The fraction of the snow by weight that is plastic (ppm, or whatever), especially with a comparison to the amounts historically and to levels that are known or

The fraction of the snow by weight that is plastic (ppm, or whatever), especially with a comparison to the amounts historically and to levels that are known or reasonably suspected of having bad consequences. This, for instance, is how particulate matter is quantified for assessing air quality in urban areas (one of the more harmful and less appreciated environmental damages).