Experts said it's likely from the decaying Salton Sea, near Indio. The body of water is known to cause a bad smell this time of year when large amounts of fish die off and decay.
"The problem that you get is, last night we had a major wind storm out here and the Salton Sea is very shallow," said Andrew Schlange of the Salton Sea Authority. "The wind can stir the Salton Sea up and it brings up gases off the bottom of the sea."
However, Schlange said that may be a stretch because the Salton Sea is 50 miles away from some of the areas reporting the odor. He said the wind would not likely carry smell that far west.
In Sylmar, teachers at Osceolo Elementary School kept students indoors as a precaution due to the noxious smell. Some students were reporting minor stomach aches.
Los Angeles fire officials said they were aware of the odor in the valley and foothill communities, but have not yet identified the source. They do not believe there are any hazards associated with it.
South Coast Air Quality Management District said it has received more than 100 calls about the smell since midnight. Field inspectors are trying to determine the exact cause of the sulfur odors.
Some of the causes of foul odor from the Salton Sea include high levels of salt from irrigation runoff, the sea's high evaporation rate and the inflow of untreated water from Mexico.
When the seasons change, the wind direction reverses, increasing the potential for the odor.
"Sulfur smells could come from a large body of water when you have an algae bloom or a fish kill, it could come from a wastewater treatment plant. Those are the kinds of things that you would think of with this kind of smell," said Sam Atwood of the AQMD.