Posted: 2001
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Article SummaryThis manual on how to use risk comparisons and risk statistics was commissioned to help chemical plant managers explain air emissions to their neighbors. Chapter III on risk comparisons, especially, is still relevant. Later research hasn’t borne out all its seat-of-the-pants conclusions, but the advice at the end of the chapter about the worst risk comparisons holds firm – in my terms these comparisons fail (especially when people are outraged) because they try to compare the hazard of high-outrage and low-outrage risks. The other chapters are also useful and not really outdated, I think. The appendices are both outdated and all too likely to be misused. They’re what the client originally wanted most. Vincent Covello, Paul Slovic, and I wrote the rest of the manual to soften them.

Risk Communication, Risk Statistics,
and Risk Comparisons:
A Manual for Plant Managers

Washington, DC: Chemical Manufacturers Association, 1988

Appendix A
Concentration and Quantity Comparisons


number 1
The data in the tables are designed to stress how small small really is. By minimizing magnitude, the comparisons tend to minimize risk.
number 2
When the data in the tables are used by those with an obvious interest in minimizing risk, they often lack credibility. They are also likely to provoke the same sort of backlash provoked by self-serving risk comparisons.
number 3
The data often lack relevance and meaning unless coupled with data on the toxic potency of a particular chemical.
number 4
Efforts to minimize small quantities through the comparisons given in the tables may backfire by making the quantity easier to visualize. While an image such as “a drop of vermouth in 500 barrels of gin” may be intended to convey smallness, for some in the audience it serves as an easily visualized and memorable metaphor.

Table A.1
Concentration Comparisons
Organized by Unit Categories

Unit  1 part per million1 part per billion1 part per trillion     
Length1 in./16 mi. 1 in./16,000 mi.1 in./16,000,000 mi. (a 6-in. leap on a journey to the sun)
Time 1 min./2 years1 sec./32 years1 sec./320 centuries (or 0.06 sec. since the birth of Jesus Christ)
Money1 cent/$10,0001 cent/$10,000,0001 cent /$10,000,000,000
Weight 1 oz./31 tons1 pinch salt/10 tons
of potato chips
1 pinch salt/10,000 tons of
potato chips
Volume1 drop vermouth/
80 “fifths” of gin
1 drop vermouth/
500 barrels of gin
1 drop of vermouth in a pool of gin covering the area of a football field 43 ft. deep
Area 1 square ft./
23 acres
1 square in./
160-acre farm
1 square ft./the state of Indiana; or 1 large grain of sand on the surface of Daytona Beach
Action1 lob/1,200
tennis matches
1 lob/1,200,000
tennis matches
1 lob/1,200,000,000 tennis matches
Quality1 bad apple/
2,000 barrels
1 bad apple/
2,000,000 barrels
1 bad apple/
2,000,000,000 barrels

Source: Adapted from Warren B. Crumett, Dow Chemical Company (as reproduced in Rowe, W.D. et al., 1984, Evaluation Methods for Environmental Standards. CRC Press Inc., Boca Raton, Florida.


Use of data in this table for risk comparison purposes can damage your credibility (see text).

Table A.2
Miscellaneous Concentration Comparisons

Parts per million:

  • One automobile in bumper-to-bumper traffic from Cleveland to San Francisco
  • One drop of gasoline in a full-size car’s tankful of gas
  • One facial tissue in a stack taller than the Empire State Building
  • One pancake in a stack four miles high

Parts per billion:

  • One silver dollar in a roll of silver dollars stretching from Detroit to Salt Lake City
  • One kernel of corn in enough corn to fill a 45-foot silo, 16 feet in diameter
  • One sheet in a roll of toilet paper stretching from New York to London

Parts per trillion:

  • One square foot of floor tile on a kitchen floor the size of Indiana
  • One drop of detergent in enough dish-water to fill a train of railroad tank cars ten miles long
  • One mile on a two-month journey at the speed of light

Parts per quadrillion:

  • One postage stamp on a letter the size of California and Oregon combined
  • The palm of one’s hand resting on a table the size of the United States
  • 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

Source: Adapted from data supplied by Jim Callaghan, Hill and Knowlton, Inc., as reproduced in P. Sandman, D. Sachsman, and M. Greenberg, 1987, Risk Communication for Environmental News Sources. Industry/University Cooperative Center for Research in Hazardous and Toxic Substances: New Brunswick, New Jersey.


Use of data in this table for risk comparison purposes can damage your credibility (see text).

Figure A.1
Air Emissions From Chemical Production
Tons per Year Projected through 1988
for the
Dow Chemical Company, Midland Division

(Image not available)

Source: Adapted from Midland Daily News, June 5, 1987.

Copyright © 1988 by Chemical Manufacturers Association

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