In early October of 1992 during which the weather was changing from rainy season to winter season, the first sulfur dioxide episode caused by the Mae Moh power plant was reported. The maximum hourly ground level ambient SO2 concentration of as high as 3,418 g/m3 was observed (hourly average standard of SO2 is 780 g/m3). Back then, there were only 11 units in operation with a total installed generating capacity of 2,025 MW. The episode occurred after Unit 10 and Unit 11 with installed generating capacity of 300 MW each were brought into operation in early 1992 resulting in additional SO2 emission of 160,000 ton/year.