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

"Silo gas, or nitrogen dioxide, is a natural byproduct 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 causes 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 for several hours or even days. If you think you've been exposed, see your doctor. To avoid silo gas exposure, follow these recommendations:

* Don't allow anyone near the silo or in attached buildings for two to three weeks after filling.

* Post big warning signs 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 any gas is present.

* 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 a potentially deadly confined space. If you have employees entering the silo at any time, specific regulations apply.