The city of Fort Worth, Texas, one of the biggest beneficiaries in the natural-gas boom, is questioning its largely supportive stand of the industry after a study found high levels of hazardous chemicals in the air near production sites.

On Tuesday, Fort Worth’s mayor said the city would follow up on the state-sponsored study with its own air-quality tests and could consider rewriting rules that allow drilling in residential neighborhoods.

“It’s time we had some answers,” Mayor Mike Moncrief said at a City Council meeting Tuesday evening.

The concerns over emissions come at a delicate time for natural-gas producers. New technologies have opened up huge new gas fields across the country, boosting U.S. supplies but also bringing drilling to areas such as Pennsylvania and New York that have seen little such activity for decades.

Companies have been forced to defend themselves against accusations that their drilling practices threaten the environment, especially drinking-water supplies. Industry leaders argue those concerns are baseless and have said opposition comes mostly from people who are being exposed to the industry for the first time.

But Fort Worth is a different story. The city has a long history with the oil industry, and when companies discovered a huge gas field in the area in the early 2000s, thousands of homeowners sold the right to drill beneath their properties.

A massive drilling boom brought thousands of jobs and millions of dollars of investment to Fort Worth. Concerns focused mostly on quality-of-life issues like noise and increased truck traffic.

“If you’re a Texan, you grow up with the oil industry,” said Libby Willis, president of the Fort Worth League of Neighborhoods. “You tend not to second-guess it.”

Attitudes began to change last year when air-quality tests commissioned by the nearby town of Dish showed elevated levels of benzene and other chemicals. The industry and some independent experts have questioned the study’s methodology, but the results prompted the Texas Commission on Environmental Quality to conduct its own tests late last year.

The state commission tested 94 sites outside Fort Worth and found what it called “extremely high” levels of benzene, considered an immediate health concern, near two gas-production facilities. The highest level was comparable to levels experienced by drivers while filling their cars with gasoline.

The state study said the emissions were caused by mechanical problems that were quickly addressed. An additional 19 sites had elevated benzene levels that weren’t an immediate health concern but that could cause problems over years of exposure.

Benzene is a carcinogen that has been shown to cause leukemia in workers exposed to high levels over extended periods.

The other 73 sites tested didn’t show high emission levels, and the study didn’t collect air samples farther away from the well sites where residents would be more likely to breathe in the chemicals.

A separate, smaller study didn’t find any problems within the city of Fort Worth.

The Texas Commission on Environmental Quality said energy companies have been cooperating with its investigation.

Michael Honeycutt, director of the commission’s toxicology division, said the study didn’t suggest that gas production poses an immediate risk to residents’ health. But he said the results were concerning because hundreds of wells are being drilled in heavily populated areas, meaning residents could be exposed for years.

“That could turn into an issue, especially with the density of those wells and associated equipment in areas of such population density,” Mr. Honeycutt said.

The industry argues air-quality concerns are overblown and says companies quickly fix any leaks or other mechanical problems that lead to high emissions.

John Satterfield, director of environmental and regulatory affairs for gas producer Chesapeake Energy Corp., says people are overreacting to the study.

“I think a lot of people have taken that information out of context and have run with it,” he said. Mr. Satterfield says Chesapeake supports further testing but doesn’t see the need for new regulations, as some local lawmakers have proposed.

Still, the political winds appear to be changing. City Councilor Kathleen Hicks on Tuesday night said she was reluctant to approve any more drilling permits until the city can determine the extent of the air-quality issues.

Mr. Moncrief said he didn’t see any need to stop drilling without more evidence that there was a problem. But the controversy may already be having an impact.

On Tuesday, gas producer XTO Energy Inc. withdrew a request for City Council permission to drill several wells on a site that some residents said was too close to an elementary school.

XTO will instead drill four wells on four different nearby sites, which doesn’t require City Council approval. The company, which last year agreed to be acquired by Exxon Mobil Corp., didn’t return calls seeking comment.

Write to Ben Casselman at ben.casselman@wsj.com