This study builds on our prior work as participating center in the ISAAC phases one and three by confirming the previous findings in another age group (10 year old children) using ISAAC phase II protocols. Consistent with our previous finding in ISAAC phase one (1995) and phase three (2001) surveys conducted on 6–7 and 13–14 year old children, the prevalence rates of asthma symptoms and diagnosis in these 10 year old children were significantly higher in South Sharqiyah region [4, 5]. More importantly, this study explores the role of the Arabian incense burning "bakhour" which is commonly used in Oman and other Gulf countries, as a potential indoor environmental risk factor for asthma symptoms. In this study, we evaluated the relationship between the frequency of bakhour use and asthma symptoms and its interaction with other potentially relevant factors.
After controlling for potential confounding factors in multivariate analysis (Table 2), we found a significant dose-response association between the frequency of bakhour use and its effect on these children breathing especially asthmatics. Parents who reported regular use (more than twice per week) of bakhour at home were three times more likely to report that their child breathing was affected by bakhour compared to parents who rarely used bakhour, and this was more 2.5 times higher in children with current asthma. The effect of bakhour was also significantly higher in girls and in children from Sharqiyah. In addition, bakhour was reported (Figure 1) as a cause of worsening wheeze in nearly 40% of children with current asthma, making it the fourth most common precipitating factor of wheeze (after dust, weather and viral upper respiratory tract infections). However, we found no significant association between bakhour use and the prevalence of current asthma (Table 3). There was also no significant difference between bakhour users and non users in the prevalence of any of the surveyed respiratory symptoms (Table 4). These findings suggest that bakhour is a common trigger factor of respiratory symptoms among Omani school children especially asthmatics and females. However, it is not be associated with higher prevalence of current asthma.
As there is no data on the contribution of incense burning to indoor air pollution in Oman, it is reasonable to assume that a higher frequency of incense burning produces a higher concentration of incense smoke pollutants. In general, most people spend approximately 90% of their time indoors . Due to the very hot weather in Oman, it is expected the time spent indoors to be even higher especially for women and children. This may explain the observed greater susceptibility of girls to the effect of bakhour. The cause of the higher effect of bakhour on children from Sharqiyah region is not clear. Possible explanations include greater susceptibility , presence of other confounding factors not controlled for in the present analysis  and/or the difference in the composition of bakhour which is known to vary between regions within Oman. Further studies are necessary to determine the factors which influence the susceptibility to bakhour.
In the three studies that looked at the relationship between bakhour and asthma in the Arabian Gulf region, exposure to different types of Arabian incense was also reported as a significant precipitating factor of asthma symptoms in children [10, 13, 14]. However, none of these studies evaluated the relationship between bakhour and the prevalence of asthma. In Qatar, a case control study, found history of exposure to bakhour in 80% of asthmatic children compared to 66% of normal controls,  and in another study, bakhour was reported as precipitating factor of asthma symptoms in 19% of children with asthma . Exposure to bakhour was also reported as trigger for asthma in 26% of 135 consecutive asthmatics attending a pediatric asthma clinic in a tertiary referral centre in Kuwait .
Several other studies from Asian populations, where different types of incense are burnt for religious purposes, reported significant associations between exposure to incense smoke and respiratory symptoms [12, 19, 20]. In contrast, one large survey of school children, showed incense burning at home was negatively associated with the prevalence of asthma . The authors suggested that incense use might have been decreased in families with asthmatic children. In another study of 346 primary school children, there was no association between exposure to incense burning and respiratory symptoms . Likewise, a case control study of 50 non smoking Saudi women with chronic obstructive pulmonary disease, reported no significant difference in the use of bakhour between cases and control .
Similarly our study did not find any significant association between bakhour use and the prevalence of current asthma. In fact there was an inclination towards a negative association which was significant in simple regression analysis, but disappeared after adjusting for other factors in the multivariate analysis. There was also no significant difference between bakhour users and non users in the prevalence of core asthma symptoms or any of the surveyed respiratory symptoms. Since our study is a cross-sectional, it is possible that families with asthmatic children avoid the use of bakhour after learning that bakhour might worsen asthma symptoms. Such avoidance behavior may influence the relationship between the reported prevalence of asthma and exposure to bakhour leading to underestimation or even reversal of any association. In our study, this was supported by the finding that parent who reported that their children were affected by bakhour exposure were less likely to report its use in the presence of child or in the child room.
The mechanisms by which bakhour provokes respiratory symptoms are unknown. People who are exposed to incense smoke inhale a complex mixture that contains particulate matter, gas products and many organic compounds, making it difficult to single out the health effects contributed by a certain component in the smoke. Furthermore, there appears to no standardized constituents of different types of bakhour even within the same country . Our study has its limitations. The use of symptoms questionnaire to distinguish asthmatics and non asthmatics has potential problems arising from subjective symptom recognition and recall. In addition, as they were new to the Arabic version of ISAAC phase II, bakhour questions needed to have more scrutinized measures of validity and reliability. In ISAAC studies, current asthma is defined by wheeze in the past 12 months (current wheeze), regardless of the response to "ever had asthma?" question . In addition, we included children with positive response to "ever had asthma?" together with positive responses to exercise wheeze and/or night cough not related to cold in the past 12 months in current asthma category. Like any cross-sectional study, this report lacks clear temporal association between exposure and its effect. We tried to overcome this problem with the question "does exposure to bakhour affect your child breathing?" In addition, the finding of bakhour exposure as one of the most frequent triggers of wheeze in our children suggests a temporal association. Finally, because of our primary focus, only few of asthma risk factors were evaluated.