254 Int. J. Environ. Res. Public Health 2011, 8 1. Introduction Little is known about tobacco use prevalence rates among emergency department (ED) patients, yet this is an important population to understand from a public health perspective and to target with tobacco cessation interventions [1,2]. The few studies that have reported ED tobacco prevalence show great variability (26%–41%) and rates tend to be much higher than the general population [3-6] but firm conclusions have been precluded primarily by two methodological limitations. First, none of the studies have systematically tracked tobacco use on all patients. Data have been based on small samples and surveys with recruitment eligibility criteria representing only a subset of the ED patient population versus the entire ED population, thereby introducing sample error. Second, none of the studies have taken age into account. Tobacco use decreases with age, especially after 55+ years [7] so the smoking prevalence for a given population will be directly affected by the proportion of younger to older people included in the sample [8]. If patients <55 years of age represent a relatively larger proportion of the ED population, the tobacco use rates would be expected to be higher than the general population. Even when tobacco use status is electronically recorded as part of standard practice in the ED, the quality of tobacco use data can be poor and data extraction can be difficult due to the inconsistency of asking and recording status by different healthcare providers, lack of standardization in terms of how tobacco use is coded and where it appears in the chart, and various errors such as tobacco status occurring in multiple places in a patient’s chart with discrepant information for the same visit [9]. The current study was designed to move the field forward by addressing methodological issues that have previously hindered conclusions about tobacco use rates in the ED population and by comparing age-stratified prevalence rates with the general population. A standardized tobacco use question that has been used in previous inpatient tobacco cessation studies [8] was chosen and integrated into the ED electronic registration software and ED clerks’ workflow so that the tobacco-use status of the entire ED population could be systematically tracked using the same metric in the same place in the chart. To enhance the specificity of tobacco use prevalence data for public health application, the data were stratified by age and compared to national, provincial, and regional tobacco use prevalence rates for the general population. The long-term perspective of the study was to provide an ongoing assessment of the magnitude of the problem of tobacco use in the participating hospitals and to help inform decision-making about providing tobacco cessation services to patients. 2. Experimental Section 2.1. Setting Participating sites were acute care hospitals in the northwest (NW) region of the province of Ontario, Canada. For funding purposes under the provincial single-tier universal healthcare system, the region is defined by the geographical healthcare regionalization structure of the Local Health Integration Network (LHIN). The participating sites represented all 11 of 12 hospitals in LHIN 14 that used Meditech electronic admitting software. The geographical catchment of these hospitals covers a landmass of 526,355 square kilometers, almost 60% of the province, and includes over 30 northern and remote First Nations communities, but it represents only 2% of the provincial population (approximately 235,000 of which 20% are Aboriginal peoples) with a population density of