Complaints of a strange stink began to flood Louisville’s air pollution control agency. Much of the city, locals said, was rank with a musty, mildew smell that no one could immediately explain. Theories ranged from sewer sludge to industrial pollution—but, as it turns out, the answer was right under everyone’s feet.
Louisville Metro Air Pollution Control District announced that the culprit of the odd odor is most likely a chemical occurring naturally in soil, called geosmin. The organic compound is made by microorganisms including the streptomyces family of bacteria, which are responsible for most of our antibiotics. And in addition to grossing out Kentuckians, geosmin is in large part responsible for that earthy taste you get from beets and for making dirt smell like, well, dirt.
The human nose can pick up geosmin in even very small concentrations, as low as 0.7 parts per billion. And so, even when just a little of the chemical is around, it makes its presence known with an earthy, dank odor. Some scientists have theorized that humans evolved sensitivity to the smell as a way to help us identify sources of water, since geosmin’s stink can be amplified in moist conditions.
And that’s just what seems to have happened in Louisville. The area recently experienced snow and rain, leading to a wet and muddy city. Pollution Control District spokesman Tom Nord said that the natural geosmin smell was “exacerbated” by these conditions.
Though tests haven’t been conducted to definitively prove the chemical as the smelly source, Louisville’s water treatment department has encountered the stench before and pinpointed geosmin as the cause. In dry conditions, a spokeswoman told one news source, the chemical can make drinking water smell and taste weird. But geosmin doesn’t pose any health concerns in the air or liquid so, besides their olfactory sensibilities, Louisville locals should be safe.
Maybe they can take comfort in knowing that they are far from the only city to be confronted by a seemingly inexplicable funk. Sudden stink problems are actually relatively common, and reports of them may be getting more frequent thanks to social media’s ability to connect those wondering who’s to blame for a certain smell.
Take, for example, the bizarre cat pee smell that plagued a town in Western Pennsylvania last November. Officials said the stink wasn’t toxic, but required testing to figure out the cause. And let's not forget the “maple syrup events” that hit New York City in 2005, 2006, and 2009. After much speculation on the cause, officials finally nailed down the source: a fragrance factory in NJ using the seeds of a clover-like plant called fenugreek. If only every smelly event made our homes smell like someone is making waffles.