The oil spill in the Gulf of Mexico is unprecedented. From the origin of the leak, to the amount of oil released into the environment, to the spill’s duration and ongoing nature, the Gulf oil spill poses unique challenges to human health. On April 20, 2010, the Deepwater Horizon offshore drilling rig exploded in the Gulf of Mexico, killing 11 workers. The explosion led to a sea-floor oil leak 1 mile beneath the ocean’s surface. The gusher’s depth has made it difficult to accurately measure how much oil is being discharged, with officials estimating 35,000 to 60,000 barrels1 a day and some scientists estimating 40,000 to 100,000 barrels per day (MacDonald et al., 2010). Located approximately 40 miles off the coast of Venice, Louisiana, but threatening the entire Gulf and Atlantic coastlines, the Deepwater Horizon oil spill, also known as the Gulf of Mexico oil spill or disaster, is the largest offshore oil spill in U.S. history.
The clean-up effort has been the most demanding on-water response in U.S. history, calling for the use of more than 1 million gallons of oil spill dispersants (Judson et al., 2010) and the deployment of more than 2,500 skimming vessels (Deepwater Horizon Response, 2010), including approximately 2,000 “vessels of opportunity” (local boat operators assisting with containment and response activities), by mid-June.2 The
potential physical, psychological, and socioeconomic impacts of the Gulf oil spill and clean-up response on the short- and long-term health of individuals in the affected region—including land- and sea-based clean-up workers, fishermen and -women, other commercial workers, residents, visitors, and communities as a whole—are unknown.
Assessing the Effects of the Gulf of Mexico Oil Spill on Human Health: An Institute of Medicine Workshop
Appropriate monitoring is one key to detecting the spill’s short- and long-term physical and psychological health effects on at-risk individuals and to assessing appropriate preventive and health care services. To inform this endeavor’s development and implementation, Secretary Kathleen Sebelius of the U.S. Department of Health and Human Services (HHS) asked the Institute of Medicine (IOM) to convene a public workshop to provide expert, scientific input on surveillance needs and directions for future research. (Box 1-1 provides HHS’s charge to the IOM.) Nancy Adler chaired the six-member planning committee.3
Drawing on the best scientific expertise, the workshop examined a broad range of health issues potentially related to the Deepwater Horizon oil disaster and response with a focus on informing health-monitoring and health care efforts. Workshop presenters and attendees examined a broad range of potential health issues related to the Gulf oil disaster and response. The 2-day workshop comprised four half-day sessions that included expert presentations, six panel discussions, and an open-microphone dialogue with the audience. This report summarizes the panel presentations, discussions, and public comments that occurred during the workshop. (See Appendix C for the workshop agenda.)
Given the gravity of the impact of the Gulf oil spill, it was important to both the IOM and the planning committee members to afford abundant opportunity to hear from members of the public. To accomplish this goal, the planning committee designed four methods for members of the public to submit their questions and comments to the workshop: (1) submitting electronic comments through the IOM website; (2) submitting a written comment sheet during the workshop; (3) completing question cards for
Charge to the IOM from HHS
individual panels; and (4) speaking during the workshop’s public comment session. Appendix D includes a brief summary of the public comments and questions submitted to the IOM through all four methods.
During the 2-day workshop, several individuals made personal recommendations for moving forward. Importantly, the personal recommendations and the other statements and opinions that were expressed do not reflect group consensus and should not be construed as such. None of the ideas and suggestions published in this report have been endorsed or verified by the National Academies, the Institute of Medicine, or workshop participants as a whole. Furthermore, although the current affiliations of panelists are noted in the report, many speakers qualified their comments as personal opinion that did not necessarily represent the positions of their affiliated organizations (unless specifically noted). Finally, readers should keep in mind that the workshop and writing of this summary report occurred in mid-June through mid-July, 2010, just after British Petroleum installed a temporary cap on the broken wellhead.
The IOM workshop covered a wide range of complex and sometimes sensitive issues. To avoid confusion, it is important to define a few key terms that will be used throughout this summary. First, “health” is a very broad concept. Lynn Goldman reminded the workshop that the World Health Organization defines “health” as a complete state of physical, mental, and social well-being, and not merely the absence of disease or infirmity. Additionally, a number of participants noted the impact that a failing economy can have on health. Although the summary sometimes
categorizes health as physical or psychological for purposes of organization, the subsequent discussions acknowledge the multifaceted nature of health, which is dependent on physical, psychological, social, and economic factors.
Second, because a broad definition of health necessarily implies an even broader range of factors that can affect health status, risks to health include more than simple physical or chemical exposures. In this report, a “hazard” is anything that can cause harm, such as chemical and physical exposures, dangerous working or living conditions, and loss of livelihoods. “Risk” is also used more broadly to mean the chance of a harm occurring. To the extent possible, this summary limits the use of “exposure” to chemical or heat exposures. However, speakers differed in their definitions of “exposed” individuals or populations. Some speakers referred only to individuals who had direct bodily contact with oil spill contaminants whereas others used a broader definition that included individuals affected by the oil spill’s socioeconomic impacts.
Finally, a few participants argued that the term “oil spill” did not adequately describe the inception of, or the current situation in, the Gulf of Mexico, arguing that “disaster” may be more appropriately used. For example, Maureen Lichtveld stated that the Gulf oil spill was a disaster “regardless of who declares it, when it is declared, or how it is declared.” When referring to the oil in the Gulf of Mexico, this report uses both terms, which may be preceded by “the Gulf of Mexico,” “the Gulf,” or “the Deepwater Horizon.” This decision is not meant to imply that the situation in the Gulf is anything less than disastrous for communities affected by the oil and response activities.
The workshop provided the opportunity for attendees to hear and to provide a rich array of experiences, diverse perspectives, and a variety of fresh ideas. Over the course of the workshop, certain refrains were often repeated by individuals. These themes represent some of the characteristics and considerations that workshop participants discussed in the context of developing a surveillance system to monitor the spill’s effects on human health. Box 1-2 summarizes the emerging themes.
Emerging Workshop Themes
Complexity. Assessing the effects on human health of oil spills and response activities is complex.
Multiple dimensions. Human health is multidimensional and includes physical, psychological, and socioeconomic dimensions.
Uncertainty. Information about the specific hazards related to the Gulf oil spill and the range of potential acute and long-term effects of oil spills on human health is incomplete and leads to uncertainty.
Immediacy. Understanding the current state of knowledge can guide immediate actions to mitigate known risks and to fill existing knowledge gaps.
Community engagement. Community involvement and collaboration are essential when designing surveillance systems, related research activities, and effective risk communication strategies.
Coordination. Coordination can strengthen existing and developing surveillance and monitoring systems.
Commitment. Long-term surveillance and related research activities are critical to identifying acute, chronic, and long-term health effects of oil spills.
Assessing the effects on human health of oil spills and response activities is complex. Individual workshop participants identified a number of factors, such as oil composition and weathering, a diverse range of exposures and potential adverse health effects, the unique characteristics of affected populations, and the ongoing nature of the oil spill, that increase the complexity of assessing the effects of the Gulf oil spill on human health.
Human health is multidimensional and includes physical, psychological, and socioeconomic dimensions. These dimensions are not independent, but interact to influence the overall well-being of individuals
and communities, said Maureen Lichtveld. Describing numerous acute physical symptoms from oil-spill-related exposures and other hazardous conditions, a number of workshop participants, including Howard Osofsky, predicted that the Deepwater Horizon disaster will likely have an even greater effect on the psychological health of affected communities because of serious and prolonged disruptions to the social environment and local economies.
Information about the specific hazards related to the Gulf oil spill and the range of potential acute and long-term effects of oil spills on human health is incomplete and leads to uncertainty. A number of speakers, such as Nancy Adler, Blanca Laffon, and Nalini Sathiakumar, explained that very little is known about the potential short-term and especially long-term health consequences of the Gulf oil spill, despite lessons learned from past large oil spills, such as the Exxon Valdez and Prestige spills. Moreover, the exact nature and extent of hazards, risks, and populations vulnerable to adverse health effects are ill-defined, contributing to the uncertainty. As a result, it is difficult to develop and communicate strategies for action, which can contribute to elevated levels of anxiety and fear among affected populations, said Howard Osofsky.
Understanding the current state of knowledge can guide immediate actions to mitigate known risks and to fill existing knowledge gaps. Although many aspects of the Deepwater Horizon disaster remain uncertain, public health officials can draw from what is known to prevent and mitigate harm from identified hazards, said Paul Lioy and other participants. Additionally, there is an urgent need to begin data-collection and surveillance activities immediately, in order to accurately assess the relationships between exposures and conditions related to the Gulf oil spill and adverse health outcomes, argued speakers including John Bailar.
Community involvement and collaboration are essential when designing surveillance systems, related research activities, and effective risk communication strategies. A number of participants, such as Maureen Lichtveld, noted that local residents and communities have unique experience and expertise that can improve surveillance-related activities, especially if community engagement begins early. A number of participants, including David Abramson, said that community engagement can also strengthen surveillance activities by increasing community participation, encouraging surveillance activities that target “actionable” information, and improving the message and manner of risk communications.
Coordination strengthens existing and developing surveillance and monitoring systems. In light of limited resources and the oil spill’s scope and magnitude, coordination between and among all interested parties is essential, said Linda Rosenstock and others. Some presenters spoke about the need for centralized oversight to coordinate the activities across sectors. Others, like Lynn Goldman, asserted that coordination could also extend to shared platforms or standards.
Long-term surveillance and related research activities are critical to identifying acute, chronic, and long-term health effects of oil spills. Scott Barnhart, among other speakers, emphasized the importance of continuous reassessment based on real-time monitoring of exposure data. A number of participants suggested that long-term investment should focus on public health infrastructure, rather than individual disasters or outbreaks.
In addition to these recurring themes, participants offered throughout the workshop a number of suggestions on data collection, research methods, and components of an effective surveillance system. This summary covers these suggestions in greater detail in later chapters.
Harvey V. Fineberg, Institute of Medicine
Nancy E. Adler, University of California, San Francisco
In his opening remarks, Harvey Fineberg, president of the IOM, welcomed workshop participants, expressing a sense of unity with the people in the Gulf region. Observing that many participants were both directly and intimately involved in current response efforts to the Gulf oil spill, he thanked planning committee members, local hosts, and every participant for attending the workshop on such short notice. He also expressed his appreciation to Secretary Sebelius and HHS for sponsoring the workshop.
Describing the Gulf oil catastrophe as being distinctive in its scope, magnitude, and duration, Fineberg said that the workshop’s primary purpose was “to join together to share our best thinking, our experience, our ideas, our expertise, our concerns, and our strategies” to develop a clearer, more comprehensive, and more focused sense of how to assist, and monitor the health of, people from affected regions.
Stating what would very quickly emerge as a major overarching theme of the 2-day conference, Fineberg remarked that the complexity of the assessment derives from the reality that it will involve more than scientifically evaluating the medical consequences of exposure to chemical (and physical) substances; it will also require evaluating the social consequences of the Gulf oil catastrophe and developing a better understanding of the culture and needs of the people in the Gulf region.
Nancy Adler, the chair of the workshop planning committee, stated that the amount of uncertainty defines the Deepwater Horizon spill. From oil on the beaches, to oil fumes, to oil dispersants, to controlled burns, to extreme heat, to possible effects within the food chain, to the, perhaps, permanent disruption of people’s livelihood, Adler said that this oil spill’s effects on human health is uncertain not only because of the lack of evidence from prior oil spills but also because of the unprecedented magnitude and scope of the Deepwater Horizon spill. She said, “For people of this region, which have had more than [their] share of disasters, I wish we could bring more definitive data to say, ‘These are the known health effects.’ Unfortunately, we are not able to do that right now.” Instead of reaching consensus recommendations, Adler emphasized that the workshop is an important step toward identifying the critical questions and laying out a range of options for answering those questions.
Charge to Workshop Participants
Nicole Lurie, U.S. Department of Health and Human Services
Nicole Lurie, Assistant Secretary for Preparedness and Response,4 summarized the charge to the IOM (see Box 1-1). Differentiating the Gulf oil spill from other environmental disasters, Lurie noted that developing and relying on the best available science was a critical factor to responding to a major environmental catastrophe. “Unfortunately,” said Lurie, “we still don’t have the kind of evidence base we need.” Beyond guiding the HHS response to the oil spill, Lurie stated that the workshop’s goal was also to explore “a shared framework for thinking about … the health issues and the work that needs to be done” to prepare the U.S. public health community for this and future disasters, especially those characterized by a high degree of uncertainty.5 Moreover, said Lurie, the response to this oil spill will be as much a communication issue as it will be a scientific issue.
THE COMPELLING NEED TO UNDERSTAND THE POTENTIAL EFFECTS OF OIL SPILLS ON HUMAN HEALTH
Bernard Goldstein, Blanca Laffon, and Edward Overton set the stage for the workshop by providing scientific overviews of what is known (and not known) about oil, the relationship between oil spill exposures and human health, and lessons learned from previous oil spill surveillance and research activities. Specifically, Goldstein discussed the current level of scientific understanding regarding the effects of oil spills on human health, emphasizing that, while disaster response has improved over the past 20 years (since the Exxon Valdez spill), it still “has a long way to go.” Laffon summarized results of the handful of previous studies on the human health effects of exposure to oil spills and described in detail the human health biomonitoring activities that occurred (and are still occurring) following another major oil disaster (the Prestige spill).
Finally, Overton described the chemical makeup of oils and oil dispersants and explained what happens when oil spills into the environment; he emphasized the dynamic nature of the composition of oil spilled into the environment and how exposure varies across time and space.
Understanding the Effects of Oil Spills on Human Health
Bernard D. Goldstein, Pittsburgh Graduate School of Public Health
In discussing the current level of scientific understanding regarding the effects of oil spills on human health, Goldstein stated that, while disaster response has improved over the past 20 years (since the Exxon Valdez spill), it still “has a long way to go.” He described the complex cause-and-effect relationship between oil spill exposures and human health and the lack of evidence around how best to respond to a disaster of this magnitude. Goldstein used three themes to organize his remarks.
The first theme was the unity of human health and the environment. When oil spills into the physical environment, it also spills into the social environment, creating a very complex situation with many unanswered questions about the impact of oil and oil dispersants on human health. Goldstein stated that a growing body of knowledge has been building around the inextricable link between health and the environment. The Gulf oil spill is not just an environmental disaster—it is also a human health disaster. Access is needed to data on the toxicological effects of oils and dispersants, but data may qualify as proprietary information, which may prevent individuals or companies from releasing the data to the public. Goldstein called for legislative reform to limit litigation secrecy and to increase accessibility to proprietary toxicology data.6 The situation is even more complex because of the possibility that oil or dispersants might contribute to the hypoxic “dead zone” in the Gulf, resulting in human health effects from algal blooms, such as red tides.
Goldstein’s second theme stated that, although our capacity to respond to the public health consequences of disasters is greater than it has been in the past, there is still “a long way to go.” Citing several Exxon Valdez studies by Lawrence Palinkas and colleagues (see e.g., Palinkas et al., 1992) and referring to the more than 100 additional studies on the
health effects of exposure to the World Trade Center attack in 2001, Goldstein stated that there is a growing body of knowledge on ways to improve disaster response and to prevent adverse human health effects. Much of this past research—and, therefore, the knowledge gained—is in the area of mental health. (Palinkas spoke about some of this research in a later session; see Chapter 2 for a summary of his remarks.) Additionally, there are several new federal programs relevant to the clean-up response that were not in place 20 years ago. For example, the National Institute of Environmental Health Sciences (NIEHS), an organization typically associated with basic research, has a very effective hazardous waste training program that has trained more than 1 million workers since its inception, many of whom were involved in the 9/11 clean-up response. Other programs that did not exist 20 years ago include the Centers for Public Health Preparedness (funded by the Centers for Disease Control and Prevention [CDC]) and Preparedness and Emergency Response Research Centers programs.
The third theme was that the majority of the expertise needed for disaster response, just as the majority of the impact, will always be local. Goldstein described the importance of involving affected communities and engaging local authorities in the Deepwater Horizon response. He discussed the concept of environmental justice and its three “indisputable truisms”: (1) there are more environmental hazards in disadvantaged communities; (2) there are more individuals with poor health in disadvantaged communities; and (3) individuals with poor health tend to be more susceptible to environmental pollutants.
Recognizing these truisms, Goldstein suggested three levels of community participation. First, environmental health research should focus on disadvantaged communities. Second, to be successful, environmental health research requires the cooperation of disadvantaged communities. Too often in academia, community participation involves either communicating the results of a study after the study has been completed or allowing community members to participate in the study. The current situation, Goldstein said, calls for a third level of participation: working with the community and together deciding what research should be done and how to do it. This will involve going beyond studying the effects of exposure to one or more specific chemicals and will require considering how to assess objective and subjective outcomes, such as job loss or disruptions to a quality and way of life—things that matter to the population or culture being affected by the Gulf oil spill.
Research on the Human Health Effects of Exposure After Oil Spills: The Prestige Experience
Blanca Laffon, University of A Coruña, Spain
The human health effects of exposures during oil spills have been studied following only 7 of the 38 major oil spills over the past half century, said Blanca Laffon. These studies have examined various acute symptoms (e.g., eye and throat irritation, respiratory symptoms, dermatological symptoms), physiological functions (e.g., renal function, liver function), and psychological symptoms (e.g., depression, anxiety, post-traumatic stress). Generally, the results have shown evidence of effects, but those effects diminish with time and are largely reversible.
Laffon discussed the Prestige accident, which occurred 130 miles off the coast of France and Spain. The Prestige spill involved about 44,000 tons initially and 22,000 tons over the course of the next few weeks (125 tons/day) and attracted a total of 327,476 volunteers from throughout Europe to help clean the sea, beaches, rocks, and oil-contaminated birds and other fauna. The oil was a complex mixture of compounds, including three groups of compounds with known human health consequences of exposure (e.g., genotoxicity, carcinogenicity, endocrine disrupting): (1) volatile organic compounds (VOCs); (2) polyacylic aromatic hydrocarbons (PAHs); and (3) heavy metals.
Before the 2002 Prestige spill off the Galician coast of Spain, there was no evidence on the potential chronic human health effects of exposure to spilled oils. Because of the large number of people involved in the Prestige clean-up effort, Laffon and colleagues saw an opportunity to fill some gaps in knowledge around the potential adverse, long-term health effects of exposure, especially genotoxicity. They designed a study with two key objectives: (1) to evaluate the genotoxic effects of oil exposure during the handling of oil-contaminated birds and the cleaning of beaches and rocks; and (2) to determine the influence of physiological factors, consumption habits, and the use of protective devices on genotoxicity. Laffon described the study in detail.
The first part of the study involved examining genotoxicity in individuals who had performed autopsies and cleaned oil-contaminated birds (34 exposed individuals, 35 controls). Briefly, according to the results of one type of genotoxicity test (the “comet assay,” which measures DNA damage at the cellular level), the first part of the study demonstrated significant genotoxic damage in the exposed individuals, with genotoxicity being greater among individuals exposed for longer periods of time.
However, according to a second type of genotoxicity test (the “micronuclei assay,” which measures permanent structural chromosomal alterations), there was no significant genotoxicity effect among the exposed individuals. Laffon and her team concluded that, because the results of the first test were positive, there was significant DNA damage, but the negative results of the second test suggest that whatever damage occurred initially did not persist.
Additionally, because not all of the individuals in the first part of the study wore protective masks, the researchers were also able to assess whether the effects of exposure on genotoxicity are the same or different between clean-up workers who wear masks and those who do not. Again, although there appeared to be more damage among individuals who did not wear the protective masks, the damage subsided over time.
The second part of the study involved examining genotoxicity in individuals who had participated in the beach and rock clean-up effort. According to the results of the comet assay, all three exposed groups demonstrated significant DNA damage, with the most damage occurring in volunteers. Laffon and colleagues surmised that, because the clean-up workers had been exposed for several months before testing, they may have developed an adaptive response over time. According to results of the micronuclei assay, both the manual workers and workers using high-pressure water machines experienced significant DNA damage, suggesting that a long period of exposure was necessary to induce significant genotoxic damage. According to results of a third genotoxicity assay (the “sister chromatid exchange [SCE] assay”), only workers using high-pressure machines showed significant DNA damage. Based on results of the three assays, the researchers concluded that the genotoxic effects of exposure change over time and vary depending on the type of clean-up work being done.
Also in the second part of the study, the researchers examined levels of exposure to VOCs and heavy metals and found that, with respect to VOCs, volunteers were the most exposed and workers using high-pressure water machines were the least exposed. With respect to heavy metals, all the different types of workers showed significant exposure to aluminum compared to the controls, but only some of the different types of workers showed significant exposure to nickel and zinc (volunteers and workers who used high-pressure water machines) and lead (clean-up workers who worked manually). None of the workers showed significant exposure to cadmium.
Additionally, the researchers evaluated the value of wearing protective devices, given that most bodily contact with oil is with the hands, followed by the head, neck, arms, legs, and feet. The researchers found that neither clothes nor masks correlated with significant reductions in symptoms, which could suggest that the protective devices may not have been suitable or that workers may not have been using the protective devices correctly. Laffon and colleagues are still studying the chronic genotoxic effects of the spill.
What Exactly Are People Being Exposed to During the Deepwater Horizon Oil Disaster Cleanup?
Edward Overton, Louisiana State University
The sparse data on exposure to oil spills and the human health effects of such exposure, coupled with the reality that not all oil spills are the same (e.g., oil acts differently in different circumstances—the Exxon Valdez oil was a heavy, thick, “gunky” material, compared to the Deepwater Horizon oil, which is already emulsified by the time it reaches the ocean surface), has led to a great deal of uncertainty around exactly what it is that workers and other populations have been and continue to be exposed to during the Deepwater Horizon oil disaster and response. Edward Overton explored some of this uncertainty.
What Is Oil?
Oil acts differently in different environments, but there are some “rules of thumb” when you are designing surveillance frameworks to evaluate exposures to oil, Overton explained. First, oil contains many thousands of compounds. Moreover, all oils contain the same compounds and molecular structures, regardless of their source. Differences among oils (e.g., between the Exxon Valdez oil, and the Deepwater Horizon oil) are caused by differences in quantities of those compounds in the oils. Overton and his colleagues analyzed the content of the Deepwater Horizon oil and detected nearly 2,000 identifiable compounds.
Second, the quantity of different hydrocarbons determines an oil’s chemical and physical properties. For example, an oil that contains lighter, smaller molecules is less viscous. The Deepwater Horizon oil is an extremely light oil, which affects evaporation at the ocean’s surface.
The structure of the hydrocarbon (e.g., straight-chain, branched, and nonaromatic and aromatic cyclical) also affects how an oil interacts with its environment. Aromatic compounds include benzene and PAHs or PAH homologs, and may include sulfur or heavy metals within the chain. Finally, oil contains a class of residue, asphaltenes, which together form what we call road tar.
Third, most aromatic hydrocarbons in oils are alkyl homologs of “parent” polycyclic compounds, and most studies on the human health effects of oil exposure have involved parent polycyclic compounds (the Environmental Protection Agency’s [EPA’s] standard oil analytic test, the 8270 GCMS method, detects only the parent compounds). The Deepwater Horizon oil has almost no parent polycyclic compounds, which means that the standard 8270 GCMS method would not be useful.
Lastly, because there are so many compounds present in oil, it is difficult to evaluate toxicity. While the aromatic hydrocarbons are responsible for much of the toxicity, Overton cautioned that they are not the only compounds in oil with potentially toxic effects on human health. Oil also contains a series of saturated compounds, including hydrocarbons, that are called biomarkers. Because biomarkers do not degrade quickly, they can be used to trace oil as it moves through the environment.
What Happens to Oil in the Environment?
It is extremely difficult to quantitatively analyze oil in the environment—both fresh oil in the water and weathered oil on the beach—because of oil’s heterogeneous distribution. Oil starts changing immediately after it leaves the wellhead. Overton explained how, at the time of the workshop, the Gulf Oil disaster involved two main types of oil: a very light, floating oil (source oil) and a very heavy, sinking oil (weathered oil). Initially, the more toxic, lighter compounds (e.g., benzene and other aliphatic hydrocarbons) break off and enter the water column. As oil continues to weather, it becomes heavy and sticky, which Overton stated is likely a high-asphaltenic oil. The weathered oil is neutrally buoyant and can easily wash down into the water column and settle on the bottom of the ocean floor. He listed several weathering steps that oils can undergo and commented on their relevance to the Gulf oil spill:
Adsorption (sedimentation). The process by which one substance is attracted to and adheres to the surface of another substance without actually penetrating its internal structure. This is not a significant problem in the Gulf.
Biodegradation. The degradation of substances resulting from their use as food energy sources by certain microorganisms. The Deepwater Horizon oil is an “imminently degradable” oil, Overton said, and the bacteria “love it.” He stated that biodegradation and evaporation together led to about 30-50 percent of the Deepwater Horizon oil weathering away within the first week.
Dispersion (either naturally or through chemicals). The distribution of spilled oil into the upper layers of the water column by natural wave action or application of chemical dispersants. Corexit 9500, the dispersant being used at the time of the workshop, is composed of six components. Information about the components is available on the Nalco Company website (Nalco, 2010).
Dissolution (or subsurface weathering). The act or process of dissolving one substance in another. In the Gulf oil spill, many of the small molecules dissolve within the water column as it rises to the surface. This creates oil plumes. Although the concentration of dissolved chemicals within an oil plume is relatively low when compared to freshly released oil, toxicity of certain dissolved chemicals may still be a concern.
Emulsification. The process whereby one liquid is dispersed into another liquid (emulsifies) in the form of small droplets. Because this is a deep spill, the oil has plenty of time to emulsify within the water column, turning the oil into what looks like a “red mousse” (see cover photograph).
Evaporation. The process whereby any substance is converted from a liquid state to become part of the surrounding atmosphere in the form of a vapor. Again, biodegradation and evaporation together led to about 30-50 percent of the Deepwater Horizon oil disappearing within the first week.7
Photo-oxidation. In the context of the Gulf oil spill, photo-oxidation occurs when sunlight facilitates a chemical reaction between the air and oil. As Overton explained, photo-oxidation basically takes a nonpolar molecule and makes it polar, which enhances the production of stable emulsions. Additionally, photo-oxidation can enhance phototoxicity. Although photo-oxidation has not been shown to have much of a direct effect on the human population, it does affect the ecology of our environment, which can indirectly impact human health.
Overton concluded by summarizing the types of issues that need to be considered when evaluating the complexity of exposure analysis. In addition to the effects that weathering has on the large number of existing chemicals and chemical structures within oil, the ongoing nature of the Gulf oil spill further complicates monitoring activities because not all the oil within the environment is at the same stage of the weathering process. Therefore, the exposure risks are constantly changing as the oil itself evolves within the environment.