Background:
Children are particularly vulnerable to the effects of secondhand smoke (SHS), mainly due to presenting a respiratory and immune systems in development and a faster respiratory rate. The objective of this study is to describe the potential social inequalities in SHS exposure in households with children under 12 in Spain.

Methods:
Cross-sectional study in a representative sample of population under 12 years of age in Spain. The sample size was 2,411 individuals, being proportional by regions, size of municipality of residence, sex and age groups. A telephone survey was administered to parents or legal tutors in 2016. It included sociodemographic variables and variables related to SHS exposure at home. A descriptive bivariate analysis of the main SHS exposure variables according to educational level and social class (based on occupation) was carried out.

Results:
25.8% of children are exposed to SHS at home (smoking reported either inside or on balconies or terraces). An exposure gradient is observed according to the level of studies of the household's main earner, being the prevalence of exposure higher as the educational level decreases (37.9% in primary studies or lower, 27.8% in secondary studies and 20% in university studies, p < 0.05). This reverse gradient is maintained by social class (31.1% in class V-VII (most deprived), 23.4% in class III-IV and 21.7% in class I-II (most affluent), p < 0.05). There are no significant differences according to the sex of the child.

Conclusions:
In Spain, one out of every four children is exposed to SHS at home. This exposure presents a clear socioeconomic gradient, being children living in more deprived families at higher risk of SHS exposure. Interventions aiming to reduce SHS exposure in children, with a perspective of equity, are urgently needed.