Ooh that smell. Salton Sea stink is back. What causes it?

You're not imagining it.

The funky odor that signifies summer at the Salton Sea is wafting in tiny but detectable amounts over portions of the Coachella Valley on Wednesday, air monitors confirm. Politely referred to as a "rotten egg" smell, the odor at full strength is more like manure combined with sewage. It's actually from hydrogen sulfide that forms in large quantities in the drying water body.

When temperatures soar and combine with early morning humidity or other sticky weather, the putrid compound rises off the water and blows where the breeze takes it. The levels are low: As of 8 a.m. Wednesday, 11.2 parts per billion were measured in Mecca, and 8.3 parts per billion were measured at Indio's Amisted High School, per the South Coast Air Quality Management District's automatic monitors.

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Though small, those levels are detectable by the human nose but not risky, said Kim White, a spokeswoman for AQMD, the region's air regulator, in an email about similar levels a few weeks ago. "While levels were likely high enough to smell, they were still well below the California hourly ... standard."

The hourly measurements have not hit 30 parts per billion, at which point state law mandates a public advisory must be issued. Headaches, nausea and respiratory or nasal burning are all possible temporary side effects from those levels.

For now, there's just that faint but still unmistakable odor in parts of Mecca, Indio, Palm Desert, La Quinta and other communities in the path of prevailing breezes.

AQMD monitors hydrogen sulfide at three locations in the Coachella Valley: Indio, Mecca, and at a monitor in an unpopulated area on the northwest side of the Salton Sea. Residents can see daily measurement data at https://www.saltonseaodor.org/ and sign up for alerts at the same link, to be notified when levels exceed the state maximum.

Janet Wilson is senior environment reporter for The Desert Sun and co-authors USA Today Climate Point. She can be reached at jwilson@gannett.com