Describing Contaminants in Environmental Media
Naturally occurring materials such as air, water, soil, and sediment are referred to as environmental media. In environmental health we are interested in exposures (through the skin, lungs, and gastrointestinal tract) to the chemical or physical agents that are present in these media.
In order to determine how much contaminant (typically a chemical or physical agent) is present in the soil adjacent to a home, in the food we eat or in the air that we breathe, tests are conducted on samples of the environmental media to measure how much contaminant is present in the environment. Usually concentrations are reported in weight units, such as the weight of the impurity compared to the weight of the total (weight of contaminant/weight of total sample).
Parts per million, billion, trillion, and gazillion!
Sometimes concentrations of a contaminants are reported as parts impurity per whole of the sample (parts contaminant/parts total sample). Parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt) are commonly used terms to describe the concentration of contaminants or impurities in our environmental media. One part per million means one part contaminant for every million parts of soil or air or water. You might think of concentrations expressed in terms of percentages as "parts per hundred." For example 3.2% = 3.2/100 = 0.032.
How much is a part per million? provides a a detailed explanation of how to interpret expression, such as parts per million.
Expressing the level of contamination in terms of concentration allows a comparison between different samples or between a sample and a regulatory standard. For example, scientists can measure the concentration of a chemical in the soil near a home by looking at a small amount of soil and they can then compare these concentrations to those present at another location (like at the local playground or at a hazardous waste site) or to a standard which mandates some action (i.e. removal of soil) at a particular concentration.
The Metric System and PPM
The metric system is a very convenient way to express parts per million, etc. since metric units go by multiples of ten. 1 ppm = 1 µg/g.
- a gram is a thousandth of a kilogram (1g = 1,000kg)
- a milligram is a thousandth of a gram (1mg = 0.001g)
- a milligram is also a millionth of a kilogram (1mg = 1,000,000kg)
- a milligram is one part per million of a kilogram, which is the same as one milligram per kilogram: 1 ppm = 1 mg/kg
With the ability to detect even smaller amounts of contaminants, the terms part per billion and part per trillion are becoming more common.
Parts per million is often used to express a concentration in terms of mass, but it can also be used to describe a concentration in terms of volume or numbers. For example, if you had one can of tuna fish, and you divided the can equally into 10 pieces, then each piece would be a part per ten. If, instead, you divided the tuna into a million pieces, then each portion would represent a millionth of the amount of tuna or one part per million of the original can of tuna. Likewise, if you have 1 million people, one person in the group represents 1 part per million people.
What About Solid Media (soil, sediment or food)?
Contaminant concentrations in soil, sediment and food are typically expressed as mass of contaminant per mass of media, e.g., 40 mg mercury per kg tuna. As explained above this is equivalent to 40 ppm on a weight/weight basis.
Equivalents to PPM and PPB
1 mg/kg = 1 µg/g = 1 ppm
1 µg/kg = 1 ng/g = 1 ppb
What about liquids?
Contaminant concentrations in liquids are typically expressed on a weight per volume basis. This can be checked using the unit conversion method described above.
How 2 µg of trichloroethylene per liter (L) water = 2 ppb
- The density (weight/volume) of water is 1 kg/L (or 1 g/mL)
- 2 µg trichlorethylene/L water = 2 µg trichlorethylene/kg water.
- That in turn is equivalent to 2 ppb
- This can be checked using the conversion method:
How about gases (air)?
For air, we typically express concentrations as mass of contaminant per volume of air, e.g., 20 µg/m3. Sometimes air concentrations may be reported as ppm or ppb. For air this is done on a volume per volume basis (i.e., volume of contaminant per volume of air). Conversion between the two systems typically uses the Ideal Gas Law. You will not need to do this for EH717-B1.
