One example is, PPM means "components for every million". If you find out which the concentration of NO2textual content NO _2NO2​ is equivalent to 1 ppm, it ensures that if you're taking a "sample" of air and divide it into 1,000,000 elements, one of them will encompass NO2text NO _2NO2​. https://franciscoehgfz.blogoxo.com/36948844/how-much-you-need-to-expect-you-ll-pay-for-a-good-free-ppm-calculator