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Public Health Assessment
Air Pathway Evaluation,
Isla de Vieques Bombing Range,
Vieques, Puerto Rico

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August 26, 2003
Prepared by:

Federal Facilities Assessment Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry

IV. Exposure Pathway Analysis

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:

  • If people are not exposed to a site's environmental contamination, then the contaminants cannot pose a public health hazard and additional analyses are not necessary.
  • If people are exposed to site-related environmental contamination, then further analysis is needed to characterize that exposure. Just because exposure occurs does not mean that people will have health effects or get sick. In fact, for many chemicals, environmental exposures are far lower than the exposures that people experience through their diets and perhaps through their occupations. Several issues must be considered to understand the public health implications of exposure: exposure concentrations, the frequency and duration of exposure, and the route of exposure by which people may be exposed. These issues must be carefully evaluated to determine if harmful health effects might result from exposure.

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:

  • Source of contamination. A source of contamination must exist in order for exposures to occur. Many sources of air contamination are found at Vieques, most notably releases that previously occurred during the military training exercises.
  • Environmental media and transport. People cannot be exposed unless contaminants move from their source or origin through the environment to an exposure point. ATSDR has identified two dispersion modeling studies (Cruz Pérez 2000; IT 2001) that suggest contaminants previously released during the military training exercises might have transported downwind in the air to the residential areas of Vieques. These contaminants dispersed greatly over the 7.9 miles that separated the LIA and residential areas. Further, during certain times (e.g., when exercises were not occurring, when rainfall removed contamination from the air, when winds blew air pollutants away from the residential areas), no contamination from the LIA reached the residential areas. This element of the exposure pathway, therefore, is not always present. However, ATSDR considers the various pathways reviewed in this document to be completed exposure pathways during limited time periods, specifically, when the wind was blowing toward the residential areas during training exercises.
  • Point of exposure. Exposure cannot occur unless contaminants reach a location where people have access. The two modeling studies predict that some contaminants from the LIA might have crossed into the residential area of Vieques in low quantities.
  • Route of exposure. For exposure to occur, people must contact chemicals in a contaminated media, either through inhalation, ingestion, or dermal contact. Inhalation exposures clearly occur if air contaminants are present.
  • Potentially exposed population. Ultimately, people must come into contact with chemicals at the point of exposure in order to conclude that exposure has taken place. Recognizing again that dispersion modeling studies suggest that some contaminants from the LIA reached the residential areas of Vieques, a potentially exposed population is clearly present for this site.

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:

  • Exposures to wind-blown dust on days when military training exercises did not take place.
  • Exposures to contaminants released during military training exercises that involved use of only practice bombs (i.e., the types of exercises that took place between April 19, 1999, and May 1, 2003).
  • Exposures to contaminants released during the military training exercises that involved the use of live bombs (i.e., the types of exercises that took place prior to April 19, 1999, primarily those taking place between the early 1970s and April 19, 1999).
  • Exposures to contaminants released during open burning and open detonation of selected wastes and to materials (depleted uranium and chaff) used sporadically on Vieques between the early 1970s and May 1, 2003.

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.

B. Assessment Methodology

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.

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:

  • If the contaminant has comparison values published in ATSDR's "Air Comparison Values" (ATSDR 2002), the lowest of these comparison values was selected.
  • If no ATSDR comparison values are available, the EPA risk-based concentration for ambient air was selected, if available. These values are published by EPA Region 3.
  • If neither of the previous sources have comparison values, ATSDR researched other sources, such as EPA's National Ambient Air Quality Standards and occupational exposure limits.
  • If no appropriate health-based comparison value is available, ATSDR automatically selected the contaminant for further evaluation and reviewed relevant toxicologic and epidemiologic studies to put the measured levels of contamination into a public health context.

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.

1 "Small arms ranges" are designated areas where military personnel fire small arms (e.g., guns) at stationary and moving targets. Bombs are not dropped on the small arms ranges.
2 For this calculation, ATSDR considered all wind directions between northeast (45º) and southeast (135º) as "from east to west."
3 Though the Navy owns the property where military training exercises take place, various parties used this property prior to 1999. These parties included the Navy, the U.S. Marine Corps, and military forces from some foreign countries.


 
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