Authorities say an odor prompted a voluntary evacuation of about 100 homes in suburban Philadelphia Sunday.
Officials scrambled to figure out what was causing the smell as utility companies reported no issues with their systems.
Early Monday authorities announced the smell is due to an organic compound coming from the sump pumps in the basements.
The Montgomery County hazardous response team, state Department of Environmental Protection and federal Environmental Protection Agency are working to identify the "volatile organic compound" involved, the county Department of Public Safety said in a statement Monday.
Officials said a strong order was reported at a home in the Skippack Township neighborhood at about 5:45 p.m. Sunday. The township fire company responded with meters that "went into alarm" in the basement of the residence, and more homes were tested and found to have levels of the organic compound as well, the public safety department said.
Fire Chief Haydn Marriott had earlier told WPVI-TV in Philadelphia that crews testing the homes had gotten high readings of a poisonous gas called hydrogen cyanide. But he said the crews didn't actually think the odor in the homes was from hydrogen cyanide, but some other chemical.
The Times-Herald reports hydrogen cyanide gas has a distinctive bitter almond odor (others describe a musty 'old sneakers smell'), but a large proportion of people cannot detect it;
An emergency shelter was set up at the local elementary school, and the American Red Cross of southeastern Pennsylvania said 10 people from four families were put up for the night. Some residents went to a hospital as a precaution and were released.
Residents are being asked to contact the township and have their homes tested for the presence of the compound before returning home.
Volatile organic compounds are found in products such as gasoline, paints and paint thinners and solvents used for dry cleaning and metal degreasing, according to the U.S. Geological Survey. They can find their way into ground water from spills or leaking storage tanks or in stormwater runoff from roads and parking lots. The compounds are usually used in liquid form and are called volatile because many can readily evaporate.
—The Associated Press Contributed
