One Part Per Trillion is a Very Finely-Split Hair

Just what is a part per million, a part per billion or even a part per trillion?

Extremely low measurements and laws and regulations based on them, are growing more commonplace as the scientific community perfects and uses increasingly sophisticated sensors. Such minute figures probably don't register with most of us though it is important to understand their meaning since the maximum contaminant levels for many of the substances that can be found in water, and which are reported in consumer confidence reports, are expressed in parts per million and parts per billion.

So let's make it easier. Dr. Warren B. Crumwell of the Dow Chemical Co. has made some comparisons that put the figures in their proper perspective.

1 Part Per Million (ppm):

• 1 inch to 16 miles
• 1 minute to 2 years
• 1 cent to $10,000
• 1 ounce to 31 tons
• 1 bad apple in 2,000 barrels

1 Part Per Billion (ppb):

• 1 inch to 16,000 miles
• 1 second to 32 years
• 1 cent to $10 million
• 1 pinch of salt to 10 tons of potato chips
• 1 bad apple in 2 million barrels

1 Part Per Trillion (ppt):

• 1 inch to 16 million miles (A six-inch leap on a journey to the Sun)
• 1 second in 320 centuries
• 1 cent to $10 billion
• 1 pinch of salt to 10,000 tons of potato chips
• 1 bad apple in 2 billion barrels