Introduction:
Epidemiological studies that use surveys in the field of exposure to environmental tobacco smoke (ETS) are common because they have a reasonable cost and obtain detailed and retrospective information. Although, it should be noted that there is no agreement about which questions should be included. The aim of the study was to analyze how exposure to ETS is ascertained in studies that assessed exposure to ETS in childhood, in adulthood and in Health Surveys carried out in Spain or in its Autonomous Communities (AC).

Material and Methods:
A PubMed search was conducted for original articles in English and Spanish published between 01/01/2012 and 31/12/2021. The variables analyzed were the scenarios of exposure to ETS, the verification of exposure, the intensity of exposure and present the wording of questions. The search was completed with a manual review of the wording the questionnaires included in the European Health Surveys (EES), National Health Survey (ENSE) and in surveys developed in the AC of Spain.

Results:
A total of 64 studies were identified, 13 studying exposure to ETS in childhood and 51 in the adult population. In childhood, wording of questions was described in 5 out of 13 studies, being the only scenario ascertained by all home. The verification of exposure was evaluated unevenly, being more frequent questions oriented to cohabitation with smokers. In the adult population, the most evaluated scenario was home followed by work or place of studies; 15 of 51 studies included the question wording, 19 of 51 studies also evaluate exposure to ETS through biological markers, 18 through cotinine. The ENSEs carried out in 2006, 2011-2012 and 2017 as well as the EES of 2009, 2014 and 2020 included questions oriented to ascertain the prevalence of exposure to ETS. In the last year 9 AC asked about exposure to ETS. The most individually valued scenario was home followed by work or place of studies and closed places. The questions varied among the different surveys. Ascertaining exposure to ETS through questionnaires is study-dependent. This emphasizes the need for a standard set of valid and comparable questions to include in epidemiological studies.