Fire crews responded at 8:43 a.m. to a report of an alarm sounding at the refinery at 6880 Smith Ave., fire spokeswoman Aisha Knowles said.
The crews were told that a pipe containing a superheated oil had leaked, and that about 70 employees at the refinery had safely evacuated, Knowles said.
Evergreen Oil handles hazardous waste materials, including used motor oil, antifreeze and other toxic waste, according to the company's website.
A county hazardous materials team responded and determined there was no health hazard posed by the leak, according to Knowles.
Newark police also responded and said in a statement that there may be a strong chemical smell near the refinery but it is not considered hazardous.
One person at the scene had a medical complaint and was treated at the scene. No other injuries were reported, Knowles said.
She said one fire engine remained at the refinery as of 10:30 a.m. as a precaution, but that the rest of the crews had left and the refinery's employees had been sent home for the day.