National Academies Press: OpenBook

Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan (2011)

Chapter: Appendix B: Review of Air Monitoring Data from Joint Base Balad

« Previous: Appendix A: Committee Biographical Sketches
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

Appendix B

Review of Air Monitoring Data from Joint Base Balad

This appendix provides supplemental information on the air monitoring data collected at Joint Base Balad (JBB) and discussed in Chapter 4. The main focus of the appendix is on the strengths and limitations in the sample collection and analysis. A discussion of the sampling issues for specific chemical classes detected at JBB—polycyclic aromatic hydrocarbons (PAHs), particulate matter (PM), metals, volatile organic compounds (VOCs), and polychlorinated dibenzo-para-dioxins/furans (PCDDs/Fs)—is also given.

STRENGTHS AND LIMITATIONS OF THE AMBIENT DATA

In each monitoring campaign at JBB, ambient air concentrations of PM10, metals, speciated VOCs, PAHs, and PCDDs/Fs were determined using standard EPA methodologies. In the 2009 campaign, PM2.5 was also measured. The following standard EPA sampling methods for toxic organics (TO) were used (EPA 1990):

  • TO-9 sampling for PCDDs/Fs;
  • TO-13A sampling for PAHs;
  • TO-14 sampling for VOCs in 2007;
  • TO-15 sampling for VOCs in 2009; and
  • MiniVol sampling for particulate matter with aerodynamic diameter less than 10 mm (PM10) in 2007 and also less than 2.5 mm (PM2.5) in 2009; the PM10 samples were also analyzed for individual metals by inductively coupled plasma mass spectrometry (ICPMS; Method 200.8).

For each method, samples are obtained by passing ambient air at a constant rate through a sampling apparatus for a fixed time, either to collect the materials (generally air pollutants) on a filter and/or sorbent (TO-9, TO-13A, Mini-Vol methods) or to collect a sample of the air itself (TO-14, TO-15 methods). The TO-9, TO-13A, TO-14, and TO-15 methods are designed to capture both vapor-phase and particulate-phase material. Photographs of the monitoring equipment are given in the CHPPM report and each instrument appears to have collected samples at about 4 feet above the ground (Taylor et al. 2008). The collected material is subsequently analyzed for specific chemicals. The measurements at JBB are the average concentration of the measured material in the air over the (approximately) 24-hour sampling period. Any spikes in concentration that may occur because of variations in

Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

source conditions or meteorology during the measurement period are incorporated into the 24-hour average, and are not discernible in the data except insofar as they affect the average.

The measurements also include all amounts of the material in ambient air regardless of the source. They do not directly measure emissions from the burn pit or from any other particular source. There are likely to be multiple sources of all the measured chemicals either on or in the vicinity of JBB, and the measurements include background concentrations due to those other sources as well as concentrations from the burn pit. The background concentrations from those other sources vary between sample times as a result of variations in time, number of sources, or location of those sources, as well as variations in meteorological conditions.

Several details of the sampling design and methodology affect the committee’s ability to analyze the sampling data. Such details have different effects on different analyses, so the following comments should not be interpreted as criticisms of the sampling design or methodology, since the samples were collected for a different purpose.

  • Samples were not collected simultaneously at all locations for each sampling event. An effort was made to sample all the individual pollutant types (VOCs, semivolatiles, PM, PCDDs/Fs) on the same day, and there is substantial overlap in the sampling start times and the length of time for each sample, but they vary by as much as several hours both within and between pollutant types. This limits the comparability of samples, since the weather conditions and sources of pollutants are likely to be different in character, number, and strength at different times of day. Consequently, comparisons between the different sites are made more difficult, and identification of distinct sources from the sample data is compromised to some extent.
  • Samples were not collected on any planned schedule. Instead a “convenience sampling” approach was used in which samples were collected when logistically possible. As a result, averages of the sample results may be unrepresentative of long-term average exposures because of unrecognized patterns in exposure (e.g., daily, weekly, monthly, or seasonal variations) not compensated by averaging the measurements.
  • The PM methodology used is not suited for collecting samples with high concentrations of PM, such as occur during dust storms, or even on many non-dust-storm days at JBB (NRC 2010). As a result, some of the filter samples probably suffer from sampling artifacts (underestimates or overestimates of PM concentration). The 2009 samples, where PM10 and PM2.5 samples were collected simultaneously at the same locations, show evidence of such problems. The ratio of PM2.5 to PM10 was between 0.25 to 0.6 for most samples, but a few showed ṣubstantially higher or lower ratios. In particular, for some samples the ratio exceeded unity (the ratio should always be less than unity except for measurement errors, since PM2.5 is a component of PM10). Similar problems are likely to have occurred with PM10 sampling in 2007, and may have been masked in some 2009 samples by occurring simultaneously in both PM10 and PM2.5 samples. Further evidence of these problems is provided by some large discrepancies in measurements of the same parameter (PM2.5 or PM10) at the different sampling sites on the same day.
  • Monitoring at different areas of the site would have been helpful. A site closer to the burn pit might have been more strongly and clearly affected by this source, allowing better characterization of the burn pit emissions. Sites in all the housing areas would allow better evaluation of exposures to all personnel on base rather than just those initially thought to be most highly exposed. Use of paired sites relatively close together, rather than single sites, may have compromised the analytic methods the committee attempted to use, since the paired sites were sufficiently far apart to be affected differently by local sources. Moreover, since these closely spaced sites were not sampled simultaneously, they do not provide any useful information on the variation of exposures within the housing area.
  • The TO-14/TO-15 methodology (in which a sample of air is collected in an initially evacuated stainless steel flask) is not well suited to measure polar or reactive organics such as acrolein and 1,3-butadiene. Such materials may react on the walls of the stainless steel flask or in the gas phase during storage and transport of the sample. For example, 1,3-butadiene will decompose inside the canister during storage, mostly by reactions with nitrogen oxides. For risk assessments this is an important issue, because acrolein and butadiene are often the major risk contributors in screening risk assessments.
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

TABLE B-1 Average PAH Concentrations (ng/m3) at JBB

Analyte Guard Tower/ Transportation Field H-6 Housing/CASF Mortar Pit
Spring 2007 Fall 2007 2009 Spring 2007 Fall 2007 2009 Spring 2007 Fall 2007 2009
Acenaphthene 3.3 5.6 1.8 5.6 3.0 2.2 3.2 2.4 2.0
Acenaphthylene 5.7 24.4 3.5 12.1 14.2 8.3 4.9 8.7 2.7
Anthracene 2.8 6.2 1.3 3.4 2.8 1.0 2.0 2.0 0.5
Benz[a]anthracene 1.6 2.8 0.9 1.7 1.4 0.7 1.6 0.8 0.6
Benzo[a]pyrene 1.0 2.7 1.0 0.9 1.7 1.0 0.9 1.7 1.0
Benzo[b]fluoranthene 2.8 5.5 1.9 2.2 3.0 1.9 2.8 2.2 1.8
Benzo[e]pyrene 1.6 3.0 1.1 1.2 1.5 1.1 1.5 1.2 1.0
Benzo[g,h,i]perylene 1.5 3.3 1.3 1.2 2.7 1.7 1.4 1.7 1.4
Benzo[k]fluoranthene 0.6 1.2 0.5 0.5 0.7 0.4 0.6 0.5 0.5
Chrysene 2.5 3.7 2.5 2.4 2.3 1.6 2.1 1.3 1.4
Dibenz[a,h]anthracene 0.3 0.7 0.3 0.2 0.3 0.1 0.3 0.3 0.1
Fluoranthene 7.8 10.9 4.8 7.9 6.6 4.5 5.5 4.3 4.1
Fluorene 12.3 24.1 9.3 16.1 11.7 8.0 8.7 8.1 6.6
Indeno[1,2,3-cd]pyrene 1.4 3.2 1.1 1.1 2.0 1.2 1.3 1.7 1.3
Naphthalene 200.0 536.9 205.3 242.9 348.4 283.8 133.7 335.5 201.9
Phenanthrene 29.4 45.1 19.5 34.6 23.2 17.4 20.5 15.8 15.1
Pyrene 6.4 9.1 3.4 6.9 6.1 3.5 4.6 3.6 2.9
Number of samples 9 10 19 11 7 17 10 6 18
  • The sampling locations were chosen to evaluate concentrations downwind of the burn pit (Taylor et al. 2008). The committee was initially informed that no military personnel serviced the burn pit at JBB, but subsequent information indicated that military personnel worked at or very near the pit. The concentration of contaminants in or near the pit cannot be inferred from any of the samples obtained, so there could be a subpopulation of military personnel who were highly exposed but whose exposure cannot be estimated from available data.

Polycyclic Aromatic Hydrocarbons

Measurements of PAHs were obtained at five sampling sites at JBB, but for 2007 these were reported as three locations—the guard tower/transportation field (20 samples),1 H-6 housing/CASF (18 samples) and mortar pit (16 samples) (Table B-1). In 2007, samples were collected on each of 22 days, but on only 12 days were all three locations sampled. The 2009 measurements were again taken at the five sampling sites, and were reported separately for the five sites; but they are treated here as the three sampling locations as was done for the 2007 samples—the guard tower/transportation field (19 samples), H-6 housing/CASF (17 samples) and mortar pit (18 samples). Some samples were collected on each of 20 days, but only on 15 days were all three locations sampled. The nominally 24-h samples were not obtained simultaneously at the three locations on the common sampling days, but began within 2.5 hours in 2007, and within 1.6 hours in 2009; the sample times varied from 17.4 hours to 25.4 hours in 2007, and 21.1 hours to 25.2 hours in 2009. In both 2007 and 2009 each sample was analyzed for 17 PAH analytes, with all of the analytes detected in most samples.2

_____________

1Twenty-one samples results are reported in the data provided to the committee and used by Taylor et al. (2008), but one clearly corresponds to an unexposed sample and is omitted from further consideration here.

2The omitted sample was nondetect for every PAH except naphthalene, which was measured at a level 50 times lower than the next lowest sample. The committee considers this to be an unexposed sample.

Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

Particulate Matter and Metals

The 2007 PM measurements included 28 to 32 samples at each of the three sampling locations (total 90 samples) on a total of 32 sampling days. PM10 samples were obtained by specifically collecting particles with aerodynamic diameters less than 10 mm on filters. The measurement consists of a careful weighing of the filter before and after the collection period; then knowledge of the amount of air directed through the filter allows computation of the average concentration of PM10 in the air. The PM on the filter was then analyzed for the following metals: antimony, arsenic, beryllium, cadmium, chromium, lead, manganese, nickel, vanadium, and zinc. For the CHPPM samples at JBB in 2007, the measurements of individual metals were not useful, because the detection limit of the method used was too high to detect the metals of interest in the PM10 material in the great majority of samples. Only 6 detections, all of lead, were made in the 90 samples; these detections were all made on 2 consecutive days in November 2007 and all were less than 0.7 mg/m3. The committee therefore disregarded the metals measurements as not providing useful information.

In 2009, 50 measurements were available for 19 days of near-simultaneous PM10 or PM2.5 samples, together with another 8 samples of PM10 or PM2.5 individually, although four of the PM10 measurements were clearly affected by measurement artifacts and were ignored by the committee. The collected PM10 material was analyzed for metals, and again the detection limits were sufficiently high that most of the samples had nondetectable levels of the metals of interest. There were, however, many more detections than in 2007—25 of 108 samples had at least one metal detected—despite detection limits that were very similar; the concentrations of the metals varied, with individual concentrations up to 5 mg/m3. The third highest PM10 measurement was discounted because it suffered from measurement artifact (by comparison with the simultaneous PM2.5 measurement), and the consistency of the two available PM2.5 measurements on the same day strongly suggests that the second highest PM10 measurement also suffered from the same problem.3

Volatile Organic Compounds

VOCs were detected in 66 samples in 2007 and in 55 samples in 2009. The likely sources were considered to be combustion (including petroleum fuel combustion), fuel additives, solvents, and refrigerants. The 2007 measurements of VOCs included 66 samples on 26 days, using TO-14 methodology for sample collection. Each sample was analyzed for 78 VOCs. Fifty-five of these analytes were detected in six or fewer samples. The frequency of detection and likely major sources or uses for the other 28 analytes are shown in Table B-2.

The 2009 measurements included 57 samples on 20 days. There was a change to TO-15 methodology, so each sample was analyzed for up to 62 VOCs, including some not measured in 2007, and some of those measured in 2007 were omitted from the analyte list in 2009. Forty of the VOCs were detected in six or fewer samples. The frequency of detection and likely major sources or use for the other 22 analytes are shown also in Table B-2.

The differences between these lists largely arise from the different analytes measured or different detection limits in 2007 and 2009. Octane, isooctane, chlorodifluoromethane and pentane were analyzed in 2007 but not in 2009, while the xylenes were analyzed in different combinations. Isopropyl alcohol and cyclohexane were analyzed in 2009 but not in 2007. The detection limits for acrolein, 2-butanone, methylene chloride, and 1,4-dichlorobenzene were lower in 2009 than in 2007, although as discussed earlier the method used for acrolein was inadequate. The detection limit for MtBE was higher in 2009 than in 2007, and the average detected concentration in 2007 was half the 2009 detection limit. However, 4-ethyltoluene was detected less frequently and at lower concentrations in 2009 (even with a lower detection limit).

Table B-3 presents average concentrations of the twelve most frequently detected VOCs by location and sampling campaign, with nondetects assumed to contribute one-half the detection limit (for these VOCs, setting nondetects to zero alters the mean estimate by a factor of 1 to 2.4). Like PM10, VOC concentrations were similar

_____________

3This assumes negative artifacts (underestimation of concentrations). Since positive artifacts are also quite likely at high PM concentrations with the methodology used for measurement at JBB (NRC 2010), this evaluation of artifact could be entirely backward. In that case the majority of high concentration measurements could be artifacts.

Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

TABLE B-2 Number of Detects and Likely Source for Analytes Detected More than Occasionally in 2007 and 2009

Analyte Number of detectsa Likely major source or use
2007 2009
Acetone 66 55 Solvent, combustion
Benzene 66 56 Combustion
Chloromethane 66 54 Natural sources (ATSDR 1998, 1999)
Toluene 63 56 Petroleum-based fuel combustion
Hexane 65 53 Petroleum-based fuel combustion
Pentane 65 NA Petroleum-based fuel combustion
Dichlorodifluoromethane 64 55 Refrigerant
Propylene 32 55 Petroleum-based fuel combustion
2-Butanone (MEK) 20 53 Solvent, combustion
Methylene chloride 27 52 Solvent
o-Xylene 51 52 Combustion
n-Heptane 60 36 Petroleum-based fuel combustion
Ethylbenzene 49 49 Combustion
m,p-Xylene 55 NA Combustion
Xylenes, total NA 45 Combustion
1,2,4-Trimethylbenzene 51 41 Combustion
Octane 46 NA Petroleum-based fuel combustion
Trichlorofluoromethane 42 37 Refrigerant
Chlorodifluoromethane 41 NA Refrigerant
Acrolein 4 34 Combustion
4-Ethyltoluene 38 12/30 Combustion
Isopropyl alcohol NA 27 Solvent, disinfectant
Methyl tert-butyl ether (MtBE) 29 0 Anti-knock fuel additive
1,3,5-Trimethylbenzene 24 11 Combustion
Isooctane 17 NA Anti-knock fuel additive
Styrene 17 14 Combustion
1,4-Dichlorobenzene 4 9 Mothballs, pesticide
Cyclohexane NA 8 Petroleum-based fuel combustion

NOTE: NA = not analyzed.
aTotal possible detects 66 in 2007 and 57 in 2009 except 4-ethyltoluene.

for many analytes at all the measurement locations at JBB, and there did not appear to be any consistent gradients in concentration, although differing gradients exist for some analytes at some times.

Polychlorinated Dibenzo-Para-Dioxins/Furans

During 2007, 18, 21, and 21 PCDD/F nominal 24-hour samples were collected at the guard tower/transportation field, H-6 housing/CASF, and mortar pit locations, respectively, on 20 sampling days (TO-9 method; 60 samples total). Two of the samples for the H-6 housing/CASF and mortar pit locations were obtained on the same day (about 2 hours and 20 minutes apart in starting time, respectively), and on 2 of the 20 sampling days the guard tower/transportation field was not sampled. All samples were analyzed for the seventeen 2,3,7,8-chlorinated PCDD/F congeners, with all congeners detectable in 41 or more of the 60 samples except 1,2,3,7,8,9-hexaCDF (detected in 14/60 samples).

The 2009 sampling data for PCDD/PCDFs included 19, 17, and 18 PCDD/F nominal 24-hour samples at the

Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×

TABLE B-3 Average Concentration (mg/m3) of the 12 Most Frequently Detected VOCs at JBB

Analyte Guard Tower/ Transportation Field H-6 Housing/CASF Mortar Pit
Spring 2007 Fall 2007 2009 Spring 2007 Fall 2007 2009 Spring 2007 Fall 2007 2009
Benzene 6.8 9.1 4.9 4.8 5.0 3.8 3.3 8.3 2.7
Acetone 37.9 10.8 29.6 25.7 9.4 42.7 13.8 6.8 29.7
Chloromethane 2.0 1.6 1.9 1.9 1.1 1.7 1.5 1.2 1.8
Dichlorodifluoromethane 4.3 2.5 2.5 3.0 2.6 2.6 2.7 2.3 2.5
Toluene 11.8 23.0 10.7 8.6 30.2 13.1 5.0 55.2 9.8
Hexane 5.1 4.5 5.3 2.9 5.4 31.6 2.1 16.2 7.2
Xylenes 7.2 22.4 7.7 9.6 18.3 8.9 4.6 52.6 6.4
Ethylbenzene 4.0 4.8 2.9 3.2 5.3 2.7 2.4 9.6 1.7
n-Heptane 3.6 2.9 2.1 4.1 3.5 2.8 1.8 6.5 1.9
1,2,4-Trimethylbenzene 2.6 18.0 2.0 3.7 9.3 2.9 2.2 11.9 1.8
Propylene 2.7 3.8 2.2 1.6 2.8 2.0 1.2 4.8 1.3
Methylene chloride 10.1 2.7 17.3 2.3 2.1 17.9 2.5 3.0 8.3
Number of samples 15 8 19 12 9 18 15 7 19

guard tower/transportation field, H-6 housing/CASF, and mortar pit locations, respectively, on 19 sampling days, with 3 different days missing a sample at one location (TO-9 method; 54 samples total). The concentrations measured in May and June of 2009 were consistently lower for all congeners, and at all measurement locations, than those measured in January, February, April, October, and November 2007, with a larger proportion of nondetects than in 2007 (206/918, 22%, versus 126/1173, 11%). Again, 1,2,3,7,8,9-hexaCDF was the most frequent nondetect (only 4/54 samples measurable).

CHPPM also provided the committee with PCDD/PCDF sample data for 9 samples collected on 9 dates in 2006, but no location for these samples was given. At JBB, the distributions of concentrations of total PCDD/Fs (the sum of the seventeen 2,3,7,8-chlorinated congener concentrations) and individual congeners at each location are approximately lognormal with high correlations between congeners. In the 2007 measurements, there is no significant difference in individual congeners or in total PCDD/F between the January–April and October–November measurements at each location, although there is a slight trend for the January–April concentrations to be higher than the October–November concentrations at the guard tower/transportation field and H-6 housing/CASF locations.

REFERENCES

ATSDR (Agency for Toxic Substances and Disease Registry). 1998. Toxicological profile for chloromethane. Tox profiles. Atlanta, GA: Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp.asp?id=587&tid=109 (accesed March 3, 2011)

EPA (U.S. Environmental Protection Agency). 1990. Technical assistance document for sampling and analysis of toxic organic compounds in ambient air. Washington, DC: Atmospheric Research and Exposure Assessment Laboratory.

NRC (National Research Council). 2010. Review of the Department of Defense Enhanced Particulate Matter Surveillance Program report. Washington, DC: The National Academies Press.

Taylor, G., V. Rush, A. Peck, and J. A. Vietas. 2008. Screening health risk assessment burn pit exposures Balad Air Base, Iraq and addendum report. IOH-RS-BR-TR-2008-0001/USACHPPM 47-MA-08PV-08. Brooks City-Base, TX: Air Force Institute for Operational Health and U.S. Army Center for Health Promotion and Preventative Medicine.

Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 133
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 134
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 135
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 136
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 137
Suggested Citation:"Appendix B: Review of Air Monitoring Data from Joint Base Balad." Institute of Medicine. 2011. Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan. Washington, DC: The National Academies Press. doi: 10.17226/13209.
×
Page 138
Next: Appendix C: Epidemiologic Studies Cited in Chapter 6: Health Outcomes »
Long-Term Health Consequences of Exposure to Burn Pits in Iraq and Afghanistan Get This Book
×
Buy Paperback | $45.00 Buy Ebook | $35.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Many veterans returning from the conflicts in Iraq and Afghanistan have health problems they believe are related to their exposure to the smoke from the burning of waste in open-air "burn pits" on military bases. Particular controversy surrounds the burn pit used to dispose of solid waste at Joint Base Balad in Iraq, which burned up to 200 tons of waste per day in 2007. The Department of Veterans Affairs asked the IOM to form a committee to determine the long-term health effects from exposure to these burn pits. Insufficient evidence prevented the IOM committee from developing firm conclusions. This report, therefore, recommends that, along with more efficient data-gathering methods, a study be conducted that would evaluate the health status of service members from their time of deployment over many years to determine their incidence of chronic diseases.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!