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tremor, headaches, chest pain, respiratory and cardiovascular effects, and visual and other central nervous system (CNS) effects.

The respiratory, cardiovascular, and CNS effects of CO2 are related to the decreases in blood and tissue pH that result from exposures (Eckenhoff and Longnecker 1995; Yang et al. 1997; HSDB 2004). Changes in pH act directly and indirectly on those systems. The pH changes also trigger various compensatory mechanisms, including increased ventilation to reduce excess CO2 in the bloodstream, increased renal acid excretion to restore acid-base balance, and sympathetic nervous system stimulation to counteract the direct effects of pH changes on heart contractility and vasodilation (Eckenhoff and Longnecker 1995; HSDB 2004). The key effects for setting EEGL and CEGL values are tremor, headache, hyperventilation, visual impairment, and CNS impairment.

Effects in Humans

Accidental Exposures

In a case report of two men who lost consciousness in a wellhead chamber as a result of exposure to a “high concentration” of CO2 in the atmosphere, one man exhibited constricted visual fields, enlarged blind spots, photophobia, loss of convergence and accommodation, deficient dark adaptation, headaches, insomnia, and personality changes (Freedman and Sevel 1966). The other man died of asphyxia. In a similar incident, one overexposed man died after 11 months in a coma; he exhibited retinal atrophy and gliosis as well as loss of all ganglion cells (Sevel and Freedman 1967). In addition to that delayed fatality, three men died immediately from asphyxia. These studies are not relevant to establishing EEGL and CEGL values, but they are consistent with the findings that CO2 affects vision.

Experimental Studies

Experimental studies have shown that CO2 causes a variety of effects, ranging from nonspecific signs and symptoms, such as tremor, dyspnea, intercostal pain, and headache, to cardiovascular and CNS effects. Each of the effects mentioned is addressed in the following discussion.

Tremor was noted at a CO2 concentration of 60,000 ppm after several hours of exposure in a review that lacked documentation of specific methods (Schulte 1964). It was also noted in 10 of 12 subjects exposed at 7,000-

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