An odor advisory for the Coachella Valley was extended three times, due to the continuous elevated levels of hydrogen sulfide which smells like rotten eggs wafting up from the Salton Sea.

The South Coast Air Quality Management District first issued the odor advisory on Aug. 18, after detecting hydrogen sulfide concentrations at 239 parts per billion, exceeding the state standard of 30 parts per billion, in a sparsely populated area immediately downwind from the Salton Sea. The advisory was originally set to expire on Aug. 20, however, the district extended it for two more days after another test showing even higher amount that the original readings.

On that day, the district picked up hydrogen sulfide concentrations were at 253 parts per billion near the Salton Sea shore and 63 parts per billion in Mecca, extending the advisory through Aug. 24. Yet again, after another test, despite the amount being lower than the earlier readings, it was still above the state standard, and the advisory the advisory was extended yet again through Aug. 27.

Elevated levels of the gas near the lake are relatively common and are a product of natural processes in the water.

There is increased potential for the foul-smelling odors as winds shift, especially during the summer in the early morning and late afternoon, or as thunderstorms occur over the southwestern U.S. deserts, according to the AQMD.

At 30 parts per billion, “most individuals can smell the odor and some may experience symptoms such as headaches and nausea,” according to an AQMD statement. “However, the symptoms associated with this level of exposure are temporary and do not cause any long-term health effects. Humans can detect hydrogen sulfide odors at extremely low concentrations, down to a few parts per billion.’”