Research paper RESULTS Venue level information We were able to collect data on 585 venues during the pre-ban period and on 559 venues during the post-ban period. No signiﬁcant difference in pre-ban indoor CO measurements between venues that were or were not visited in the post-ban period was observed (mean value in venues visited twice 4.56 (3.69); mean value in venues visited only in the pre-ban period 4.42 (4.97); p value ¼ 0.86). The main reason for not obtaining a paired visit was because some venues went out of business during the follow-up period. A wide range of venue types was ascertained. In table 1 we present information regarding venue type and pre-ban and post- ban CO measurements in the three different environments sampled. We observed a signiﬁcant difference between pre-ban measured CO levels and venue type only regarding closed envi- ronments (p¼0.04), and not in semi-open and open environ- ments (p¼0.08 and p¼0.23, respectively). This difference was due to higher mean CO levels in closed environments in bars and nightclubs compared with restaurants (5.02, 4.91, and, 4.01, respectively). Regardless of venue type, a signiﬁcant difference was veriﬁed regarding closed, semi-open, and open environments (p<0.001 for mean difference between environments, adjusted for venue type). As expected, signiﬁcant correlations were observed between the studied environments in the pre-ban period (Pearson coefﬁcient between closed and semi-opened: 0.84; closed and opened: 0.57; and semi-opened and opened: 0.70, all with a p value <0.001). In the post-ban period, signiﬁcant reductions in all study environments were observed (ﬁgure 1, table 1). As can be observed, and expected, although all reductions were highly signiﬁcant, the absolute and relative reductions observed for closed environments, regardless of venue type, were higher than those observed for open spaces. In closed environments, in the pre-ban period 180 (30.8%) venues had CO concentrations above the level of 4.5 ppm (associated with increased overall health risk). In the post-ban period, only eight (1.4%) venues had CO measurements above this threshold. Interestingly, for closed environments a signiﬁcant interaction between CO reduction and venue type was identiﬁed (p¼0.01 for the interaction term). This was observed owing to a higher CO reduction observed in bars and nightclubs compared with that observed in restaurants. One interpretation of this result is that in the post-ban period, no signiﬁcant difference was observed between venue type and closed environment CO levels (p¼0.39), as opposed to the pre-ban period, where bars and nightclubs had signiﬁcantly higher values. In fact, in the post- ban period survey no signiﬁcant difference between CO levels and environment (open, semi-open and closed), or venue type (restaurant, bar, nightclub or other) was observed any longer, thus suggesting equalisation with outdoor levels (table 1). Figure 1 Absolute values on measured environment CO concentrations before and after ban implantation according to venue type and environment type. The quality of the air in the city during the same period of our study through the air quality databank from the Environmental Agency of São Paulo (CETESB) was as follows: in study days of the pre-ban period: 0.94 ppm (0.075 SE, CI 95% 0.77 to 1.12, minimum 0.31, maximum 1.59 for mean levels, and min 0.1, max 2.4 for individual data points), in the study days of the post-ban period 0.66 ppm (0.073 SE and CI 95% 0.49 to 0.83, min 0.09, max 2.40 for mean levels, and min 0.0, max 3.2 for individual data points). Individual level information Our survey design was able to collect information of CO level on 627 employees in the pre-ban period, 359 non-smoking and 268 smoking employees. Employees were mainly male (147 vs 39 for smoking; 232 vs 45 for non-smoking). Mean age was 33.8 (10.1) years for smoking and 34.2 (11.5) years for non-smoking employees. In the post-ban period, we were able to re-ascertain the CO levels of 464 employees, 277 non-smoking and 187 smoking employees. The mean number of cigarettes in the pre-ban period was of 14 (SD 10), and 12 in the post-ban period (SD 9), p¼0.02. The main reason for not obtaining a higher rate of repeated measures of CO level was because of the survey design, in which venues were re-visited at the same time and day of the week of Table 1 Pre-ban and post-ban mean CO level according to venue type and sampled environment Venue type Restaurant Bar Nightclub Other Mean value 158 Environment Closed pre-ban Mean 4.01 5.02 4.91 4.64 4.57 SD 3.39 4.27 3.21 3.94 3.7 Closed post-ban Mean 1.29 1.35 1.30 1.67 1.35 SD 1.66 1.22 .82 2.87 1.66 p Value <0.001 <0.001 <0.001 <0.001 Semi-opened pre-ban Mean 3.49 4.14 4.01 3.58 3.79 SD 2.09 3.02 2.70 2.32 2.49 Semi-opened post-ban Mean 1.11 1.21 1.15 1.21 1.16 SD 1.06 1.37 .73 1.16 1.14 p Value <0.001 <0.001 <0.001 <0.001 Open pre-ban Open post-ban Mean 3.16 3.23 3.50 3.74 3.31 SD 1.98 1.74 2.33 3.23 2.2 Mean 1.22 1.43 1.33 1.31 1.31 SD 1.30 1.68 0.97 1.25 1.39 p Value <0.001 <0.001 <0.001 <0.001 Tobacco Control 2011;20:156e162. doi:10.1136/tc.2010.037614