Have vehicle registration restrictions improved urban air quality in Japan?

About 2.6 million noncompliant vehicles were removed from designated metropolitan areas in Japan after the introduction of vehicle registration restrictions under the 1992 Automobile NOx Control Law. Based on a difference-in-differences framework and using a monitor-level panel dataset for the period January 1981-December 2015, we find that the intervention led to a 3%-6% reduction in the monthly mean ambient concentration of nitrogen dioxide (NO2) in the treated areas. Back-of-the-envelope calculations identify benefits equal to about US$104 million as a result of reduced mortality from asthma. (JEL Q53, Q58).