Research paper that the correlation coefﬁcient was much higher in non-smoking than in smoking employees (0.79 and 0.32, respectively), thus suggesting that the indoor environment was important in predicting expired CO levels in both smoking and, especially, in non-smoking employees (ﬁgure 2). Table 2 presents data on the pre-ban and post-ban periods for both smoking and non-smoking employees. Overall, a signiﬁcant reduction in CO levels was observed for both smoking and non- smoking employees after the smoking-ban (p<0.001). This occurred irrespective of venue type (p¼0.58 for the interaction term between survey time and venue type). Interestingly, the degree of CO reduction was also independent of the smoking status (ﬁgure 3). If the correlation structure of smoking status and environ- mental CO levels is dissected, an interesting pattern can be seen in the post-ban period. For non-smoking employees a signiﬁcant, but much weaker, correlation existed between environment CO levels and expired CO levels (p¼0.001, correlation coefﬁcient 0.22). On the other hand, for smoking employees one could no longer observe any correlation between environment CO levels and expired CO levels (p¼0.17, correlation coefﬁcient 0.11). This can be better observed in ﬁgure 2 (A and B for the correlation structure in the pre-ban period, and C and D for the correlation structure in the post-ban period). Another important point regarding exhaled CO concentra- tions after the ban is that CO reductions were veriﬁed even in smoking employees that continued to smoke after the ban implementation (although a higher reduction was observed, as expected, in smoking employees who reduced the number of cigarettes). This suggests that reduction of expired CO after the ban was regardless of changing smoking habits in venue’s employees (ﬁgure 4). DISCUSSION The association between active smoking and cardiovascular disease has been known for many decades, and more recently SHS has also been recognised as a risk factor for coronary disease. Reducing public exposure to cigarette smoke can have immediate implications on human health. In the last two decades, some studies5e9 have shown a signiﬁcant reduction in cardiovascular hospital admissions after the implementation of smoking-ban ordinances. The current study analysed the effects of the smoking ban on levels of CO in restaurants, bars and similar venues in the city of São Paulo, Brazil. There was a clear reduction in the concen- tration of CO in almost all visited places. Measured CO reduc- tions were especially signiﬁcant in indoor areas, but were also seen in semi-open and open areas of these sites. Regarding cooking stoves, although they can inﬂuence CO measurement, a signiﬁcant difference between the ﬁrst and the second measurement was observed even in restaurants, where this Pre-ban and post-ban CO measurements at the individual level Pre-ban period Post-ban period Table 2 Venue type Restaurant Bar Nightclub Other 160 Smoking status Smoking Non-smoking Smoking Non-smoking Smoking Non-smoking Smoking Non-smoking Mean 15.67 6.32 16.47 7.22 17.73 6.94 10.61 7.91 SD 9.8 3.49 10.39 5.85 11.61 6.69 3.79 3.32 Mean 11.48 3.44 10.75 3.75 13.49 3.29 7.18 3.5 SD 8.38 2.02 7.25 2.85 6.97 1.68 3.03 2.01 Figure 4 Mean expired CO levels in smoking employees before and after ban implementation according to changes in smoking behaviour after the ban. Tobacco Control 2011;20:156e162. doi:10.1136/tc.2010.037614 Figure 3 Exhaled CO reduction after ban implementation in venue employees according to smoking status. confounding effect is more common. An interesting fact is that restaurants were the places with the lowest pre-ban CO average among all venues types. These data were corroborated by Erazo et al16 who analysed the air nicotine concentration in bars and restaurants in Chile and noticed a lower concentration of nicotine in restaurants compared to bars in Santiago, Chile. Some studies have used other biological markers of SHS pre-smoking ban and post-smoking ban.17e19 They analysed other components of SHS like particulate matter concentrations, volatile organic compound and cotinine levels, and they found