IQR increases in mean PM ( 2.5 ) ( 7.0 g/m ) and CO ( 161.7 parts per billion ) concentrations in the first 24h before the plasma nitrite measurement were associated with increased plasma nitrite concentrations ( PM ( 2.5 ) : 15.5 nanomolar ; 95 % confidence interval ( CI ) : 2.4 , 28.5 ; CO : 15.6 nanomolar ; 95 % CI : 2.4 , 28.9 ) .