Conversion factors and comparisons of units

Most of the chemical data that is reported for waterbodies is expressed as a concentration: a mass of chemical per unit volume of water. Most of the total dissolved solids content of ordinary water consists of common salts with the predominant ions being calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride and silicate. These ions collectively are usually in the range of about 20 to 1000 mg/L (milligrams per liter) - the low end being very soft water and the high end being very hard (high in minerals - usually calcium and magnesium carbonates).

A milligram per liter of water is equivalent to 1 ppm (part-per-million) because a liter of water weighs 1000 grams and a milligram is 1 one thousandth of a gram.

The various forms of nitrogen and phosphorus most available to plants (nitrate-N, ammonium-N and phosphate-P) are typically present at concentrations or levels of only 0.001 to 0.500 mg/L . These are typically expressed as micrograms-per-liter or ug/L . A microgram /L is 1 one thousandth of a milligram/L . It is also equivalent to 1 ppb (part-per-billion).

Toxic pollutants such as heavy metals like cadmium and mercury usually exist at sub - ppb levels and can be considered to be a problem at ppb levels. Some organic contaminants, a diverse group of chemicals that includes pesticides, PCBs and dioxins, may be measured at sub- ppb levels and may be expressed as ng/L (nanograms-per-liter = parts-per-trillion) or even 1 pg/L
(picograms- per-liter = parts-per-quadrillion).

These are all very dilute concentrations and below we list some comparisons to provide some intuitive feel for how low these levels are. By the way, seawater has a salt content of about 32 g/L (32 parts-per-thousand which is the same as 3.2 % since 1 percent = 1 part-per-hundred). This is also equivalent to 32,000 ppm (part-per-million).

Concentration Analogies: Explaining chemical concentrations (parts per million, parts per billion) by using analogies (from : Michael A. Kamrin, Delores J. Katz and Martha L. Walter , for National Sea Grant Program, 199? . Taking the Risk out of Reporting Risk Assessment.

These people wrote a book to explain relative risk assessment to journalists writing for the general public. They found that:

1 ppm = 1 drop of gas in an auto gas tank appeals to the imagination and helps people understand the magnitude of a concentration.

Like risk comparisons, however, analogies can cause anger if used merely to minimize the magnitude, and thus the risk. They suggest that analogies be accompanied by information on the significance of the concentration-its effect on human health, the environment, etc.

One-Part-Per-Million
one automobile in bumper-to-bumper traffic from Cleveland to San Francisco
one inch in 16 miles
one minute in two years
one ounce in 32 tons
one cent in $10,000

One-Part-Per-Billion
one 4-inch hamburger in a chain of hamburgers circling the earth at the equator 2.5 times
one silver dollar in a roll of silver dollars stretching from Detroit to Salt Lake City
one kernel of corn in a 45-foot high, 16-foot diameter silo
one sheet in a roll of toilet paper stretching from New York to London
one second of time in 32 years

One-Part-Per-Trillion
one square foot of floor tile on a kitchen floor the size of Indiana
one drop of detergent in enough dishwater to fill a string of railroad tank cars ten miles long
one square inch in 250 square miles
one mile on a 2-month journey at the speed of light

One Part Per Quadrillion
one postage stamp on a letter the size of California and Oregon
one human hair out of all the hair on all the heads of all the people in the world
one mile on a journey of 170 light years

Here are some more from the New Orleans' Stormwater Education Resource Packet:
1 ppm:
one drop of food dye in 16 gallons of water
one large mouthful in a lifetime of eating
one ounce of sugar in 7,813 gallons of Kool-Aid
one dollar bill in a stack of new dollar bills 250 feet high
one inch in 16 miles
1/32 of an inch of a football field
one minute of 2 years
one penny of $10,000

And also from Neal Stephenson (1995. Zodiak , Bantam Books)
1 ppb (part-per-billion) = a drop in a railroad tanker car
2 ppm (part-per-million)= area of a banana peel relative to a football field