For example, PPM means "elements for each million". If you find out that the focus of NO2text NO _2NO2​ is equal to one ppm, it ensures that if you're taking a "sample" of air and divide it into one million elements, one of them will consist of NO2text NO _2NO2​. https://laneocmvd.bloggip.com/36800978/a-review-of-online-ppm-calculator