On scorching days when winds blow across the California desert, the Salton Sea regularly gives off a stench of decay resembling rotten eggs.

New research has found that the shrinking lake is emitting the foul-smelling gas hydrogen sulfide more frequently and at higher levels than previously measured.

The findings document how the odors from the Salton Sea add to the air quality problems and health concerns in communities near the lake, where windblown dust drifts from exposed stretches of lakebed and where people suffer from high rates of asthma and other respiratory illnesses.

“The communities around the Salton Sea are on the front lines of a worsening environmental health crisis,” said Mara Freilich, a co-author of the study and assistant professor in Brown University’s Department of Earth, Environmental and Planetary Sciences.

The Salton Sea is California’s largest lake, covering more than 300 square miles in Imperial and Riverside counties. Hydrogen sulfide is released as a byproduct of decaying algae and other organic material in the lake, where accumulating fertilizers and other nutrients from agricultural runoff and wastewater feed the growth of algae.

Hydrogen sulfide, or H2S, is toxic and studies have found that health effects of exposure at certain levels can include dizziness, headaches, vomiting, cough, chest tightness and depression. Although being exposed to high levels in the workplace is a widely known health hazard, less is known about the health effects of chronic exposure to the gas at lower levels.

People who live near the Salton Sea, many of them farmworkers, have complained for years that the stench, which tends to emerge most strongly in August and September, can give them headaches, nausea and nosebleeds.

Freilich and other researchers analyzed existing air-quality data from three monitoring stations maintained by the South Coast Air Quality Management District in Indio, Mecca and the reservation of the Torres Martinez Desert Cahuilla Indians.

They worked with the local nonprofit group Alianza Coachella Valley to install an additional air-quality sensor on a wooden piling protruding from shallow water near the north shore. The sensor has often detected hydrogen sulfide at high levels.

Examining data from different monitoring sites between May 1 and July 25, 2024, they found a striking contrast. Although the monitor on the Torres Martinez reservation detected hydrogen sulfide at levels exceeding the state air-quality standard for only four hours during that time, the sensor over the water found 177 hours with levels above the threshold.

The scientists said their results indicate that a significant portion of the gas that’s being released by the Salton Sea isn’t being measured, even as the stench drifts through the area’s predominantly Latino communities.

“These findings highlight the need for improved air quality monitoring and more effective environmental management policies to protect public health in the region,” the researchers wrote in the study, which was published May 31 in the journal GeoHealth.

The Salton Sea lies about 242 feet below sea level in the Salton Trough, which over thousands of years has cycled between filling with Colorado River water and drying out. The lake was formed most recently in 1905-07, when the Colorado flooded the region, filling the low-lying basin.

In the 1950s and ‘60s, the Salton Sea became a popular destination where tourists flocked to go fishing, boating and waterskiing. Celebrities including Frank Sinatra and Lucille Ball visited the lake during its heyday.

But lakeside communities deteriorated after flooding in the 1970s. Fishing waned as the lake grew too salty for introduced species such as corvina, and people stopped boating as the water quality worsened.

The lake has for more than a century been sustained by water draining off farms in the Imperial Valley, but it has been shrinking since the early 2000s, when the Imperial Irrigation District began selling a portion of its Colorado River water to growing urban areas under an agreement with agencies in San Diego County and the Coachella Valley.

The lake’s level has declined about 13 feet since 2003. Its water is now about twice as salty as the ocean and continues to get saltier with evaporation. Bird populations have declined.

Hydrogen sulfide builds up in the lower, oxygen-deprived layer of water in the lake as decaying algae and other material decompose. During the hottest times of year, a warm upper layer of water forms. Then, when winds churn up the lake, some of the deeper water can rise to the surface and release the stinky gas into the air.

California’s ambient air-quality standard is 30 parts per billion, averaged over one hour. The study found that under certain wind conditions, hydrogen sulfide levels were on average 17 parts per billion higher at the newly installed sensor over the water compared with an existing monitor near the shore.

Sometimes, the sensor detected levels as high as about 200 parts per billion.

People can detect the smell of the gas, however, at levels as low as 1 or 2 parts per billion.

“Residents exposed to hydrogen sulfide are impacted not only in their physical health — experiencing respiratory irritation, headaches and fatigue — but as well in their quality of life,” said Diego Centeno, the study’s lead author, who conducted the research while studying at Brown University and is now a doctoral student at UCLA.

“If you want to be active outside, go on a run or do something, and it smells like rotten eggs, you’d be more inclined not to,” Centeno said. “Especially during summertime, nobody wants to go outside.”

Centeno grew up within sight of the Salton Sea in the low-income community of North Shore. He said he was always fascinated by the immense body of water, not knowing why he never saw anyone bathing or boating in it.

“As water levels continue to decline, if nothing is done, this hydrogen sulfide gas really has the potential to grow,” Centeno said. “So the more we understand, the more we can learn how to mitigate and restore the Salton Sea.”

The researchers said their findings highlight the need for increased air-quality monitoring around the Salton Sea, and for regulators to focus on hydrogen sulfide as a pollutant that affects people’s health.

Freilich said regional water regulators should prioritize setting of water-quality standards for the Salton Sea, a step that could lead to efforts to treat or reduce the nutrient levels of water flowing into the lake.

“The water quality in the sea is affecting the air quality,” she said. “It requires the attention of multiple agencies, because it is something that connects water quality and air quality, which are typically handled separately.”

The South Coast Air Quality Management District, or AQMD, regulates air pollution in the Coachella Valley, including the northern portion of the Salton Sea. In May, the agency installed a fourth monitor for hydrogen sulfide on the northeastern side of the lake.

“This H2S monitoring network is very comprehensive,” said Rainbow Yeung, an AQMD spokesperson, adding that there are currently only a few other monitors reporting such data in the country.

Yeung said in an email that the sensor installed by the researchers is of a different type than the agency’s monitors and “may have higher H2S readings as the location of the sensor is over the source of likely emissions, which can be dispersed and therefore may not be representative of levels experienced by the community.”

AQMD issues alerts whenever hydrogen sulfide levels reach the state standard of 33 ppb at any of the monitoring sites. (Residents can sign up to receive these air-quality alerts at www.saltonseaodor.org.)

The California Office of Environmental Health Hazard Assessment has established a chronic exposure threshold for hydrogen sulfide of 8 parts per billion, a level at which long-term exposure over many years may begin to result in health effects.

The highest annual average concentration at any of the AQMD monitoring sites since 2016 has been 5.5 parts per billion, and annual averages have typically been less than 3 parts per billion, levels at which health effects are not expected, Yeung said.

The water that drains from Imperial Valley farmland and feeds the sea comes from the Colorado River. A quarter-century of mostly dry years compounded by climate change has prompted difficult negotiations among seven states over how to use less water from the dwindling river.

As these talks examine water-saving solutions, Freilich said, policymakers should “account for the health impacts on communities” and prioritize steps that will help mitigate the problems.

California officials recently sent water flowing from a pipe onto hundreds of acres of dry lake bed near the south shore, filling a complex of shallow ponds in an effort to create wetland habitat for fish and birds, and help control lung-damaging dust.

It’s not known how these new wetlands might affect emissions of hydrogen sulfide, and Freilich said she and her team plan additional studies focusing on wetlands and shallow-water areas.

Consuelo Márquez, a Coachella resident who has helped with the research, said she lived for several years as a child in North Shore, where she got nosebleeds and experienced the rotten egg odor, a “really strong fishy smell.”

“I would wake up with blood on my pillow,” she said. When she asked her father about it, she recalled him saying: “This happens because of the lake, because of the air.”

She said the study’s results validate the concerns many people have been raising for years.

Aydee Palomino, a co-author of the study and environmental justice project manager for the group Alianza Coachella Valley, said the study shows people are “breathing in pollutants that are under the radar of traditional monitoring systems.”

“This has the potential to have really far-reaching ramifications if it’s not addressed,” Palomino said.

Funding for the research came from the Burroughs Wellcome Fund, Google’s Environmental Justice Data Fund and NASA. But Freilich learned in March that the Trump administration had terminated the NASA grant under an order targeting diversity, equity and inclusion programs.

The researchers have an ongoing appeal of that decision, which Freilich said has been disruptive to ongoing work.

“The community is who’s going to suffer at the end of the day,” Palomino said. “And it is unfortunate because now it comes back to us to fill in the research gaps.”