Snoring occurs when there is an obstruction to the free flow of air through the airways at the back of the mouth and nose. The prevalence of habitual snoring in children has been reported to vary between 3.2 and 11%. Infrequent snoring is present in 17–27% of all children 123. A study of young Australian children (2–5 years old) found the prevalence of snoring to be 10.5% 4.

Approximately one third of children who snore regularly have obstructive sleep apnea syndrome (OSAS) 5. A few studies have claimed that snoring in children can affect neurocognitive function, behaviour and blood pressure to some extent even in the absence of apnea 67. Thus, concerns about causes of snoring and prevention strategies for children have arisen among both professional medical workers and parents.

Numerous risk factors for snoring and OSAS have been reported including enlarged adenoids and/or tonsils, obesity, allergies or other causes of nasal obstruction, and exposure to environmental tobacco smoke (ETS) 8910. However, there has been very little research on exposure to environmental irritants, other than ETS, as contributing factors for snoring in children. The home is the predominant environment for exposure to many environmental irritants such as allergens and air pollutants. We hypothesized that high levels of exposure to common indoor irritants including volatile organic compounds, formaldehyde and nitrogen dioxide, could increase the risk of snoring. The aim of this study, therefore, was to investigate domestic environmental factors associated with snoring in children.

Of the 996 participants, 985 children (98.9%) had intact records for snoring. There were 248 children (24.9%; 95% CI: 21.2%–28.6%) reported infrequent snoring and 151 children (15.2%; 95% CI: 12.6%–17.8%) suffered from habitual snoring.

Snoring is an important symptom and major risk factor for obstructive sleep apnea 5. Several studies in Italy and Thailand reported that the prevalence of habitual snoring varied from 4.9 to 34.5% in primary school children 51617, while the prevalence of snoring was 10.5% according to a study of Australian children aged 2–5 years 4. The present study found the prevalence of habitual snoring among primary school children in Perth was 15.2%, and 24.9% of the children had infrequent snoring. The total prevalence of snoring was 40.1%. That the participants had high rates of current asthma (18.7%) and allergy (44.0%) (allergic rhinitis or hay fever) may explain the apparently high snoring prevalence taking into account the link between snoring and asthma and allergy.

The prevalence of snoring among older children was significantly lower than that of younger children. No significant difference in snoring prevalence between boys and girls was observed, which appeared to be consistent with the literature 1618.

Strong associations were also found between snoring and respiratory symptoms, asthma and other allergic conditions, as in previous studies 101920.

In relation to the domestic environment, passive smoking was identified as a major risk factor for habitual snoring and consistent with other studies 1021. An interesting finding was the observed inverse relationship between snoring and pet ownership. There is evidence in the literature suggesting that pet ownership in early life can protect against the development of allergic disease 22. Although the protective effect of pet ownership on habitual snoring was significant after controlling for allergic diseases, the mechanism that led to a lower risk of snoring remains to be investigated.

Unlike TVOCs and HCHO, it appears that domestic exposure to NO₂ was significantly associated with snoring. It should be remarked that the low exposure threshold was set below the annual value of 40 μg/m³ recommended by WHO 23, whereas the high exposure cut off is higher than the guideline value. Our results suggested that high exposure to NO₂could increase the risk of snoring by 4.5 times. A previous study reported that children aged 5–12 years had a 20% increased risk of respiratory symptoms and disease for each increase of 28.3 μg/m³ in NO₂concentrations (2-week average), when the weekly average concentrations were in the range 15–128 μg/m³ or possibly higher 23. Another study in Australia confirmed the link between NO₂ exposure from gas appliances and the prevalence of respiratory symptoms 24. Our results also suggested that exposure to NO₂ was related to gas heating during winter.

Although the effect of NO₂ exposure on snoring was significant even after adjustment for asthma, atopy and other confounding factors, caution must be taken when interpreting the NO₂ findings and further investigation is required before they can be generalized to the pediatric population at large. A limitation of this cross-sectional study is that only 9% of the study sample was monitored for environmental testing due to budget and other constraints. Nevertheless, this subgroup of children did not differ significantly from the whole sample or other populations of young children in Perth 2526 with respect to home environment, respiratory symptoms and atopy. Secondly, the causal effects of NO₂ could not be determined because the measurements of exposure and illness were taken at the same time. The assessment of snoring was retrospective in relation to the time of environmental monitoring. Moreover, the significant association between snoring and NO₂ exposure in winter may be attributed to NO₂ emission from gas heaters in conjunction with low ventilation during the winter season.

As for potential mechanisms for this association, there is little in the literature that can directly explain how exposure to NO₂ might result in snoring. Although there is evidence to suggest that exposure to NO₂ is associated with development of allergic disease 27, the observed association between NO₂ exposure and snoring is independent of atopy. Snoring occurs due to upper airway obstruction during sleep. The obstruction commonly occurs at the level of the nasal turbinates as with anterior rhinitis or the nasopharynx due to adenoid hypertrophy. Exposure of airway epithelium in vitro results in the release of inflammatory cytokines and adhesion molecules 28. Therefore, it is possible that exposure to NO₂ increases upper airway inflammation, resulting in mucosal oedema and airway obstruction. Alternatively, upregulation of ICAM1 the primary ligand for rhinovirus 28 could increase the susceptibility to, or severity of upper respiratory tract infection, resulting in upper airway oedema and/or adenoid hypertrophy. Finally, it has been suggested that NO₂ increases lipid membrane fluidity 29 that in turn can alter receptor-ligand interactions. Thus NO₂ exposure might produce changes in cell-cell and cell-pathogen interactions that could result in altered upper airway physiology. Given the high prevalence of snoring in our population and the knowledge that snoring is a significant risk factor for obstructive sleep apnea, the mechanisms that might underpin the association between NO₂ exposure and snoring require further study.

In conclusion, the present study shows that snoring is common among primary school children in Perth, and snoring is associated with other respiratory symptoms. Passive smoking increases the risk of snoring in children but pet ownership may decrease the risk. The level of nitrogen dioxide in domestic environment is positively associated with the prevalence of snoring in children.

GZ, JS, KR, SS Study design, coordination and management

GZ, JS, KR Field measurement and laboratory work

AHL, GZ Data analysis and interpretation of results

GZ, JS, KR, AHL, SS Preparation and revision of the manuscript