Heavy haze" was first reported at 7:33 p.m. in the San Francisco-bound track of the tunnel, BART spokeswoman Alicia Trost said.
Trains went to single-track service, where only one train could enter the tunnel at a time, causing delays in both San Francisco and East Bay-bound directions.
Crews went into the tunnel to inspect where the haze was coming from, but as of 8:05 p.m. reported that they had not found any evidence of the haze.
By 8:32 p.m. BART service resumed normally, Trost said.
