Hapner then turns to a set of standards that have been generally accepted as representative of how much of each chemical it takes for your average citizen to notice the smell. That's called the "detection threshold." He takes the sum of all the chemicals detected by the instrument, and divides it by the threshold number, and that yields the odor index.
If the chemical is hydrogen sulfide, for example, it only takes one part per billion to reach the detection threshold, so just a tiny bit of that rotten egg smell will trigger a very high index number.
The system could have widespread applications, going far beyond the pig farm and the local sewage plant. Odors are really big these days.
Stink Bomb Science
The Department of Defense, for example, is trying to develop non-lethal weapons based on smells that would drive anyone out of an area.
According to a report in Chemical & Engineering News, the weekly magazine of the American Chemical Society, the military's "odor bomb" smells like rotting garbage, human waste and burning hair. It's putrid enough to make anyone want to flee in disgust, according to the article.
Apparently, it's a lot more effective than tear gas.
Anyone expecting to encounter the odor bomb might want to stop by Johns Hopkins University's School of Medicine and check out their research on something called the "nose plug."
There's a switch in the brain that turns off the olfactory system if the smell becomes unbearable, according to researchers there. Maybe someday we'll have a pill that will turn off the nose when something really stinks.
But until that day, people will still have to work in smelly landfill projects, and cultivate fields overlain with aged solid wastes, and figure out when it's safe to dump that load of fertilizer near the neighbor's yard.
Maybe an odor index will help. At least we would really know how bad it is, even if we couldn't make it go away.
And finally, this note to the guys at the Department of Defense. Could we rethink that odor bomb bit?
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.