Some Examples of Important Small Amounts:He wasn't driving drunk, he just had a trace of blood alcohol; 800 ppm (0.08%) is the limit (PDF) in all 50 U.S. states, and limits are lower in most other countries.Don't worry about your iron deficiency; iron is only 4.4 ppm of your body's atoms.Ireland isn't important; it's only 660 ppm (0.066%) of the world population.That ibuprofen pill can't do you any good; it's only 3 ppm of your body weight (200 mg in a 60 kg person).The Earth is insignificant; it's only 3 ppm of the mass of the solar system.Your children can drink that water; it only contains a trace of arsenic (0.01 ppm is the WHO and U.S. EPA limit).Ozone is only a trace gas: 0.1 ppm is the exposure limit established by the U.S. National Institute for Occupational Safety and Health. The World Health Organization (WHO) recommends an ozone limit (PDF) of 0.051 ppm.A few parts per million of ink can turn a bucket of water blue. The color is caused by the absorption of the yellow/red colors from sunlight, leaving the blue. Twice as much ink causes a much stronger color, even though the total amount is still only a trace relative to water.
