This paper examines the knowledge and understanding of Hong Kong secondary school students about the composition of unpolluted and polluted air, and the nature and effects of air pollutants. A number of misconceptions are highlighted, including the common belief amongst younger students that oxygen was more common in unpolluted air than nitrogen. Along with their cognitive understanding, students were questioned about their views concerning what they and others could and should do to reduce air pollution. In this, they seemed agreed, amongst other things, that financial penalties were less acceptable than legal compulsion - especially on a personal level. It is proposed that learning about the constituents of unpolluted air and air pollutants might be enhanced by linking the individual gases involved to their biological roles and pathological effects, respectively. Similarly, it is suggested that students should be encouraged more to reflect, in terms of environmental cost, on their role as consumers of goods and services. (Contains 7 figures.)