Farm workers always need to be aware of dangerous silo gas at silo-filling time, but this year presents an especially dangerous threat.

Nitrogen dioxide, commonly called silo gas, is apt to be at higher levels in plants that have been drought-stressed, says John Shutske, University of Minnesota Extension professor and safety and health specialist.

"Stay out of a newly filled silo and surrounding areas for two to three weeks after it's been filled," Shutske said. "When it's safe to enter, run the blower for at least an hour to assure that the gas has been removed and there's a supply of fresh air."

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Farm workers who must enter the silo need to do it immediately after filling, according to Shutske.

"But even then, there could be significant levels of gas, so maximum ventilation is essential," he said. "The silo must be treated as a dangerous confined space."

Most farmers who make silage are somewhat familiar with the gas, but don't always understand the true risks, says Shutske. Silo gas is formed as a natural by-product of silage production when chopped-up plant material ferments.

Silo gas dangers aren't limited to upright silos, says Shutske.

"We typically associate silo gas with upright, concrete silos," he says. "But silage in bag systems and bunker silos also produce silo gas. The risks with these other storage systems are lower, however, since they are more easily ventilated with ambient outside airflow."

Shutske says silo gas has a bleach-like odor and a yellow-brown, hazy color, but it can be hard to see in dim lighting. It's heavier than air, causing it to settle into low-lying areas. These may include the bottom of silo chutes, between silage bags or in low spots within a bunker silo.

"Silo gas is highly toxic, even at low levels," said Shutske. "Toxic exposure can and does occur to producers, children, livestock and pets that are exposed to the gas."

A farmer or family member exposed to low levels of silo gas might only notice some mild irritation or intermittent coughing, says Shutske. With higher gas levels, people can become unconscious. If not removed to fresh air, high exposure can be fatal.

At the lower levels of exposure, the nitrogen dioxide will react with the moisture in a person's airway to form various acids. These compounds are highly irritating and corrosive, says Shutske. The body responds by trying to dilute the acids with more fluid. Thus, a person can die several hours or even days after an initial exposure to silo gas, due to excess fluid build-up in the lungs. Even if a person recovers, they can still face a lifetime of respiratory health problems from a single exposure.

Shutske recommends checking with your local silo salesperson or builder for the latest information on how to safely enter silos.