BOSTON — An unknown amount of natural gas is seeping into the atmosphere from aging underground pipes, fouling the environment, creating public safety hazards and costing the state’s energy consumers.

Utility companies are required to report the so-called “lost and unaccounted-for gas” to state regulators annually, but environmental groups say the industry estimates belie the full scope of the problem.

Proposed rules being considered by the state Department of Public Utilities, which go before a public hearing Aug. 15, would require publicly regulated utilities to report lost gas in a more uniform and transparent manner.

The rules, set to go into effect in January, would require private and municipal gas companies to report to the state how much gas is lost, why it is lost, and how much it is costing consumers.

“Nobody really knows how much gas is lost from leaks, some of which are literally gushing,” said Audrey Schulman, executive director of the Home Energy Efficiency Team, a Boston-based nonprofit that advises communities about gas leaks.

“Right now, the utilities are doing it their own way, so we don’t know how big the problems are or where to put resources toward fixing them,” she said.

Schulman said researchers believe the estimates of lost gas provided by the companies to regulators are only a small portion of what is actually seeping out of the aging distribution systems.

“They claim almost all of the lost gas is accounting errors,” she said. “Meanwhile, the companies estimate the lost gas using methods they don’t explain and that vary company to company.”

Massachusetts utilities reported more than 34,000 gas leaks in 2017, about 7,500 of which were classified as “Grade 1” leaks, meaning they should be repaired immediately, according to state data.

A 2015 Harvard University study estimated the gas leaking from aging pipes and loose connections is costing utilities more than $90 million a year — a cost passed on to ratepayers.

Meanwhile, environmental groups say methane seeping from gas pipes complicates state efforts to meet ambitious goals to curb carbon emissions.

A recent Supreme Judicial Court ruling criticized the Baker administration for not doing enough to hit those benchmarks. It cited gas leaks as a major contributor.

Utilities say they are working to reduce gas leaks by replacing aging pipes — some of the oldest in the nation — and argue that environmental concerns about lost gas are overstated.

In comments submitted to the DPU, several gas companies that operate in Massachusetts — including National Grid, Eversource, Unitil and Columbia Gas — said they are willing to adopt more transparent and uniform reporting but argued that unaccounted-for gas “is an accounting mechanism, not a measure of methane emissions.”

A utility defines lost and unaccounted-for gas as the difference between what it purchases and how much it provides to customers. Besides leaks, loss can involve factors such as venting of gas systems, theft, third-party damage or billing issues.

The companies said a number of factors contribute to unaccounted-for gas, including a “mismatch between large, sophisticated pipeline meters and small, less-sophisticated customer meters.”

Concerns about gas leaks were heightened as a result of the gas explosions and fires in the Merrimack Valley in September. The disaster killed a teenage boy, injured dozens of others, and left thousands without heat or hot water in Lawrence, North Andover and Andover.

The Baker administration has taken a number of steps since the Merrimack Valley disaster to ensure the safety of gas pipelines, including hiring new inspectors and a private firm to review the systems.

The DPU review of lost gas reporting is required under a clean energy bill, signed by Baker last year, aimed at increasing wind power and making it easier for solar farm operators to produce electricity.

“We need more uniformity and transparency in the reporting of lost and unaccounted-for gas,” said Rep. Lori Ehrlich, D-Marblehead, who helped craft the legislation. “Not only is the lost gas expensive for consumers, but it’s also extremely damaging to the environment.”

Groups such as the Sierra Club welcome the tougher rules but want the state to accelerate a shift away from its reliance on natural gas to renewable energy such wind, solar and hydropower.

They have asked the DPU to direct gas companies to “phase out the use of natural gas in favor of other sources of heat in order to meet Massachusetts’ goals under the Global Warming Solutions Act.”

“Ultimately, the most effective and cost-effective way to reduce lost gas is to switch building heating and appliances to electricity,” David Zeek, a member of the Massachusetts Sierra Club’s energy committee, wrote in public comments to state regulators, “and to reduce our dependence on natural gas altogether.”

A hearing on the proposed rules will be held Aug. 15 at 10 a.m. at the Department of Public Utility’s offices, which are located on the fifth floor of Boston’s South Station.

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites.