45ppm

45ppm denotes a concentration of 45 parts per million, meaning 45 units of a substance per one million units of the surrounding medium. In substance terms, 1 ppm is equal to 1 × 10^-6 of the mixture, so 45 ppm corresponds to 4.5 × 10^-5, or about 0.0045 percent.

ppm is used in two common contexts. For gases and vapors, ppm is usually a volume/volume measure (v/v), describing how many molecules of the substance are present per million molecules of air or other gas. For liquids, including water, ppm is often treated as a mass/volume ratio, so 45 ppm is roughly 45 milligrams of solute per liter of solution, assuming the density of the solvent is close to that of water.

In practice, 45 ppm is a relatively dilute concentration and can be significant depending on the substance and the medium. It is commonly used in environmental monitoring, industrial hygiene, water treatment, and manufacturing to express contaminant levels, residuals, or additives. Measurement methods include gas chromatography, infrared spectroscopy, mass spectrometry, and various field sensors or colorimetric tubes for quick readings. It is important to note that conversions between ppm, mg/L, and other units depend on context and, for gases, on temperature and pressure due to the gas-phase nature of ppm.

Because ppm is a dimensionless ratio that depends on the matrix and conditions, specifying the substance and measurement basis is essential when interpreting a value like 45 ppm.