Kehl et al. BMC Public Health 2010, 10:424 http://www.biomedcentral.com/1471-2458/10/424 may indicate exposure to ETS over the last 24 hours due to its half-life of approximately 20 hours [11]. By the use of cotinine-to-creatinine ratio (CCR) dilution of the urine may be considered [12]. There is little research about the relationship between smokers’ self-reports of smoking indoors and their chil- drens’ exposure to tobacco smoke. Children in house- holds with home smoking bans had lower cotinine than those in homes without a smoking ban [5]. Additionally, smoking and the number of cigarettes per day (cpd) smoked in child’s presence were associated with urinary cotinine [13]. Among children exposed to more than 10 cpd, the median CCR was 2.4 times higher than among those unexposed to smoke [14]. Children in households with one or more current daily cigarette smokers, com- pared to their counterparts in households without cigar- ette smokers, were more likely to have a urinary CCR greater than 32 ng/mg [15]. In addition, consistency and variability of CCR has been assessed every other month from birth to two years of age and how well self reported ETS data predict multiple CCR measurements was analyzed [15]. The findings revealed that variation in ETS exposure over time is reflected in the CCR values. This raises the question whether the CCRs fluc- tuate if daily measurements, including weekends, are taken. It also raised the question of whether daily mea- sured CCRs are related with parents’ reports of smoking. Limitations of the current research are that CCR has not been taken daily over one week, including a week- end, or longer. Hence, it seems to be unknown whether the CCRs vary if daily measurements are taken. The aim of the present study was to examine relationship between infants’ daily exposures to ETS, as assessed by urinary CCR, and parents’ self-reported smoking over a seven day period. We also wanted to describe CCR over seven days among infants’ at age three years or younger. Methods Sample A convenience sample of 27 households in Northern Germany was drawn. Households could be included in the study if they had a child aged three years old or younger and if there was at least one daily smoker pre- sent at home, i. e. a person that had smoked at least one cigarette per day for the last four weeks prior to the study [16]. Average age of the 28 study infants was 18.04 (SD = 12.07) months, with a range from 2 to 43 months. During the study period, only one of the chil- dren was breast-fed, and this child was breast-fed by a mother who did not smoke at the time of study. Half of the infants were girls. In one household lived twins. Among the households, 22 were recruited proactively in one pediatrician’s office, and five were recruited through advertising in pediatricians’ offices, in a newspaper or Page 2 of 8 through friends. Informed written consent was obtained from all parents of the children prior to their participa- tion in the study. Participants were offered 30 Euro that they received at the end of the data collection. Data were gathered in the city and suburbs of Rostock, Germany. The study was part of the project about reduction of ETS in families and was conformed to the principles of the Helsinki Declaration as reflected by an a priori approval of the Ethics committee of the University of Greifswald (project GESA: reference number BB 64/07). Assessments Parents took part in a 10- to 15-minute interview about their smoking status, household smoking habits (includ- ing smoking on the balcony or outside, instead of inside the home) and socio demographic characteristics. The interview took place in the respondents’ homes. Parents’ age, sex, native language and presence of a partner were recorded. Level of education was grouped into the fol- lowing categories: less than or equal to 9 years, 10 to 11 years, and 12 to 13 years of school. Employment status was classified as “employed,” “unemployed,” “parenting time,” and “other.” The smoking status of the adult respondents was recorded. Individuals were classified as ever smokers if they smoked more than 100 cigarettes in their lifetime. They were classified as current daily smokers if they had smoked one or more cpd throughout the last 30 days. Parents who smoked during the last 30 days but not daily were classified as less than daily smoker. Former smokers were those who had smoked more than 100 cigarettes in their lifetime but did not smoke during the last 30 days. Mean cpd was obtained from current daily smokers. Additionally, respondents were asked whether they had smoked tobacco other than cigarettes. Smoking status was determined for each adult who lived in the household. Furthermore, participants were asked whether the smoking occurred in the home, and if so, in what rooms. Participants were also asked if they smoked in the living room during the seven days prior to the interview. Additionally, the infant’s sex and age (in months) were recorded, as well as whether the infant was breast-fed. For the study period of seven days, participants had to complete a “smoking diary” that included the following information: cpd smoked in the presence of the infant at home, on the balcony, in the car or outside home. Presence was defined as the possibility for the smoke to reach the child via air flow. If parents smoked outside the room where the child was, with the doors closed, this was considered not smoking in the presence of the infant, even though particles of tobacco smoke likely existed in the parents’ clothes. For the study period of