Odor Complaints: Can You Smell That Smell?

“The first condition of understanding a foreign country is to smell it.”  — Rudyard Kipling

A few weeks ago, a lot of folks in this area found themselves obsessively scraping the bottoms of their boots. Based on the smell that seemed to be following them around, they were sure that they had stepped in something unpleasant.

The odor was so pervasive, though, that meteorologist Garry Frank, on KSDK, felt compelled to explain it. It turned out that they hadn’t stepped in dog poop. Instead, people in Missouri and Illinois were smelling air that had passed over cattle and hog farms in Iowa and Nebraska and then was trapped in a temperature inversion. A cold front moving through the region pushed the stink south and southeast.

People complained, but I haven’t heard of any class action lawsuits against the sources of the smell. That wouldn’t be true if the smell had come from a chemical plant.

If It Smells Bad, Is It Bad For You?

Most people accept the idea that if something smells bad, then it must be bad for you. Even the American Chemical Society shares this bit of folk wisdom with teachers to use in their classrooms. “More often than not, the nose knows.” In large part, this is related to the widely held belief that our sense that something stinks is an evolutionary survival trait, which it might well be. A commonly cited example is that of hydrogen sulfide—the gas that gives rotten eggs their terrible smell.

The common explanation for our aversion to the smell of hydrogen sulfide, H₂S, is that it signals that organic matter has started to decay and so would be bad to eat. Not because of the H₂S itself, but because of the decomposition it signals. A similar, but simpler explanation that I have encountered is that we have evolved an aversion to the odor of H₂S because H₂S is bad for us.

OSHA’s permissible exposure limit (PEL) for H₂S is 20 ppm (8-hour time weighted average). NIOSH has established that the concentration that is Immediately Dangerous to Life and Health (IDLH) is 100 ppm, primarily because olfactory fatigue sets in at that level and we can no longer smell it. A level of 700 ppm or higher results in “rapid unconsciousness, cessation of respiration, and death.” A recent release of H₂S at the Pemex refinery in Deer Park, Texas, killed two and injured 35 others.

Although it varies depending on the person, the odor threshold for hydrogen sulfide is 0.005 to 0.05 ppm, about 400 to 4000 times lower than the PEL. If our nose is an alarm system for H₂S, it would seem that the set point is much too low.

Other Odor Thresholds For Toxic Gases

There are two other toxic gases that are characterized by their smell: hydrogen cyanide and phosgene. Hydrogen cyanide, aka cyanide gas, has the odor of almonds and phosgene has the odor of mown hay—both odors that I find pleasant. The odor threshold for hydrogen cyanide, HCN, is 2 to 10 ppm. The OSHA PEL for HCN is 10 ppm. If you can smell it, it’s already too high. The NIOSH IDLH for HCN is 50 ppm.

Phosgene, COCl₂ is even more dangerous. The odor threshold for phosgene is 0.5 to 1.5 ppm, but the OSHA PEL COCl₂ is for 0.1 ppm and the NIOSH IDLH for COCl₂ is 2 ppm. Both are too low for smell to be any use as a warning.

Complaints About Odors

It is not HCN or COCl₂ that people complain about, however. It’s the smell of sewers, or livestock waste, or landfills, or in our neighborhood, the smell from the brewery. Smells that people find unpleasant, but that are not hazardous. That doesn’t mean that we can ignore these or other unpleasant odors for which our facilities are the source.

If your organization performs process hazard analyses, PHAs, you’ve established risk tolerance criteria. Somewhere in those risk tolerance criteria there is a consequence severity level described as “complaint from the community”. Few things will generate complaints faster than a stink.

What can be especially frustrating is when you get a complaint about an odor that is not even coming from your plant. Due to temperature inversions and unseasonal wind directions, it could be coming from cattle and pig farms in Iowa and Nebraska.

Fenceline Monitoring

To defend against complaints, or to comply with local odor regulations, some facilities want to turn to fenceline monitoring. There are devices called “field olfactometers” available that are used for this purpose. While it may sound like these are scientific instruments, capable of calibration with NIST-traceable standards, they ultimately depend on a person’s nose. The device dilutes ambient air with odor-free air and the operator, or “panelist”, determines how much the ambient air must be diluted to go below the odor threshold. The measurement is then reported as a dilution-to-threshold, D/T, ratio.

Better than nothing, I suppose. Or perhaps not.

What To Do

Odor complaints can be a nettlesome issue for any facility. They are especially frustrating when the odors are not coming from your facility. Instead, they could be coming from the plant next door, or if the conditions are right, from hundreds of miles away. What do we do? Clearly, it is important to avoid releases that neighbors can smell. If the odors are coming from your facility, it won’t help to explain that they are non-hazardous. Address them as best you can and just as important as working on the odors will be working with the complainers. Complaints, like the odors themselves, depend on a person’s nose.