# Chapter 4. Dosimetry and Pharmacokinetics of Carbon Monoxide

4.1. Introduction

Inhaled ambient CO elicits various health effects by binding with and altering the function of a number of heme-containing molecules, mainly Hb. Traditional concepts for CO pathophysiology have been based on the high affinity of CO for hemoglobin, resulting in COHb formation and consequent reduction in O 2 -carrying capacity of blood and impaired O 2 delivery to tissues. Research on CO pharmacokinetics dates back to the 1890s, but since the late 1970s has become limited. Current literature primarily focuses on endogeno us CO produced by the metabolic degradation of heme by heme oxygenase (HO) and its role as a gaseous messenger. This chapter reviews the physiology and pharmacokinetics of CO. The chapter draws heavily from Chapter 5 of the previous AQCD (U.S. EPA, 2000, 000907 ). Relevant new data are included when available. Recent models of Hb binding are characterized, as well as measurem ents of tissue CO concentrations using new methods of extraction. CO binds with a number of heme-containing molecules including Mb and cytochromes, but none have been studied as extensively as Hb. The primary focus of this chapter is placed on the models and kinetics of such binding and the factors influencing this event. The chapter discusses effects at ambient or near ambient levels of CO leading to low COHb levels ( ≤ 5%); however few studies are available at ambient CO concentrations . Both human and animal studies using higher CO exposure concentrations, resulting in moderate to high COHb levels (<20%), are discussed where needed to understand CO kinetics, pathophysiologic processes, and mechanisms of cytotoxicity. Where human studies could not experimentally test certain hypotheses or were unavailable, animal experiments were used as surrogates. CO uptake and elimination has been shown to be inversely proportional to body mass over environmentally rele vant exposure levels, meaning the smaller the animal, the faster the rate of absorption and elimination (Klimisch et al., 1975, 010762 ; Tyuma et al., 1981, 011226 ). However, the basic mechanisms of CO toxicity between experimental animals and humans are similar and are thus extrapolated from an imals to humans in this chapter, keeping in mind a number of interspecies differences.



# 4.2. Carboxyhemoglobin Modeling

4.2.1. The Co burn-Forster-Kane and Other Models

Investigators have modeled the effect of CO bi nding to Hb in a number of ways. Empirical and mechanistic models are two distinct approaches that have been taken to model in vivo COHb formation after CO exposure. First, empirical models were used to predict COHb by regressing concentration and duration of exogenous CO exposure with observed COHb, with or without the inclusion of physiological predictors such as initial COHb levels and alveolar ventilation (V A ). These methods were reviewed in depth in the previous AQCD (U.S. EPA, 2000, 000907 ). It is important to note that CO empirical regression models are lim ited to estimating COHb in the exact conditions on which the models were based. These simple models include those by Peterson and Stewart (1970,

Note: Hyperlinks to the reference citations throughout this doc ument will take you to the NCEA HERO database (Health and Environmental Research Online) at http://epa.gov/hero . HERO is a database of scientific litera ture used by U.S. EPA in the process of developing science assessments such as the Integrated Science Assessments (ISAs) and the Integrated Risk Information System (IR IS).

January 2010 4-1012416 ) and Ott and Mage (1978, 011124 ), as well as various others (Chung, 1988, 012749 ; Forbes et al., 1945, 012850 ; Selvakumar et al., 1992, 013750 ; Sharan et al., 1990, 003798 ; Singh et al., 1991, 013583 ). Using a linear differential equation where ambient CO concentrations varied, it was shown that the presence of brief ambient CO conc entration spikes averaged over hourly intervals may lead to underestimating the COHb concentration by as much as 21% of the true value. To avoid this problem, it was suggested that ambient CO measurements be monitored and averaged over 10- to 15-min periods (Ott and Mage, 1978, 011124 ). Other empirical models predict COHb as a function of exposure time (Sharan et al., 1990, 003798 ; Singh et al., 1991, 013583 ) or exposure time and altitude (Selvakumar et al., 1992, 013750 ). A comparison of empirical model predictions showed a wide disparity in predicted COHb values, highlig hting the inaccuracy of these models outside of the conditions on which they were presented (Tikuisis, 1996, 080960 ). Secondly, mechanistic models use physical and physiological processes and an understanding of biological processes to predict COHb production. The most commonly used mechanistic method for predicting levels of blood COHb after CO i nhalation is the Coburn-Forster-Kane equation or CFK model developed in 1965 (Coburn et al., 1965, 011145 ). This differential equation was developed to examine endogenous CO production, using the major physiological and physical variables influencing this value. Since then, it has been shown to provide a good approximation to the COHb level at a steady level of inhale d exogenous CO (Peterson and Stewart, 1975, 010696 ; Stewart et al., 1973, 012428 ). The CFK model describes a four-element, physical system containing an exogenous CO source, a transfer interface, an endogenous CO source, and a storage compartment. The linear CFK model assumes O 2 Hb concentration is constant and is as follows in Equation 4-1 :



### [ ] [ ] [ ] ⎟ ⎟ ⎟ ⎟ ⎟

⎠ ⎞

⎜ ⎜ ⎜ ⎜ ⎜

⎝ ⎛

− + +
⎟ ⎟ ⎟ ⎟ ⎟

⎠ ⎞

⎜ ⎜ ⎜ ⎜ ⎜

⎝ ⎛

− + −=

• • •

A O2HB

L I

A O2HB

L 2 2 C 0
CO t b

V PP

COD 1 COP

V PP

COD 1 1

MHbO OPCOHb V dt COHbd V

Equation 4-1

V b blood volume in milliliters (mL)

[COHb] t COHb concentration at time t in mL CO/mL blood, at standard temperature and pressure, dry (STPD)

V CO endogenous CO production rate in mL/min, STPD

[COHb] 0 COHb concentration at time zero in mL CO/mL blood, STPD

[O 2 Hb] O 2 Hb concentration in mL O 2 /mL blood, STPD

P C O 2 average partial pressure of O 2 in lung capillaries in mmHg

M Haldane coefficient representing the CO chemical affinity for Hb

D L CO lung diffusing capacity of CO in mL/min/mmHg, STPD

P B barometric pressure in mmHg

P H2O saturation pressure of water vapor at body temperature in mmHg (47 mmHg)

V A alveolar ventilation in mL/min, STPD

P I CO CO partial pressure in inhaled air in mmHg

The linear CFK model assumes instant equilibration of COHb concentration between venous and arterial blood, gases in the lung, and COHb concentrations between blood and extravascular tissues, which is not physiologically representativ e. The nonlinear CFK equation extends the linear CFK equation to incorporate the interdependence of COHb and O 2 Hb levels since they are derived from the same pool of blood Hb. This interdepende nce can be modeled by substituting (1.38 Hb [COHb]) for O 2 Hb, where TH b refers to the number of grams of Hb per mililiter of blood (Peterson and Stewart, 1975, 010696 ). The nonlinear CFK differential equation is as follows in Equation 4-2 :

January 2010 4-2⎟ ⎟ ⎟ ⎟ ⎟

⎠ ⎞

⎜ ⎜ ⎜ ⎜ ⎜

⎝ ⎛

− + + ⎟ ⎟ ⎟ ⎟ ⎟

⎠ ⎞

⎜ ⎜ ⎜ ⎜ ⎜

⎝ ⎛

− + − −=

• • •

A O2HB

L A O2HB

L t b 2 C 0 CO t b

V PP

COD 1 1

V PP

COD 1 1
M)]COHb[]TH[38.1( OP]COHb[ V dt ]COHb[d V

Equation 4-2

The nonlinear equation is more physiologically accurate; however the linear CFK equation gives a good approximation to the nonlinear soluti on over a large range of values during CO uptake and during low levels of CO elimination (Smith, 1990, 013164 ). The linear equation prediction of COHb concentration at or below 6% will deviate by no more than ± 0.5% COHb from the nonlinear equation prediction. Sensitivity analysis of the CFK equations has shown that alterations in each variable of the equation will affect the outcome va riably at different times of exposure, so that the relative importance of the CFK variables will change with the experimental conditions (McCartney, 1990, 013162 ). Figure 4-1 illustrates the temporal changes in fractional sensitivities of the principal physiological determinants of CO uptake for the linear form of the CFK equation, where TH b is the total blood concentration of Hb in g Hb/mL blood and F I CO is the fractional concentration of CO in ambient air in ppm. The fractional sensitivity of unity means that, for example, a 5% error in the selected variable induces a 5% error in the pr edicted COHb value by the nonlinear model. As Figure 4-1 demonstrates, a constant or given percent error in one variable of the model does not generally produce the same error in the calcu lated blood COHb, and the error is time dependent. Thus, each variable influencing CO uptake and elimination will exert its maximal influence at different times of exposure. This analysis found that only F I CO (shown in Figure 4-1 ) and V CO will not affect the rate at which equilibrium is reached (McCartney, 1990, 013162 ).

January 2010 4-3Source: Adapted with Permission of the American I ndustrial Hygiene Association from McCartney (1990, 013162 )



### Figure 4-1. Plot of fractional sensitivities of selected variables versus time of exposure.

The mechanistic CFK model contains a number of assumptions under which the model is solely applicable, including: (1) ventilation is a co ntinuous process; (2) equ ilibrium between plasma CO concentration and COHb concentration is obtai ned in the pulmonary system; (3) percent COHb can exceed 100% saturation in the linear model; and (4) it does not account for the shape of the O 2 or CO saturation versus pO 2 or pCO relation (McCartney, 1990, 013162 ). Estimations outside of these assumptions have been attempted but with less pr edictive agreement. For example, transient exposures such as those that would simulate ever yday conditions would violate the assumption of a single, well-mixed vascular compartment. COHb levels during exposure of subjects exposed to frequent but brief high CO exposures (667-7,500 ppm for 75 s to 5 min) were not accurately predicted by CFK modeling (Benignus et al., 1994, 013908 ; Tikuisis et al., 1987, 012219 ; Tikuisis et al., 1987, 012138 ). Consistently, the COHb value predic ted by the nonlinear CFK overpredicted observed venous COHb (0.8-6%) and underpredicted arterial COHb (1.5-6.1%) and this disparity increased after exercise. Individual differences between arterial and venous COHb varied from 2.3-12.1% COHb (mean, 6.2 ± 2.7% COHb), where the observed steady state COHb averaged ~14% and the observed arterial peak COHb averaged ~17.5% (Smith et al., 1994, 076564 )(Benignus et al., 1994, 013908 ). These inaccuracies between measured and predicted COHb values disappeared after simulated mixing of arterial and venous blood and thus are likely due to delays in mixing of arterial and venous blood and differences in cardiac output and lung wash-in. This discrepancy in predicted and observed COHb suggests that over a short period (< 10 min) the arterial COHb levels that are delivered to tissues could be higher than what is predicted by the CFK equation. A modified CFK was created to adjust for these issues and produce a more accurate COHb prediction (Smith et al., 1994, 076564 ). This expanded CFK model used multiple compartments to model the lung, arm circulation, and the rest of the body (quickly a nd slowly perfused tissues). This model was more

January 2010 4-4accurate than the nonlinear CFK in predicting the indi vidual peak or maximal values of arterial and venous COHb during CO uptake in the first 10 mi n after exposure. However, both the nonlinear CFK and this expansion produced accurate predic tions several minutes after the 5-min exposure ended. The expanded model required the use of two parameters, V A and V b , that were not measured individually or derived from the literature, and i nstead were estimated by adjustments between the simulations and experimental subject data. In addition to the limitations discussed a bove, the CFK model does not account for extravascular storage sites for CO, such as m uscle Mb. CO will undergo reversible muscle Mb binding, similar to Hb, as well as uptake into other extravascular tissues (Vreman et al., 2006, 098272 ). The most recent adaptation to the CFK equation incorporates alveoli-blood and bloodtissue CO exchanges and mass conservation of CO at all times (Gosselin et al., 2009, 190946 ). This model has a single free parameter whose value is estimated from one data set; however, it better predicted COHb formation over a wide range of CO levels and several temporal scenarios (Stewart et al., 1970, 013972 ; Tikuisis et al., 1987, 012138 ; Tikuisis et al., 1987, 012219 ; Tikuisis et al., 1992, 013592 ) compared to the linear CFK model. Like th e linear CFK model, this modified model assumes a constant level of oxyhemoglobin. Sensitiv ity analysis of the model showed that the most important parameter influencing the level of COHb in this model is M, followed by P C O 2 and V A

. Ambient exposure scenarios were simulated with this model to determine the CO concentrations needed to reach certain COHb leve ls in humans from 3 months of age to 40-yr-old adults (Gosselin et al., 2009, 190946 ). The CO concentrations needed to achieve 2% COHb vary from 24.4-48.1 ppm for a 1-h exposure, from 11.1-13.1 ppm for an 8-h exposure, and from 9.8-10.1 ppm for a daily exposure. Infants (1 yr old) were most sensitive to CO concentrations, whereas newborns (3 mo old) required the highest CO concentration to reach 2% COHb. Newborns required a higher CO exposure partially because the values used in the model for the newborn blood Hb concentration (170 g Hb /
L blood ) is higher than at infancy (115 g Hb /L blood ) or adulthood (150 g Hb /L blood ). The model was also used to simulate time profiles of COHb formation for two work week exposure scenarios in a healthy 40-yr-old man. Figure 4-2 A represents a high exposure scenario where the work period is spent at 35 ppm and the rest of the time at 3 ppm. Figure 4-2 B represents a lower exposure scenario where there is a constant 3 ppm exposure. Both figures consist of 5 days where 24 h are broken up into 3 consecutive 8-h periods: sleeping from 12 a.m. to 8 a.m.; working with light exercise from 8 a.m. to 4 p.m.; and sitting from 4 p.m. to 12 a.m..

January 2010 4-5Source: Reprinted with Permission of Inform a Healthcare from Gosselin et al. (2009, 190946 )



### Figure 4-2. Simulated COHb formation for two 5-day workweeks. “The 24-h day consists of 3 consecutive 8-h periods: sleeping from 12 a.m. to 8 a.m.; working (light exercise) from 8 a.m. to 4 p.m.; and sitting from 4 p.m. to 12 a.m. (A) High exposure: work period at 35 ppm and the rest of the time at 3 ppm. (B) Low daily exposure at 3 ppm. The CO exposure periods are represented by dotted lines (----) and the COHb simulations by solid lines (—).”



## 4.2.2. Multic ompartment Models

A third approach applied more recently to model COHb formation is the use of multicompartment or physiologically-based pharm acokinetic (PBPK) models. Cronenberger et al. (2008, 194085 ) described a two-compartment population-ba sed model to describe and predict COHb pharmacokinetics from smoking. This model required a compartment for extravascular binding of

January 2010 4-6CO to accurately predict COHb formation during multiple short and rapid inhalations followed by a period of no exposure, as occurs in smoking. A five-compartment PBPK model has been propos ed to predict CO uptake and distribution from acute inhalation exposure and contains compone nts for lung, arterial blood, venous blood, muscle tissue, and nonmuscle tissue (Bruce and Bruce, 2003, 193975 ; Bruce and Bruce, 2006, 193980 ; Bruce et al., 2008, 193977 ). This model structure is illustrated in Figure 4-3 and includes the dynamics of CO storage in the lung and its dependence on ventilation and CO pressure of mixed venous blood, relaxes the assumption that Hb is saturated by including the role of CO in altering the O 2 dissociation curve, includes a subcompartmentalized muscle tissue compartment, accounts for dissolved CO in blood and tissue, and predicts COHb based on age and body dimensions. This multicompartment model is limited by its exclusion of cellular metabolism or Mb diffusion, simplification of within tissue bed spatial variab ility, and assumption that ventilation and average partial pressure of alveolar O 2 (P A O 2 ) are constant. Another limitation of this model is that some of the physiological parameters used in simulations are estimated through visual fits to the COHb profile and not from experimental or published data . This model better predicts COHb levels when inspired CO levels change rapidly or when incomplete blood mixing has occurred, and better predicts the CO washout time course compared to the CFK equation. Bruce and Bruce (2003, 193975 ) compared the two models and found similar results for long-duration exposure settings (1,000 min); however, the multicompartment m odel predicted somewhat lower COHb levels compared to the CFK model during transient CO uptake conditions when using data taken from Peterson and Stewart (1970, 012416 ).

Source: Adapted with Permission of Elsevi er Science from Bruce and Bruce (2008, 193977 )



### Figure 4-3. Overall structure of the Bruce and Bruce (2008, 193977 ) multicompartment model of storage and transport of CO. Includes compartments for lung, arterial blood, venous blood, muscle tissue, and nonmuscle tissue. The muscle compartment is divided into two subcompartments for diffusion of gases within the tissue.

January 2010 4-7A multicompartment model of the human respiratory system was developed using characteristics of the tissue repres entation of Bruce and Bruce (2003, 193975 ), and the lung representation described in Selvakumar et al. (1992, 013750 ) and Sharan (1999, 194673 ), which considered the exchanges of CO, O 2 , and CO 2 (Neto et al., 2008, 194672 ). The model contains six compartments including: alveolar, pulmonary ca pillaries, arterial, venous, tissue capillary, and tissues (muscular and non-muscular). The model was applied to four simulated physical activity levels, resting, sitting, standi ng, and walking, in a healthy subject exposed to the urban atmosphere of a metropolitan area of Brazil. The highest and lo west COHb levels were simulated in the walking individual, suggesting that greater variability in COHb occurs at higher physical activity levels.



## 4.2.3. Model Comparison

A number of models have been presented which predict COHb formation over numerous exposure scenarios. These models are often compared to the CFK equation to determine the most accurate predictive model under certain exposur e conditions. As was mentioned in Section 4.2.1

, Tikuisis (1996, 080960 ) conducted a comparison of empirical model predictions that showed a wide disparity in predicted COHb values, highlighting the inaccuracy of these models outside of the conditions on which they were presented. Smith et al. (1990, 013164 ) compared the linear and nonlinear CFK equations and concluded that the linear CFK equation gives a good approximation (within 1%) to the nonlinear solution over a large range of values during CO uptake and over a somewhat smaller range during CO eliminati on. The linear equation prediction of COHb concentration at or below 6% will only differ ± 0.5% COHb from the nonlinear equation prediction. Additionally, the most recently modi fied CFK model (Gosselin et al., 2009, 190946 ) better predicted COHb formation over a wide range of CO levels (50-4,000 ppm) and several temporal scenarios (Stewart et al., 1970, 013972 ; Tikuisis et al., 1987, 012138 ; Tikuisis et al., 1987, 012219 ; Tikuisis et al., 1992, 013592 ) compared to the linear CFK model. Linear regression slopes between the simulated COHb values from Gosselin et al. (2009, 190946 ) and the observed experimental values were closer to 1 in all experimental scenarios, indicating a better fit to the observed data. When evaluating all validation studies the modified model had an estimated slope of 0.996 (95% CI: 0.986-1.001) compared to 0.917 (95% CI: 0.906-0.927) using the linear CFK model. Bruce and Bruce (2003, 193975 ) compared their model to the CFK and found similar results for long-duration exposure settings (1,000 min [16.5 h]), however, thei r multicompartment model predicted somewhat lower COHb levels over transient CO uptake conditions when using data taken from Peterson and Stewart (1970, 012416 ). The Bruce and Bruce model better predicts However, there has not been a quantitative comparison of the recent multicom partment models (Bruce and Bruce, 2003, 193975 ; Neto et al., 2008, 194672 ) and the improved CFK equation models (Gosselin et al., 2009, 190946 ; Smith et al., 1994, 076564 ) to determine which is most accura te in predicting COHb levels under exposure scenarios that include occasional peak concentrations. The nonlinear and linear CFK equations remain the most extensively validated and applied models for COHb prediction. COHb levels when inspired CO levels change rapidly or when incomplete blood mixing has occurred, and better predicts the CO washout time course compared to the CFK equation. However, there has not been a quantitative comparison of the recent multicompartment models (Bruce and Bruce, 2003, 193975 )(Neto et al., 2008, 194672 ) and the improved CFK equation models (Smith et al., 1994, 076564 )(Gosselin et al., 2009, 190946 ) to determine which is most accurate in predicting COHb levels under exposure scenarios that include occas ional peak concentrations. The nonlinear and linear CFK equations remain the most extensive ly validated and applied models for COHb prediction.



## 4.2.4. Mathem atical Model Usage

As no new data have become available on the distribution of COHb levels in the U.S. population since large-scale nationwide surveys – e. g., National Health and Nutrition Examination Survey II (NCHS; et al., 1982, 011442 ) – and human exposure field studies – e.g., Denver, CO, and Washington, DC (Akland et al., 1985, 011618 ) – were conducted in the 1970s and 1980s, mathematical models are used to predict the resulting COHb levels from various CO exposure scenarios. Table 4-1 illustrates the predictions of venous COHb after 1, 8, or 24 h of CO exposure at a range of concentrations in a healthy adult human at rest (V A = 6 L/min; D L CO = 20

January 2010 4-8[mL/min]/mmHg), during light exercise (V A = 15 L/min; D L CO = 34 [mL/min]/mmHg), and during moderate exercise (V A = 22 L/min; D L CO = 43 [mL/min]/mmHg). The Quantitative Circulatory Physiology (QCP) model, which integrates human physiology using over 4,000 variables and equations based on published biological interactions, wa s used to predict these values (Abram et al., 2007, 193859 ; Benignus et al., 2006, 151344 ). This dynamic whole body model uses the nonlinear CFK equation with modifications presented in Smith et al. (1994, 076564 ). The contribution of alveolar ventilation and lung diffusion to the cha nges in COHb levels is discussed in Section 4.3.1.2

. Increased ventilation leads to an increased rate of CO uptake, causing COHb levels to reach equilibrium earlier. Also, increased ventilation leads to a decrease in steady state COHb levels due to increased CO expiration. For example, 35 ppm CO exposure at moderate exercise (22 L/min) results in a lower 24-h COHb saturation (4.73%), compared to COHb saturation from 35 ppm CO at rest (5.03%) (Table 4-1). Whereas, after 1 h, COHb le vels are still increasing following exposure at all levels of exercise and have not reached stea dy state, thus the greater uptake from increased ventilation leads to initially elevated CO Hb in higher ventilation situations. Endogenous CO production varies as described in Section 4.5 but generally results in <1% COHb, with a QCP modeled value of 0.27% at time zero. The rate of endogenous CO production was set at 0.007 mL/min for this simulation, whereas both higher and lower values have been reported (Coburn et al., 1966, 010984 ) (Section 4.5 ). Table 4-1 illustrates that 35 ppm CO for 1-h results in between 0.9-1.9% COHb and 9 ppm CO for 8-h results in between 1.1-1.3% COHb, depending upon activity level. Also, this table shows that low concentration CO exposure over several hours can result in equivalent COHb le vels compared to higher concentration, acute exposure. For example, in a resting condition w ithout additional baseline COHb, COHb resulting from 35 ppm for 1 h (0.89%) is approximately equivalent to 6 ppm for 8 h (0.83%) or 4 ppm for 24 h (0.82%).



### Table 4-1. Predicted COHb levels resulting from 1, 8, and 24 h CO exposures in a modeled human
at rest (V A 

### = 6 L/min; D L 

### CO = 20 (mL/min)/mmHg; V CO 

### = 0.007 mL/min; initial COHb = 0.27%; Hb = 0.15 g/mL), during light exercise (V A 

### = 15 L/min; D L 

### CO = 34 (mL/min)/mmHg), and during moderate exercise (V A 

### = 22 L/min; D L 

### CO = 43 (mL/min)/mmHg). The QCP model used a dynamic nonlinear CFK with the Smith et al. (1994, 076564 ) COHb algorithm and affinity constant M = 218.

1 h 8 h 24 h

CO (ppm) 6 L/min 15 L/min 22 L/min 6 L/min 15 L/min 22 L/mi n 6 L/min 15 L/min 22 L/min

2 0.30 0.30 0.29 0.45 0.38 0.35 0.54 0.40 0.36

3 0.31 0.33 0.34 0.54 0.51 0.48 0.68 0.54 0.49

4 0.33 0.36 0.62 0.64 0.64 0.62 0.82 0.69 0.63

6 0.36 0.44 0.48 0.83 0.90 0.88 1.10 0.97 0.91

9 0.42 0.55 0.63 1.12 1.29 1.27 1.52 1.39 1.31

15 0.53 0.77 0.92 1.69 2.05 2.06 2.35 2.22 2.12

24 0.70 1.10 1.35 2.55 3.19 3.22 3.57 3.45 3.31

35 0.89 1.50 1.89 3.58 4.55 4.60 5.03 4.91 4.73

The QCP model incorporating the Smith et al. (1994, 076564 ) COHb algorithm was also used to simulate population exposure scenarios including various commuting concentrations ( Figure 4-4 ) and endogenous production rates ( Figure 4-5 ). Commuting concentrations were modeled since the highest ambient CO exposure levels are generally observed during transit (Section 3.6.6.2). Figure 4-4 presents simulated COHb levels in a healthy adult throughout the second of 5 modeled days containing a 60-min commute at various CO concentrations. The U.S. Census Bureau estimates that 5% of the population commutes in automobiles for 60 or more minutes to work daily (U.S. Census Bureau, 2008, 194013 ) and exposure studies have reported in-vehicle transit concentrations up to 50 ppm (Abi-Esber and El-Fadel, 2008, 190939 ; Duci et al., 2003, 044199 ). However, U.S. studies have reported in-vehic le concentrations of <6 ppm, although peak

January 2010 4-9concentrations in congested urban areas have been reported to be higher than 50 ppm (Rodes et al., 1998, 010611 )(Riediker et al., 2003, 043761 ). CO concentrations during commuting lead to spikes in COHb in this model scenario with a 1% COHb increase over the initial CO Hb (0.3%) after 50 ppm

exposure. Figure 4-4 also illustrates that the COHb saturation after CO exposure from commuting is not fully eliminated by the next commuting period. Modeling successive days results in the same pattern and degree of COHb formation, i ndicating no accumulation of COHb over time.

0 0.2 0.4 0.6 0.8

1 1.2 1.4 1.6

0 360 720 1080 1440
Time (min)COHb (%) 2 ppm 10 ppm 20 ppm 50 ppm



### Figure 4-4. Predicted COHb levels in healthy commuters exposed to various CO concentrations over a 60-min commute twice a day. Ambient CO concentration not during commuting time was 1 ppm. The activity pattern simulated: (1) sleeping for 8 h; (2) standing and light e xercise for 30 min; (3) sitting during a 60-min commute; (4) light exercise for 8. 5 h; (5) sitting during a second 60-min commute; (6) moderate exercise for 60 min; and (7) sitting for 4 h. The graph illustrates the second day simulated under these conditions. 1

Figure 4-5 presents simulated COHb levels in adults with various endogenous CO production rates throughout the second of 5 modeled days containing a 60-min commute at 20 ppm CO. The normal endogenous rate of CO production in young adult males with an average COHb of 0.88% averages 0.007 mL/min (18.7 ± 0.8 μmol/h) (Coburn et al., 1963, 013971 ). However, a number of diseases and conditions described in Section 4.5 can affect this production rate. Patients with

1 Sleeping/lying human parameters: V A - 3.8 L/min, V T - 467 mL, V D - 147 mL, V CO - 0.007 mL/min, D L CO- 17.9 mL/min/mmHg, M- 218, initial. COHb- 0.27%. Sitting human parameters: V A - 5.2 L/min, V T - 560 mL, V D - 155 mL, V CO - 0.007 mL/min, D L CO- 18 mL/min/mmHg. Standing human parameters: V A - 6.4 L/min, V T - 636 mL, V D - 161 mL, V CO - 0.007 mL/min, D L CO- 19.3 mL/min/mmHg. Light exercise (1 MPH, 32 W) human parameters: V A - 13.4 L/min, V T - 994 mL, V D - 218 mL, V CO - 0.007 mL/min, D L CO- 30.4 mL/min/mmHg. Heavy exercise (3 MPH, 96 W) human parameters: V A - 31.4 L/min, V T - 1642 mL, V D - 241 mL, V CO - 0.007 mL/min,

D L CO- 49.6 mL/min/mmHg.

January 2010 4-10hemolytic anemia have endogenous CO production rates ranging from 0.012 to 0.053 mL/min (31-143 μmol/h) (Coburn et al., 1966, 010984 ). The venous COHb levels in these same patients ranged from 0.77 to 2.62%.

0 0.5

1 1.5

2 2.5

3 3.5

0 360 720 1080 1440

Time (min)COHb (%) 0.007 mL/min 0.02 mL/min 0.04 mL/min 0.06 mL/min



### Figure 4-5. Predicted COHb levels due to various endogenous CO production rates. The activity pattern presented in Figure 4-4 was used. Ambient CO concentration not during commuting time was 1 ppm and commuting CO concentration was 20 ppm. The graph illustrates the second day simulated under these conditions.



# 4.3. Absorption, Distribution, and Elimination

4.3.1. Pulmon ary Absorption

Pulmonary uptake of CO accounts for all environmental CO absorption and occurs at the respiratory bronchioles and alveolar ducts and sacs. CO and O 2 share various physico-chemical properties, thus allowing for the extension of the knowledge about O 2 kinetics to those of CO despite the differences in the reactivity of the gases. The exchange of CO between the air and the body depends on a number of physical (e.g., mass transfer and diffusion), as well as physiological factors (e.g., alveolar ventilation and cardiac output), which are controlled by environmental conditions, physical exertion, and other processes discussed in Section 4.4 . The ability of the lung to take up inhaled CO is measured by D L CO, and CO uptake representing the product of D L CO and the mean alveolar pressure (P A CO). The importance of dead space volume, gas mixing and homogeneity, and

January 2010 4-11ventilation/perfusion matching were discussed in depth in the 2000 CO AQCD (U.S. EPA, 2000, 000907 ).



## 4.3.1.1. Mass Tr ansfer of Carbon Monoxide

Mass transfer refers to the molecular and c onvective transport of CO molecules within the body stores, driven by random molecular motion from high to low concentrations. CO enters through the airway opening (mouth and nose) and transfers in a gas phase to the alveoli. CO transport is due to convective flow, the mechanical action of the respiratory system, and diffusion in the acinar zone of the lung (Engel et al., 1973, 014336 ). Then, CO diffuses across the air-blood interface into plasma and subsequently into red blood cells (RBC), bi nding RBC Hb. At environmental CO levels, CO uptake into RBC is limited by the reaction rate of binding of CO to O 2 Hb forming COHb. Pulmonary capillary RBC CO diffusion is rapidly achieved (Chakraborty et al., 2004, 193759 ; Gibson and Roughton, 1955, 193941 ; Reeves and Park, 1992, 193847 ; Roughton and Forster, 1957, 193862 ). The formation rate and level of COHb depends upon pCO, pO 2 in the air, time of exposure, and the ventilation rate (Roughton and Forster, 1957, 193862 ). Most of the body CO is bound to Hb; however, 10-15% of the total body CO is located in extravascular tissues primarily bound to other heme proteins (Coburn, 1970, 013916 ). Considerable concentrations of CO have been measured in spleen, lung, kidney, liver, muscle, and heart (Vreman et al., 2005, 193786 ; Vreman et al., 2006, 098272 ), whereas less CO is localized to fatty tissues, such as adipose and brain. The transfer of CO occurs by a partitioning of CO between Hb and tissue. Less than 1% of the total body CO stores appear as dissolved in body fluids, due to the i nsolubility and small tissue partial pressure of CO (Coburn, 1970, 013916 ). Transport pathways and body stores of CO are shown in Figure 4-6

.

Source: Adapted with Permission of Wiley-Blackwell from Coburn (1967, 011144 )



### Figure 4-6. Diagrammatic presentation of CO uptake and elimination pathways and CO body
stores.



## 4.3.1.2. Lung Di ffusion of Carbon Monoxide

Lung diffusion of CO is an entirely passive process of gas diffusion across the alveolocapillary membrane, through the plasma, across the RBC membrane an d into the RBC stroma, where

January 2010 4-12CO binding to Hb rapidly occurs. Membrane an d blood phase transfer are governed by physicochemical laws, including Fick’s first law of diffusion. The diffusing capacity of the lung for CO, represented as D L CO, is a measurement of the partial pressure difference between inspired and expired CO. Due to the rapid binding of CO to Hb, a high pressure differential between air and blood exists when CO air levels are increased. Inhalati on of CO-free air reverses the pressure differential (higher CO pressure on the blood side than the alveolar side), and then CO is released into the alveolar air. Since CO is also produced endogenously, CO release will also be affected by this production pressure. However, the air-blood gradient for CO is usually higher than the blood-air gradient; therefore, CO uptake will be a propor tionately faster process than CO elimination. A number of factors have been found to affect D L CO including Hb concentration, cardiac output (Q), erythrocyte flow, COHb concentration, P A CO 2 , body position, exercise, time of day, age, etc. (Forster, 1966, 180430 ; Hsia, 2002, 193857 ). D L CO consistently decreases after intense bouts of exercise, likely due to the redistribution of blood volume to the periphery (Hanel et al., 1997, 193918 ; Manier et al., 1991, 193979 ). However, in going from rest to exercise, D L CO can increase linearly from: lung expansion leading to unfolding and distension of alveolar septa, opening and/or distension of capillaries as Q in creases, increased capi llary hematocrit, an d more homogeneous distribution of capillary erythrocytes (Hsia, 2002, 193857 ). D L CO is less dependent upon lung volume at mid-range vital capacity, but at extreme volumes the diffusion rate is varied, higher than average at total lung capacity and lower at residual volume (McClean et al., 1981, 012411 ).
D L CO is also altered by a number of diseases. Decreased D L CO is evident in patients with restrictive lung disease (i.e., decreased lung volumes ) since a loss of lung tissue leads to a loss of functional lung units. D L CO also shows a good correlation with the severity of restrictive lung disease (Arora et al., 2001, 186713 ). Conditions affecting D L CO vary and include chronic obstructive pulmonary disease (Terzano et al., 2009, 108046 ), ulcerative colitis (Marvisi et al., 2000, 186703 ; Marvisi et al., 2007, 186702 ), severe gastroesophageal reflux (Schachter et al., 2003, 186707 ), beta thalassemia (Arora et al., 2001, 186713 ), thoracic or abdominal aortic aneurysm (Sakamaki et al., 2002, 186706 ), pulmonary arterial hypertension (Proudman et al., 2007, 186705 ), and chemotherapy for breast cancer (Yerushalmi et al., 2009, 186711 ). Diseases affecting CO kinetics and D L CO are also discussed in Section 4.4.4

.



## 4.3.2.

4.3.2.1. Tissue Uptake

Respira tory Tract

The upper respiratory tract contributes little to the overall CO uptake. The lung has nearly constant exposure to CO; however, relatively little CO diffuses into the tissue except at the alveolar region en route to the circulation. No detectable uptake of CO was observed in the human nasal cavity or upper airway (Guyatt et al., 1981, 011196 ) or in the monkey oronasal cavity after high CO exposure (Schoenfisch et al., 1980, 011404 ).



## 4.3.2.2. Blood

The blood is the largest reservoir for CO, where it reversibly binds to Hb. The chemical affinity of CO for adult human Hb is approximately 218 times greater than that of O 2 , meaning one part CO and 218 (210-250) parts O 2 would form equal parts of O 2 Hb and COHb (Engel et al., 1969, 193914 ; Rodkey et al., 1969, 008151 ; Roughton, 1970, 013931 ). This would happen when breathing air containing 21% O 2 and 960 ppm CO. This concept was presented by Haldane and Smith (1895, 010538 ) and later represented as the Haldane constant M (210-250) in the Haldane equation by Douglas, Haldane, and Haldane (1912, 013965 ). M is relatively unaffected by changes in physiological pH, CO 2 , temperature, or 2,3-diphosphoglycerate:



### ) ( 2 2 

### pOpCOMHbOCOHb ÷ × = ÷
Equation 4-3

The Hb association rate for CO is 10% slower than O 2 and occurs in a cooperative manner (Chakraborty et al., 2004, 193759 ; Sharma et al., 1976, 193766 ). Hb is composed of four globin

January 2010 4-13chains, each containing a heme group capable of binding CO or O 2 . The associative reaction rates become faster with successive heme binding, attrib uted to interactions within the protein and to strains imposed on the heme and its ligands (Alcantara et al., 2007, 193867 ). More simply, the greater the number of heme sites bound to CO, the greater the affinity of free heme sites for O 2 , thus causing Hb to bind and retain O 2 that would normally be released to tissues. Cooperativity is greatly reduced in CO dissociation, but the rate of dissociati on of CO from Hb is orders of magnitude slower than O 2 ( k CO = 4 × 10-4 k O2 ), which accounts for the high affinity values (Chakraborty et al., 2004, 193759 ). The half-time of dissociation reaction is about 11 s at 37°C (Holland, 1970, 193856 ). In general, CO uptake to COHb equilibrium is slower in humans and large animals, requiring 8-24 h, than in smaller species such as rats, wh ich will equilibrate in 1-2 h (Penney, 1988, 012519 ). Also, COHb equilibrium within the blood stream is not instantaneous. Men exposed to brief (~5 min) high-dose CO had an initial delay of 1-2 min in the appearance of venous COHb after the start of CO inhalation (Benignus et al., 1994, 013908 ; Smith et al., 1994, 076564 ). Additionally, arterial COHb concentrations were considerably higher than venous concentrations during CO exposure; however, they converged within 2-10 min after the end of exposure, as venous and arterial blood mixed. CO binding to Hb also has effects on the O 2 dissociation curve of the remaining Hb by shifting the curve progressively to the left and altering the normal S-shaped curve to become more hyperbolic due to increased cooperative O 2 binding (Roughton, 1970, 013931 ). This is referred to as the “Haldane effect” and causes tissues to have more trouble obtaining O 2 from the blood, even compared to the same extent of reduced Hb resulting from anemia. For example, Figure 4-7 (as explained in the 2000 CO AQCD) illustrates that in an acute anemia patient (50% of Hb) at a venous pO 2 of 26 mmHg (v ′ 1 ), 5 vol % of O 2 (50% saturation) was extracted from the blood. In contrast, for a CO poisoned person with 50% COHb, the venous pO 2 will have to drop to 16 mmHg (v ′ 2 ) to release the same 5 vol % O 2 . This more severe effect on O 2 pressure may lead to brain O 2 depletion and loss of consciousness if any higher demand of O 2 is needed (e.g., exercise).

January 2010 4-14Source: U.S. EPA (1991, 017643 )



### Figure 4-7. O 2 

### Hb dissociation curve of normal human blood, of blood containing 50% COHb, and of blood with only 50% Hb because of anemia.



## 4.3.2.3. Heart a nd Skeletal Muscle

Mb is a globular heme protein that facilitates O 2 diffusion from the muscle sarcoplasm to mitochondria, acting as an O 2 supply buffer to maintain adequate pO 2 for mitochondria when the O 2 supply changes, as in exercise. O 2 has a greater affinity for Mb than Hb, which allows small changes in tissue pO 2 to release large amounts of O 2 from O 2 Mb (Wittenberg et al., 1975, 012436 ). Small reductions in O 2 storage capacity of Mb, due to CO binding, may have a profound effect on the supply of O to the tissue. 2 Like Hb, Mb will undergo reversible CO binding, however the affinity constant is approximately eight-times lower than Hb (M = 20-40 versus 218, respectively) (Haab, 1990, 013359 ). The association rate constant of CO and Mb is approximately 27 times lower than O 2 ; however, the dissociation rate constant is approximately 630 times lower than O 2 (Gibson et al., 1986, 016289 ), causing CO to be retained and possibly st ored in the muscle. CO levels have been measured in human muscle and heart tissues with <2% COHb concentrations at background levels

January 2010 4-15(15 and 31 picomole [pmol] CO/mg ww, respectively) (Vreman et al., 2006, 098272 ) ( Table 4-2 ). Under conditions of CO asphyxiation, tissue concentrations increased 17-18 fold (265 and 527 pmol CO/mg ww muscle and heart tissue, respectively) ; however, heart tissue concentrations varied widely between individuals. Mouse muscle did not show this increase after exogenous CO exposure (Vreman et al., 2005, 193786 ). This may be due to the fact that human muscle has a 15-fold higher concentration of myoglobin protein than mouse muscle (Weller et al., 1986, 187298 ). The capacity for diffusion of CO into the muscle is represented by the coefficient D

m CO and is generally larger in males than in females, likely due to the differen ces in muscle mass and capillary density (Bruce and Bruce, 2003, 193975 ). COMb concentrations in the heart and skeletal muscle increase with work load, due to a higher relative rate of CO binding to Mb relative to Hb. This causes an increase in COMb/COHb that is not seen at rest (Sokal et al., 1984, 011591 ). Subjects with 2% COHb but not those with 20% COHb levels showed a significant upt ake of CO from the blood to the muscle with increasing work intensity of the quadriceps muscle (Richardson et al., 2002, 037513 ).



### Table 4-2. CO concentration in pmol/mg wet we ight tissue and fold ti ssue CO concentration changes (normalized to background tissue concentrations) – human.

Exposure Adipose Brain Muscle Hear t Kidney Lung Spleen Blood % COHb

Background 3 ± 1 3 ± 3 15 ± 9 31 ± 23 23 ± 18 57 ± 59 79 ± 75 165 ± 143 1.5 ± 1.2

Fire 5 ± 4 [1.7] 7 ± 5 [2.3] 24 ± 16 [1.6] 54 ± 33 [1.7] 27 ± 11 [1.2] 131 ± 127 [2.3] 95 ± 69 [1.2] 286 ± 127 [1.7] 3.8 ± 3.2 [2.5]

Fire + CO 18 ± 29 [6.0] 17 ± 14 [5.7] 168 ± 172 [11.2] 128 ± 63 [4.1] 721 ± 427 [31.3] 1097 ± 697 [19.2] 2290 ± 1409 [29.0] 3623 ± 1975 [22.0] 40.7 ± 28.8 [27.1]
CO asphyxiation 25 ± 27 [8.3] 72 ± 38 [24.0] 265 ± 157 [17.7] 527 ± 249 [17.0] 885 ± 271 [38.5] 2694 ± 1730 [47.3] 3455 ± 1347 [43.7] 5196 ± 2625 [31.5] 56.4 ± 28.9 [37.6]

Source: Reprinted with Permission of W iley-Blackwell from Vreman et al. (2006, 098272 )



## 4.3.2.4. Other T issues

CO binds with other hemoproteins, such as cytochrome P450, cytochrome c oxidase, catalase, and peroxidase, but the possibility of this binding influencing CO-O 2 kinetics has not been established. CO transfers between COHb and tissue, the extent of which varies between organs. Blood-to-tissue flux causes less CO to be expired following CO exposure than what is lost from the blood in terms of COHb (Roughton and Root, 1945, 180418 ). This value is estimated to be 0.3-0.4% min -1 or 0.24 mL/min (Bruce and Bruce, 2003, 193975 ; Prommer and Schmidt, 2007, 180421 ). The equilibration rate from blood to tissue is uncerta in. Newly modeled CO trafficking kinetics shows that CO continues to be taken up by the muscle and extravascular tissues well beyond the end of exposure because of a less than instant equilibration (Bruce and Bruce, 2006, 193980 ). Table 4-2 contains tissue CO concentrations from humans under different CO exposure conditions. The distribution of CO between the different human organs was shown to follow the same pattern versus percent of the blood CO concentration, irrespectiv e of the level of blood CO (Vreman et al., 2006, 098272 ). Consistently, the spleen, lung, and kidney ha d the highest measured CO concentration and the most dramatic increases over basal levels ; the brain and adipose had the lowest CO concentrations. In addition to these fatty tissues, the muscular tissues including the heart and skeletal muscle had similarly low increases over backgrou nd CO levels. This pattern was also found in rodents exposed to exogenous CO; however, increased endogenous CO produced after heme administration did not follow this pa ttern of uptake (Vreman et al., 2005, 193786 ). Increased endogenous CO production led to moderately increased CO present in the lung, heart, liver, and spleen and no change in CO concentration in the testes, intestine, muscle, brain, and kidney. The spleen and liver have an abundance of HO-1 expres sion and are involved in the catabolism of heme, thus it is expected to have elevated CO concentrations in these organs after heme treatment. Also, elevated CO in the lung is not surprising since it is the site of CO excretion. The tissues analyzed in these studies were blanched before analysis; however, contamination of the tissue sonicates with blood from the vessels within each organ is a possi ble source of error. The measurements were presented by the authors as minimum tissue CO concentrations, due to the possibility of rapid loss of CO from blood and tissue exposed to the atmosphere , light, and elevated temperature (Chace et al., 1986, 012020 ; Ocak et al., 1985, 011641 ). These results are not consistent with older papers,

January 2010 4-16suggesting that negligible retention of CO occu rs in the liver or brain (Sokal et al., 1984, 011591 ; Topping, 1975, 193784 ).



## 4.3.3. Pulmon ary and Tissue Elimination

Blood COHb concentrations are generally consi dered to have a monotonically decreasing, second-order (logarithmic or exponential) eliminati on rate from equilibrium. However, more recent reports have presented evidence for a biphasic washout curve, especially after brief CO exposure ( Figure 4-8 ) (Bruce and Bruce, 2006, 193980 ; Shimazu et al., 2000, 016420 ; Wagner et al., 1975, 010989 ). This event is modeled by a two-compartment system where the initial rapid decrease is the washout rate from the blood, followed by a slower phase due to CO flux from the muscle and extravascular compartments back to the blood. Tissue elimination rates have been reported as slower than those for blood (Landaw, 1973, 010803 ). The biphasic curve is more obvious after shortduration CO exposure (<1 h), whereas longer CO exposure ( ≥ 5 h) results in a virtually monoexponential elimination, which could account for the historical findings. However, this elimination curve also follows a biphasic curve with a slightly higher rate of elimination initially (Shimazu et al., 2000, 016420 ). Differences in elimination kinetics could also be a result of the variation in CO exposure duration (Weaver et al., 2000, 016421 ). The elimination of COHb is affected by a number of factors, including duration of exposure,
P
a O 2 , minute ventilation, the time post-exposure for anal ysis due to extravascular stores, as well as inter-individual variability (Bruce and Bruce, 2006, 193980 ; Landaw, 1973, 010803 ; Shimazu, 2001, 016331 ). The elimination rate does not seem to be dependent upon the CO exposure source (e.g., fire, non-fire CO exposure) (Levasseur et al., 1996, 080895 ). In addition, in a series of poisoning cases, the COHb elimination half-lif e was not influenced by gender, age, smoke inhalation, history of loss of consciousness, concur rent tobacco smoking, degree of initial metabolic acidosis (base excess), or the initial COHb level (Weaver et al., 2000, 016421 ). On the contrary, in modeling the nonlinear kinetics of CO, a subject with a higher initial COHb will detoxify and eliminate CO more rapidly (Gosselin et al., 2009, 190946 ). Similarly, it has been shown that the absolute elimination rates are associated positively with the initial concentration of COHb, however the relative rate of elimination, expressed as a percentage decline in COHb% after a measured time, is independent of the initial COHb concentration (Wagner et al., 1975, 010989 ). COHb elimination half-life falls as the fractional inspired O 2 concentration increases. While breathing air at sea level pressure, the expected half-life in adult males is approximately 285 min, but may be shorter in adult females. With inhalation of normobaric 40% O 2 , the half-life falls to 75 min and further to 21 min when breathing 100% O 2 because of greater competition for Hb by O 2 (Landaw, 1973, 010803 ). Another study reports the half-life falls to 74 min (mean) after breathing 100% O 2 , although the range in this particular study was 26-148 min (Weaver et al., 2000, 016421 ). In addition, COHb halflife will fall further after no rmocapnic hyperoxi c hyperpnea (i.e., hyperventilation while maintaining normal CO 2 pressure in high O 2 ) (Takeuchi et al., 2000, 005675 ).

January 2010 4-171 10 100

0 30 60 90 120 150 180

Time (min)COHb (%) A

B

Source: Adapted with Permission of Lippincott Williams & Wilkins from Shimazu et al. (2000, 016420 )



### Figure 4-8. Changes in blood COHb after exposure to CO for a few minutes (A) or several hours (B), representing the bi phasic nature of CO elimination. Note: y-axis is logscale.



## 4.3.4. COHb A nalysis Methods

Blood COHb saturation can be analyzed usi ng numerous methods with various benefits and limitations. The most popular current tec hniques include gas chromatography (GC) and spectrophotometry, specifically using CO-oximeters. CO-oximeters are commonly used because they require little sample preparati on and simultaneously measure COHb, O 2 Hb, methemoglobin, and total hemoglobin concentration. However, at low concentrations of COHb relevant to ambient exposure (<5%), CO-oximeters overestimate COHb levels determined by GC (Mahoney et al., 1993, 013859 ; Widdop, 2002, 030493 ). Conversely, at higher COHb levels (>5%), CO-oximeters will underestimate COHb concentrations. In addition to the inaccuracy of the CO-oximeters, some studies report considerable imprecision in the results. Also, numerous substances or conditions can interfere with CO-oximeter measurements (i.e ., temperature, bilirubi n, fetal hemoglobin). Alternatively, GC is an accurate, precise, highly sp ecific analysis method and is generally used as the reference method for COHb analysis. GC requires th e CO incorporated into blood or tissue samples to first be released using a liberating agent such as potassium ferricyanide or sulfosalicylic acid (Vreman et al., 2005, 193786 ; Vreman et al., 2006, 098272 ), and then measured directly or indirectly. This methodology is more complex and time-consuming than spectrophotometry. In either analysis method, it is important to remember that COHb measured at one site in the body does not necessarily represent whole body CO distribution. CO can also be measured directly in air or br eath samples by using an electrochemical sensor that depends on the electrical signal generated by th e oxidation of CO. There are conflicting reports on the correlation of exhaled CO (COex) with COHb . Multiple reports present positive correlation coefficients (r) ranging from 0.92 and 0.98 in smoking subjects (Jarvis et al., 1980, 011813 ; Jarvis et al., 1986, 012043 ; Landaw, 1973, 010803 ). Positive linear correlations have also been shown in diseased patients with increased COHb (De las Heras et al., 2003, 194087 ). Others have reported no correlation between low level COHb and COex and have suggested less correlation exists at the lower levels of COex relevant to ambient exposures (Horvath et al., 1998, 087191 ; Scharte et al., 2000, 194112 ). Finally, CO is endogenously produced in the nose and paranasal sinus which may contribute to COex concentrations (Andersson et al., 2000, 011836 ).

January 2010 4-18

# 4.4. Conditions Affecting Uptake and Elimination

4.4.1. Physic al Activity

Exercise is an important determinant of CO ki netics and toxicity due to the extensive increase in gas exchange. O 2 consumption can increase more than 10 fold during exercise. Similarly, ventilation, membrane and lung diffusing capacity , pulmonary capillary blood volume, and cardiac output increase proportional to work load. Also, exercise will improve the ventilation/perfusion ratio in the lung and mobilize RBC reserves from the spleen. The majority of these changes facilitate CO uptake and transport, by increasing gas exchange ef ficiency. Likewise, the COHb elimination rate increases with physical activity, causing a decrea se in COHb half-life (Joumard et al., 1981, 011330 ). During a transition period from rest to exerci se while exposed to CO (500 ppm/10 min), the diffusing capacity and CO uptake were reported to rise faster than O 2 consumption for each exercise intensity (Kinker et al., 1992, 086328 ). The two physiological variables that are most influential in the formation of COHb are alveolar ventilation an d cardiac output. However, exercise did not affect the ability of the CFK equation to predict COHb saturation as long as appropriate variables were used for model analysis (Tikuisis et al., 1992, 013592 ).



## 4.4.2. Altitu de

Increased altitude changes a number of factors that contribute to the uptake and elimination of CO. The relationship between altitude and CO exposure has been discussed in depth in the 2000 CO AQCD and other documents (U.S. EPA, 1978, 086321 ). In an effort to maintain proper O 2 transport and supply, physiological changes occur as compensatory mechanisms to combat the decreased barometric pressure and resulting altitude indu ced hypobaric hypoxia (HH). HH, unlike CO hypoxia, causes humans to hyperventilate, which reduces arterial blood CO 2 (hypocapnia) and increases alveolar partial pressure of O 2 . Hypocapnia will lead to difficulty of O 2 dissociation and decreased blood flow, thus reducing tissue O 2 supply. HH increases blood pressure (BP) and cardiac output and leads to redistribution of blood from skin to organs and from blood vessels to extravascular compartments. Generally these changes will favor increased CO uptake and COHb formation, as well as CO elimination. In hypoxic conditions both CO and O 2 bind reduced Hb through a competitive-parallel reaction (Chakraborty et al., 2004, 193759 ). Sea level residents exposed to high altitude (3,658-5,800 m) for short or long visits (< 1 year) experience negligible or minor changes in
D L CO, although these changes in D L CO can be accounted for by pol ycythemia or increased red blood cell count and by the increased rate of reac tion of carbon monoxide with hemoglobin due to hypoxia (West, 1962, 199513 )(Guleria et al., 1971, 199518 ). Breathing CO (9 ppm) at rest at altitude produced higher COHb compared to sea level (McGrath et al., 1993, 013865 ), whereas high altitude exposure with exercise caused a decrease in COHb levels versus similar exposure at sea level (Horvath et al., 1988, 012725 ). This decrease could be a shift in CO storage or suppression of COHb formation, or both. Altitude also increases the baseline COHb levels by inducing endogenous CO production. Initial HH increased lung HO-1 protein and activity, whereas chronic HH induced endogenous CO production in nonpulmonary sites (see Section 4.5 ) (Carraway et al., 2000, 021096 ). As the length of stay increases at high altitude, acclimatization occurs, inducing hyperventilation, polycythemia, and increased tissue capillarity and Mb content in skeletal muscle, which could also favor increased CO uptake. The D L CO of sea level natives who are long-term residents at altitude (3,100 m) increases from sea level values (Cerny et al., 1973, 199736 ). Additionally, natives of high altitude (3,100-3,658 m) have increased D L CO compared to natives of sea level or sea level natives that stay at high altitude (DeGraff et al., 1970, 199737 ) (Guleria et al., 1971, 199518 ). This has been attributed to high pulmonary capillary blood volume and membrane diffusing capacity, and altered lung structure. Most of the early adaptive changes gradually revert to sea level values after individuals return to sea le vel. However, differences in people raised at high altitude persist even after reacclim atization to sea level (Hsia, 2002, 193857 ). For example, altitude natives (3,658 m) staying at sea-level still have increased D L CO compared to sea level natives which suggests a permanent change in the lung structure resulting in a larger diffusing surface area (Guleria et al., 1971, 199518 ).

January 2010 4-19

## 4.4.3. Physic al Characteristics

Certain physical characteristics (e.g., age, sex, pr egnancy) can alter the variables that influence the uptake, distribution, and elimination of CO. Va lues of CO uptake and elimination change with age. Young children eliminate COHb more rapidly than adults after CO exposure (Joumard et al., 1981, 011330 ; Klasner et al., 1998, 087196 ). After infancy, the COHb half-life increases with age, nearly doubling between 2 and 70 yr (Joumard et al., 1981, 011330 ). The rate of this increase in CO elimination is very rapid in the growing years (2-16 yr of age), but slows beyond adolescence. Alveolar volume and D L CO increase with increasing body length of infants and toddlers (Castillo et al., 2006, 193234 ), suggesting a further degr ee of lung development and faster CO uptake. After infancy, increasing age decreases D L CO and increases V A /Q mismatch, causing it to take longer to both load and eliminate CO from the blood (Neas and Schwartz, 1996, 079363 ). COHb concentrations are generally lower in fema le subjects than in male subjects (Horvath et al., 1988, 012725 ), and the COHb half-life may be longer in healthy men than in women of the same age, which may be partially explained by differences in muscle mass or the slight correlation between COHb half-life and increased height (Joumard et al., 1981, 011330 ). However, women do have a higher rate of endogenous production while in the progesterone phase of the menstrual cycle and during pregnancy (Section 4.5 ). The rate of decline of D L CO with age is lower in middle-aged women than in men; however, it evens out towards older age (Neas and Schwartz, 1996, 079363 ). Women also tended to be more resistant to altitude hypoxia (Horvath et al., 1988, 012725 ). Ethnicity does alter physiological variables that determine CO uptake and kinetics. Lung volumes are 10-15% less in both Asian and African-American populations when compared to Caucasians. This causes a reduced alveolar surface area (20% less than estimated values) for gas exchange, leading to a 13% difference in D L CO (Pesola et al., 2004, 193842 ; Pesola et al., 2006, 193855 ). Certain factors, such as socioeconomic status (SES), were not controlled for in these studies. SES has been shown to affect pulmonary function, including decreasing D L CO (Hegewald and Crapo, 2007, 193923 ).



## 4.4.3.1. Fetal P harmacokinetics

Inhaled CO by pregnant animals quickly passes the placental barriers and enters the fetal circulation (Longo, 1977, 012599 ). Fetal CO pharmacokinetics do not follow the same kinetics as maternal CO exposure, making it difficult to estim ate fetal COHb based on maternal levels. Fetal COHb will vary as a function of maternal exposure but will also depend upon the rate of endogenous fetal CO production (Section 4.5 ), placental diffusing capacity of CO, the relative affinity of fetal Hb for CO compared to O 2 , and the affinity of fetal blood for O 2 (Longo, 1970, 013922 ). Human fetal Hb has a higher affinity for CO than adult Hb, wh ere the ratio of fetal COHb to maternal COHb at steady state in humans is approximately 1.11 (Longo, 1970, 013922 )(Di Cera et al., 1989, 193998 )(Hayde et al., 2000, 201602 ). Maternal and fetal COHb concentrations have been modeled as a function of time using a modified CFK equation (Hill et al., 1977, 011315 ). At steady-state conditions, the fetal COHb is up to 10-15% higher than the maternal COHb levels. For example, exposure to 30 ppm CO results in a maternal COHb of 5% and a fetal COHb of 5.75%. The fetal CO uptake lags behind the maternal for the first few hours but later may overtake the maternal values ( Figure 4-9 ). Fetal COHb equilibrium may not be reached for 36-48 h after exposure. Similarly, during washout, the fetal COHb levels are maintained for longer, with a half-life of around 7.5 h versus the maternal half-life of around 4 h (Longo and Hill, 1977, 010802 ).

January 2010 4-20Source: Reprinted with Permission of the American Physiological Society from Hill et al. (1977, 011315 )



### Figure 4-9. Predicted maternal and fetal COHb during periodic exposure to CO (50 ppm for 16 h followed by 0 ppm for 8 h ).



## 4.4.4. Health Status

Health status can influence the toxicity involve d with CO exposure by influencing the severity of hypoxia resulting from CO exposure. Any condition that would alter the blood O 2 carrying capacity or content will result in a greater ri sk from COHb induced hypoxia and decreased tissue O 2 delivery. The severity of this effect depends upon the initial level of hypoxia. Anemias are a group of diseases that result in insufficient blood O 2 or hypoxia due to Hb deficiency through hemolysis, hemorrhage, or reduced hematopoiesis. Anemia may result from pathologic conditions characterized by chronic infl ammation, such as malignant tumors or chronic infections (Cavallin-Ståhl et al., 1976, 086306 ; Cavallin-Ståhl et al., 1976, 193239 ). The bodies of people with anemia compensate, causing cardiac output to increase as both heart rate and stroke volume increase. The endogenous production of CO, thus COHb, is increased in patients with hemolytic anemia due to increased heme catabolism, causing an increased baseline COHb concentration. One of the most prevalent anemias arises from a single-point mutation of Hb, causing sickle cell diseases. The Hb affinity for O 2 and O 2 carrying capacity is reduced causing a shift to the right in the O 2 dissociation curve. It is well documented that African-American populations have a higher incidence of sickle cell anemia, wh ich may be a risk factor for CO hypoxia. Chronic obstructive pulmonary disease (COPD) is often accompanied by a number of changes in gas exchange, including increased deadspace volume (V D ) and ventilation-perfusion ratio (V A /Q) inequality (Marthan et al., 1985, 086334 ), which could slow both CO uptake and elimination. Patients with pulmonary sarcoidosis, a restrictiv e lung disease, may also have a decrease in lung volumes, a loss of D L CO, and gas exchange abnormalities during exercise, including decreased arterial oxygen pressure (P

a O 2 ) and increased alveolar-arterial oxyge n pressure difference (Lamberto et al., 2004, 193845 ). Individuals with heart disease may be at a greater risk from CO exposure since they may already have compromised O 2 delivery. Time to onset of angina was reduced after exposure to

January 2010 4-21100 ppm CO, compared to clean air (Kleinman et al., 1998, 047186 ). Hyperlipidemic patients may have decreased CO diffusion capacity, a loss of V A /Q gradient, and a decrease in P

a O 2 (Enzi, 1976, 195794 ) (Section 5.2).



# 4.5. Endogenous CO Production and Metabolism

Humans breathing air containing no environmen tal sources of CO will still have a low measurable level of circulating COHb due to endogenous CO production from heme protein catabolism. In the normal degradation of RBC Hb, the porphyrin ring of heme is broken at the

α -methene bridge by HO. HO is co-localized with NADPH-flavoprotein reductase and biliverdin reductase on the endoplasmic reticulum, where it catabolizes heme in an O 2 and NADPH-dependent manner to biliverdin, ferrous iron, and CO. Biliverdin is then further br oken down by biliverdin reductase into bilirubin, a powerful endogenous an tioxidant. HO mediated metabolism functions as the rate-limiting enzyme step in heme degrad ation and endogenous CO production (Wu and Wang, 2005, 180411 ). Three isoforms of HO exist, but HO-1 is the only inducible form (Maines and Kappas, 1974, 193976 ; Maines et al., 1986, 193978 ; McCoubrey et al., 1997, 016715 ). Endogenous CO production can be increased by the up-regulation of HO-1 expression and activity by inducers such as oxidative stress, hypoxia, heavy metals, sodi um arsenite, heme and heme derivatives, various cytokines, and also exogenous CO (Wu and Wang, 2005, 180411 ). The major site of heme catabolism, and thus th e major organ of CO production, is the liver, followed by the spleen, brain, and erythropoietic system (Berk et al., 1976, 012603 ). These rates of CO formation may be due to higher levels of HO activity in these tissues. The whole body production rate of CO is approximately 18.8 μ mol/h (0.42 mL/h or 0.007 mL/min) and produces between 400-500 μ mol CO per day (Coburn et al., 1963, 013971 ; Coburn et al., 1964, 013956 ; Coburn et al., 1966, 010984 ) ( Figure 4-10 ). The endogenous rate of production varied somewhat within individuals measured on multiple days (±4.5 μ mol/h and ±0.35% COHb) (Coburn et al., 1966, 010984 ). However, these measurements of day-to-day CO production variability were comparable to the equipment me asurement error reported (±3.1 μ mol/h). The endogenous rate of CO formation varies between different tissues, ranging from 0.029 nmol/mg protein/h in chorionic villi of term human placentas to 0.28 nmol/mg protein/h in cultured rat olfactory receptor neurons and rat liver perfusate (Marks et al., 2002, 030616 ). However, these estimations are uncertain since CO is quickly scavenged in the cytosol of living ce lls. CO is endogenously produced in the nose and paranasal sinus which may contribute to exhale d CO concentrations (Andersson et al., 2000, 011836 ). It is also important to note that increas ed endogenous CO production does not universally lead to an increase in COHb saturation.

January 2010 4-22Study Condition

Coburn et al. (1963, 013971 ) Adult male

Delivoria-Papadopoulos et al. (1974, 086316 ) Female - Progesterone phase

Delivoria-Papadopoulos et al. (1974, 086316 ) Female - Estrogen phase

Longo (1970, 013922 ) Pregnancy

Longo (1970, 013922 ) Fetal

Longo (1970, 013922 ) Postpartum - 24 h

Longo (1970, 013922 ) Postpartum - day 4

Coburn et al. (1966, 010984 ) Hemolytic anemia min

Coburn et al. (1966, 010984 ) Hemolytic ane mia max

Zegdi et al. (2002, 037461 ) Sepsis survivors

Scharte et al. (2006, 194115 ) Cardiac disease patients max

Scharte et al. (2006, 194115 ) Critically ill on dialysis max

0 0.01 0.02 0.03 0.04 0.05 0.06

Endogenous CO production (mL/min)



### Figure 4-10. Representative estimates of en dogenous CO production rates resulting from various conditions and diseases.

Not all endogenous CO production is derived from Hb breakdown. Other hemoproteins, such as Mb, cytochromes, peroxidases, and catalase, contribute 20-25% to the total amount of endogenous CO (Berk et al., 1976, 012603 ). All of these sources result in a normal blood COHb concentration between 0.3 and 1% (Coburn et al., 1965, 011145 ). The level of endogenous production can be changed by drugs or a number of physiological conditions that alter RBC destruction, other hemoprotein breakdown, or HO-1 expression and activity ( Figure 4-10 ). Nicotinic acid (Lundh et al., 1975, 086332 ), allyl-containing compounds (acetamids and barbiturates) (Mercke et al., 1975, 086303 ), diphenylhydantoin (Coburn, 1970, 010625 ), progesterone (Delivor ia-Papadopoulos et al., 1974, 086316 ), contraceptives (Mercke et al., 1975, 086308 ), and statins (Muchova et al., 2007, 194098 ) can increase CO production. Compounds such as carbon disulfide and sulfur-containing chemicals (parathion and phenylthiourea) will in crease CO by acting on P450 system moieties (Landaw et al., 1970, 012605 ). The P450 system may also cause large increases in CO produced from the metabolic degradation of dihalomethanes, leading to very high (>10%) COHb levels (Bos et al., 2006, 194084 ; Manno et al., 1992, 013707 ) that can be further enhanced by prior exposure to hydrocarbons or ethanol (Pankow et al., 1991, 013551 ; Wirkner et al., 1997, 082642 ). Minor sources of endogenous CO include auto-oxidation of phenols, flavonoids, and halomethanes, photo-oxidation of organic compounds, and lipid peroxidation of cell membrane lipids (Rodgers et al., 1994, 076440 ). Women experience fluctuating COHb levels throughout menstruation when endogenous CO production doubles in the progesterone phase (0.62 mL/h versus 0.32 mL/h in estrogen phase) (Delivoria-Papadopoulos et al., 1974, 086316 ; Mercke and Lundh, 1976, 086309 ). Similarly, endogenous CO production increases during pregnanc y (0.92 mL/h) due to contributions from fetal endogenous CO production (0.036 mL/h) and altered hemoglobin metabolism (Hill et al., 1977, 011315 ; Longo, 1970, 013922 ). Any disturbance in RBC hemostasis by accelerated destruction of hemoproteins will lead to an increased production of CO ( Figure 4-11 and Figure 4-12 ). Pathologic conditions such as anemias, hematomas, thalassemia, Gilbert’s syndrome with hemolysis, and other hematological diseases and illness will accelerate CO pr oduction (Berk et al., 1974, 012386 ; Hampson and Weaver, 2007, 190272 ; Meyer et al., 1998, 047530 ; Solanki et al., 1988, 012426 ; Sylvester et al., 2005, 191954 ). Patients with hemolytic anemia exhibit COHb levels at least two- to threefold higher than healthy individuals and CO production rates two- to eightfold higher (Coburn et al., 1966, 010984 ). Recent studies report COHb levels measured by CO-oximeter that are elevated to levels between 4.6% and 9.7% due to drug-induced hemolytic anemia (Hampson and Weaver, 2007, 190272 ) and between 3.9% and 6.7% due to an unstable hemoglobi n disorder (Hb Zürich) (Zinkham et al., 1980, 011435 ).

January 2010 4-23Endogenous CO production rate varied from 0.70 to 3.18 mL/h in anemic patients (Coburn et al., 1966, 010984 ).

Study Condition

Hampson and Weaver (2007, 190272 ) Hemolytic ane mia max

Hampson and Weaver (2007, 190272 ) Hemolytic anemia min

Zinkham et al. (1980, 011435 ) Hb Zürich min

Zinkham et al. (1980, 011435 ) Hb Zürich min

Coburn et al. (1966, 010984 ) Hemolytic anemia max

Coburn et al. (1966, 010984 ) Hemolytic anemia min

Tran et al. (2007, 194120 ) Liver transplant

De Las Heras et al. (2003, 194087 ) Peritonitis (SBP)

De Las Heras et al. (2003, 194087 ) Cirrhosis

Yasuda et al. (2005, 191953 ) Exacerbated COPD

Morimatsu et al. (2006, 194097 ) Critically ill

De Las Heras et al. (2003, 194087 ) Healthy

02468

COHb (%) 1 0



### Figure 4-11. Representative COHb saturation resulting from various diseases and conditions. Measurements of COHb taken using CO-oximeter, except in Coburn et al. (1966, 010984 ), where COHb was measured using GC. SBP: Spontaneous bacterial peritonitis

Critically ill patients exhale more CO a nd have higher endogenous CO production than healthy controls, likely due to both increased heme turnover as well as upregulation of the expression and activity of HO-1 (Morimatsu et al., 2006, 194097 ; Scharte et al., 2000, 194112 ; Scharte et al., 2006, 194115 ) ( Figure 4-12 ). CO production weakly correlates with the multiple organ dysfunction score (MODS), which es timates severity of organ dysfunction; however, it did not correlate with the Acute Physiology and Chronic He alth Evaluation II score (APACHE II) (Scharte et al., 2006, 194115 ) or the sequential organ failure assessment score (SOFA) (Morimatsu et al., 2006, 194097 ). Critically ill patients that survived ha d a higher exhaled CO (COex) concentration than nonsurvivors (median 3.9 ppm versus 2.4 ppm) (Morimatsu et al., 2006, 194097 ). Similarly, patients that survived severe sepsis had a higher CO production than those that did not survive (14.7 ± 5.3 versus 8.5 ± 3.3 μl/kg/h) (Zegdi et al., 2002, 037461 ).

January 2010 4-24Study Condition

Yamaya et al. (1998, 047525 ), Zayasu et al. (1997, 084291 ) Smokers

De Las Heras et al. (2003, 194087 ) Peritonitis (SBP)

De Las Heras et al. (2003, 194087 ) Cirrhosis

Sylvester et al. (2005, 191954 ) Sickle Cell Anemia

Yamaya et al. (1998, 047525 ) URTI

Zayasu et al. (1997, 084291 ) Asthma w/ steroids

Zayasu et al. (1997, 084291 ) Asthma

Paredi et al. (1999, 118798 ) Cystic fibrosis

Horvath et al. (1998, 087190 ) Bronchiectasis

Paredi et al. (1999, 118798 ) Type 2 diabetes

Paredi et al. (1999, 118798 ) Type 1 diabetes

Morimatsu et al. (2006, 194097 ) Critically ill

Morimatsu et al. (2006, 194097 ) Healthy

0 3 6 9 12 15

Exhaled CO (average fold change from healthy)



### Figure 4-12. Representative exhaled CO concentrations (ppm) resulting from various conditions plotted as fold increases over healthy human controls from each study. SBP: Spontaneous bacterial peri tonitis; URTI: Upper respiratory tract infection

Diseases involving inflammation and infection result in increased endogenous CO production. For example, patients with severe sepsis or se ptic shock have a higher COex and endogenous CO production compared to control patients, which wa s reduced with treatment of the disease (i.e., antibiotics, surgery) (Zegdi et al., 2002, 037461 ). Similarly, patients with pre-existing cardiac disease, as well as patients with renal failure who undergo dialysis, produced higher amounts of endogenous CO compared to other critica lly ill patients (Scharte et al., 2006, 194115 ). High plasma COHb levels measured by CO-oximeter were found in nonsmoking patients evaluated for liver transplantation (mean 2.1%); however, this increase was not correlated with the Model for End Stage Liver Disease (MELD) score or the Child Turcotte Pugh score, used to assess the degree of liver impairment (Tran et al., 2007, 194120 ). Further investigation in cirrhotic patients, with and without ascites, provided evidence for increased pl asma CO concentrations, HO-1 activity in polymorphonuclear cells, exhaled CO, and blood COHb (De las Heras et al., 2003, 194087 ; Tarquini et al., 2009, 194117 ). COex, plasma CO, and COHb levels were correlated with the Child-Pugh score, and thus the severity of disease. These pa rameters were significantly higher in patients with ascites or with spontaneous bacterial peritonitis (SBP) (COHb, healthy: 0.6 ± 0.1%; cirrhosis: 1.0 ± 0.1%; with ascites: 1.6 ± 0.2%; with SBP: 1.9 ± 0.2%; measured by CO-oximeter). Both COex and COHb levels decreased after resolution of the infect ion in patients with SBP, reaching values similar to noninfected patients within 1 mo (De las Heras et al., 2003, 194087 ). Endotoxin concentration was correlated with plasma CO levels, sugge sting a link between systemic endotoxemia and increased activity or expression of the HO/CO system (Tarquini et al., 2009, 194117 ). COex concentrations are also elevated in patients with di abetes (Type 1: 4.0 ± 0.7 ppm; Type 2: 5.0 ± 0.4 ppm; healthy: 2.9 ± 0.2 ppm), and correlated with blood glucose levels and duration of disease (Paredi et al., 1999, 194102 ). Likewise, obese Zucker rats, a model of metabolic syndrome with insulin resistance, have increased respiratory CO excretion and COHb levels compared to lean Zucker rats (3.9 ± 0.1% versus 3.0 ± 0.1% COHb), which is decreased by HO inhibition (Johnson et al., 2006, 193874 ). Endogenous CO is also increased in airway inflammatory diseases. Patients with upper respiratory tract infections exhaled higher CO conc entrations than normal controls and this increase was attenuated after recovery (Yamaya et al., 1998, 047525 ). Arterial COHb levels have been related

January 2010 4-25to disease severity in COPD patients (Yasuda et al., 2005, 191953 ). Bronchiectasis patients had higher COex; however, anti-inflammatory treatment did not decrease the CO levels (Horvath et al., 1998, 087191 ). Patients with cystic fibrosis had higher COex than normal controls (6.7 ± 0.6 ppm versus 2.4 ± 0.4 ppm), and patients treated with ster oids had a decrease in CO levels (8.4 ± 1.0 ppm versus 5.1 ± 0.5 ppm) (Paredi et al., 1999, 118798 ). Increased arterial COHb measured by COoximeter was reported in patients with bronchial as thma, pneumonia, idiopathic pulmonary fibrosis, pyelonephritis, and active rheumatoid arthritis (Yasuda et al., 2002, 035206 ; Yasuda et al., 2004, 191955 ). Similarly, asthmatic patients exhibited an elevation of COex that decreased with corticosteroid therapy (nonsmoking controls: 1.5 ± 0.1 ppm; asthmatics without corticosteroids: 5.6 ± 0.6 ppm; with corticosteroids: 1.7 ± 0.1 ppm; smoking controls: 21.6 ± 2.8 ppm) (Zayasu et al., 1997, 084291 ). These results were confirmed and associa ted with increased expression of HO-1 in airway macrophages (Horvath et al., 1998, 087190 ). Also, COex was increased in patients with allergic rhinitis during the pollen season; however, their COex was similar to control subject levels out of season (Monma et al., 1999, 180426 ). Similarly, endogenous CO production and HO-1 expression in nasal mucosa was correlated with allergic rhinitis in guinea pigs as described in Section 5.1 (Shaoqing et al., 2008, 192384 ). Altitude is also positively associated with baseline COHb concentrations (McGrath, 1992, 001005 )(McGrath et al., 1993, 013865 ). This increase in COHb with altitude-induced hypoxia is associated in rats and cells with increases in th e mRNA, protein, and activity of HO-1 leading to enhanced endogenous CO production (Carraway et al., 2002, 026018 ; Lee et al., 1997, 082641 ). Whether other variables such as an accelerated metabolism or a greater pool of Hb, transient shifts in body stores, or a change in the elimination rate of CO, play a role has not been explored. Because of the sensitivity of COHb to changes in the metabolic state, ranges of endogenous COHb levels in the population are uncertain. Ho wever, baseline levels of COHb, which reflect exposure to ambient and non-ambient CO and endoge nous production of CO, have been measured in the population. COHb levels measured by CO-oximeter in packed red blood cell units reserved for use between 2004 and 2005 averaged 0.78 ± 1.48%, w ith 10.3% of samples having COHb levels of 1.5% or greater and a maximum measurement of 12% (Ehlers et al., 2009, 194089 ). This study reported a decrease from a study conducted in 1982-1983 in the number of units with elevated COHb; at that time, 49% of units had COHb levels >1.5% (Aronow et al., 1984, 194083 ) versus 10.3% in 2004-2005. Another study calculated that 23% of donated blood units had COHb levels exceeding 1.5%, with the highest measur ement being 7.2% (Aberg et al., 2009, 194082 ). Smoking is the main factor causing increased blood concen trations of CO. A dose-response relationship was shown to exist between COHb concentration and the number of cigarettes smoked a day (nonsmoker: 1.59 ± 1.72%; 1-5 cig/day: 2.31 ± 1.94 %; 6-14 cig/day: 4.39 ± 2.48%; 15-24 cig/day: 5.68 ± 2.64%; ≥ 25 cig/day: 6.02 ± 2.86% COHb). The mean baseline COHb value for former smokers was higher than that of never smokers in this prospective cohort study (1.96 ± 1.87 versus 1.59 ± 1.72%) (Hart et al., 2006, 194092 ). Endogenous CO is removed from the body mainly by expiration and oxidation. CO diffuses across the alveolar-capillary membrane and is exhaled. This event has been used as a noninvasive measurement of both endogenous and body load CO (Stevenson et al., 1979, 193767 ). CO can also be oxidized to CO 2 by cytochrome c oxidase in the mitochondria (Fenn, 1970, 010821 ; Young and Caughey, 1986, 012091 ). However, the rates of CO metabolism are much slower than the rates of endogenous CO production, with the rate of consump tion representing only 10% of the rate of CO production in dogs (Luomanmäki and Coburn, 1969, 012319 ).



# 4.6. Summary and Conclusions

CO elicits various health effects by binding with and altering the function of a number of heme-containing molecules, mainly Hb. The formation of COHb reduces the O 2 -carrying capacity of blood and impairs the release of O 2 from O 2 Hb to the tissues. Venous COHb levels have been modeled mainly by the CFK equation, but more recent models have included venous and arterial blood mixing and Mb and extravascular storage compartments, as well as other dynamics of CO physiology. The CFK equation remains the most exte nsively validated and applied model for COHb prediction. Recent models have indicated that CO has a biphasic elimination curve, due to initial washout from the blood followed by a slower flux from the tissues. The flow of CO between the

January 2010 4-26blood and alveolar air or tissues is controlled by diffusion down the pCO gradient. The uptake of CO is governed not only by this CO pressure differen tial, but also by physiological factors, such as minute ventilation and lung diffusing capacity, that can, in turn, be affected by conditions such as exercise, age, and health. Susceptible populations, including health compromised individuals and developing fetuses, are at a greater risk from C OHb induced health effects due to altered CO kinetics, compromised cardiopulmonary function, and increased baseline hypoxia levels. Altitude may also significantly affect the kinetics of COHb formation. Compensatory mechanisms, such as increased cardiac output, compensate for the decrease in barometric pressure. Altitude also increases the endogenous production of CO through upregulation of HO-1. CO is considered a second messenger and is endogenously produced from the cat abolism of heme proteins by enzymes such as HO-1. A number of diseases and conditions af fect endogenous CO production, possibly causing a higher endogenous COHb level. Finally, CO is rem oved from the body by expiration or oxidation to CO 2

.

January 2010 4-27

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