In the late 1980s, Health and Environment Authorities in Australia raised concerns about the indoor air quality and the extent that particularly older style unflued gas heaters were contributing to higher than acceptable levels of nitrogen dioxide (NO2). Consequently in 1989 the New South Wales Department of School Education initiated an extensive investigation of nitrogen dioxide in schools throughout New South Wales. As an interim measure the Health Authorities advised that a level of 0.3 ppm NO2 should become the upper limit for classrooms. The Australian Gas Association in turn reduced the indoor emission rate of NO2 for unflued gas heaters from 15 to 5 ng/J and this remains the current limit. The New South Wales government, through the Public Works Department, also re-evaluated alternative methods of heating classrooms, to ensure a safe and healthy environment for students.