## (3) Nitrogen Oxides

Nitrogen oxides (NOx) are mainly generated by combustion from stationary sources, such as factories, and mobile sources, such as motor vehicles. High concentrations of NOx can damage the respiratory system.

The average annual values for nitrogen dioxide concentrations have remained steady over a long time. Its concentration achieved a 99.0% compliance with the environmental quality standard for nitrogen dioxide in ambient air pollution monitoring stations in FY 2001. Since compliance with the environmental quality standard remained low in large cities, which were specified as target areas for countermeasures by the Law Concerning Special Measures for Total Emission Reduction of Nitrogen Oxides from Automobiles in Specified Areas (Automobile NOx Law), in 2001, the Law was revised and strengthened to become the Law Concerning Special Measures for Total Emission Reduction of Nitrogen Oxides and Particulate Matter from Automobiles in Specified Areas (Automobile NOx/PM Law).

## (4) Suspended Particulate Matter (SPM)

Suspended particulate matter in the air with a diameter of $ 10\mu m $ or less is categorized as primary particles or secondary particles. Primary particles include soot and dust from factories, diesel exhaust particles (DEP) generated from diesel vehicles, and soil particles dispersed in the air. Secondary particles are those formed by chemical reaction within the atmosphere from gaseous substances, such as nitrogen oxides (NOx). Because SPM is minute, it remains in the air for extended periods of time. An accumulation of SPM in high concentrations in the lungs or the trachea can have damaging effects on the respiratory system.

The annual average for SPM concentration in recent years has shown signs of a shift from a steady state to a steady decline. However, the compliance rates to EQS have been in decline since FY 2000. In FY 2001, under the Automobile NOx/PM Law, particulate matter was added in the list of subjects to be controlled. Furthermore, studies have been conducted on fine particulate matter with a diameter of $ 2.5 \mu m $ or less and diesel exhaust particles, for their impact on human health have recently raised concerns.

Changes in the Total Number of Days Warnings Were Issued and Number of Sufferers Reported (1993–2002)

Year	1993	1994	1995	1996	1997	1998	1999	2000	2001	2002
Changes in the total number of days warnings were issued (days)	71	175	139	99	95	135	100	259	193	184
Number of sufferers reported (persons)	93	564	192	64	315	1,270	402	1,479	343	1,347

Source: Compiled from Information related to Air Pollution by Photochemical Oxidants in 2002, Ministry of the Environment

Changes in the Annual Average of Nitrogen Dioxide Concentrations (FY 1970–2001)

Source: Compiled from FY 2001 Report on the State of Air Pollution, Ministry of the Environment

Changes in the State of Compliance with the Environmental Quality Standard (EQS) for Nitrogen Dioxide in Specified Areas (RAPMSs) (FY 1992–2001)

2. Evaluation of air pollution by the EQS for nitrogen dioxide is carried out with the annual 98-percentile of daily averages of nitrogen dioxide taken at each monitoring station.

Source: Compiled from FY 2001 Report on the State of Air Pollution, Ministry of the Environment

Changes in the Annual Averages of the Concentration of Suspended Particulate Matter (FY 1974–2001)

Source: Compiled from FY 2001 Report on the State of Air Pollution, Ministry of the Environment

Changes in the State of Compliance with the Environmental Quality Standard (EQS) for Suspended Particulate Matter (FY 1997–2001)

Source: Compiled from FY 2001 Report on the State of Air Pollution, Ministry of the Environment