https://blog.gongquiz.com/2022/06/07/%eb%b0%94%ea%b2%8c%ed%8a%b8-%eb%b0%b0%ed%95%a9%eb%a5%a0%ec%97%90%ec%84%9c-%eb%b9%84%ed%83%80%eb%af%bc-c%eb%a5%bc-30ppm-%ec%82%ac%ec%9a%a9%ed%95%98%eb%a0%a4%ea%b3%a0-%ed%95%a0-%eb%95%8c-%ec%9d%b4/