Chapter 2. Risk and Life-Cycle Frameworks for Sustainability¹

2.1. Introduction

Sustainability issues are complex, and understanding their interactions with equally complex engineered systems is best done through well-structured analysis frameworks. Risk-based frameworks have traditionally been used in the characterization and prioritization of environmental issues, and the first section of this chapter describes those frameworks. Increasingly, however, life-cycle frameworks are gaining prominence as a means of characterizing and understanding sustainability. The second major section of this chapter introduces life-cycle frameworks and their uses.

2.2. Risk

Understanding risk frameworks for environmental and sustainability issues requires an understanding of the language of risk, the tools used to quantify risk, and the use of risk in regulation. This section addresses each of these prerequisites for understanding the use of risk frameworks.

2.2.1. Definitions

Risk is defined as the potential for an individual to suffer an adverse effect from an event. The concept of risk is used in many disciplines, such as finance, engineering, and health, and the precise definition of risk can vary, depending on the application. Environmental risks, which are the focus of this chapter, can be grouped into three general categories:

• Risks involving voluntary exposure: Activities done for a living or for enjoyment (firefighting, skydiving, mountain climbing, bungee cord jumping, etc.). The risk (danger) is usually obvious and the activity is usually done by free will.

• Risks associated with natural disasters: Floods, hurricanes, earthquakes, meteorite hits, and other disasters beyond human control. Exposure to the effects of certain natural disasters can be exacerbated by actions such as living on a known earthquake fault or the slope of a volcano.

• Risks involving involuntary exposure: An individual or entity releases a compound into the environment (pesticides, known carcinogens), potentially harming workers or members of the public, who cannot directly control the exposure.

The magnitudes of involuntary environmental risks are often quite different from the magnitudes of the risks associated with voluntary activities or natural disasters. Table 2-1 lists one assessor’s evaluation of various risks.

Table 2-1. Loss of Life Expectancy from Various Societal Activities and Phenomena

In this chapter, the focus is on the concept of involuntary environmental risks associated with material, radiation, or other releases into the environment. Environmental risk is a function of hazard and exposure:

Environmental risk = f (hazard, exposure)

The hazard depends on the toxicity of the material, radiation, or other release. Exposure depends on the concentration of the material that a person experiences. Engineers can reduce environmental risk by using less toxic material (minimizing hazard) or by engineering systems that do not allow materials to escape into the environment (minimizing exposure).

Environmental risk assessment is used to quantitatively determine the probability of the adverse effects of environmental releases. A common application of risk assessment methods is to evaluate human health and ecological impacts of chemical releases to the environment. Information collected from environmental monitoring or modeling is incorporated into models of human or worker activity, and estimates of the likelihood of adverse effects are formulated, as illustrated in Example 2-1. Risk assessment is an important tool for making decisions with environmental consequences. Almost always, when the results from environmental risk assessment are used, they are incorporated into the decision-making process along with the economic, societal, technological, and political consequences of a proposed action.

Use the following exposure properties:

where RR is the efficiency of the lungs to retain benzene and ABS is the efficiency of the lung tissue to absorb the retained chemical. These values were to set to maximum values (1.0) for this problem and may actually be much lower.

a. Calculate the inhalation dose of benzene to a typical resident using the following equation:

b. Calculate the inhalation carcinogenic risk for this scenario using the following equation:

Inhalation Carcinogenic Risk (dimensionless) = Inhalation Dose × SF

c. Is the risk greater than the recommended range of <10^-4 to 10^-6 (1 in 10,000 to 1 in 1,000,000 additional people contracting cancer due to inhaled benzene) for carcinogenic risk?

d. Discuss possible reasons why this methodology might overpredict the actual risk.

Solution:

a. Inhalation Dose, I_INH:

Note that the number of days in the numerator and denominator is the same, since it is assumed that the exposed individual remains in the same location, the point of maximum concentration, for 70 years.

b. Inhalation Carcinogenic Risk

c. Risk of cancer due to inhalation is greater than the recommended range of <10^-4 to 10^-6.

d. This calculation might overpredict the risk because RR and ABS were assumed to be 1 when their actual value would be less than 1 but greater than 0 (the bloodstream will not absorb all of the benzene inhaled into the lungs). Also, it was assumed that exposure to outside air, at the point of maximum concentration, would occur for 24 hours per day, 365 days per year, for 70 years. In practice, people move about over a 70-year lifetime.

2.2.2. Risk Assessment

In 1983, the National Research Council (NRC, 1983) developed an environmental risk assessment framework that, updated (NRC, 2009), is still in place today. The framework consists of four major components: hazard assessment, dose response, exposure assessment, and risk characterization.

1. Hazard assessment: What are the adverse health effects of the chemical(s) in question? Under what conditions? Toxicologists usually perform this analysis. Sources of hazard information are provided in the appendix to this chapter.

2. Dose response: How much of the chemical causes a particular adverse effect? Are some individuals more sensitive to a particular dose than others? There may be multiple adverse health effects, or responses, for the same chemical, and each adverse effect has a unique relationship between dose and response. For our purposes, dose is defined as the quantity of a chemical that crosses a boundary to get into a human body or organ system. The term applies regardless of whether the substance is inhaled, ingested, or absorbed through the skin. Dose response, then, is a mathematical relationship between the magnitude of a dose and the magnitude of a certain response in the exposed population.

3. Exposure assessment: Who is exposed to this chemical? How much of the chemical reaches the boundary of a person, and how much enters the person’s body? Exposure may be measured, estimated from models, or even calculated from measurements of biomarkers taken from exposed individuals.

4. Risk characterization: How great is the potential for adverse impact from this chemical? What are the uncertainties in the analyses? How conclusive are the results of these analyses?

The risk assessment process can be iterative. That is, if a cursory or screening risk assessment identifies concerns, a more rigorous process may be called for. This process may in turn illustrate that there are important, specific data gaps that need to be filled to render the risk assessment process conclusive enough for risk management. The data gaps may be filled with recommendations for special studies of varying cost and time requirements, such as

• Proceeding with testing for health effects

• Evaluating the effectiveness of engineering controls, to limit exposure to chemicals, and of personnel protective equipment

• Defining the degradation kinetics and decomposition products of a waste stream and the impact of the chemical waste and its degradation products on local flora and fauna

If it is reasonably clear from the risk assessment that a risk exists, the next step is risk management.

Risk management is the process of identifying, evaluating, selecting, and implementing actions to reduce risk to human health and to ecosystems. The goal of risk management is scientifically sound, cost-effective, integrated actions that reduce or prevent risks while taking into account social, cultural, ethical, political, and legal considerations (Presidential Commission, 1997).

Risk managers must answer many questions, some of which are:

• What level of exposure to a chemical risk agent is an unacceptable risk?

• How great are the uncertainties and are there any mitigating circumstances?

• Are there any trade-offs between risk reduction, benefits, and additional cost?

• What are the chances of risk shifting, that is, shifting risk to other populations?

• Are some of the risks worse than others?

The answers to these questions can depend on culture and values, as illustrated by the use of risk assessment in environmental law.

2.2.3. Risk-Based Environmental Law

Many of our environmental laws are based on risk frameworks. Table 2-2 lists selected U.S. safety, health, and environmental statutes that require or suggest human health risk assessment before regulations are promulgated. These regulatory risk assessments are made complex because not all environmental statutes (laws) are developed using the same types of standards. For example, the provisions of the Clean Air Act pertaining to National Ambient Air Quality Standards (NAAQS) call for values that “protect the public health allowing an adequate margin of safety.” That is, these standards mandate protection of public health based only on risk, without regard to technology or cost factors. In contrast, the Clean Water Act requires industries to install specific treatment technologies. These have descriptions like “best practicable control technology” and “best available technology economically achievable.” Pesticides are licensed if they do not cause “any unreasonable risks to man or the environment taking into account the economic, social, and environmental costs and benefits of the use of any pesticide.” In this case, economic and other factors may or may not be combined with risk issues as regulations are developed. So, the details of the risk frameworks used in environmental regulations vary, but the basic principles of the framework remain the same.

Table 2-2. U.S. Safety, Health, and Environmental Statutes That Imply Risk Assessment

Section 812 of the Clean Air Act Amendments of 1990 provides a case study of a statute that requires a particularly detailed risk assessment. The goal of this section of the Clean Air Act is to assess the costs and the benefits of the Clean Air Act. It requires the Environmental Protection Agency to estimate the hazards associated with air pollutants covered under the act, use dose-response curves to estimate health effects (and ecological effects to the extent possible), use exposure estimates to determine health impacts in the U.S. population, characterize uncertainties, and perform a form of risk management (cost-benefit analysis) (for more details, see U.S. EPA, 2011a). These steps are outlined here:

• Hazard identification: Mortality and disease associated with exposure to air pollutants are the primary hazards considered in Section 812 analyses; some risks to ecosystems, such as the nitrification of bays, have been considered on a case-by-case basis.

• Dose response: Quantitative relationships are developed for increased mortality and disease associated with exposure to air pollutants; the primary analyses are for exposure to particulate matter and ground-level ozone.

• Exposure assessment: The exposure of the entire U.S. population to these pollutants is estimated, based on geographical variability of pollutant concentrations (for example, ozone concentrations are higher in Los Angeles than in Houghton, Michigan).

• Risk characterization: Mortality and the prevalence of disease caused by ozone and particulate matter are estimated; uncertainties are estimated and reported; and the estimated reductions in mortality and disease due to the regulations in the Clean Air Act are estimated.

• Risk management: To help inform decisions about whether Clean Air Act regulations are too strict, not strict enough, or at about the right level, the costs and benefits of the regulations are estimated. Costs are expressed in dollars and are estimated based on the costs of equipment (e.g., exhaust controls on vehicles) and the costs of operations (e.g., energy needed to run some types of pollution control devices such as electrostatic precipitators to capture particles). The benefits of the regulations are avoided early deaths (death cannot be avoided, but it can be hastened) and avoided disease. The risk manager could make decisions based on the estimates of monetary costs of the controls, weighed against avoided early deaths and avoided disease, but this would involve an implicit evaluation of the monetary value of disease and early death. An interesting feature of the Section 812 analyses performed by EPA is that the benefits of the regulation are explicitly monetized. The benefit of disease reduction is monetized by calculating direct health care costs associated with treating the diseases. Monetizing avoided early deaths is more problematic. One approach to monetizing the costs of early deaths has been to estimate lost wages due to decreased life expectancy. This means that the life of a well-educated college student is worth more than the life of that student’s grandparent, because if the student were to die prematurely because of air pollution, the lifetime loss in expected wages would be greater than if the grandparent were to die early from air pollution exposure. But is one life worth more than another? Currently, in estimating the monetary benefit of avoided early mortality, the EPA values all avoided early deaths equally—at about $7 million to $9 million. These assumptions lead to estimated benefits of the Clean Air Act (about $2 trillion) that far outweigh the estimated costs (about $85 billion) (U.S. EPA, 2011b).

2.3. Life-Cycle Frameworks

Understanding life-cycle frameworks for environmental and sustainability issues requires an understanding of the language of life-cycle assessment, the tools used to quantify life-cycle impacts, and the use of life-cycle assessment in regulation. This section addresses each of these prerequisites for understanding the use of life-cycle frameworks and provides case studies of the use of life-cycle assessment tools.

2.3.1. Defining Life Cycles

While risk assessment and risk management have proven to be useful frameworks for managing environmental issues, one shortcoming of these approaches is that they tend to examine individual events (e.g., a single pollutant being emitted from a single source or group of sources) rather than systems. Decisions made to address environmental issues or sustainability often have cascading impacts through complex engineered systems. Consider, as an example, the use of biofuels to reduce the net emissions of greenhouse gases.

Burning a biofuel (such as ethanol derived from sugarcane or corn) releases carbon dioxide, but that carbon was originally withdrawn by the biomass from the atmosphere during the process of photosynthesis. When the atmospheric withdrawal due to photosynthesis is combined with the combustion emissions, the net emissions of carbon dioxide to the atmosphere are lowered for biofuels. In contrast, burning petroleum-based fuels releases carbon to the atmosphere that was originally (at least over the timescale of centuries) sequestered in geological formations. As a consequence, the use of biofuels is often seen as a strategy for reducing net greenhouse gas emissions.

While use of biofuels may reduce net carbon dioxide emissions to the atmosphere if just photosynthesis and combustion are considered, biofuels are the product of a complex engineered system. Growing biomass for biofuels requires fertilizer. Fertilizers require energy to manufacture and may cause the release of N₂O (a potent greenhouse gas) when applied to soils. Growing biomass for biofuels requires land. As land is converted from nonagricultural uses into agricultural production, carbon in the soil and in the original vegetation covering the land may be lost. Growing biomass for biofuels sometimes requires irrigation. Pumping water requires energy. So, the decision to grow biomass for biofuels will change a complex agricultural system in complicated ways. Frameworks are needed to understand these systems. Life-cycle frameworks are one possibility.

Products, services, and processes all have a life cycle. For products, the life cycle begins when raw materials are extracted or harvested. Raw materials then go through a number of manufacturing steps until the product is delivered to a customer. The product is used, then disposed of or recycled. These product life-cycle stages are illustrated in Figure 2-1, along the horizontal axis. As shown in the figure, energy is consumed, and wastes and emissions are generated in all of these lifecycle stages.

Figure 2-1. Product life cycles include raw material extraction, material processing, and use and disposal steps and are illustrated along the horizontal axis. Process life cycles include planning, research, design, operation, and decommissioning steps and are shown along the vertical axis. In both product and process life cycles, energy and materials are used at each stage, and emissions and wastes are created.

Processes also have a life cycle. The life cycle begins with planning, research, and development. The products and processes are then designed and constructed. A process has an active lifetime, then is decommissioned, and, if necessary, remediation and restoration may occur. Figure 2-1, along its vertical axis, illustrates the main elements of this process life cycle. Again, energy consumption, wastes, and emissions are associated with each step in the life cycle.

Traditionally, product designers have been concerned primarily with product life cycles up to and including the manufacturing step. Process designers have been primarily concerned with process life cycles up to and including the manufacturing step. That focus is changing in the design process. Increasingly, product designers must consider how their products will be recycled. They must consider how their customers will use their products and what environmental hazards might arise. Process designers must avoid contamination of the sites where their processes are located. Simply stated, engineers must become stewards for their products and processes throughout their life cycles.

2.3.2. Life-Cycle Assessment

There is some variability in life-cycle assessment terminology, but the most widely accepted terminology was originally codified by international groups convened by the Society for Environmental Toxicology and Chemistry (SETAC) (see, for example, Consoli et al., 1993) and has since been codified in the 14040 series of life-cycle assessment standards, issued by the International Standards Organization (ISO, 2006). To begin, a life-cycle assessment (LCA) is the most complete and detailed form of a life-cycle study. A life-cycle assessment consists of four major steps.

Step 1. The first step in an LCA is to determine the scope and boundaries of the assessment. In this step, the reasons for conducting the LCA are identified; the product, process, or service to be studied is defined; a functional unit for that product is chosen; and choices regarding system boundaries, including temporal and spatial boundaries, are made. But what is a functional unit, and what do we mean by system boundaries? Let’s look first at system boundaries.

The system boundaries are simply the limits placed on data collection for the study. The importance of system boundaries can be illustrated by a simple example. Consider the problem of choosing between incandescent lightbulbs and fluorescent lamps for lighting a room. During the 1990s the U.S. EPA began its Green Lights Program, which promoted replacing incandescent bulbs with fluorescent lamps. The motivation was the energy savings provided by fluorescent bulbs. Like any other product, however, a fluorescent bulb is not completely environmentally benign, and a concern arose during the Green Lights Program about the use of mercury in fluorescent bulbs. Fluorescent bulbs provide light by causing mercury, in glass tubes, to fluoresce. When the bulbs reach the end of their useful life, the mercury in the tubes might be released to the environment. This environmental concern (mercury release during product disposal) is far less significant for incandescent bulbs. Or is it? What if we changed our system boundary? Instead of just looking at product disposal, as shown in the first part of Figure 2-2, what if the entire product life cycle were considered, as shown in the second half of Figure 2-2? In a comparison of the incandescent and fluorescent lighting systems, if the system boundary is selected to include electric power generation as well as disposal, the analysis changes. Mercury is a trace contaminant in coal, and when coal is burned to generate electricity, some mercury is released to the atmosphere. Since an incandescent bulb requires more energy to operate, the use of an incandescent bulb results in the release of more mercury to the atmosphere than the use of a fluorescent bulb. Over the lifetime of the bulbs, more mercury can be released to the environment from energy use than from disposal of fluorescent bulbs. Thus, the simple issue of determining which bulb, over its life cycle, results in the release of more mercury depends strongly on how the boundaries of the system are chosen.

Figure 2-2. The importance of system boundaries in life-cycle assessment is illustrated by the case of lighting systems (see the text).

As this example illustrates, the choice of system boundaries can influence the outcome of a life-cycle assessment. A narrowly defined system requires less data collection and analysis but may ignore critical features of a system. On the other hand, in a practical sense it is impossible to quantify all impacts for a process or product system. Returning to our example, should we assess the impacts of mining the metals and making the glass used in the bulbs we are analyzing? In general, we would not need to consider these issues if the impacts are negligible, compared to the impacts associated with operations over the life of the equipment. On the other hand, for specific issues, such as mercury release, some of these ancillary processes could be important contributors. What is included in the system and what is left out are often based on engineering judgment and a desire to capture any parts of the system that may account for 1% or more of the energy use, raw materials use, wastes, or emissions (other approaches to defining system boundaries are possible; see Allen et al., 2009).

Another critical part of defining the scope of a life-cycle assessment is to specify the functional unit. The choice of functional unit is especially important when life-cycle assessments are conducted to compare products. This is because functional units are necessary for determining equivalence between the choices. For example, if paper and plastic grocery sacks are to be compared in an LCA, it would not be appropriate to compare one paper sack to one plastic sack. Instead, the products should be compared based on the volume of groceries they can carry. Because fewer groceries are generally placed in plastic sacks than in paper sacks, some LCAs have assumed a functional equivalence of two plastic grocery sacks to one paper sack. Differing product lifetimes must also be evaluated carefully when using life-cycle studies to compare products. For example, a cloth grocery sack may be able to hold only as many groceries as a plastic sack but will have a much longer use during its lifetime that must be accounted for in performing the LCA. As shown in Example 2-2, the choice of functional unit is not always straightforward and can have a profound impact on the results of a study.

Step 2. The second step in a life-cycle assessment is to inventory the outputs that occur, such as products, by-products, wastes, and emissions, and the inputs, such as raw materials and energy, that are used during the life cycle. This step, shown conceptually in Figure 2-3, is called a life-cycle inventory and is often the most time-consuming and data-intensive portion of a life-cycle assessment.

Figure 2-3. Life-cycle inventories account for materials use, energy use, wastes, emissions, and co-products over all of the stages of a product’s life cycle.

An example of the level of detail that can be associated with a life-cycle inventory is shown in Tables 2-3 and 2-4. The data, drawn from a publicly available database of life-cycle inventory information (NREL, 2011, described later in this chapter), shows the inputs required to produce a kilogram of corn and a kilogram of corn stover (the residual after the corn is harvested). The table illustrates both the detail and the diversity of information available in a typical life-cycle inventory.

Table 2-3. Life-Cycle Inventory Data for the Production of 1 kg of Corn and 1 kg of Corn Stover

Table 2-4. Life-Cycle Inventory Data for the Production of 1 kg of Crude Oil at the Production Site

Table 2-4 shows similar information for crude oil production (U.S. average data) from the same database. The entries are equally detailed but account for very different types of inputs and releases to the environment. Comparing Tables 2-3 and 2-4 can be done for some types of inputs and releases but is difficult for many others.

Step 3. The output from a life-cycle inventory is an extensive compilation of specific materials used and emitted, as illustrated in Tables 2-3 and 2-4. Converting these inventory elements into an assessment of environmental performance requires that the emissions and materials use be transformed into estimates of environmental impacts. Thus, the third step in a life-cycle assessment is to assess the environmental impacts of the inputs and outputs compiled in the inventory. This step is called a life-cycle impact assessment.

This step often involves multiple types of impacts, just as the inventory involved multiple inputs and emissions. The impacts can include natural resource use, such as energy use and water use. They can include environmental impacts, such as acid deposition, smog formation, and solid waste generation. Sometimes social impacts, such as employment, are also quantified. While calculating impacts can sometimes facilitate comparisons between disparate products or materials, often the interpretation can be challenging, as illustrated in Example 2-3.

Step 4. The fourth step in a life-cycle assessment is to interpret the results of the impact assessment, suggesting improvements whenever possible. When life-cycle assessments are conducted to compare products, for example, this step might consist of recommending the most environmentally desirable product. Alternatively, if a single product were analyzed, specific design modifications that could improve environmental performance might be suggested. This step is called an improvement analysis or an interpretation step. While comparisons between disparate products or materials can sometimes be done effectively, often the interpretation can be challenging, as illustrated in Example 2-3.

2.3.3. Life-Cycle-Based Environmental Law

Life-cycle concepts are increasingly becoming a part of environmental regulations, especially those involving greenhouse gas emissions. For example, as this text is being written, a variety of governmental agencies are developing approaches for regulating the emissions of greenhouse gases associated with the production and use of transportation fuels. As an example, the Energy Independence and Security Act (HR 6) of 2007 states (EISA, 2007):

No Federal agency shall enter into a contract for procurement of an alternative or synthetic fuel, including a fuel produced from nonconventional petroleum sources, for any mobility-related use, other than for research or testing, unless the contract specifies that the lifecycle greenhouse gas emissions associated with the production and combustion of the fuel supplied under the contract must, on an ongoing basis, be less than or equal to such emissions from the equivalent conventional fuel produced from conventional petroleum sources.

States are also implementing greenhouse gas emission regulations. For example, the California Global Warming Solutions Act of 2006 (2006) has resulted in draft regulations that establish a limit for life-cycle greenhouse gas emissions of transportation fuels. Both the California low-carbon fuel standard and Section 526 of EISA, cited previously, require a life-cycle evaluation of the greenhouse gas emissions of transportation fuels, and this is becoming a common approach to considering greenhouse gas emissions. Employing a life-cycle approach in estimating greenhouse gas emissions from the production and use of transportation fuels means assessing all emissions from field to vehicle tank and from tank to vehicle exhaust. This scope of emissions assessment is frequently referred to as a “well-to-wheels” analysis.

The steps involved in performing a life-cycle assessment (scope and boundaries definition, life-cycle inventories, life-cycle impact assessment, life-cycle interpretation) can be applied to the problem of compliance with regulations such as EISA. The U.S. Air Force, the largest consumer of fuel in the federal government, must comply with Section 526 of EISA in purchasing fuels. To provide guidance for producers of fuel seeking to sell Section 526—compliant fuels, the Air Force convened a working group to develop guidance on procedures for estimating life-cycle greenhouse gas emissions for aviation fuels. This group defined the following steps for an LCA in this application (adapted from Allen et al., 2009):

Step 1: Determine the goal and scope of the assessment. Choices made at the goal and scope-setting stage can significantly impact the results of a life-cycle assessment. For example, an LCA could be based on the operation of a specific refinery, or the average operations of all refineries in a state, region, or nation. Also, the analysis could include not only industrial and combustion-related greenhouse gas emissions but also the implications of changes in land use at local, regional, national, or global scales.

Step 2: Develop an inventory of the greenhouse gas emissions throughout the life-cycle system. Multiple choices made at this stage also have the potential to significantly impact the results of the analysis. Time periods and spatial scales for data gathering and strategies for filling data gaps are among multiple factors influencing the results of the analyses. For example, EISA Title II, Subtitle A, Section 201, requires the greenhouse gas emissions of alternative fuels to be compared to the greenhouse gas emissions of average petroleum-based fuels in 2005. In 2005, disruptions due to Hurricanes Katrina and Rita had substantial impacts on refining operations. Other years without these disruptions may have different greenhouse gas emission characteristics, highlighting the importance of the selection of time periods for analysis. Furthermore, choices made at this stage regarding how emissions are assigned to processes that produce multiple products have a significant impact. For example, soy grown to make soy oil for biodiesel also results in the production of soy meal. How are the emissions associated with growing the soybeans distributed among the soy oil and the soy meal products? By mass? By economic value? By some other measure? The choice can substantially influence the results of the analysis (Allen et al., 2009).

Step 3: Assess the climate change impacts of the life-cycle inventory. Although methods for assessing global warming potentials (GWPs) of emissions are available from the IPCC (IPCC 2007), choices made at this stage still influence results of life-cycle assessments. For example, many LCAs consider only high-volume emissions (e.g., emissions of CO₂, CH₄, and N₂O), omitting other emissions that influence the energy balance of the atmosphere (such as sootlike “black carbon”).

Step 4: Interpret the LCA results. Interpretation of LCA results needs to consider the types of uncertainties outlined in steps 1 through 3. A detailed case study of a life-cycle assessment of a fuel that could be used to determine EISA compliance is provided by Allen et al. (2011).

This example of the application of life-cycle approaches illustrates that, while life-cycle frameworks for analyzing environmental issues can be extremely useful, there is a need to develop detailed guidance for the implementation of the frameworks. Risk-based frameworks for assessing and managing environmental risks evolved over decades. Life-cycle frameworks will similarly need to evolve.

2.4. Life-Cycle Assessment Tools

A number of quantitative tools are emerging that enable life-cycle assessments and analyses. These tools fall into two general types: process-based analysis tools and input-output analysis tools. This section will examine the types of analyses enabled by both of these types of tools, using tools that are in the public domain.

2.4.1. Process-Based Life-Cycle Assessments

Process-based life-cycle assessments follow supply chains, as illustrated in Figure 2-1. For example, for a biofuel, a process-based life-cycle assessment would link together process steps including corn growing, transporting the corn to a refiner, refining cornstarch or corn stover into a fuel, transporting the fuel to the point of sale, and combusting the fuel. As shown in Table 2-3, life-cycle inventory data can be assembled for each of these steps.

The steps are linked by accounting for mass flows. For example, assume that 500 kg of corn and 500 kg of corn stover were grown. The initial inputs and emissions would be equal to 500 times the amounts listed in Table 2-3. If the harvested material now had to be transported to a refiner 100 km from the field where the corn plant was grown, 1 metric ton of material would need to be transported 100 km for a total transportation burden of 100 ton-km (tkm). If the fuel requirement for transporting 1 tkm of freight by truck is 0.027 L of diesel fuel (NREL, 2011), then the 2.7 L of diesel would be added to the diesel requirements of the biofuel produced from 500 kg of corn and 500 kg of stover. Similar additions would be made to all of the inputs and emissions. Then, the next process step would be added to the analysis, and this process would continue until the entire life cycle was modeled.

Tools exist in the form of data on individual process steps, and in the form of software packages that facilitate the linking of individual processes. An example of the former category is the U.S. Life Cycle Inventory (LCI) Database, maintained by the National Renewable Energy Laboratory (NREL, 2011). An example of the latter category, which is in the public domain (there are multiple software packages that can be licensed), is the GREET model. GREET, the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation model, is maintained by Argonne National Laboratory (GREET, 2011).

There are several major challenges associated with performing process life-cycle assessments. One challenge is availability of data. A review of the U.S. LCI database (NREL, 2011) reveals that while data are available for many commodity materials, there are many data gaps. A second issue is that of system boundaries. For example, the data for corn growing includes the fuel used for the tractor that plowed the fields. But what about the steel used to make the tractor? What about the materials used to construct the steel mill that made the steel that went into the tractor that plowed the fields? Where do we draw the boundary? The answers are not simple, but recently another form of life-cycle assessment that does not have these challenges has emerged: input-output LCA.

2.4.2. Input-Output LCA

An input-output LCA relies on tools that have become widely used by economists. These economic input-output tools segment national and regional economies into sectors and follow the flows of money. Consider a simple example. Imagine that a consumer purchases an automobile for $20,000. The automaker might spend $10,000 purchasing parts from first-tier suppliers. Those first-tier suppliers then might spend $1000 on steel. The steelmaker in turn might purchase coal. The transactions would continue, with the initial purchase leading to more than $50,000 eventually changing hands. Economists have built models that define these financial linkages between sectors of the economy. Typically, an economy is broken into hundreds of sectors and the financial flows between each sector and all of the other sectors are tracked, creating an economic input-output model (EIO). The EIO can be used as a life-cycle assessment tool by recognizing that for each sector, parameters such as energy use per dollar of sales can be tracked. If dollar flows between sectors are known, and if energy use per dollar is known for each sector, energy use across the economy can be tracked. This approach to modeling economy-wide flows of energy, materials, and emissions is known as an EIO-LCA. EIO-LCA models are relatively recent developments, but online tools are available (e.g., see Problem 8 at the end of the chapter).

The advantage of EIO-LCA approaches is that they track all flows up to the point of purchase. Thus, they avoid problems of system boundaries. Only limited types of flows are tracked (e.g., energy, greenhouse gases, certain toxic compounds), so this method also suffers from data gaps. It also has the disadvantage of representing products at a relatively coarse level. So, for example, all automobiles, including electric vehicles, small sedans, and large luxury cars, are averaged in the same economic sector.

2.4.3. Hybrid Approaches

Process-based and EIO-LCAs have complementary strengths and are beginning to be used in sophisticated ways. It is beyond the scope of this chapter to describe these emerging, advanced tools, but the problems at the end of this chapter will provide an introduction to the types of analyses that can be done using various analysis tools.

2.5. Summary

Complex environmental and sustainability issues are best managed through structured analysis frameworks. This chapter has provided summaries of both traditional (risk-based) and emerging (life-cycle-based) frameworks. The basic principles, methodologies, and applications in environmental regulation have been summarized for both methods.

Problems

1. Voluntary risk Each year, approximately 45,000 persons lose their lives in automobile accidents in the United States (population 281 million according to the 2000 census). How many fatalities would be expected over a three-day weekend in the Minneapolis–St. Paul, Minnesota, metropolitan area (population 2 million)?

2. Involuntary risk Lurmann et al. (1999) have estimated the costs associated with ozone and fine particulate matter concentrations above the NAAQS in Houston. They estimated that the economic impacts of early mortality and morbidity associated with elevated fine particulate matter concentrations (above the NAAQS) are approximately $3 billion/year. Hall et al. (1992) performed a similar assessment for Los Angeles. In the Houston study, Lurmann et al. examined the exposures and health costs associated with a variety of emission scenarios. One set of calculations demonstrated that a decrease of approximately 300 tons/day of fine particulate matter emissions resulted in a 7 million person-day decrease in exposure to particulate matter concentrations above the proposed NAAQS for fine particulate matter, 17 fewer early deaths per year, and 24 fewer cases of chronic bronchitis per year. Using estimated costs of $300,000 per case of chronic bronchitis and $7,000,000 per early death, estimate the social cost per ton of fine particulate matter emitted.

3. Life cycles of cups In evaluating the energy implications of the choice between reusable and single-use cups, the energy required to heat wash water is a key parameter. Consider a comparison of single-use polypropylene (PP) and reusable PP cups. The reusable cup has a mass roughly 14 times that of the single-use cup (45 g versus 3.2 g), which, in turn requires petroleum feedstocks.

a. Calculate the number of times the reusable cup must be used in order to recoup the energy in the petroleum required to make the reusable cup.

b. Assuming that the reusable cup is washed after each use in 0.27 L of water, and that the wash water is at 80^°C (heated from 20^°C), calculate the energy used in each wash if the water is heated in a gas water heater with an 80% efficiency. Calculate the number of times the reusable cup must be used in order to recoup both the energy required to make the reusable cup and the energy used to heat the wash water. Assume that 1.2 kg of petroleum are required to produce 1 kg of polypropylene and that the energy of combustion of petroleum is 44 MJ/kg.

c. Repeat Part b, assuming that an electric water heater is used (80% efficiency) and that electricity is generated from fuel at 33% efficiency.

C_p of water = 4.184 J/g K

4. Durability versus efficiency improvements in newer products In minimizing the environmental footprints of products, there is tension between product durability and rapidly replacing older products with newer products that have less environmental impact associated with their use. Consider this question: When is it most energy-efficient to replace my vehicle?

a. The production of a 1995 vehicle consumed 125,000 MJ of energy, and the energy intensity of the materials used in manufacturing automobiles (energy required per kilogram of material) decreases by 1% to 2% per year. Assuming that the energy intensity of automobile manufacturing decreased by 1.5% per year between 1990 and 2010, calculate the energy required to produce a new automobile during the model years 1990, 2000, 2005, and 2010.

b. The projected average fuel economy of light-duty automobiles is expected to increase from 27.5 to 32.5 mpg between 1990 and 2020. Assume that this increase occurs in step changes, with an average fuel economy of 27.5 mpg between 1990 and 1999, 30 mpg between 2000 and 2009, and 32.5 mpg between 2010 and 2019. Calculate the amount of energy used (assuming an energy content for gasoline of 124,000 BTU/gal, 1.3 * 10⁸ J/gal) for vehicles traveling 12,000 miles per year for the decades of the 1990s, 2000s, and 2010s.

c. Is it more efficient to replace a vehicle every 10 years or every 15 years?

5. Options for moving energy Approximately 1 billion tons of coal are burned annually in the United States, providing 50% of the country’s electricity consumption. The coal may be either moved by train from the mine to power plants near where the power is used, or combusted near the mine mouth to generate electricity that can be transmitted over long distances to the users. As a case study of this trade-off, consider electricity use in Dallas, which is generated, in part, using coal from the Powder River Basin (PRB) in Wyoming. Power plants using PRB coal supply 6.5 billion kWh of power per year to the Dallas area, at an average conversion efficiency (energy in the generated electricity per energy in the fuel burned) of 33%. The coal mined at the PRB has a heat content of 8340 BTU/lb coal (1 kWhr = 3412 BTU).

a. Determine the amount of coal required from the PRB to support consumers in Dallas.

b. If the energy required to transport coal by train is 0.0025 gallons of diesel per ton mile, and the distance from the PRB to Dallas is 1000 miles, calculate the amount of energy required to transport the coal to Texas, and the total energy consumed in combustion and transport. What fraction of the total energy consumption is due to transport? Assume that diesel fuel has an energy content of 124,000 BTU/gal.

c. Calculate the amount of coal consumed if the electricity were generated at the mine (assume a 33% power plant efficiency) and if the transmission losses for the electricity, from the mine to Dallas, were 7%.

d. Which option (transporting coal or transporting electricity) would be more efficient?

6. Functional unit in life-cycle assessment: personal mobility Mobility is one of the measures of quality of life that citizens of many developed nations value highly, ranked behind only food and shelter as necessities for life. Mobility is also a key factor in sustainability because of the cumulative effects of providing mobility on the environment, on resource depletion, and on the economy.

In the table below, data are presented on two modes of transportation, automobile and bus. Use these data to answer the questions that follow.

Annual Average Personal Transportation Data for the United States

Other data and conversion factors: 150,000 BTU/gal gasoline, 163,000 BTU/gal diesel (includes production energy and feedstock energy over the fuel life cycle):

4.3 + 19.4 lb CO₂ e/gal gasoline (production + combust.)

3.6 + 22.2 lb CO₂ e/gal diesel (production + combust.)

a. Define an appropriate functional unit for a comparison of the bus and car transportation table for personal mobility.

b. Calculate the gallons of fuel needed to satisfy the transportation functional unit, and then convert gallons to energy (BTU per functional unit). Also, calculate the CO₂ emissions per functional unit (pounds of CO₂ emitted per functional unit).

c. Compare bus and auto transport based on energy consumption and greenhouse gas emissions.

7. Functional unit in life-cycle assessment: transport of goods Transport of goods is another important energy-consuming and greenhouse-gas-emitting activity, and, as for personal mobility, there are choices in modes of freight transportation.

In the table below, data are presented on three modes of freight transportation: by road (heavy trucks), by rail, and by ship (oceanic freighter). Use these data to answer the questions that follow.

Annual Average Freight Transportation Data for the United States

Other data and conversion factors: 190,000 BTU/gal heavy oil, 163,000 BTU/gal diesel

3.7 + 26.0 lb CO₂ e/gal heavy oil (production + combust.)

3.6 + 22.2 lb CO₂ e/gal diesel (production + combust.)

a. Define an appropriate functional unit for a comparison of the transportation modes shown in the table for freight transportation.

b. Calculate the gallons of fuel needed to satisfy the freight transportation functional unit, and then convert gallons to energy (BTU per functional unit). Also, calculate the CO₂ emissions per functional unit (pounds of CO₂ emitted per functional unit).

c. Rank the transportation modes from the least energy and greenhouse-gas-intensive to the most energy and greenhouse-gas-intensive.

8. Transport of goods: truck or air? Use the U.S. Life Cycle Inventory Database (www.nrel.gov/lci) to determine the relative amount of diesel fuel required to transport 1 ton of freight 1000 km by truck and by air.

9. Economic input-output life-cycle assessment Review the input-output model for life-cycle assessment, developed by Carnegie Mellon University. This model, available at the Web site www.eiolca.net, allows you to estimate the overall environmental impacts from expending a user-defined dollar amount in any of roughly 400 economic sectors in the United States. It provides rough guidance on the relative impacts of different types of products, materials, services, and industries, up to the point of purchase.

Use the model to answer these questions:

a. What is the most energy-intensive sector of the chemical industry (resin, rubber, artificial fibers, agricultural chemicals, and pharmaceuticals sector in the EIOLCA model; measured as total life cycle energy use per million dollars of sales in the sector)?

b. What suppliers to the automotive manufacturing sector have the greatest emissions of greenhouse gases?

c. How much energy is used to manufacture a passenger vehicle costing $20,000? How does this compare to the energy consumption from driving the vehicle? Assume that the car is driven 200,000 miles and gets 30 mpg of gasoline consumed. Assume that the gasoline has a heating value of 124,000 BTU/gal and that it takes 26,000 BTU of energy to produce the gasoline.

Appendix: Readily Available Hazard References

Although the list is not comprehensive, listed below are references commonly used to inform hazard assessment. The list is intended as a starting point for the engineer charged with hazard assessment.

1. MSDS. The Material Safety Data Sheet is a document developed by chemical manufacturers. The MSDS contains safety and hazard information, physical and chemical characteristics, and precautions on safe handling and use. MSDS may include hazards to animals, especially aquatic species. The manufacturer is required to keep it up-to-date. Any employer that purchases a chemical is required by law to make the MSDS available to employees. Development of an MSDS is required under OSHA’s Hazard Communication Standard.

2. NIOSH Pocket Guide to Chemical Hazards. NIOSH is the National Institute for Occupational Safety and Health; this is the organization that performs research for OSHA, the Occupational Safety and Health Administration. The Pocket Guide may be found online at www.cdc.gov/niosh/npg/. It includes safety information, some chemical properties, and OSHA Permissible Exposure Limit concentrations, or PELs. The lower the permissible concentration, the greater the hazard to human health.

3. IRIS. IRIS is a database maintained by the U.S. Environmental Protection Agency. IRIS stands for Integrated Risk Information System. It is available through www.epa.gov/ngispgm3/iris/index.html. IRIS is a database of human health effects that may result from exposure to various substances found in the environment.

4. Hazardous Substances Data Bank (HSDB). The data bank is available from the National Library of Medicine. The Web address is http://toxnet.nlm.nih.gov. The HSDB is a toxicology data file that focuses on the toxicology of potentially hazardous chemicals. It is enhanced with information on human exposure, industrial hygiene, emergency handling procedures, environmental fate, regulatory requirements, and related areas.

5. Toxnet. Toxnet is also available from the National Library of Medicine. The Web address is http://toxnet.nlm.nih.gov. Toxnet is a cluster of databases on toxicology, hazardous chemicals, and related areas. Both IRIS and the HSDB are available through Toxnet.

6. Casarett and Doull’s text Toxicology: The Basic Science of Poisons, Fifth Edition (1996). This is the classic text in the field for interested readers. It is published by McGraw-Hill.

7. Patty’s Industrial Hygiene and Toxicology. This set of volumes is a starting point for readers who want more information than exposure limits but who are not experts in toxicology. It is published by John Wiley & Sons.

References

Allen, D. T., and D. R. Shonnard. 2002. Green Engineering: Environmentally Conscious Design of Chemical Processes. Upper Saddle River, NJ: Prentice Hall.

Allen, D. T., N. Bakshani, and K. S. Rosselot. 1992. Pollution Prevention: Homework and Design Problems for Engineering Curricula. New York: American Institute of Chemical Engineers.

Allen, D. T., C. Allport, K. Atkins, J. S. Cooper, R. M. Dilmore, L. C. Draucker, K. E. Eickmann, J. C. Gillen, W. Gillette, W. M. Griffin, W. E. Harrison III, J. I. Hileman, J. R. Ingham, F. A. Kimler III, A. Levy, C. F. Murphy, M. J. O’Donnell, D. Pamplin, G. Schivley, T. J. Skone, S. M. Strank, R. W. Stratton, P. H. Taylor, V. M. Thomas, M. Wang, and T. Zidow. 2009. The Aviation Fuel Life Cycle Assessment Working Group, Framework and Guidance for Estimating Greenhouse Gas Footprints of Aviation Fuels (Final Report). Prepared for Universal Technology Corporation and the Air Force Research Laboratory. April Interim Report. AFRL-RZ-WP-TR-2009-2206.

Allen, D. T., C. Allport, K. Atkins, D. G. Choi, J. S. Cooper, R. M. Dilmore, L. C. Draucker, K. E. Eickmann, J. C. Gillen, W. Gillette, W. M. Griffin, W. E. Harrison III, J. I. Hileman, J. R. Ingham, F. A. Kimler III, A. Levy, J. Miller, C. F. Murphy, M. J. O’Donnell, D. Pamplin, K. Rosselot, G. Schivley, T. J. Skone, S. M. Strank, R. W. Stratton, P. H. Taylor, V. M. Thomas, M. Q. Wang, and T. Zidow. 2011. Life Cycle Greenhouse Gas Analysis of Advanced Jet Propulsion Fuels: Fischer-Tropsch Based SPK-1 Case Study. Final Report from the Aviation Fuel Life Cycle Assessment Working Group to the U.S. Air Force. AFRL-RZ-WP-TR-2010-XXXX. Draft, February 4.

California Global Warming Solutions Act of 2006. 2006. Assembly Bill 32 (AB 32). Available at www.leginfo.ca.gov/pub/05-06/bill/asm/ab_0001-0050/ab_32_bill_20060927_chaptered.pdf. Accessed March 2011.

Consoli, F., D. T. Allen, I. Boustead, J. Fava, W. Franklin, A. A. Jensen, N. deOude, R. Parrish, R. Perriman, D. Postlewaite, B. Quay, J. Sequin, and B. Vigon. 1993. Guidelines for Life Cycle Assessment: A Code of Practice. Pensacola, FL: SETAC Press.

EISA (Energy Independence and Security Act of 2007). 2007. Available at http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=110_cong_bills&docid=f:h6enr.txt.pdf. Accessed March 2011.

Fan, A. M., and L. W. Chang, eds. 1996. Toxicology and Risk Assessment: Principles, Methods, and Applications. New York: Marcel Dekker, Inc., p. 247.

Federal Focus, Inc. 1991. Towards Common Measures: Recommendations for a Presidential Executive Order on Environmental Risk Assessment and Risk Management Policy. Washington, DC: Federal Focus, Inc., and the Institute for Regulatory Policy.

Fort, D. D. 1996. “Environmental Laws and Risk Assessment.” In Toxicology and Risk Assessment: Principles, Methods, and Applications, edited by A. M. Fan and L. W. Chang. New York: Marcel Dekker, Inc., pp. 653–77.

GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model). 2011. Argonne National Laboratory. Available at http://greet.es.anl.gov/. Accessed July 2011.

Hall, J. V., A. M. Winer, M. T. Kleinman, F. W. Lurmann, V. Brajer, and S. D. Colome. 1992. “Valuing the Health Benefits of Clean Air.” Science 255:812–17.

IPCC (Intergovernmental Panel on Climate Change). 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IPCC. Cambridge: Cambridge University Press.

ISO (International Standards Organization). 2006. 14040 series of standards. Available at www.iso.org/iso/iso_14000_essentials.

Lurmann, F. W., J. V. Hall, M. Kleinman, L. R. Chinkin, V. Brajer, D. Meacher, F. Mummery, R. L. Arndt, T. H. Funk, S. H. Alcorn, and N. Kumar. 1999. Assessment of the Health Benefits of Improving Air Quality in Houston, Texas. Final Report by Sonoma Technologies to the City of Houston. STI-998460-1875-FR. November.

NRC (National Research Council). 1983. Risk Assessment in the Federal Government: Managing the Process. Committee on Institutional Means for Assessment of Risks to Public Health. Washington, DC: National Academy Press.

___________. 2009. Science and Decisions: Advancing Risk Assessment. Washington, DC: National Academy Press.

NREL (National Renewable Energy Laboratory). 2011. U.S. National Life Cycle Inventory Database. Available at www.nrel.gov/lci. Accessed July 2011.

Presidential/Congressional Commission on Risk Assessment and Risk Management. 1997. Final Report, Vol. 1, p. 1.

Roberts, W. C., and C. O. Abernathy. 1996. “Risk Assessment: Principles and Methodologies.” In Toxicology and Risk Assessment: Principles, Methods, and Applications, edited by A. M. Fan and L. W. Chang. New York: Marcel Dekker, Inc., pp. 245–70.

U.S. EPA (U.S. Environmental Protection Agency). 2011a. Clean Air Act, Section 812 Analyses on the Benefits and Costs of the Clean Air Act from 1990 to 2020. March. Available at www.epa.gov/oar/sect812.

___________. 2011b. The Benefits and Costs of the Clean Air Act from 1990 to 2020. March. Available at www.epa.gov/oar/sect812/feb11/summaryreport.pdf and www.epa.gov/oar/sect812/feb11/fullreport.pdf.