Whenever we are talking about water quality or the amount of CO2 in the atmosphere, the term “ppm” or “part per million” is used. Now, unless you are an analytical chemist, you probably don’t understand what this means. So here is information that I hope will help.

Parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt), are the most commonly used terms to describe very small amounts of contaminants in our environment.

But what do these terms represent? They are measures of concentration, the amount of one material in a larger amount of another material; for example, the weight of a toxic chemical in a certain weight of food. They are expressed as concentrations rather than total amounts so we can easily compare a variety of different environmental situations. For example, scientists can measure the concentration of a chemical in the Great Lakes by looking at small samples. They do not have to measure the total amount of chemicals or water in all of the lakes.

An example might help illustrate the part per … idea. If you divide a pie equally into 10 pieces, then each piece would be a part per ten; for example, one-tenth of the total pie. If, instead, you cut this pie into a million pieces, then each piece would be very small and would represent a millionth of the total pie or one part per million of the original pie. If you cut each of these million minute pieces into a thousand little pieces, then each of these new pieces would be one part per billion of the original pie. To give you an idea of how little this would be, a pinch of salt in ten tons of potato chips is also one part (salt) per billion parts (chips).

In this example, the pieces of the pie were made up of the same material as the whole. However, if there was a contaminant in the pie at a level of one part per billion, one of these invisible pieces of pie would be made up of the contaminant and the other 999,999,999 pieces would be pure pie. Similarly, one part per billion of an impurity in water represents a tiny fraction of the total amount of water. One part per billion is the equivalent of one drop of impurity in 500 barrels of water.

### Part per hundred

One part per hundred is generally represented by the percent (%) symbol and denotes one part per 100 parts, one part in 10^{2}, and a value of 1 × 10^{–2}.

This is equivalent to one drop of water in 5 milliliters (one spoonful) or about fifteen minutes out of one day.

### Part per thousand

One part per thousand is generally spelled out in full and not as “ppt” (which is usually understood to represent “parts per trillion”). It may also be denoted by the permille (‰) symbol. Note however, that specific disciplines such as the analysis of ocean water salt concentration and educational exercises occasionally use the “ppt” abbreviation. “One part per thousand” denotes one part per 1000 parts, one part in 10^{3}, and a value of 1 × 10^{–3}.

This is equivalent to one drop of water diluted into 50 milliliters (ten spoonful’s), or about one and a half minutes out of one day.

### Part per ten thousand

One part per ten thousand is denoted by the permyriad (‱) symbol. It is used almost exclusively in finance, where it is known as the basis point and is typically used to denote fractional changes in percentages. For instance, a change in an interest rate from 5.15% to 5.35% would be denoted as a change of 20 basis points or 20 ‱. Although rarely used in science (ppm is typically used instead), one permyriad has an unambiguous value of one part per 10,000 parts, one part in 10^{4}, and a value of 1 × 10^{–4}.

This is equivalent to one drop of water diluted into half a liter, or about nine second out of one day.

### Part per million

One part per million (ppm) denotes one part per 1,000,000 parts, one part in 10^{6}, and a value of 1 × 10^{–6}.

It is often used when measuring levels of pollutants in air, water, body fluids, etc. One ppm is 1 part in 1,000,000. The common unit mg/liter is equal to ppm. Four drops of ink in a 55-gallon barrel of water would produce an "ink concentration" of 1 ppm.

A ppm is equivalent to one drop of water diluted into 50 liters (roughly the fuel tank capacity of a compact car), or about thirty seconds out of a year.

A part per million is equal to:

- one penny in $10,000
- one minute in two years
- one dime in a one-mile-high stack of pennies

### Part per billion

One part per billion (ppb) denotes one part per 1,000,000,000 parts, one part in 10^{9}, and a value of 1 × 10^{–9}.

One part per billion is 1 part in 1,000,000,000. One drop of ink in one of the largest tanker trucks used to haul gasoline would represent 1 ppb.

The difference between 1 ppm and 1 ppb is important. A prestigious scientific journal recently reported the concentration of a substance as 0.5-1.5 ppm. The real value was 0.5-1.5 ppb. The difference between $1 and $1000!

A ppb is equivalent to 1 drop of water diluted into 250 chemical drums (50 m^{3}), or about three seconds out of a century.

A part per billion is equal to:

- one penny in $10,000,000
- one pinch of salt in 10 tons of potato chips
- one second in 32 years

### Part per trillion

One part per trillion (ppt) denotes one part per 1,000,000,000,000 parts, one part in 10^{12}, and a value of 1 × 10^{–12}.

A unit of concentration used to measure vanishingly small levels of pollutants or contaminants in, for example, body fluids. One ppt is 1 part in 1,000,000,000,000. One drop of ink distributed through the water contained in a total of 4 of the 3-million-gallon reservoirs pictured would result in a final concentration of 1 ppt.

A ppt is equivalent to 1 drop of water diluted into 20, two-meter-deep Olympic-size swimming pools (50,000 m^{3}), or about three seconds out of every hundred thousand years.

### Part per quadrillion

One part per quadrillion (ppq) denotes one part per 1,000,000,000,000,000 parts, one part in 10^{15}, and a value of 1 × 10^{–15}.

This is equivalent to 1 drop of water diluted into a cube of water measuring approximately 368 meters on a side (fifty million cubic meters, which is a cube about as tall as the roof of the Empire State Building), or two and a half minutes out of the age of the Earth (4.5 billion years). Although relatively uncommon in analytic chemistry, measurements at the ppq level are performed.

A part per quadrillion is equal to:

- one penny in $10,000,000,000,000
- one second in 320,000 centuries

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The interesting thing about ppm is that if it’s an aqueous pollutant, 1 ppm is equivalent to 1 mg per liter of solution, which will weigh about 1 kg or 1 million mg, But for things like CO2 in the atmosphere or any solute in air, ppm is a mole fraction. For example 400 ppm, the present concentration in our atmosphere, corresponds to 400 moles of CO2 per 1 000 000 moles of air. (That’s because a liter of air does not have a mass of a million mg.) Since under the same conditions of pressure and temperature, mole ratio is proportional to a volume ratio, 400 ppm of CO2 in air is equivalent to 400 L per million L or 0.04% (V/V).