part per trillion
ConcentrationDimensionlessexact by definition
A part per trillion is 10⁻¹⁰ percent, one part in 10¹². It is the reporting level for the most potent trace contaminants: dioxins, PCBs and PFAS in drinking water are regulated at single-digit ppt, which is roughly one drop in twenty Olympic swimming pools.
Watch out: "ppt" is genuinely ambiguous and this catalog reads it as parts per TRILLION, the dominant meaning in environmental chemistry. Some European and older sources use ppt for parts per thousand, a factor of 10⁹ apart. If a number written ppt is near 35, it is salinity and it means thousand.
| 1 ppt (Concentration) | 1.0000000e-10 % |
| 1 ppt (Dimensionless) | 1.0000000e-12 — |
One part per trillion is exactly 1e-12 of the whole, at the edge of what routine analytical chemistry can measure. Per- and polyfluoroalkyl substances in drinking water are regulated here, with US limits for PFOA and PFOS set at 4 ppt. The factor is exact by definition.