As an example, PPM means "elements for every million". If you find out the focus of NO2text NO _2NO2​ is equivalent to 1 ppm, it ensures that if you are taking a "sample" of air and divide it into 1,000,000 sections, one of these will consist of NO2text NO _2NO2​. https://dallascpzhs.mybloglicious.com/56730033/a-secret-weapon-for-ppm-calculator