An air quality advisory has been issued for Southern San Diego after elevated levels of hydrogen sulfide has been detected. 

The gas causes a strong rotten egg smell and cause headaches and nausea. Officials say it’s in the air from the trans-boundary wastewater and sewage flows near the Tijuana Valley. 

While the county is monitoring the levels of hydrogen sulfide locally, Airnow.gov monitors real-time air quality across the U.S. Residents most impacted by the odors can apply to see if they qualify for a free home air purifier through sdapcd.org/AIRE.