Air quality deteriorated across the nation last year in terms of particulate matter, while nitrogen dioxide levels were largely unchanged from 1999, according to Environment Ministry data released Tuesday.
The levels of photochemical smog recorded were also dismal, the data show.
Levels of suspended particulate matter -- sooty black dust like that expelled from diesel engines and factories -- met government-designated environment standards at 84.4 percent of the nation's monitoring stations, a 6 percent drop from the previous year.
Levels met government standards at 66.1 percent of the roadside checkpoints, a 10 percent drop from 1999.
Government standards for suspended particulate matter were met at 45.7 percent of the roadside monitoring sites in Tokyo, compared with 80.7 percent of the capital's general monitoring stations.
Suspended particulate matter can accumulate in the lungs and has been implicated as a carcinogen that causes respiratory disorders.
Levels of nitrogen dioxide pollution were almost on par with the previous year's readings, with environment standards met at 99.2 percent of general and 80 percent of roadside monitors around the nation.
Nitrogen oxides react with other compounds to create secondary pollutants, such as smog.
The situation generally remained dismal in the Kanto and Kansai regions. These areas have historically been plagued by poor air quality and are subject to special legislation enacted in 1992 in an effort to bring their nitrogen oxide pollution levels under control. Environmental standards were met at just 45.7 percent of roadside monitors in the Kanto region and at 67.4 percent of monitors in the Osaka-Hyogo area.
In addition, 259 smog warnings were issued. This was up considerably from the 100 logged in 1999 but still a far cry from the more than 1,400 issued during the early 1970s.
Although air quality slipped in some areas in 2000, abnormally strong winter winds helped quality overall, keeping it from falling to dismal pre-1999 levels.