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Update: Louisville Officials Have Maybe Figured Out the Source of That Smell

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Update 3:35 p.m.: It Might Be Geosmin
The Louisville Metro Air Pollution Control District's staff thinks its figured out what's behind that reported odor—geosmin.

In a news release sent Monday afternoon, APCD said:
As part of our investigation, we consulted with other government agencies, including MSD and the Louisville Water Company. The Louisville Water Company says the odor is likely to be caused by a naturally occurring chemical in the soil called “geosmin” that has been exacerbated by the wet, muddy conditions in the wake of the recent snowfall and rain. While this is not a water quality issue, LWC has had experience with geosmin in the past and is familiar with its characteristics. Our noses are extremely sensitive to geosmin and there doesn’t need to be much of it in the air at all for us to detect it. It’s been estimated humans can detect geosmin at concentrations as low as 5 parts per trillion, which means it takes only a very, very small amount to become noticeable.
In the news release, the APCD said it's not certain that the odor is caused by geosmin, but it is "reasonably confident." That said, the city agency will continue looking into reports.

The Associated Press reported earlier:
There's a bad smell in parts of the Louisville area but nobody knows where it's coming from—and it's not thought to be toxic, according to Louisville Metro Air Pollution Control District spokesman Tom Nord. City crews have been trying for nearly a week to pin down the odor that some residents say smells like mildew, but so far they've had no luck. Nord said his agency is doing its best to track the odor, which has been reported by dozens of people who have called from different areas of the city that aren't normally associated with bad smells. Crews are driving around the city and searching the old-fashioned way—with their noses, said engineering manager Matt King.