1. Introduction
In Japan, increasing automobile traffic caused considerable
increases in air pollutant levels, including nitrogen oxides (NOx)
and particulate matter (PM). The potential effect of these
concentrations on the health of children who live in urban areas
was a matter of concern. Associations between exposure to air
pollution and respiratory disorders have been observed in
population-based studies in traffic congested areas (Nitta et al.,
1993; Shima and Adachi, 1996; Shima et al., 2003). In 1992, “The
Law Concerning Special Measures for Total Emission Reduction of
Nitrogen Oxides (Automobile NOx law),” was enacted to reduce the
concentration of NOx derived from automobiles in large cities. In
June 2001, the law was amended to “The Automobile NOx/PM law,”
to also include PM. On the basis of these laws, measures such as
“vehicle type restrictions”, banning the use of vehicles not conforming
with the criteria of NOx and PM emissions, were enforced
in designated areas (PM-law-enforced areas), leading to improvements
in air quality over the past decade.