Odors Aren't Likely to Awaken Us From Sleep

June 10, 2004 -- Anyone who expects the stench of leaking natural gas or propane to wake them up from a deep sleep may be, quite literally, flirting with death.

New research from Brown University in Providence, R.I., dispels the myth that additives in gases like propane smell bad enough to wake someone up. The findings come as thousands of vacationers hit the road in their recreational vehicles, nearly all which use propane for heating water, or warming the interior of the vehicle, or cooking.

Propane, like natural gas, is odorless, so various chemicals are added to make it smell really awful. It's an unmistakable odor, so if the user is awake, the warning of a leak is instantaneous, even for a very small leak. That has probably saved thousands of lives.

But, "the idea that a smell will wake you up is not true," says psychologist Rachel S. Herz, who has spent nearly 15 years researching smells. She teamed up with Mary A. Carskadon, professor of psychiatry at Brown and a sleep expert, for the research. Their findings were published in a recent issue of the journal Sleep.

Which Comes First

Some might argue with the findings, because they remember awakening to the smell of bacon or another aroma, but the researchers say something else woke them up.

"We wake up and smell the coffee, not the other way around," Carskadon says.

Herz points out that in the early morning "you are more likely to be waking up, and you go through brief stages of actually being awake." During those short "awakenings" you may smell the bacon cooking, for example, but you smell it because you are awake, even if only for a few micro-seconds, not the other way around, she says.

Participants in the study could not be aroused from deep sleep by two distinctively different odors, but they all woke up with an auditory alarm. That has led the researchers to call for audible devices to warn of hazardous leaks.

"There has to be another stimulator, like auditory, which seems to be quite powerful, if you are at all concerned," Herz says.

Clearing the Air

Three men and three women, 20 to 25 years old and in good health, participated in the study. That's a small group to draw such sweeping conclusions from, but the researchers say the results were so clear they are confident the findings are true. The study lasted two nights, and the participants were tested over and over again, always with the same results.

During the experiment the participants were subjected to two odors, one pleasant and one nasty, during various stages of sleep. They were monitored several ways, including changes in breathing and heart rate.

Participants showed some response during the earliest stages of sleep, but once they were really snoozing neither the pleasant smell of peppermint nor the acrid odor of pyridine woke them up. The researchers chose those two odors because they wanted both good and bad, and because much research has already been done on both so it gave them some basis for comparing their results with other findings.

The participants responded slightly more to pyridine than peppermint, but the difference was not enough to be significant, Herz says. Pyridine is used in various manufacturing processes. It is commonly found in firewood because it is used as a herbicide, so the familiar smell of smoke wasn't enough to alert the sleepers of possible danger.

All it took was a moderate tone from a nearby speaker to wake them up, every time, regardless of how deeply they were sleeping.

The lesson in this, the researchers contend, is don't count on your nose to save your life. At least not if you're asleep.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.