As an example, PPM usually means "parts per million". If you find out which the focus of NO2text NO _2NO2​ is equivalent to one ppm, it ensures that if you're taking a "sample" of air and divide it into 1,000,000 sections, one of these will encompass NO2text NO _2NO2​. PPT https://claytonjnmje.blogzet.com/how-much-you-need-to-expect-you-ll-pay-for-a-good-online-ppm-calculator-51468194