The correlation between CO and cigarettes per day is statistically significantly lower ( z = 3.43 ; P = 6.07 10 ( -4 ) ) in African Americans ( r = 0.14 ; 95 % CI , 0.02-0 .26 ; P = 0.003 ) than in European-Americans ( r = 0.36 ; 95 % CI , 0.31-0 .40 ; P = 0.0001 ) .