One example is, PPM usually means "pieces per million". If you find out that the focus of NO2textual content NO _2NO2​ is equivalent to one ppm, it means that if you're taking a "sample" of air and divide it into one million pieces, one of them will include NO2textual content https://freeppmcalculator74936.snack-blog.com/36757966/ppm-calculator-no-further-a-mystery