The most common and most dangerous gas present in a conventional silo (typically constructed of concrete staves held together by steel hoops or made of reinforced concrete, steel, galvanized tile, or brick) is nitrogen dioxide, which has a bleach-like odor and accompanying low-lying yellow, red, or dark brown fumes or haze. Nitrogen dioxide starts forming within hours of forages being ensiled. Gas formation peaks about three days after harvest and then begins to decrease in concentration. Nitrogen dioxide is heavier than air, so it tends to settle on top of the silage or to flow down the chute and collect in adjoining feed rooms or other low-lying areas in the barn, near the base of the silo.
Nitrogen dioxide is dangerous because it causes severe irritation to the nose and throat, which can result in lung inflammation. Even minimal exposure to nitrogen dioxide is hazardous because a person exposed to the gas may not have any immediate pain or discomfort. However, hours later, he or she can suffer from fluid collection in the lungs that may be serious enough to cause death. Ongoing or reoccurring pneumonia-like symptoms can persist for two to six weeks after the person's initial exposure to silo gas. Anyone who has been exposed to any level of nitrogen dioxide, for even a short period of time, should seek immediate medical attention.
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