For example, PPM usually means "pieces for every million". If you discover out that the focus of NO2textual content NO _2NO2​ is equivalent to one ppm, it ensures that if you take a "sample" of air and divide it into 1,000,000 areas, one of them will encompass NO2text NO _2NO2​. https://freeppmcalculator15690.blogs100.com/37268410/ppm-calculator-secrets