each subject was evaluated on four separate occasions , a qualifying visit and three blinded test visits , which involved exposure ( in random order ) to air without added carbon monoxide and to air that contained carbon monoxide concentrations calculated to produce approximately 2.2 % or 4.4 % carboxyhemoglobin , measured by gas chromatography , at the end of the exposure period .
C1874024
air
C0007018
carbon monoxide