https://blog.gongquiz.com/2021/01/19/so2-20ppm%ec%9d%80-%ec%95%bd-%eb%aa%87-g-m3-%ec%9d%b8%ea%b0%80-%eb%8b%a8-so2-%ec%9d%98-%eb%b6%84%ec%9e%90%eb%9f%89%ec%9d%80-64%ec%9d%b4%ea%b3%a0-%ec%82%b0%ec%97%85%ec%95%88%ec%a0%84/