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Hidden dangers

Tina Kohlman Published on 14 August 2013

Exposure to deadly silo gases continues to occur wherever silos exist and will continue to be a hazard on the farm as long as silage remains a common feedstuff for livestock.

Exposure to silo gas can cause serious injury: severe respiratory distress, permanent lung damage and, in some cases, death.

During late summer and early fall, the danger for silo gas is at its peak as farmers fill silos with fresh feed. Exposure to silo gas is highest in upright silos, but gas will also be generated in horizontal and bag silage storage systems.

How gases form
Gases are produced during the natural fermentation of chopped silage shortly after it is placed in storage. Carbon dioxide is always produced during the fermentation process as the limited oxygen is converted into small amounts of water and carbon dioxide.

Additional carbon dioxide is produced as acid-forming anaerobic microbes feed on the sugars and starches in the silage, releasing organic acids such as lactic and acetic acid and carbon dioxide.

The acidity of the silage continues to increase over a period of several weeks, eventually reaching a pH level that will kill the acid-forming bacteria. Even though carbon dioxide is not toxic, it is potentially lethal as it is heavier than air and displaces oxygen, which can cause asphyxiation.

The gas of greater concern is nitrogen dioxide, often called silo gas, which can cause serious respiratory injury to people. Not to be confused with nitrous oxide, or “laughing gas,” nitrogen dioxide is a highly corrosive, toxic gas.

Silo gas appears as a reddish to yellowish-brown haze with an acrid or bleach-like odor. Since nitrogen dioxide is 58 percent heavier than air, silo gas will settle on the surface of the silage, flow down silo chutes and accumulate in low-lying areas in and around storage structures.

During the ensiling process, the nitrates in silage crops are converted to oxides of nitrogen. Nitrates react with the organic acids in the silage to form nitrous acid. As the temperature of the silage increases during fermentation, nitrous acid is broken down to form mixtures of various oxides of nitrogen (NO, NO2, N2O3 and others).

The oxides of nitrogen are formed within hours and up to three weeks after the silo is filled. The percent of nitrate in the plant material ensiled determines how much oxides of nitrogen are produced. Weeds have the highest percentage of free nitrate, but corn silage forms more nitrogen dioxide silo gas than any other crop.

Environmental conditions such as prolonged summer drought followed by rain just before ensiling and high soil nitrogen levels can cause excess nitrates to accumulate in the plant, which in turn can increase the risk for production of this deadly silo gas.

What happens to your lungs
Nitrogen dioxide is of great concern because it is deadly when inhaled. Silo-filler’s disease is the term given the injury resulting from exposure to silo gas. When inhaled, nitrogen dioxide comes in contact with the moisture in the lungs, where it is converted to nitric acid.

Nitric acid acts very quickly, causing a chemical burn within the respiratory system. The chemical burn causes severe burning and scarring of the lungs and other parts of the respiratory system, which limits or altogether stops oxygen transfer from the lungs to the bloodstream.

Tiny blood vessels within the lungs are broken down, causing bleeding. Inhaling even a small amount can result in serious, permanent or fatal lung injury. High concentrations of nitrogen dioxide may make a person helpless in two to three minutes. Symptoms of silo-filler’s disease include coughing, burning, shortness of breath, chills, fever, headaches, nausea and vomiting.

While a person may not immediately experience the symptoms from a mild exposure, in three to 30 hours there is a slow, progressive inflammation of the lungs that results in fluid build-up. This can be fatal. A unique characteristic of this disease is that there may be a relapse in two to six weeks after the original episode.

Preventing what you can’t see
The only way to prevent exposure to silo gas is to stay out of the silo during the first three to four weeks after filling or to use a self-contained breathing apparatus approved for confined space entry.

Label the area with signage to warn of the gas hazards. If it is necessary to enter a silo within the first three weeks after filling, Cheryl Skjoolas, a farm safety and health specialist with UW Extension, recommends the following precautions for upright silos:

• Be alert for bleach-like odors or yellowish-brown gases in or near silos.

• If you must enter the silo (e.g., to set up a silo unloader), do so immediately after the last load is in. Don’t wait several hours, after supper or overnight. If you need to wait until the next day, save the last load to add before entering.

Ventilate by running a forage blower at least 15 to 20 minutes before entering and keep it running while inside.

• When opening a chute door for the first time after filling, if possible, go a door above the silage level. Have someone keep in contact with you from outside the silo chute.

• Keep a door open down to the silage surface, and have someone keep in contact with you from the outside.

• Ventilate the silo room adequately for three weeks after filling, keeping windows and doors open.

• Keep the door between the silo room and the barn closed to prevent silo gas from killing livestock.

Silo gases are not limited to traditional, upright silos. Even though risk for exposure is greatest with upright silos, there is also risk for exposure when using horizontal silos or bags for feed storage.

• Consider where nitrogen dioxide gas may drift from horizontal silos and silage bags, as this gas is heavier than air and may collect in other buildings or low areas.

• Cover immediately when done harvesting.

• Watch for signs of gas when repairing plastic or working in the area.

• Do not puncture bubbles in plastic that may release gas directly into an individual’s face. Provide distance with the individual and the bag or plastic by using a hay spear or other long object to puncture bubbles.

• Use caution when opening the plastic during the first three weeks after covering or sealing a horizontal silo or silage bag.

The effects of silo gas can occur quickly and are deadly. Be observant at all times when working around feed storage that has been filled within the past three weeks. Educate and warn family members and employees of the dangers of silo gas and symptoms.

If you experience throat irritation or coughing in the silo, get to fresh air immediately. Seek medical attention immediately; exposure to silo gas is dangerous and can be lethal.   FG

Exposure to deadly silo gases continues to occur wherever silos exist and will continue to be a hazard on the farm as long as silage remains a common feedstuff for livestock. Photo by Paul Marchant



Tina Kohlman
Dairy and Livestock Specialist
University of Wisconsin Extension