Part per quadrillion (ppq) is a unit of measurement that expresses one part of a substance in every one‑quadrillion parts of a total sample, equivalent to a concentration of 10⁻¹⁵. Used primarily in ultra‑trace analysis, ppq quantifies contaminants such as heavy metals, pesticides, or radioactive isotopes in air, water, food, and pharmaceutical products where even infinitesimal levels can impact health and safety. The symbol “ppq” follows the same convention as ppm (parts per million) and ppb (parts per billion), providing a standardized way to report extreme dilution or purity across scientific literature, regulatory guidelines, and industry specifications. By enabling detection of substances at the parts‑per‑quadrillion scale, this metric is crucial for environmental monitoring, quality control, and research that demand the highest sensitivity and accuracy.