Yet while there is probably some correlation between this percentage and losses to the atmosphere, it is difficult to tease out which part of that is the part that escapes, said Boston University's Phillips.
Some losses are impossible
"Right now we can't say that 'unaccounted for' means leaks," Phillips said. "It's some black box that includes leaks, accounting errors and meter errors."
Additionally, the reported numbers on lost and unaccounted for gas often seem unreliable. In 2012, PHMSA data on lost and unaccounted for rates from gas companies included a range of numbers that defied the possible.
One company, Indiana Utilities Corp., responsible for 139 miles of total pipeline, was listed as having a 563 percent lost and unaccounted for rate. Many others, mostly small systems of less than 200 pipeline miles, were listed at improbably high percentages of lost and unaccounted for gas.
In an emailed statement responding to a query on why the lost and unaccounted for numbers appeared to have so many errors, PHMSA said the responsibility for the data's accuracy lay with the reporting company.
"The accuracy of the data provided for unaccounted for gas is dependent on the operator's ability to understand how to calculate the formula. PHMSA data staff will be following-up with operators to verify data accuracy," the statement read.
In a telephone interview, Frank Czeschin, the president of Indiana Utilities Corp., opened up the electronic file of his 2012 report to PHMSA in order to look over his numbers.
"I just pulled my actual report, and it indicated 0.563 percent. I do not know how the report you pulled was missing the decimal," said Czeschin, who added that PHMSA had not contacted him about the supposed 563 percent loss.
Industry experts say the lost and unaccounted for rate should be no higher than 3 percent. PHMSA recommends contacting the company if the rate is more than 10 percent.
In the PHMSA database, which lists more than 1,400 gas companies, 72 companies reported lost and unaccounted for rates of 10 percent or higher. Two-hundred-and-seventy-five companies had a rate between 3 and 9.9 percent.
How much goes up? The jury remains out
The natural gas industry, represented by the American Gas Association, says the reported lost and unaccounted for percentages should not be used as a proxy for leaks. "It doesn't have anything to do with emissions or with what actually is emitted into the atmosphere," said Pamela Lacey, AGA's senior managing counsel for environment.
For its part, AGA is quick to highlight U.S. EPA's estimates of methane emissions from natural gas. EPA has said that, from the gas well to your stovetop, the industry leaks 1.4 percent of the gas it produces.
For the pipeline distribution system, the agency calculates this loss based on leak rates calculated from a 1996 study, conducted in collaboration with the Gas Research Institute (now the Gas Technology Institute). That study took leak measurements from participating gas companies for different kinds of pipes: cast iron, unprotected and protected steel, and plastic.
To determine total leaks, the agency multiplies the leak rate by the miles of pipe, subtracts any emissions reductions techniques reported by gas companies and comes up with a final emissions number. Cornell's Howarth has argued that this study underestimates emissions from natural gas.
New research to re-examine distribution system leak rates has been funded by the Environmental Defense Fund as part of a larger project to quantify lost methane from the natural gas system. Much of the field work on that is being done this year, headed up by Brian Lamb, a researcher at Washington State University.