By Kevin Beck – Updated August 30, 2022

What Are ppm and ppb?

Parts per million (ppm) and parts per billion (ppb) quantify trace concentrations of substances in air, water, soil, and food. They express a ratio of one part of a contaminant to a million or a billion parts of the total mixture.

Units Matter – How “Parts” Are Defined

Because the meaning of “part” depends on the physical state and the medium, it is essential to know what is being measured.

• Gases: ppm and ppb are expressed as volume‑to‑volume ratios. For example, 1 ppm of a gas in air is 1 µL of the gas per liter of air, while 1 ppb is 1 nL per liter.
• Liquids: ppm is typically expressed as milligrams per liter (mg/L). Since 1 mg of water occupies 1 µL, this is effectively a volume‑to‑volume comparison.
• Solids: ppm is a mass‑to‑mass ratio, such as µg/g or mg/kg.

Converting Between ppm and ppb

Because 1 billion = 1,000 × 1 million, the relationship is straightforward:

• ppm = ppb ÷ 1,000
• ppb = ppm × 1,000

For instance, 10 ppb equals 0.01 ppm.

Real‑World Applications: MCLs and Drinking‑Water Standards

Maximum Contaminant Levels (MCLs) set by the U.S. EPA define the highest concentrations deemed safe for public consumption. Common examples include:

• Arsenic: MCL = 0.010 ppm (10 ppb or 10 µg/L)
• Beryllium: MCL = 0.04 ppm (40 ppb)

These thresholds illustrate how trace amounts—often measured in parts per billion—can have significant health implications.

In practical terms, if you were to think of ppm as a ratio, a concentration of 1 ppm in a large population would translate to roughly eight individuals across all five boroughs of New York City.