What One Part per Million Means
At a well in a community in Guanajuato, the laboratory reports that the water has "3 ppm" of a contaminant. It sounds like very little, but how little is it? Should we be worried? To answer, we first need to understand what this tiny unit means.
When Percent Falls Short
A percent is "parts per hundred." If a solution is 5% salt, there are 5 g of salt in every 100 g of solution. But contaminants in water, air, or soil are usually present in tiny amounts: a percent like $0.0003\,\%$ is awkward to read and to compare.
That is why a smaller unit is used: parts per million (ppm). Instead of counting out of every 100, we count out of every 1,000,000.
Imagine a stadium of 1,000,000 people (bigger than any that exists). If only one person is wearing a green shirt, that person is "1 ppm" of the stadium. Another example: 1 drop in a barrel of about 50 L is approximately 1 ppm.
A water analysis from a well reports 3 ppm of a pollutant. Explain in your own words what that value means and express the concentration in $\text{mg/L}$. How many…
A water analysis from a well reports 3 ppm of a pollutant. Explain in your own words what that value means and express the concentration in $\text{mg/L}$. How many milligrams of that pollutant are there in a 20 L water jug filled with that water?
Ppm in Water: 1 ppm = 1 mg/L
In dilute water, the rule of thumb is very convenient. One liter of water has a mass of 1 kg, which is 1,000,000 mg. If that liter contains 1 mg of contaminant, the proportion is 1 mg out of 1,000,000 mg: 1 ppm.
This lets us calculate the total amount: if there are 3 mg in every liter, then in 20 L there will be 20 times as much.
A water analysis from a well reports 3 ppm of a pollutant. Explain in your own words what that value means and express the concentration in $\text{mg/L}$. How many…
A water analysis from a well reports 3 ppm of a pollutant. Explain in your own words what that value means and express the concentration in $\text{mg/L}$. How many milligrams of that pollutant are there in a 20 L water jug filled with that water?
Ppm in Soil and in Air
The idea is always the same (1 part in a million), but the unit used to measure the sample changes.
| Medium | Equivalence | Meaning |
|---|---|---|
| Water | $1\ \text{ppm} = 1\ \text{mg/L}$ | 1 mg of contaminant per liter of water |
| Soil | $1\ \text{ppm} = 1\ \text{mg/kg}$ | 1 mg of contaminant per kilogram of soil |
| Air | ppm by volume | 1 mL of contaminant in every $1{,}000{,}000$ mL (1 m³) of air |
A kilogram is 1,000,000 mg, which is why it works the same in soil as in water. In air, since gases are measured by volume, 1 ppm is 1 cm³ (1 mL) of the gas in 1 m³ of air. This is how, for example, gas monitoring is reported during environmental alerts in Mexico City.
Comparing Concentrations: Same Unit First
Laboratories don't always report in the same way: some write ppm and others mg/L. Since they are equivalent in water, the first step is to convert everything to the same unit and then compare.
Order these concentrations of the same pollutant in water from lowest to highest: 0.5 ppm, 8 mg/L, 2 ppm, 0.05 mg/L. First express all of them in the same unit (ppm) and…
Order these concentrations of the same pollutant in water from lowest to highest: 0.5 ppm, 8 mg/L, 2 ppm, 0.05 mg/L. First express all of them in the same unit (ppm) and then order them.
Back to the Beginning
The well with 3 ppm has $3\ \text{mg/L}$: 3 milligrams of contaminant in every liter, or 3 parts in every million parts of water. In a 20 L jug there are $3 \times 20 = 60$ mg. To know whether that is dangerous, we have to compare it with the limit set by a standard, and that is the topic of the section Comparing with the Limits in Standards.