The Southern California Gas Company’s well in Porter Ranch is still leaking methane. That's a powerful greenhouse gas, but it's not known to be harmful to humans.
However, air quality tests have detected other substances which could pose a much greater threat to people in the area.
The most noticeable substance is one called marcaptan. It's added to colorless, odorless methane to give it a stench that humans can easily pick up on. Think rotten eggs. It's meant to let people quickly detect gas leaks and avoid explosions.
In 2008, there was a mercaptan spill in rural Mobile County, Alabama.
"Some people had just no symptoms at all and were in the geographic area and just said it smells bad," said Bert Eichold, a county health officer at the time.
Others, he said, complained of irritated eyes, shortness of breath or headaches. In most cases, the symptoms went away once the pollutants were cleaned up.
However, a few residents say their ailments have persisted and are currently suing Mobile Gas, the company behind the spill.
Eichold says Mobile County health officials will follow the case and the residents closely for any clues about the long term health impacts of mercaptan. Currently there is no significant research on the lasting effects of significant exposure to it.
"We still have a lot to learn from our experience and our exposure at this time."
Also of concern is a chemical compound called benzene which is naturally occurring and often found in natural gas wells.
It's a Group 1 carcinogen according to the International Agency for Research on Cancer and known to cause anemia and other blood disorders.
“Anytime you have elevated levels of benzene, it is a concern because there is not a safe level,” said Michael Jerrett, professor and chair of the Department of Environmental Health Sciences at UCLA.
Officials with So. Cal. Gas have said in a statement online that elevated benzene levels have been detected a handful of times in the area. But they noted the levels were below the threshold of an immediate health risk.
The Associated Press and outside experts searched the company's public records and found a dozen air samples showing at least twice the amount of benzene deemed normal by local regulators.
The company was given an opportunity to explain its conclusions but couldn’t, according to the AP.
UCLA's Jerrett says without an accurate picture of the gases escaping the well it's hard to say how dangerous this leak has been.
“The monitoring efforts here have been inadequate to fully access the health risks that the community might be facing,” he noted.
So. Cal. Gas is currently digging a relief well to stop the leak, but in a statement released this week, the company says that project likely won't be done until late February or even late March.