The main problems of the global environment presently include destruction of the ozone layer, acid rain, global warming, decreases in tropical rain forests, and desertification. Industrial development in line with economic expansion and population growth in the latter half of the 20th century has been the fundamental cause of these problems, although in many ways the cause-and-effect relationships are still not completely clear. The main global environmental issues related to the steel industry are acid rain and global warming. Acid rain is attributed to emissions of sulfur oxides (SOx) and nitrogen oxides (NOx), and it has therefore become important to limit the emissions of these oxides. Of the sulfur oxides, sulfur dioxide (SO2) is produced by the combustion of petroleum or coal which contains sulfur, and can cause both acid rain and respiratory ailments when its concentration is high. In 1973, the environmental standard for SO2 in Japan was set at below 0.04 ppm as a daily average value. In steel works, sulfur oxides originate mostly from the sintering machines, coke ovens, and reheating furnaces. Various countermeasures have been taken to remove sulfur oxide, including desulfurization of the exhaust gas from the sintering machines and coke oven gases, and the use of low-sulfur coals and fuels. Consequently, the sulfur oxide emissions from steel works have decreased greatly, as can be seen from the left side of the figure. In the whole of Japan, the concentration of sulfur dioxide (SO2) has registered a flat yearly average of 0.01 ppm since 1986. Nitrogen oxides are produced when nitrogen in the air and in fuel is oxidized at high temperatures, and is generated mainly by burning fossil fuel. Of the nitrogen oxides, nitrogen dioxide (NO2) in high concentrations has a harmful effect on the respiratory organs and is one of the harmful substances in acid rain and photochemical air pollution. In 1978, the environmental standard for NO2 in Japan was set at below 0.06 ppm as a daily average value. The sources of nitrogen oxides in steel works are similar to those of sulfur oxides, being the sintering machines, coke ovens, and reheating furnaces. Various measures have been taken to eliminate nitrogen oxides, including (i) the development of burners which reduce the generation of nitrogen oxides, (ii) improvement of combustion conditions, and (iii) the development and installation of de-NOx equipment which removes nitrogen oxides from the exhaust gas. Consequently, as can be seen on the right side of the figure, the emission of nitrogen oxides from steel works is continuing to decrease. In the whole of Japan, the yearly average concentration of nitrogen dioxide (NO2) is around 0.03 ppm as measured at the generic environment monitoring stations and around 0.04 ppm at measuring stations for monitoring automobile exhaust gas. The SO2 and NO2 emissions form the steel works have not been registered by The Japan Iron and steel federation since 1994. In recent years the steel companies are disclosing their indivdual emissions in their publication such as "Annual Report on Environmental Protection". According to those the emissions in 1994 - 2000 remains almost unchanged as in 1990 - 1993.