Which notation does the AVERAGE person understand better, ppm or percent?

I would suppose that percentages are more common in their everyday use to more Americans. That being said it all depends what you do for a living, where some knowledge of this in Chemical use is common when working in places which refer to a MSDA book. I know one of the Chemical companies (ECO-LAB) sent one of their drivers out to deliver a very caustic soap to one of the plants I was assigned to in a special Security detail, and he released the soap by accident into the open lot and received 3rd degree chemical burns on 15% of his body, and damn near died. Had he had a better understanding of PPM and that it had to be mixed with water a VERY slow rate, he might have avoided this altogether.