AUSTIN (KXAN) — If you’re worried about a weird odor in the air outside on Sunday, the Austin Fire Department said it’s most likely what they like to call “The Luling Effect.”

The distinct odor comes from oil fields in Luling and Lockhart and a spokesperson with AFD said it’s normal for this time of year.

When you get a cold, still night, that sulfur smell collects at the ground instead of dissipating like normal, so in the morning the wind brings up the smell and it spreads across the city.

In a tweet the department said they’re receiving multiple calls about a natural gas odor outside all the way from Pflugerville to East of I-35 in South Austin.

You are encouraged to call 911 if the odor is inside or if you see an active leak.