An invisible gas that smells like 'rotten flesh sitting in the sun' is causing headaches and nausea in a California city

• The city of Carson, California, has smelled like vomit or rotten eggs for roughly a month.
• Local officials attribute the odor to a toxic, colorless gas called hydrogen sulfide.

Los Angeles County Supervisor Holly Mitchell was driving along the 405 freeway last month when she caught a whiff of rotten eggs. Others in the area noticed it, too. Residents of Carson, a city just south of downtown Los Angeles, compared the scent to farts, vomit, body odor, and "the stench of death" in posts on Facebook and Twitter.

The city smells like "rotten flesh sitting in the sun," Monique Alvarez, a third-generation Carson resident, told The Los Angeles Times.

The LA County Department of Public Health attributed the odor to low levels of hydrogen sulfide, a colorless gas, on October 9. The gas is emanating from the Dominguez Channel, a 15.7-mile river that empties into the ocean at the Port of Los Angeles.

At low concentrations, hydrogen sulfide can cause headaches, nausea, coughs, shortness of breath, and irritation of the eyes, nose, and throat. High concentrations, far higher than those detected at the Dominguez Channel, could potentially result in coma or death.

In a recent statement, Mitchell said some Carson residents have experienced headaches and nausea. Other residents told The Guardian and LA Times that they're experiencing respiratory problems, such as coughs or trouble breathing.

"My grandson coughs in the evening, and it's terrible," Pamela Brown, a 60-year-old realtor in Carson, told the LA Times. "There's something going on, and they want us to believe this is all OK."

In a statement, the LA public-health department said residents' symptoms "should go away when the odors are no longer present." It recommended keeping doors and windows closed and using an air conditioner. But the stench hasn't disappeared, despite the county's attempts to mitigate it.

So on October 19, LA County said it would reimburse residents for air purifiers and hotel rooms. Health officials also advised residents to keep their pets indoors and avoid outdoor exercise between 9 p.m. and 8 a.m. - when hydrogen sulfide levels in the air are higher - while county officials work to eliminate the smell. Then on October 25, the city of Carson declared a state of emergency to secure additional funding for hotel rooms and air purifiers. Carson and LA County have relocated more than 1,300 residents so far, The Guardian reported.

The hydrogen sulfide likely comes from decaying vegetation and marine life

Public-health officials are still investigating how the noxious gas came to permeate the air.

In early October, Carson Mayor Lula Davis-Holmes suggested that a leaking pipeline had emitted the gas. More recently, though, LA County public works officials said they're investigating whether a September earthquake released hydrogen sulfide from a local refinery or chemical plant. The department did not respond to Insider's request for comment.

Meanwhile, South Coast Air Quality Management District, a local air pollution agency, has attributed the gas to naturally decaying vegetation and marine life that washed onto the channel banks during low tide.

Some Carson residents think that material washed up after debris from a warehouse fire clogged the channel, causing decaying plants and animals to build up. Eight residents have filed a lawsuit alleging that to be the case. But California's ongoing drought may have also caused vegetation to dry out naturally and emit hydrogen sulfide.

Either way, LA County public works officials are now pumping oxygen into the canal to counteract that chemical process. They're also spraying a biodegradable deodorizer to mask the smell.

So far, the strategy seems to be working. LA County officials told residents at a town hall last week that hydrogen sulfide levels in the air had declined to around 1,000 parts per billion (ppb) during the day, The Guardian reported. According to the Environmental Protection Agency, hydrogen sulfide doesn't pose a risk of permanent health problems until people are exposed to 27,000 ppb for one hour, or 17,000 ppb for eight hours. At most, county officials said, hydrogen sulfide levels in the air reached 7,000 ppb at night.

But the smell still lingers, prompting some Carson residents to say the cleanup isn't quick enough.

"If you can't sleep in your home, it's not your fault," Davis-Holmes told local news station KTLA, adding, "if it was any other community, I think a community that was not of color, I think we would've gotten a better response time."

To eliminate the gas, public-works officials may eventually need to dredge the channel to remove debris from the bottom - a process that could take months.