The owner of an olive oil factory in Almyro, Greece and one of his workers have died after the employee fell in an empty oil storage container and the owner and his sons tried to rescue him, according to an Associated Press report.

The report cited police who said the 55-year-old employee was cleaning the tank yesterday when “fumes from the residue of the oil” that had been stored in the container caused him to pass out and fall in.

According to olive oil consultant Paul Vossen, however, it probably wasn’t residue from olive oil that was to blame, but a lack of oxygen. The culprit is a well-known hazard with storage tanks: even if they’re open on the top, a lack of oxygen can cause a victim to suffocate, he said. Olive oil would not give off toxic fumes of any kind.

“Air would not go in unless there was also an opening at the bottom to get air flow,” Vossen warned. “This is a common problem for storage tanks. Most places have specific protocols for entering tanks and oxygen tanks available for rescue. The report from Greece said the rescuer found oxygen, probably from a compressed oxygen tank available nearby for safety and rescue.”

The factory owner (60) whose name has not been released, and his sons, ages 29 and 32, all climbed in to help the man. They also lost consciousness.

A fifth man who found an oxygen tank rescued the sons, but the owner and one of the workers died.