Nitric acid can be deadly when it forms in human lungs. Breathing in silo gas causes nitric acid to form in the lungs, says John Shutske, farm safety and health specialist with the University of Minnesota Extension Service.

"Silo gas, or nitrogen dioxide, is a natural by-product of silage production," says Shutske. "Dangerous levels of silo gas are generated during the first several days after the silo is filled. Since this gas is heavier than air, it travels down the silo chute and collects in adjoining buildings, including feed rooms and barns."

Shutske says silo gas usually has a yellowish-brown, hazy appearance and a bleach-like smell. Even small exposures to the gas cause extensive lung damage, or even death.

After silo gas is inhaled, it dissolves in the moisture of the lungs to form the nitric acid. The acid burns and irritates the lungs, causing them to fill quickly with fluid. The results can be fatal.

Symptoms of silo gas exposure include shortness of breath, coughing, chest pain, and other breathing difficulties. Sometimes these symptoms can be delayed by several hours or even a few days. If you think you've been exposed, see your doctor.

To avoid silo gas exposure, Shutske recommends the following:

• Don't allow anyone near the silo or in attached buildings during the two to three weeks after filling. Post a big warning sign for adults, and keep kids out with locks and barriers.
• Ventilate the feed room to remove silo gas that may "spill" down through the chute or be blown out by the unloader.
• Adjust the forage distributor to level silage during the filling process. Don't enter the silo to level the material by hand any time the gas is present, even at low levels.
• Always ventilate the silo with the blower running at maximum speed for at least an hour before entry, even after the three-week period of greatest danger has ended. Keep the blower running during silo entry.
• In Minnesota, a silo is considered to be a potentially deadly confined space. If you have employees entering the silo at any time, specific regulations apply for protecting the workers. For a copy of Minnesota's confined space entry regulation, contact the U of M Farm Safety and Health Program at (612) 624-7444.