For weeks after the gas leak began near Porter Ranch, Los Angeles County health officials kept assuring residents that their complaints of headaches, dizziness and nosebleeds were similar to what had happened in a small town in Alabama.

There, residents from unincorporated Eight Mile in Mobile County complained of a rotten egg smell, of the way the foul odor messed with their breathing and their stomachs. Lightning had struck a storage tank holding thousands of tons of mercaptan, a pungent chemical added to natural gas so it can be detected by smell. The odorant seeped into a beaver pond in Eight Mile and years later residents complained of headaches and dizziness.

But the people of Eight Mile never reported nosebleeds. In Porter Ranch, however, a third of 600 households complained of bloody noses, according to the results of a Los Angeles county public health survey released on Monday. What’s more, not all households that reported nosebleeds could smell the mercaptans, according to the report.

Health officials are now raising questions about what caused the bleeding, which they acknowledge is inconsistent with the very few health studies that exist on mercaptans. A better-documented and more hazardous compound found in natural gas ­— hydrogen sulfide — could explain some of the symptoms, independent experts say. And emerging science shows this dangerous chemical compound could be more abundant than originally thought.

“The high prevalence of reported nosebleeds and low prevalence of reported odor are noteworthy, however, and somewhat inconsistent with expectations,” according to the public health report, which includes results of expanded air monitoring in Porter Ranch. “Additional testing may be warranted to investigate these observations.”

Dr. Mary McIntyre, epidemiologist for the Alabama Department of Public Health, went with a team of experts to Eight Mile in 2012 to report on health complaints from the mercaptan spill.

“No information was provided or indicated by the people who were part of the survey that they were having problems with nosebleeds,” she said.

Of the 204 people in Alabama who responded to the survey, most complained of sore throats, nasal congestion and difficulty breathing. McIntyre said what happened in Alabama should not be compared to Porter Ranch because one was a pure mercaptan spill, the other was a natural gas leak.

“I do not think it’s apples to apples,” she said.

Questions raised

While health officials and Southern California Gas Co. executives have maintained that Porter Ranch residents’ symptoms were the effects of smelling the odorants, some experts are raising other possibilities.

These are “classic symptoms” of exposure to hydrogen sulfide, a well-documented toxic compound, said Michael Jerrett, professor and chairman of the UCLA Department of Environmental Health Sciences.

More serious effects of hydrogen sulfide range from fluid in the lungs to respiratory distress and irregular heartbeat.

Angelo Bellomo, deputy director for health protection at the Los Angeles County Dept. of Public Health, said the nosebleeds could be attributed to nasal irritation from sulfur compounds in general, including hydrogen sulfide and mercaptans.

Hydrogen sulfide, though, has been detected at mostly low levels and very infrequently in Porter Ranch, Gas Co. representatives and air quality officials point out.

One Gas Co. executive said it is highly unlikely that people are smelling hydrogen sulfide.

“Hydrogen sulfide can occur in natural gas, but it is at very low levels and intermittent,” said Gillian Wright, vice president of customer services for SoCalGas, in an earlier interview.

Emerging science

But it may be more abundant in the gas than originally thought, some emerging science indicates. Researchers in Australia discovered that methyl mercaptan, one of the odorants found in Aliso Canyon gas, may convert to hydrogen sulfide under the right circumstances, said Mel Suffet, professor in the UCLA Department of Environmental Health Sciences. While the study is unpublished, Suffet reviewed it as a member of a peer panel.

The anaerobic underground conditions in a former oil field such as Aliso Canyon would be a potential environment for the mercaptan conversion, Suffet said, “that could be one of the causes of hydrogen sulfide.”

While the Gas Co. doesn’t add methyl mercaptan (it uses two other odorants), its suppliers or producers sometimes do, according to spokeswoman Kristine Lloyd. In one January air sample, the Gas Co. measured all three odorants, she said.

Utilities choose mercaptans because they can typically be smelled at lower concentrations than hydrogen sulfide, said Wright, the Gas Co. executive. But the public health survey found only 17 percent of Porter Ranch households reported smelling the odor — with 32 percent reported nosebleeds — so county health officials were puzzled.

Bellamo, from the health department, and state officials from the state Office of Environmental Health Hazard Assessment, weren’t aware of the possible conversion between the two compounds.

Found at low levels

In a summary of the air quality findings online, a Gas Co. consultant acknowledged that hydrogen sulfide in the neighborhood exceeded the state standard once, according to air tests, but said the compound was otherwise “below levels of health concern.”

The state standard for health risks from hydrogen sulfide is 30 parts per billion, averaged over one hour of exposure. Initially, the company indicated that results were below the standard, but later made a correction.

While Porter Ranch may be mostly below the state threshold, “it’s entirely possible that levels below that could be causing the symptoms reported,” Jerrett said.

The Gas Co. detected hydrogen sulfide at least 11 times from Oct. 30 through Jan. 13, according to tests compiled by UCLA researchers. The concentration ranged from 3.8 parts per billion to 29.1 parts per billion, except for one instance when it reached 183 parts per billion. In the typical American city’s air, hydrogen sulfide ranges from 0.1 to 0.3 parts per billion, according to the U.S. Agency for Toxic Substances and Disease Registry.

The Gas Co. has tried to distance itself from the findings. Harmful sulfur compounds “were not detected in the gas leaking from the well,” Dr. Mary McDaniel of Intrinsik Environmental Sciences wrote in the online summary of health effects. “Therefore, it is unlikely that the detections in air of these sulfur compounds related to the gas leak.”

Unknown effects

With this logic, Jerrett believes Gas Co. officials have discounted the possibility of hydrogen sulfide exposure. It may not have been consistently measured, he and other experts say, for a variety of reasons.

The gas was likely “pulsing” from underground, Jerrett said, and emitting hydrogen sulfide intermittently, in waves. So tests taken by the well, at a particular moment in time, might not have registered the compound.

“It’s a very volatile, rapidly moving plume,” Jerrett added.

While the hydrogen sulfide could have come from other sources — such as natural emissions from oil and gas operations — it is unlikely at those concentrations, Jerrett said.

Also, the compound breaks down quickly, sometimes before a laboratory has the chance to detect it, said Don Gamiles of Argos Scientific, an independent air testing company that recently started monitoring in Porter Ranch.

A minority of the tests conducted by SoCalGas had very high thresholds for detection — some at 50 parts per billion, much higher than levels considered safe by state regulators. In other words, the compound might not have shown up on those results, even if it was at dangerous levels.

This testing method was “of limited use,” said Sam Atwood, spokesman for the South Coast Air Quality Management District.

For several weeks his agency has been monitoring for hydrogen sulfide in Porter Ranch, using lower detection levels. Officials haven’t detected it above 3 parts per billion, he said, and on many dates it has registered around zero.

But the district wasn’t testing for hydrogen sulfide in the early weeks of the leak, when gas was escaping at a higher rate. If residents were exposed to hydrogen sulfide for weeks or months, the possible health effects are uncertain. Nobody has researched the medium or long-term exposure to humans at low levels of hydrogen sulfide, according to the World Health Organization.

“There just isn’t a lot of evidence where we know what the long-term implications are for people’s health,” Jerrett said.

Reporter Ingrid Lobet from inewssource contributed to this report.