Researchers Working to Establish an Odor Index

It's little wonder that scientists are obsessed with numbers, since so much of their work relies on measuring and quantifying. They've got a number for just about everything.

Light is measured in candelas. Sound in decibels. If you're running a pneumatic drill, chances are you're generating 80 decibels of ear-piercing noise.

But ask a scientist how bad the pig farm down the road smells, and you'll probably get a quizzical look and a vague answer. When it comes down to odor, the nose knows when something stinks, but there's no olfactory equivalent to decibels.

So along comes Seth Hapner, who is finishing up his master's degree in environmental engineering at Penn State, with a possible solution to this dilemma. Hapner, along with Bradley A. Striebig, who heads the environmental technology group at Penn State's Applied Research Laboratory, has developed what he calls an "odor index."

Current Meters Too Subjective

He used off-the-shelf technology to create an instrument-based system that can detect gases given off by any substance and in what quantities, and then put a number on it. If the odor index value is 1,000, you should be able to detect it. If the index is 10,000, and the chemical is hydrogen sulfide, you should just be able to detect the fine scent of rotten eggs. And if it's 100,000, you'll want to plug your nose and get the heck out of there.

The most commonly used system today involves submitting air samples to various labs around the country that have "odor panels." If an environmental engineer wants to know how bad something smells, he or she can submit a sample, and the folks who serve on the odor panel will take a sniff and render up their judgment. (And you thought you had it bad.)

The problem with that system, Hapner says, is it's very subjective, despite efforts by the labs to get reliable people to serve on the panels.

"We're trying to come up with a method that would be repeatable in any lab around the country," Hapner says.

Tinkering to Keep Stink Down

If a particular odor could be systematically quantified, he says, it could help folks who are in charge of such things as disposing of sewage wastes. They could determine more precisely when they can use the waste as fertilizer without whipping up a storm of protests from downwind residents, for example. Environmental agencies in Pennsylvania and New Jersey, along with the federal Environmental Protection Agency, are sponsoring Hapner's research, and they have already tried it out successfully in the field.

Sewage treated in a major Pennsylvania plant, for example, had a 99 percent reduction in odors because of treatment procedures, Hepner says. If the final product had still stunk, he adds, the system would have told officials exactly where along the treatment stream the procedures had failed.

Here, very briefly, is how the index works: Hapner collects solid samples, like clumps of dried sewage, and takes them back to the lab in sealed plastic containers. Overnight, the clumps give off gases containing chemicals. Those gases are analyzed by a gas chromatograph, which tells which gases are present, and in what quantities.

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 A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.