学位論文内容の要旨

博士の専攻分野の名称

博士（医 学）

氏 名

叢 石学 位 論 文 題 名The association between mechanical ventilation, flue use in heaters and asthma symptoms(暖房の排気、および室内換気と児童喘息との関連)
Abstract
Background and purpose: The prevalence and burden of asthma and allergic symptoms in children haveincreased in recent decades in the world. In Japan, the prevalence of asthma in children has been steadily
increasing. Epidemiological evidence suggests that environmental factors are major risk factors for asthma
in children. Children spend more time at home than adults, thus, indoor environmental health is more
important to children than adults.
According to previous studies, the use of fume emitting heaters such as gas, coal, and wood heaters forhome heating were associated with increased asthma symptoms in children. And the use of electric heaters
might be not associated with asthma symptoms in children, compared with fume emitting heaters. However,in past research, detailed classification of heating fuel systems was lacking, and one type of fuel heater was
not set as a reference when comparing other fuel heaters.
Compared with unflued heaters, the use of flued heaters is thought to decrease asthma symptoms inchildren. This is because flue use might decrease the effect of heating pollutants on asthma. Furthermore,
improved mechanical ventilation might decrease the levels of NO2 produced by gas stoves and be a usefulway to decrease indoor air pollution. However, it is unclear whether the combination of flue use in heaters
and mechanical ventilation can reduce asthma symptoms in children.
No study exists that investigates the association between heater use and asthma in children in Japan.
According to previous data, about 90% of households use kerosene or gas heaters for home heat in Sapporo.
In particular, Sapporo winters last for at least 4 months and houses in Sapporo city are typically airtight.
However, the association between these heating systems and asthma remains unknown.
This study aimed to determine whether an association exists between the combination of ventilation andflue use in heaters and asthma symptoms among 1st–6th grade (6-12 year old) Japanese school children in
Sapporo who were exposed to fume emitting fuel heaters during the winter in poorly aerated houses.
Material and Methods: This cross-sectional study was conducted in public elementary schools in Sapporo
city, Japan. A total of 6393 children in grades 1 to 6 (6-12 years old) received a questionnaire from theirteachers, with instructions for their parents to complete the questionnaires between November 29th 2008
and January 30th 2009. Classroom teachers collected the questionnaires from the children. Parents
completed questionnaires regarding their house environment and their children’s asthma symptoms. Basicdemographic data was also collected. I used the ISAAC questionnaire to assess asthma symptoms inchildren. Five combinations of heaters and ventilation were assessed: electric heaters, flued heaters withventilation, flued heaters without ventilation, unflued heaters with ventilation, and unflued heaters withoutventilation. The χ2 test was used to analyze the association between asthma, participant characteristics andhouse environment. Logistic regression analyses were used to calculate odds ratios (ORs) and 95%
confidence intervals (95% CIs) for the effects of house heating systems and ventilation on asthma. In themultivariate analyses, I controlled for relevant confounding factors. These were: sex, school grade, parental
history of allergies, schools, residence <200 meters from a main road, presence of wall-to-wall carpeting in
the house, presence of a smoker in the house, and presence of indoor dampness. Indoor dampness is an
important risk for asthma in children, and is associated with the use of unflued gas heaters and low
ventilation in the home. It is possible that the association between unflued heaters or the absence ofventilation and asthma symptoms is mediated by dampness. Therefore, a stratified analysis according toindoor dampness was also conducted.
Results: In total, the parents of 4445 (69.5%) children replied to the questionnaires. After excluding
incomplete responses, data on 3874 children (60.6%) were analyzed. The prevalence of asthma was 12.8%,
and in boys and girl was 15.6% and 10.1%, respectively. Asthma tended to occur more frequently in boys
and in children with a parental history of allergies Distribution of the prevalence of asthma was also
statistically different for each school grade or each school. Among 3874 households, 89.9% households
used fume emitting heaters (kerosene or gas), only 16.7% households used non-fume emitting electricheaters, and the remaining 3.4% used unknown fuels. The use of kerosene heaters (OR = 1.58; 95% CI,1.03-2.49) and unflued heaters (OR = 1.29; 95% CI, 1.02-1.63) was significantly associated with asthma,compared with electric heaters and the use of flued heaters, after adjusted for confounders. After adjustingfor potential confounding factors, the use of flued heaters without ventilation (OR = 1.62; 95% CI,1.03-2.64) was significantly associated with asthma. The use of unflued heaters was also significantlyassociated with asthma in the presence (OR = 1.77; 95% CI, 1.09-2.95) or absence of ventilation (OR =2.23; 95% CI, 1.31-3.85) compared with the use of electric heaters. Dampness was associated with the useof kerosene heaters and low ventilation. However, regardless of dampness, unflued heaters weresignificantly associated with asthma in both the presence and absence of ventilation.
Discussion and conclusion: Unflued heaters were significantly associated with asthma in both thepresence and absence of ventilation, regardless of dampness. Thus, while a pathway through dampness
might exist, it is possible that air pollution from fume-emitting heaters also can explain the association
between the use of unflued heaters with or without ventilation and asthma symptoms. It might be possiblethat NO2, SO2, PM, etc. from the combustion of fuel heaters leads to asthma symptoms in children. In thecases of not using flue heaters or the absence of ventilation, asthma-related NO2, SO2, PM could not beemitted to the outside. Thus, Compared with the use of electric heaters, the use of unflued fume-emitting
kerosene or gas heaters was associated with asthma, and this association was stronger among participants
who lived in houses using unflued kerosene or gas heaters in the absence of ventilation. In northern cities
like Sapporo, particular attention needs to be paid to the use of fuel heaters, especially those that have noflue or ventilation, since there might be an association with asthma in children. I plan to assess the durationand frequency of heater use in the home and the duration of time children spend at home in further studies.