reducing oxygen delivery to tissues. Hypoxemia and resulting tissue hypoxia are the best understood mechanisms of CO toxicity. A log-log plot of estimated percentage of COHb saturation and exposure duration for different CO concentrations computed from the Coburn-Foster-Kane (CFK) equation is shown in Figure 8-1 (Peterson and Stewart 1975, Penney 1999). Details of the CFK equation are presented in Appendix A.

The central nervous system (CNS) and cardiovascular (CV) system are the primary targets of CO toxicity. Adverse effects resulting from CO exposure vary widely from subtle vascular and neurologic changes to loss of consciousness and death (NRC 2007). There is no known use of CO in spacecraft; however, it is predicted to be an off-gas product (Leban and Wagner 1989, as cited by Wong 1994). CO is also a by-product of a fire event occurring aboard spacecraft. Such an event occurred aboard the Mir Spacestation since publication of the 1994 CO SMAC (Wong 1994). CO and COHb concentrations as well as resulting health effects were documented and provide direct evidence of CO toxicity to spacecraft crews (James and Garcia 1994, James 2008).

FIGURE 8-1 Prediction of CO uptake and COHb saturation using CFK equation. Loglog plot of CO uptake by humans from very low ambient CO concentrations as computed from the CFK equation. Abbreviations: DL, diffusing capacity of lungs, [COHb]0, value before CO exposure; M, equilibrium constant; PB, barometric pressure; mean partial pressure of O2 in lung capillaries; ppm, parts per million; VA, alveolar ventilation rate; Vb, blood volume; VCO, rate of endogenous CO production. Source: Peterson and Stewart 1975, as cited by Penney 1999. Reprinted with permission; copyright 1975, Applied Physiology.

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