The human sense of smell, though vastly inferior at detecting smells compared to other animals, is, nonetheless, important for informing the brain of the presence of food and, in some cases, danger. Think back to the last time you cut an onion or a time when the rotting smell of garbage made you nauseous. How do some smells like freshly baked bread evoke pleasant sensations, whereas others stimulate physical responses like crying or gagging? The answer, for irritating odors at least, is starting to become clear. Studying mice, Dr. Weihong Lin and colleagues at the University of Colorado, Denver, and the University of Maryland, Baltimore County, have identified solitary chemosensory cells scattered in the epithelium inside the front of the nose that respond specifically to high levels of offensive odors and activate trigeminal nerve fibers. These latter structures, in turn, which are also linked to bitter taste receptors, produce pain and or burning feeling and stimulate protective reflexes, such as gagging or coughing. Not surprisingly, the threshold for signaling these responses is high, meaning that trace amounts of irritating odors won’t trigger a physical response. The next steps for researchers include extending the work to humans and trying to determine if there are receptors specific for particular chemicals or whether the same cells signal the presence of all irritants. The investigators expect their research will help to understand why some people are much more sensitive to bad odors than others.