Worldwide, more than 600 000 deaths per year are attributable to second-hand smoke (SHS) exposure. Of these, 28% occur among children.1 The World Health Organization estimates that about half of the children world population is exposed to SHS.1,2

SHS is a complex mixture of gases and micro-particles containing nicotine and many other toxic and irritants and several carcinogens.3–5 SHS is considered the most preventable indoors pollutant. No level of SHS exposure can be considered safe.2,4,5

When compared with adults, children have a higher breathing frequency, their liver metabolism and other clearing mechanisms are not yet fully developed, and inhale more air per body weight, resulting in higher exposures. This explains why children are more vulnerable to SHS exposure.6

Children's exposure to SHS is associated with poor lung function and several other respiratory health hazards such as asthma, bronchitis and pneumonia, coughing, wheezing and dyspnoea. SHS is also associated with a greater risk for sudden infant death syndrome, middle ear infections and meningitis.4–6 Children belonging to families with lower socio-economic status are among those most exposed to SHS. In addition, at least one study has reported an association between SHS exposure and smoking uptake.7,8

SHS exposure in the car is particularly dangerous due to higher pollutants’ concentration in this confined space.9 Moreover, car interiors are made with materials that retain tobacco smoke pollutants which are dangerous for human health and remain in vehicles’ surfaces long time after people finish smoking.10,11

Children's exposure to SHS is mainly caused by parental smoking, or by other household smoking. Smoking parents are more permissive to indoors smoking. 12 The current study aimed to determine the prevalence of children's exposure to SHS in the family car and to compare exposure among children with smoking and non-smoking parents

This was a questionnaire-based cross-sectional study. In February-May 2011, 3187 questionnaires were administrated in the classroom to 4th grade Portuguese children (mean age 9.5± 0.7 years, 51.1% boys).

The sampling strategy combined convenience and random methods.13 Nine municipalities (Angra do Heroísmo, Braga, Covilhã, Évora, Faro, Funchal, Lisboa, Porto, Viana do Castelo) representing the seven main regions of Portugal (Alentejo, Algarve, Azores, Centre, Lisbon, Madeira and North) were convenience samples. Considering the amount of 4th grade students from these municipalities as the total population, and estimating that each class has 20 students, a proportional number of classes per municipality was randomly chosen to participate in the study. Response rate was 77.5%; range: 69.2% (Funchal) to 91.9% (Angra do Heroísmo).

The questionnaire was based on survey tools used in previous studies.13,14 Core questions assessed: 1) parental/other relative smoking [Does your father/mother/brother(s)/other(s) family member(s) smoke? Answer: no, sometimes, yes]; 2- children's SHS exposure in the family car [Does your father/mother/brother(s)/other(s) smoke in the family car? Answer: always, sometimes and never]. The questionnaire was piloted among a small group of 4thgrade children and reviewed according to their feedback.

The survey was approved by the Portuguese Ministry of Education. Schools’ principals received guidelines regarding parents’/carers’ informed consent and the questionnaire application. Trained teachers and research fellows administrated the questionnaires in the classroom following a standardized procedure.

Prevalence rates were computed and chi-square tests were performed using the statistical software IBM SPSS Statistics version 22.

Of the participants, 52.0% reported at least one smoking parent. Overall exposure in the car was 28.9%. Children's exposure was 46.9% among those reporting smoking parents and 8.6% among those reporting non-smoking parents, (χ2=547.9; p<.001). Considering the total sample, SHS exposure was more than five times likely to be higher among children with smoking parents. This trend was observed in all regions, ranging from 2.32 in Azores to 7.12 in Madeira (Table 1).

The results show that for this sample of Portuguese 4th school grade school children, more than half has at least one smoker parent and, therefore, being at high risk of SHS and third hand smoke (THS) exposure in the family car. This is caused by their parents’ smoking or by their parents’ permissiveness to others’ smoking in the family car.

The results also suggest than over one quarter of this group of Portuguese children was regularly exposed to SHS in the family car. Almost half of the subgroup with at least one smoker parent was exposed. Almost ten per cent of the children with both non-smokers parents was exposed. SHS exposure was more than five times more frequent in children with at least one parent who smokes when compared with children with no smoking parents. No statistically significant differences were observed amongst the various regions of Portugal included in this study. These findings suggest that Portuguese families are not aware of children vulnerability to SHS and are rather permissive to indoor car smoking.

These results are in line with those reported by other studies, confirming that children's exposure to SHS in the family car is high and strongly related with parental smoking behaviour.11

Nevertheless, the current study has some limitations. Firstly, the study used a narrow age range children sample (9-11 years old). This was justified by the need of a compromise between the participants’ ages being as low as possible and their capacity to answer accurately to a self-administrated questionnaire. Another limitation is that data is based on perceived self-reported SHS exposure, without parental and/or biological validation. Nevertheless, research indicates that children’ self-report SHS exposure is accurate.15 Thirdly, the sample representativeness at regional and at national level was limited by the sampling method and the small sample size in some regions. All these limitations are explained by budget constraints. Finally, there is no information on participants that do not have or do not use a family car. It is expectable that this information will potentially increase SHS exposure prevalence rates. Despite these limitations, we believe that this is the first study that evaluates Portuguese children's SHS exposure in the family car showing that this prevalence exposure is very high.

These results should be considered unacceptable regarding SHS hazards for children. This risk is totally avoidable since parents and public opinion are aware of its severity.1–6 The findings support the need for implementing comprehensive tobacco control policies to prevent children's SHS exposure, such as public health campaigns raising awareness on SHS health hazards and promoting smoke-free environments, as well as parental smoking cessation support. Banning smoking in cars, particularly when carrying children, has been considered in several countries. This tobacco control measure should be implemented to prevent children's exposure to SHS.

Ma José López

All authors significantly contributed to this research project. JP, PDV, JCM, SBR, CS, HA and EB conceived the questionnaire and the study design. PDV, SBR, ACA, MR and JP gathered the data. Data analyses were conducted by JCM, and, to a lesser extent, by PDV. EB and JP supervised data analysis. All authors analysed and interpreted the data. PDV drafted the manuscript with the input of all authors. All authors have contributed to and approved the final manuscript.

This work was supported by the Portuguese Foundation for Science and Technology (FCT - Fundação para a Ciência e Tecnologia), grant number PTDC/CPE-CED/098281/2008.

The sponsor was not involved neither on the study design, data gathering, analysis and interpretation of the findings; nor on the drafting and submission of the manuscript.

None declared.