An odor advisory for the Coachella Valley that was first issued more than a week ago due to elevated levels of hydrogen sulfide wafting up from the Salton Sea — creating a smell similar to rotten eggs — remains in effect through Monday afternoon after being extended over the weekend for the third time.

The South Coast Air Quality Management District issued the 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 odor advisory was originally set to expire last Tuesday morning, but that day, the district extended it for two more days.

On Thursday, the district detected hydrogen sulfide concentrations were even higher than the original readings, at 253 parts per billion near the Salton Sea shore and 63 parts per billion in Mecca, and again extended the advisory through Saturday afternoon.

On Saturday morning, the district detected hydrogen sulfide concentrations of 76 parts per billion near the shore and 35 parts per billion in Mecca — much lower than earlier readings, but still above the state standard — and the advisory was extended yet again through Monday afternoon.

Winds from the south are expected again during daylight hours Monday, and elevated concentrations of hydrogen sulfide are expected in the air, particularly in the morning, according to the AQMD.

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.”

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