The Texas City Office of Emergency Management says it is aware of what is described as "an upset" at the facility.
Texas City officials say that FM 519 to 605 S Loop is temporarily closed as a precaution.
The Texas City Office of Emergency Management provided a statement in an update on Saturday's incident.
"Valero is experiencing an upset, higher than normal emissions of SO2. Actively Air monitoring in the community with negative response on air monitors as of now 6:45 p.m. portions of the S Loop have been closed out of precaution."
According to the United States Environmental Protection Agency, sulfur dioxide is one of a group of highly reactive gasses known as "oxides of sulfur," and are emitted into the air as result of fossil fuel combustion and other industrial processes.
It is still unclear as to what led up to the incident.
Eyewitness News is gathering the facts on this breaking news story.