reduction in the adverse effect depends in part on the extent of a restriction or partial ban that existed before the ban under study. For example, some locations had previously implemented partial bans, and some regions within the locations studied (for example, New York City and several other large counties in the New York state study) had previously implemented comprehensive bans (Juster et al., 2007). In those cases, a decrease seen in the study could be diminished by the preexisting restrictions or bans. Similarly, in studies that have comparison populations, partial restrictions in the control locations could affect the magnitude of differences seen.

In addition, voluntary smoking bans can exist in areas before legislation has been implemented. For example, many hotel chains, some restaurant chains, airlines and other mass transit systems, office buildings, health-care facilities, schools, and individually owned establishments instituted bans long before counties, cities, or states legislated bans. Categorizing a county as not having a smoking ban may fail to reflect the fact that the average smoker could spend a substantial amount of time in an occupational setting that prohibits smoking in and outside a building, could eat dinner in a restaurant that prohibits smoking, and could shop in stores that prohibit smoking. This is increasingly the case. In 1993, 46.5% of employees in the United States were covered by smoking restrictions; by 1998–1999, 69.3% were covered by smoking restrictions (Shopland et al., 2004). Such prohibitions have increased, so it is more difficult to attribute even temporal changes in tobacco use or exposure in a defined geographic area to the lack or presence of a smoking ordinance. That could contribute to an underestimate of the actual effect had there been no prior ban. In contrast, many bans have allowed smoking outside public buildings or more than some stated distance from entrances. Although it is possible that outside smoking could attenuate the benefits of a smoking ban, the concentrations of secondhand smoke in those areas, and the safety or hazardousness of such areas in human populations has yet to be evaluated.

If smoking bans decrease acute coronary events, the inclusiveness of a ban (for example, the types of buildings and establishments included and the number of exemptions allowed) would be expected to affect the magnitude of the decrease. Different bans can cover or exempt different types of establishments or locations (such as restaurants with bars, bowling alleys, bingo halls, and outdoor seating areas). In interpreting studies of smoking bans, especially in comparing results of different studies, it is important to consider the types and extent of different bans. In addition, if a ban is not complied with or enforced, changes in health effects would not be expected. For example, the Clean Indoor Air Act was enacted in 1985 by the Florida legislature, but enforcement usually depended on filing of complaints with the Department of Health (American Lung Association, 2009).