What is parts per million (ppm)?

Short Answer

Parts per million (ppm) is a unit used to express very low concentrations of a substance in a mixture or solution. It means 1 part of a substance present in 1,000,000 parts of the total mixture. In simple words, ppm shows how many units of a substance are found in one million units of another substance.

PPM is commonly used to measure pollutants in air, contaminants in water, chemical impurities, and trace elements in food and soil. It helps scientists describe extremely small quantities accurately and clearly.

Detailed Explanation

Parts Per Million (ppm)

Parts per million (ppm) is a concentration unit used to express extremely small amounts of a substance in a mixture. It is especially useful when the amount of solute is so tiny that normal units like percent (%) or grams per litre do not clearly represent it. PPM is widely used in environmental science, chemistry, medicine, and industry because many substances appear in minute quantities that still have major effects, such as toxins, pollutants, or minerals.

PPM literally means “out of one million,” so it tells us how much of one substance exists in a total of one million parts of another substance.

Meaning of ppm

PPM stands for parts per million. It represents:

1 ppm = 1 part of substance in 1,000,000 parts of mixture

This can be expressed in many equivalent ways, such as:

  • 1 mg of solute in 1 litre of water
  • 1 g of substance in 1,000,000 g of solution
  • 1 molecule in 1,000,000 molecules
  • 1 unit mass in 1,000,000 units mass

The exact meaning depends on the context—air, water, soil, or food.

How ppm is written

PPM has no unit, because it is a ratio. It can be written as:

  • ppm
  • parts per million
  • mg/L (for dilute aqueous solutions)
  • μg/g in some cases

Although ppm has no physical unit, it is understood as a measure of tiny concentration.

Why ppm is used

In many scientific and environmental studies, substances are present in extremely small amounts but still have a strong impact. Examples include:

  • Pollutants in air (e.g., CO₂, SO₂, NO₂)
  • Contaminants in drinking water (e.g., fluoride, lead, arsenic)
  • Mineral content in soil
  • Toxic substances in food
  • Chemical impurities in industrial products

These quantities are too small to be measured in percentages. Using ppm allows scientists to express them clearly and accurately.

How ppm is calculated

To calculate ppm, we use:

ppm = (mass of solute ÷ mass of solution) × 1,000,000

OR, for water solutions:

1 ppm = 1 mg solute per litre of water

This works because the density of water is close to 1 g/mL.

Example 1: PPM in water

If 2 mg of salt is dissolved in 1 L of water:

ppm = 2 mg/L = 2 ppm

Example 2: PPM in air

If a pollutant has a concentration of 0.3 molecules per 1 million molecules of air:

ppb = 0.3 ppm

Applications of ppm

PPM is widely used in many fields:

  1. Environmental science

Used to measure:

  • Pollution levels in air
  • Chemical contamination in water
  • Soil nutrient levels
  • Toxic metals like lead or mercury

Even small ppm values can be dangerous.

  1. Chemistry

Used to express:

  • Trace impurities in chemicals
  • Concentration of dissolved gases
  • Small quantities in laboratory solutions
  1. Medicine and health

Used to express:

  • Drug concentration in blood
  • Contaminants in foods
  • Safe limits of chemicals
  1. Agriculture

Used in fertilizers and soil analysis to express mineral content.

  1. Industry

Used to maintain purity levels in manufacturing (e.g., semiconductor industry).

PPM compared with other units

PPM is used for very small concentrations. It fits into a scale of concentration units:

  • Percent (%) = parts per 100
  • Parts per thousand (ppt) = parts per 1,000
  • Parts per million (ppm) = parts per 1,000,000
  • Parts per billion (ppb) = parts per 1,000,000,000

Thus, ppm lies between ppt and ppb in terms of concentration size.

Examples to understand ppm easily

Example 1: Sugar in water

If 1 teaspoon of sugar (about 4 grams) is dissolved in a small pond containing 4,000,000 grams of water, the concentration is:

ppm = (4 ÷ 4,000,000) × 1,000,000 = 1 ppm

Example 2: Air pollution

If carbon monoxide is present at 9 ppm in air, it means:

For every 1,000,000 molecules of air → 9 molecules are CO.

Example 3: Water quality

If fluoride level is 1 ppm in drinking water, it means:

1 mg fluoride in 1 litre of water.

Conclusion

Parts per million (ppm) is a concentration unit used to describe extremely small amounts of a substance in a mixture. It tells how many parts of a solute are present in one million parts of the total mixture. PPM is essential in fields like environmental science, chemistry, health, and industry because many significant substances exist in very low concentrations. Its simplicity and accuracy make it a widely used and reliable measurement unit.