“Some people say, ‘Well, the distilleries were there before you were, so what’s the big deal?’ ” Michael Mills said, sounding resigned as he stood on his deck, his wooden furniture dotted with black mold. He and his wife noticed it not long after they moved into this home they had built for their retirement in a leafy suburban subdivision in Frankfort, Kentucky’s capital.

Their neighbors had it, too, but they were mystified. “We didn’t know what it was to even complain about,” said Mr. Mills, a former engineer in the state’s Department for Environmental Protection.

The dark residue is visible throughout neighborhoods on days when the air carries a slight yeasty smell from nearby whiskey warehouses and on days when it does not, in heat and in damp. It is difficult to get rid of, the lawsuit alleges, returning after repeated commercial cleanings.

“For it to turn black and get discolored, it takes less than a year,” said Kayleigh Count, a health aide from Frankfort, about 35 miles outside Louisville. Like many in this area, Ms. Count said she was “raised by the distilleries and the bourbon industry.” Her father, uncle and two aunts worked in whiskey production, she said, so being a plaintiff in the suit is hard. “It feels like you’re going against something that the family’s done all their lives,” she said.

The Louisville distilleries named in two suits — Brown-Forman, Diageo and Heaven Hill, manufacturers of Woodford Reserve, Bulleit Bourbon and Elijah Craig — plan to “vigorously contest” the claims, company representatives said.

“While we are sympathetic to the concerns of the plaintiffs,” the companies said in a joint statement, “the blackening of some buildings and other structures is due to a naturally occurring common mold that is found widely throughout the environment, including in areas unrelated to the production of whiskey. The companies involved do not believe that they have caused any harm to the plaintiffs or their property.” (The two distilleries named in the lawsuit in Frankfort, Buffalo Trace and Jim Beam, had no comment.)

The fungus does occur elsewhere, Dr. Scott said; commercial bakeries also produce enough ethanol for it to thrive nearby. It does not need much: less than one part per million in the air, indetectable in a sniff test. As whiskey is barrel-aged, about 4 percent of the alcohol evaporates each year, which can add up to tons in a place like Louisville. (Distillers call this evaporated amount the “angels’ share,” as in the angels’ portion of this heavenly elixir.)