Julie Bennett – PhD 2008

There is growing evidence that asthma symptoms can be aggravated, or even triggered, by adverse aspects of the indoor environment. As part of a randomised control trial the link between indoor air in the home environment and respiratory health effects in children with asthma was investigated. The study involved 409 households that used unflued gas heaters or plug-in electric heating and had a child with asthma aged between six and 12. More effective, non-polluting heaters were installed in half the homes. In order to establish if this heating intervention was effective, levels of NO2 were measured and the impact of the intervention and NO2 on children’s health was explored.

Baseline measurements of all outcomes of interest were taken over the 2005 winter (June till September). Passive diffusion tubes were installed in living room sand children’s bedrooms during the intervention winter (2006) to measure NO2 over four, four-week periods in each of the 409 homes. Outdoor NO2 was measured for one four-week period during September. Children recorded lung function measures of FEV1 and PEFR every morning and evening during the 2006 study winter, using an electronic peak flow meter. The presence and severity of lower (coughing and wheezing) and upper (sneezing, blocked nose, sore throat, head-ache, and aches elsewhere) respiratory tract symptoms were recorded by children twice daily using a four-point scale. General health questions, including the number of colds and flu and visits to health services were recorded in a questionnaire, taken at the end of the 2006 winter period.

Ambient outdoor NO2 levels were similar in the intervention and control groups. However indoor NO2 levels in intervention group homes were significantly (p<0.001) lower than those found in control group houses. In the children's bedrooms, the geometric mean level of NO2 for the intervention group was 7.2 micrograms/metercubed (range 2.4 - 34.8 micrograms/metercubed) compared to 10.8 micrograms/metercubed (range 1.8 - 128.5 micrograms/metercubed) in the control group. In the living room, the changes were even more marked with an intervention group geometric mean level of 8.3micrograms/metercubed (range 2.7 -79.6 micrograms/metercubed) and a control group geometric mean level of 15.5micrograms/metercubed(range 2.5 - 193.6 micrograms/metercubed, p<0.001)

Homes that used unflued gas heaters over the intervention winter had more than three-times the level of NO2 in living rooms than homes without unflued gas heaters(geometric mean ratio (GMR) 3.35: 95% CI; 2.83-3.96; p<0.001), while homes using gas stove-tops had significantly elevated living room NO2 levels (GMR 1.42: 95% CI; 1.05-1.93, p=0.02). Homes with heat pumps, flued gas heating or enclosed wood burners, had significantly lower levels of NO2 in living rooms and children's bedrooms.

Children in the intervention group had significantly less parent-reported poor health (odds ratio (OR) 0.44: 95% CI; 0.28-0.70, p<0.001), fewer reports of colds and flu (OR 0.77; 95% CI; 0.65-0.91, p=0.002) than children in the control group. Children in the intervention group also had reduced asthma symptom scores (cough (GMR 0.75: 95% CI; 0.62-0.91, p=0.003)), and wheeze (GMR 0.68: 95% CI; 0.50-0.92, p=0.01). However improvement in lung function was not significant, as measured by percent predicted PEFR (beta 3.67: 95% CI; -1.17 to 8.51, p=0.15) and percent predicted FEV1 (beta 3.10: 95% CI: -1.17 to 8.51, p=0.67).

After adjusting for the effect of the intervention, covariates and the baseline health results, NO2 in the living room was significantly associated with greater hospital admission (OR 2.07: 95% CI; 1.17-3.67, p=0.01), greater daily reports of upper respiratory tract symptoms (GMR 1.03: 95% CI; 1.00-1.05, p=0.04), and greater daily reports of asthma symptom scores (cough (GMR 1.14: 95% CI; 1.12-1.27, p<0.001), wheeze (GMR 1.10: 95% CI; 1.07-1.14, p<0.001), and reliever medication usage(GMR 1.14: 95% CI; 1.11-1.17, p<0.001)). NO2 was also significantly associated with decreased percent predicted morning FEV1 (beta -1.18: 95% CI; -1.77 to -0.59), p<0.001) and decreased percent predicted PFFR (beta -1.13: 95% CI; -1.62 to -0.64, p<0.001).

Spending NZ$3000 to replace unflued gas heaters with more effective heating, would achieve a two-thirds (67%) reduction in NO2 in the living rooms of homes in this study. Furthermore, installing more effective, non-polluting heating, in the homes of children with asthma reduces children’s asthma symptoms and reduces health care utilisation.