Within a week of the start of the natural gas leak in Los Angeles, Gabriel Khanlian’s 2-year-old daughter developed a rash all over her body. His sons—ages 6 and 8—got “bloody boogers” and broke out in hives, he said, while his wife became nauseous. Khanlian himself coughed up blood.

The family’s house in the Porter Ranch neighborhood is about three-quarters of a mile from the Aliso Canyon natural gas storage facility, which began leaking methane Oct. 23. They’re among hundreds of residents who have reported health problems, including headaches, vomiting and nosebleeds.

California regulators attribute the symptoms to mercaptans—sulfurous chemicals that are added to natural gas to aid in the detection of leaks. They say the health problems are temporary and will not lead to long-term damage.

Some health experts are skeptical, however, because there’s virtually no research on prolonged exposure to mercaptans. There are many different types of mercaptans, which often irritate the skin, eyes and respiratory system. Other researchers suggest the health problems may be caused by other substances in the leaking gas.

California has no public health guidelines for mercaptan exposure. Neither does the federal Environmental Protection Agency or the U.S. Centers for Disease Control and Prevention.

“There’s not been any real push to do long-term research [on mercaptans] because it’s just accepted as the thing we use as an odorizer and all is OK,” said Wilma Subra, a public health scientist in Louisiana. She frequently conducts environmental studies for communities affected by oil spills (such as BP’s Gulf of Mexico blowout) and other industrial accidents.

The gas leak has continued for three months. Southern California Gas Co., the owner and operator of the reservoir that’s leaking, is drilling a relief well to stem the flow of gas and estimates the problem will be solved in late February. The company did not respond to questions about the leak’s impact on public health.

SoCal Gas is monitoring for five separate mercaptans at Aliso Ranch: methyl mercaptan, ethyl mercaptan, propyl mercaptan, isopropyl mercaptan and t-butyl mercaptan. They’re also tracking another sulfur-containing odorant called tetrahydrothiophene.

Sam Delson, a spokesman for California’s Office of Environmental Health Hazard Assessment (OEHHA), said the lab that analyzes SoCal Gas’s air samples automatically reports the concentrations of all six chemicals. However, only two of the odorants (t-butyl mercaptan and tetrahydrothiophene) are used in the Aliso Canyon wells.

All of the odorant concentrations have been below the instruments’ detection limit, Delson said in an email. That limit is around 2.5 to 5 parts per billion, but humans can smell and react to concentrations less than one part per billion.

Most accidents involving mercaptans occur in industrial settings, when workers are exposed to concentrations thousands of times higher than what’s present near Porter Ranch.

In November 2014, a methyl mercaptan leak killed four workers at a DuPont chemical plant in La Porte, Texas. Decades earlier, a worker fell into a coma and died after inhaling methyl mercaptan from open storage tanks.

The CDC’s online fact sheet on methyl mercaptan doesn’t address low-dose, long-term exposure, but states that children and people with pre-existing medical conditions may be more susceptible to the gas.

Regulators have developed exposure limits for certain mercaptan compounds, but they were created for workers in industrial settings. Those guidelines don’t apply to more sensitive people, such as children, pregnant women, and people with respiratory illnesses.

A peer-reviewed epidemiological study from 2014 states, “Currently, no information is available on adverse health effects associated with long-term TBM [t-butyl mercaptan] exposure.”

That paper surveyed 204 Alabama residents living near the site of a mercaptan spill. The paper reached some basic conclusions, including that people who lived closer to the spill reported significantly more health symptoms than those farther away.

The Alabama spill originated from a storage tank that leaked thousands of pounds of t-butyl mercaptan into the ground. The chemical later flowed into a pond and evaporated into the air.

The storage tank was owned by Mobile Gas. Its parent company, Sempra Energy, also owns SoCal Gas.

In addition to the public health impact, the Aliso Canyon leak has left an enormous climate footprint. Methane, the main component of natural gas, is a powerful greenhouse gas dozens of times more potent than carbon dioxide. The Environmental Defense Fund estimates the well has emitted more than 88,000 metric tons of methane since October, equal to the burning of roughly 833 million gallons of gasoline.

California Gov. Jerry Brown declared a state of emergency in early January and ordered SoCal Gas to offset the leak’s climate damage. Thousands of residents have left their homes for the duration of the leak, though others have stayed put in Porter Ranch.

The evacuees include Khanlian and his family, who relocated to a house 10 miles away. Khanlian said their symptoms disappeared when they left, but re-emerge whenever they visit their old home to pick up toys, clothes and other belongings. SoCal Gas is paying the rent for their new house.

Khanlian doesn’t trust the regulators’ assurances about the health impacts. “At this point I do not trust anybody,” he said. “Every single authority figure, political figure, as well as the company, has been very untruthful.”

‘They Don’t Know’

Celeste Monforton, a public health researcher and lecturer at George Washington University in Washington, D.C., said the Porter Ranch residents’ months-long exposure and the lack of data mean regulators should “err on the side of admitting what they don’t know.”

“I’m not saying there will be long-term consequences,” she said. “I think it’s a big open question. In terms of communicating risk I think it’s always wise for health agencies to talk about what they know, and what they don’t know, and not to give the public a false sense of knowledge.”

California’s Delson said OEHHA looked at the available data on t-butyl mercaptan and tetrahydrothiophene. Despite the “large data gaps,” there are a few studies on long-term inhalation exposures in animals, he wrote in an email. The animals were affected at exposure levels “several thousands of times greater than the mercaptan air levels in Porter Ranch. Also, there is no data indicating that any of the mercaptans cause cancer.”

Monforton said it’s standard practice to extrapolate from animal studies when human-based research is unavailable. But the extrapolations aren’t perfect, partly because some people are much more sensitive to chemical exposures than others. Animal studies also don’t assess symptoms like headaches or nausea, she said.

When Monforton was a graduate student at the GWU School of Public Health, she studied a 2006 incident in Fulton County, Georgia, in which a shipment of insecticide at a wastewater treatment plant degraded and released large amounts of propyl mercaptan into the air. The odors lingered for weeks. Residents reported headaches, burning eyes, nausea and nosebleeds, the same symptoms found in California and Alabama.

The Georgia incident was more contained than the leak at Alison Canyon, Monforton said. In Aliso Canyon, mercaptans are “just oozing out” continuously, she said, “going on and on and on. For how long?”

Other Contaminants

Some experts say regulators have downplayed the significance of other contaminants that are also present in the leak.

“I don’t think it’s mercaptans [people] are reacting to. I think they’re reacting to other things in the [natural] gas,” said David Brown, a former chief of environmental, epidemiology and occupational health at Connecticut’s Department of Public Health.

The Aliso Canyon gas contains trace amounts of benzene, toluene and other toxic gases, which can cause symptoms similar to those reported by residents. So far, air monitoring results show that none of the gases exceed California’s public exposure guidelines. California’s air quality guidelines are among the most protective in the country.

The one exception is a single sample of hydrogen sulfide from November, which measured six times higher than California’s acute exposure guideline. All of the other samples have shown much lower concentrations.

Brown, who now works for a Pennsylvania health clinic for residents living near natural gas extraction sites, said the Porter Ranch symptoms match many he has seen around Pennsylvania’s Marcellus Shale. Families within half a mile of gas wells and pipelines often report headaches, wheezing, sinus problems, skin rashes, nausea and other problems, he said. None of the Marcellus facilities contain mercaptans, which are added later in the gas supply chain.

Subra, the Louisiana health expert, said regulators should pay more attention to the cumulative impact of these chemicals. California’s exposure guidelines were designed for one chemical at a time, but people are breathing in a cocktail of compounds, and scientists simply don’t know enough about the combined effects, she said.

Delson, the OEHHA spokesman, said the individual chemical guidelines are “based on the most sensitive effect for the chemical and include uncertainty factors to account for variability in the population.”

Some of the chemicals target different organs, so the effects wouldn’t all be additive, he said in an email. “OEHHA does not believe there is any evidence to indicate that the [compounds] in combination should cause a concern for public health.”