As an example, PPM usually means "components per million". If you find out the focus of NO2text NO _2NO2​ is equivalent to one ppm, it means that if you're taking a "sample" of air and divide it into a million parts, one of them will consist of NO2text NO _2NO2​. https://zanesxvsn.blogprodesign.com/57934193/the-basic-principles-of-free-ppm-calculator