The residents had sought emergency treatment for symptoms consistent with those of carbon monoxide, police said.
Warren County and Hackettstown authorities checked an apartment building in town after four residents in one unit fell ill, possibly from carbon monoxide.
The response Wednesday night turned up no elevated levels of the potentially deadly gas or anything else of concern, town police Sgt. Darren Tynan said Thursday.
"We just can't figure out what it is at this point," said Warren County Hazardous Materials Team Chief Tom Nigro. "It's a little bit frustrating."
Responders in protective gear used a variety of meters in the Greenview Garden Apartments unit where the four live at 28 Arthur Terrace, authorities said.
The four had sought emergency room treatment in recent days for symptoms consistent with carbon monoxide poisoning, including nausea, dizziness, headache and vomiting.
The residents vacated the apartment for the night, and a further check of them showed they had not been exposed to elevated carbon monoxide levels, Tynan said in a news release.
The odorless and colorless gas can be fatal when it builds up indoors. It is produced by burning fuel in cars or trucks, small engines, stoves, lanterns, grills, fireplaces, gas ranges or furnaces, according to the U.S. Centers for Disease Control and Prevention.
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Responders also checked out the apartment building's furnace room below the apartment where the four live, Nigro said.
In addition to carbon monoxide, authorities checked oxygen levels and looked for gases including hydrogen sulfide; ammonia; and volatile chemicals such as gasoline and benzene.
"We did not detect anything at that point," Nigro said.
In addition to Hackettstown police and county haz-mat, the Warren County Board of Health, Hackettstown Fire Department and Hackettstown Rescue Squad also responded.
Kurt Bresswein may be reached at kbresswein@lehighvalleylive.com. Follow him on Twitter @KurtBresswein and on Facebook. Find lehighvalleylive.com on Facebook.