in which a no-effect level was identified for the specific end point, but there was not an effect (or low) response level. Under these conditions, the no-observed-adverse-effect level (NOAEL) was multiplied by the factor √(n)/10, where n was the number of subjects in the study. The original summary table is presented in Appendix A. The end points were considered as described in the following sections (Wong 1996).

This end point was evaluated from the observation that 3% CO₂ was a NOAEL for CNS effects in two studies; one was a 5-d exposure involving 7 subjects and the other was a 2-wk exposure involving 12 subjects (Glatte et al. 1967; Storm and Giannetta 1974). The calculation was as follows:

where AC is acceptable concentration.

This AC was applied to all exposure times because CNS effects would not be acceptable even for brief periods; however, there is no basis for supposing that prolonged exposures would result in an accumulation of CO₂ that could have CNS effects.

Evidence was presented that CO₂-induced headaches are transient and that they were rare in a 30-d study of six humans exposed to 2% CO₂ and exercising periodically during their exposure (Radziszewski et al. 1988). On the basis of this observation, 2% was assigned as a NOAEL for CO₂-induced headaches (Wong 1996).

Wong used two studies to determine the AC for this adverse effect. Menn et al. (1970) found that exposure to 2.8% CO₂ for 0.5 h did not elicit intercostal pain or dyspnea in eight subjects. Likewise, Sinclair et al. (1971) found none of his four subjects experienced dyspnea or intercostal pain when exposed to 2.8% CO₂ for 1 h or 15-20 d. From these data, Wong derived a short-term AC to protect against this end point. He did not use the small n factor, because some risk of minor effects is tolerated for short-term exposures. The 1- and 24-h ACs to protect against dyspnea and intercostal pain were as follows: