V of this PHA.
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Key Questions for this PHA
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Though this document focuses on air quality issues, ATSDR has committed to evaluate other ways that contaminants from the bombing range might affect public health. ATSDR has already addressed, or plans to address, these other public health issues as follows:
ATSDR's initial approach to evaluating air quality issues at Vieques involved gathering background information on several important topics, such as specific health concerns, site history, local demographics, and meteorology. The following discussion reviews the information collected on these and other topics, which are important background material for ATSDR's technical analyses, as documented in the "Evaluation of Air Quality Issues" section (Section V).
The remainder of this section primarily presents facts and observations about Vieques, without any analysis or interpretation. Later sections of this PHA document ATSDR's interpretation of the background information presented below.
A. Site Description and Land Use
Vieques is the largest offshore island that is part of the Commonwealth of Puerto Rico. Vieques is 20 miles long, 4.5 miles at its widest point, and about 33,000 acres (or 51 square miles) in area. Figure 1 shows the location of Vieques and surrounding islands. As the figure shows, the nearest island to Vieques is the main island of Puerto Rico, which is approximately 7 miles west of Vieques; the island of Culebra is roughly 9 miles north of Vieques; and St. Thomas, St. John, St. Croix, and other islands within the U.S. Virgin Islands are all at least 20 miles from Vieques, generally to the northeast and southeast. Therefore, Vieques is several miles removed from sources of air pollution on any other island in the Caribbean Sea.
The detailed map in Figure 2 conveys critical background information on land use in Vieques. The figure depicts the island in three separate sections, each of which is described in greater detail below:
East of EMA is the second section of land formerly owned by the Navy, which was called the Atlantic Fleet Weapons Training Facility (AFWTF). AFWTF spanned approximately 3,500 acres (TAMS 1979). As Figure 2 shows, AFWTF was further divided into three smaller sections of land:
Not shown in Figures 1 or 2 are terrain features of Vieques, which are important to consider when evaluating how contaminants move through the air. The highest point on the western half of Vieques is Monte Pirata (987 feet above sea level), and the highest point on the eastern half is Cerro Matias (450 feet above sea level), where OP-1 was located. Other than these peaks, the terrain at Vieques includes low rounded hills and an east-west ridge that runs through the residential lands. The average elevation of Vieques is approximately 250 feet above sea level (Cherry and Ramos 1995; Torres-Gonzalez 1989).
ATSDR examines demographic data, or information on the local population, to determine the number of people who are potentially exposed to environmental contaminants, as well as the presence of any sensitive populations, such as women of childbearing age, children, and the elderly.
Table 1 summarizes demographic data for Vieques, according to the 1990 and 2000 US Census. As the census data show, the population of Vieques increased from 8,602 to 9,106 residents between 1990 and 2000. These figures include both those who live in the residential lands and those who live on Navy property. Table 1 also specifies the number of residents who fall into three potentially sensitive populations: women of childbearing age, children, and the elderly. The table indicates that the percentage of elderly residents in Vieques increased by 2% between 1990 and 2000; ATSDR also notes that the percentage of elderly residents in Vieques (14% in 2000) is notably higher than the percentage of elderly residents living in all of Puerto Rico (11.2% in 2000). ATSDR has received anecdotal accounts suggesting that the population of Vieques is not highly mobile and that many people are lifelong residents of the island, but the site reports that ATSDR has obtained to date do not quantify population mobility trends. ATSDR considered all of the previous demographic figures and observations when evaluating potential exposures among the Vieques residents.
As noted previously, most of the residents at Vieques live in the two largest towns on the island, Isabel Segunda and Esperanza. Although these towns are located relatively close to the Navy property line, they are several miles removed from the LIA. Specifically, the nearest point on residential lands to the geographic center of the LIA is approximately 7.9 miles (or 12.7 kilometers). Therefore, air contaminants from the LIA dispersed over a distance of at least 7.9 miles before they reached the residential populations of Vieques. This was a key issue when evaluating air pollution, as Section V describes further.
C. Climate and Prevailing Winds
The climate and prevailing wind patterns of a given location affect how contaminants move through the air. Annual climatological summaries for Vieques, provided by the National Climatic Data Center (NCDC), indicate that the annual average temperature at Vieques ranged from 77.9 to 80.0 degrees Fahrenheit over a recent 10-year period, with only modest fluctuations in monthly average temperature (NCDC 1985-1994). Annual precipitation totals were more variable, ranging from 42.91 inches in 1991 to 57.07 inches in 1993 (NCDC 1985-1994).
Regarding prevailing wind patterns, a large body of literature reports that trade winds in the Caribbean, which consistently blow from east to west, dominate the meteorology in Puerto Rico. This trend is consistent with wind speed and wind direction data collected at the US Naval Station Roosevelt Roads–the meteorological station closest to Vieques that submits hourly observations of wind speed and wind direction to NCDC. ATSDR obtained more than 10 years of hourly meteorological data for this station. Figure 3 summarizes the hourly wind speed and direction data, in a format known as a wind rose. Wind roses display the statistical distribution of wind speeds and directions in a single plot. The data in Figure 3 demonstrate that the prevailing wind direction at Roosevelt Roads, and presumably in Vieques, is indeed from east to west. In fact, the hourly data provided by NCDC indicate that winds blow from east to west(2) about 75% of the time. This trend is consistent with the influence of trade winds.
Referring to Figures 1 and 2, an easterly wind direction (i.e., winds blowing from east to west) would blow contaminants generated at the LIA toward the residential area of Vieques. This observation, however, does not indicate what levels of air contamination previously occurred. Only sampling data or modeling analyses can provide insights into this issue, as Section V discusses.
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Terminology Used in this PHA to Characterize Military Training Exercises Over the last 2 years, ATSDR has noticed that the Navy, local residents, the media, and other parties use many different terms when referring to military training exercises on Vieques. To avoid any confusion with terminology, this text box defines the terms ATSDR uses throughout this PHA to describe the Navy's military training exercises on Vieques. Air-to-ground exercises: In this PHA, air-to-ground exercises refer to all military training exercises that involve releasing or firing of ordnance from fixed wing aircraft to targets on the ground. Over the years, many different types of ordnance have been fired in these exercises, including bombs, flares, and rockets. According to detailed statistics on ordnance usage, the total weight of explosives fired during air-to-ground exercises is far greater than the amounts fired from both ship-to-shore and land-based exercises combined. Ship-to-shore exercises: ATSDR uses the term ship-to-shore exercises to refer to all firing of ordnance from Naval vessels to targets on the island. A variety of ordnance and activities fall into this category, including artillery firing exercises. In recent years, the amount of ordnance (by weight) used for ship-to-shore exercises far exceeded that used for land-based exercises. Land-based exercises: This PHA refers to all ordnance fired from the ground during military training exercises as land-based exercises. Ordnance fired on small arms ranges and during amphibious landings are included in this category. During the time frame when most detailed ordnance usage statistics are available, land-based exercises account for the lowest quantity of ordnance that the Navy and other parties have used on Vieques. Live bombing exercises: For purposes of this PHA, "live bombs" refer to all general purpose bombs that have not had their explosive content replaced with inert materials. The Navy commonly refers to these bombs and other items as explosive ordnance. The live bombs used at Vieques contain a variety of explosives, including 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), ammonium picrate (Explosive D), methyl-2,4,6-trinitrophenylnitramine (tetryl), and others. Practice bombing exercises: In this document, "practice bombs" refers to those bombs whose main explosive content has been replaced with an inert material, such as sand or concrete. The Navy commonly refers to these bombs as non-explosive ordnance. ATSDR notes, however, that practice bombs might still contain a small quantity of explosives for purposes of spotting, but this quantity is considerably lower than that contained in most live bombs. |
The Navy first began acquiring land on Vieques in 1941 and ceased operations on the eastern half of the island on May 1, 2003. Between 1941 and 2003, a wide range of military training exercises have taken place on Vieques, with the type and intensity of exercises varying from year to year. As a result, the amounts of contaminants released to the air also have changed with time. The following paragraphs note key time frames that ATSDR has defined for purposes of evaluating the extent to which the military training exercises released contaminants into the air. ATSDR's evaluation of air quality issues (see Section V) is based on these time frames.
None of the reports ATSDR has obtained documents exactly when the first air-to-ground exercises took place on Vieques. One report suggests that the Navy first established air-to-ground bombing targets on Vieques in 1960, with actual air-to-ground exercises occurring thereafter (TAMS 1979). Though the early history of air-to-ground exercises on Vieques is not entirely clear, various accounts (e.g., TAMS 1979; Navy 1977) indicate that air-to-ground bombing activity prior to 1971 was far more intense on the island of Culebra than on the island of Vieques. The frequency and intensity of air-to-ground bombing on Vieques gradually increased in the early 1970s, as the Navy slowed and eventually stopped all military training activities on Culebra by 1975.
ATSDR distinguishes between the time with limited military training activities at Vieques (i.e., from 1941 to the early 1970s) and the time with the most extensive use of the bombing range (i.e., from the early 1970s to April 19, 1999) for purposes of evaluating exposures, as Sections IV and V explain further. Note again that ATSDR has defined these time frames specifically for this PHA and no firm dates mark the transition between this time frame and the one described below.
Figures 4 and 5 summarize the extent to which the Navy and other parties(3) have conducted military training exercises on Vieques between 1983 and 1999–the time frame for which the most complete range utilization statistics are available (Navy 1999). As Figure 4 shows, range utilization statistics indicate that the Navy and other parties conducted exercises on Vieques between 159 and 228 days per year, with the total number of days not varying considerably from one year to the next. (Note, ATSDR reviewed range utilization statistics for 1974-1999, as our response to Comment #19 in Appendix E indicates.)
Though these usage statistics provide some insights into the number of days when military training exercises took place, the weight of ordnance used during these exercises is a much better indicator of the amount of contaminants that might be released into the air. The graph in Figure 5 illustrates how the total tons of ordnance used at Vieques, as well as the tons of high explosives within this ordnance, have changed from year to year. The range utilization statistics (Navy 1999) suggest that, on average, 1,862 tons of ordnance were used at Vieques annually between 1983 and 1998. This annual amount of ordnance used, on average, contained 353 tons of high explosives. In later sections of this PHA, ATSDR uses these average range utilization statistics to estimate air pollution levels that might have occurred on Vieques during the time when live bombing took place. Refer to Appendix D for the specific inputs that ATSDR considered in its modeling analysis of emissions from military training exercises.
In addition to researching the usage of ordnance at Vieques, ATSDR considered the extent to which the ordnance was used for different categories of training exercises, namely the proportions used for air-to-ground, ship-to-shore, and land-based activities. Of these activities, air-to-ground bombing accounted for the greatest proportion of high explosives used at Vieques: according to two different reports addressing two different time frames of exercises, 94% of the high explosives used at Vieques were reportedly used for air-to-ground bombing exercises, with ship-to-shore and land-based exercises accounting for the remaining 6% of high explosives (TAMS 1979; IT 2000). These figures indicate that ordnance fired from fixed wing aircraft accounted for the largest portion of air emissions that occur during military training exercises.
Later sections of this PHA consider the chemical make-up of the various ordnance used at Vieques, as well as the contaminants that might be released after these items impact the LIA.
The Navy used chaff during military training exercises only with permission from the AFWTF Commanding Officer, and the Navy prohibited chaff from being released directly over the island of Vieques and over the warning and restricted areas that extend several miles from the Vieques shoreline. Though ATSDR has identified several sources indicating that the Navy used chaff at Vieques, none of these sources documents the exact quantities of chaff that were previously used.
The analyses of potential or completed exposure pathways (see Section IV) and evaluations of air quality issues (see Section V) review the public health implications of the different activities described in this section.
E. Other Sources of Air Contaminants
When evaluating the air exposure pathway, ATSDR not only considers emissions from the sources of concern, but also emissions from other sources in the area. This is because residents ultimately are exposed to air contaminants from all local sources, not just those from one or two. At many sites, in fact, air emissions from sources throughout a community far exceed those from a particular site of concern.
When identifying air emissions sources at a given location, ATSDR typically first accesses EPA's Toxic Release Inventory (TRI), a publicly accessible database that documents amounts of toxic chemicals that certain industrial and military facilities release to the environment. As shown in Table 2, which documents the TRI data available for Vieques, only one industrial facility on the island used hazardous chemicals in large enough quantities to trigger TRI reporting. The TRI data for this facility suggest that its air emissions were relatively low, especially when compared to data reported by facilities on the national level. Observations made during ATSDR's site visits (see Section III.F) confirm that industrial operations on Vieques are extremely limited. There are no power plants, chemical manufacturing plants, or other heavy industrial operations on the island.
Though few large industrial sources of air pollution are found on Vieques, numerous small sources of air emissions exist in and near the residential lands. Key among these are transportation sources, including motor vehicles, a small airport, and local ship traffic. Other small-scale sources include gasoline stations, auto refinish shops, construction activities, and a landfill. ATSDR has not identified a representative emissions inventory for the island from any references, thus the exact extent of emissions from these sources in residential lands is not known. Potential impacts of local emissions sources, other than the Navy bombing range, are discussed further in Section V.
In addition to expressing concerns about emissions from the military training exercises, some residents of Vieques asked ATSDR to evaluate the public health implications of exposure to emissions from "African dust storms." These dust storms occur when strong winds blow over the Sahara desert in Africa and carry large quantities of dusts in the upper air winds to locations thousands of miles away, such as the Caribbean islands and the southeastern United States. Many researchers have documented this phenomenon, including those working for the US Geological Survey (USGS), the National Aeronautics and Space Administration (NASA), and the National Oceanic and Atmospheric Association (NOAA) (e.g., Griffin et al. 2001; Taylor 2002).
Some researchers have estimated that these dust storms release as much as one billion tons (1,000,000,000 tons) of dust to the air each year (Moulin et al.1997). This dust is composed of minerals commonly found in the soils and contains many naturally occurring elements, such as lead, iron, mercury, and beryllium. Recent studies have indicated that the dust storms also carry bacteria, fungal spores, and possibly viruses (Griffin et al. 2001). These storms reportedly have the greatest effect on Caribbean air quality during the months of June through October.
To date, community concerns about the African dust storms have fallen into two general categories: Is exposure to the material in African dust unhealthy? What are the relative impacts of emissions sources thousands of miles from Vieques (such as African dust storms) and sources on the island itself (such as emissions from the LIA, motor vehicles, and the limited local industry)? To address these concerns, ATSDR researched many articles on African dust storms published in the scientific literature and consulted with several authors of these studies. ATSDR's interpretations on this issue are documented in Section VI.
F. ATSDR Involvement at Vieques
Since receiving the petition in 1999 to evaluate public health issues at Vieques, ATSDR has worked extensively to characterize and respond to community needs. Many activities to date have provided ATSDR's health assessors critical perspective for evaluating the local air quality issues. Following is a summary of ATSDR's past involvement with this site:
The previous list reviews ATSDR's activities while working at Vieques. In addition, ATSDR has invested considerable effort assessing this site's environmental health issues. Most of this work has been conducted at ATSDR's headquarters in Atlanta and is documented in the PHAs listed in Section II.
G. Quality Assurance and Quality Control
To prepare this PHA, ATSDR reviewed and evaluated information provided in the documents listed in the Reference section. The environmental data presented in this PHA are from reports produced by many parties, including ATSDR, EPA, and others. The limitations of these data have been identified in the associated reports, and they are restated in this document, as appropriate. After reviewing the studies conducted to date, ATSDR determined that the quality of environmental data available in the site-related documents for Vieques is adequate to make public health decisions. Appendix C presents ATSDR's specific conclusions regarding the quality of the air sampling studies that have been conducted on Vieques and indicates how the agency factored the findings from these different studies into this document's conclusions.
ATSDR also used an extensive review process for quality control purposes. The review involved numerous parties, including ATSDR scientists, lead authors of several studies cited in this report, and internationally recognized experts in the field of air quality issues and dispersion modeling. To date, all reviewers have agreed that the approaches ATSDR used to evaluate this site are scientifically sound and the available sampling data support this document's conclusions.
This section of the PHA addresses exposure pathways to air contaminants, or the various ways that residents of Vieques might have come into contact with contaminants previously released to the air. Analyzing exposure pathways is important because:
More detail on the air exposure pathway at Vieques follows. Section IV.A identifies the specific exposure pathways by which residents of Vieques might have come into contact with air contaminants, and Section IV.B reviews the process ATSDR used to evaluate exposure pathways.
A. Exposure Pathways for Contaminants Released to the Air
In general, there are two ways that people can come into contact with contaminants released from a source into the air. People might inhale contaminants while they are still airborne (known as direct exposure), or people might come into contact with the contaminants after they have been removed from the air by deposition or precipitation and have accumulated in other media, such as soil, groundwater, or food items (known as indirect exposure). This PHA primarily addresses the issue of direct inhalation exposure to air contaminants. ATSDR's other PHAs, which examine levels of contamination in drinking water, soil, and seafood, address the issue of potential indirect exposures to air contaminants.
ATSDR reviewed five elements of exposure pathways as a first step in evaluating the air exposure pathway. These elements, and their specific applicability to Vieques, follow:
Of the five elements of an exposure pathway mentioned above, only the "environmental media and transport" element is not always present. However, this element was present during specific limited time periods, when training exercises were occurring and when the wind was blowing toward residential areas. ATSDR therefore considers the inhalation exposure pathway at the island of Vieques to be a completed exposure pathway.
To characterize these potential exposures, ATSDR identified four inhalation exposure scenarios, which Table 3 lists. These scenarios address the main ways that residents might come into contact with contamination, and they also encompass specific concerns that community members have expressed to ATSDR since 1999 (see Section VI). The exposure scenarios considered in this PHA follow:
Section IV.B presents the methodology ATSDR used to evaluate the public health implications of exposure to environmental contaminants, and Section V documents the results of ATSDR's evaluations for the four potential exposure pathways listed above.
ATSDR used established methodologies to determine the public health implications of exposure to air contaminants. Specifically, ATSDR followed a three-step approach when addressing the four exposure scenarios identified in the previous section: identify concentrations of contaminants released to the air, select chemicals for further evaluation by screening the concentrations against health-based comparison values, and perform toxicologic evaluations for those contaminants selected for further evaluation. More detailed information on these individual steps follows.
The first step in addressing the exposure scenarios is tabulating ambient air concentrations for site-related contaminants. ATSDR prefers to use actual measurements for this step (i.e., air sampling results), rather than relying on engineering calculations or predictions from air quality models. This preference results from the fact that air quality models estimate ambient air concentrations, sometimes with great degrees of uncertainty, while sampling studies measure ambient air concentrations. However, air quality models are critical tools in cases when exposures occur during time frames when no samples were collected or analyzed. Section V indicates the exposure concentrations ATSDR used in this PHA.
The second step in evaluating exposure pathways is selecting chemicals for further evaluation. This is accomplished by comparing the ambient air concentrations for site-related contaminants to health-based comparison values. Comparison values are developed from the scientific literature concerning exposure and health effects. To be protective of human health, most comparison values have large safety factors built into them. In fact, some comparison values might be hundreds or thousands of times lower than exposure levels shown to produce effects in either humans or laboratory animals. As a result, ambient air concentrations lower than their corresponding comparison values are generally considered to be safe and not expected to cause harmful health effects, but the opposite is not true: ambient air concentrations greater than comparison values are not necessarily levels of air pollution that could present a possible public health hazard. Rather, chemicals with concentrations higher than comparison values require further evaluation. Chemicals without published health-based comparison values are automatically considered as requiring further evaluation. The text box on the following page presents the approach ATSDR used to select comparison values for this PHA.
The final step in the assessment methodology is evaluating the public health implications of exposure to any contaminants identified as requiring further evaluation. For these contaminants, ATSDR puts the public health implications of exposure into perspective by considering site-specific exposure conditions and interpreting toxicologic and epidemiologic studies published in the scientific literature. Thus, this step is a state-of-the-science review of what the exposure levels mean in a public health context.
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Approach to Selecting Health-Based Comparison Values For every contaminant considered in this PHA, ATSDR attempted to identify an appropriate health-based comparison value to evaluate whether ambient air concentrations of the contaminant (whether measured or modeled) warrant a detailed public health evaluation. Concentrations of contaminants lower than comparison values are believed to be "safe" or "harmless," while those greater than comparison values need to be evaluated further. ATSDR used the following hierarchy to select appropriate health-based comparison values:
By this approach, ATSDR identified health-based comparison values from many different sources (e.g., ATSDR's Air Comparison Values, EPA Region 3's risk-based concentrations, EPA's National Ambient Air Quality Standards). Though the comparison values from these different sources may have been derived using different assumptions, most can be interpreted in the same fashion: ambient air concentrations below the comparison values are generally considered to be safe and free from adverse health effects. In cases where chemicals have health-based comparison values published for both cancer and non-cancer effects, ATSDR chose the lower value for screening purposes, thus ensuring that the initial screening protects against both cancer and non-cancer endpoints. ATSDR encourages readers interested in more information on health-based comparison values to refer to Appendix A. That appendix lists the different types of comparison values used in this PHA, as well as the assumptions made to derive them. |
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