Farm Health and Safety Alert: Understanding Silo Gas Dangers

Farm Health and Safety Alert
Understanding Silo Gas Dangers
John Shutske – Professor, Biological Systems Engineering
UW-Madison College of Agricultural and Life Sciences


  • Silo gas is a potentially deadly hazard that poses significant risks to farmers and agricultural workers. The term ‘silo gas’ generally refers to a combination of gases that include nitrogen dioxide and other oxides of nitrogen. This alert aims to educate the agricultural community on the dangers of silo gas, how to recognize its presence, and preventive measures to ensure safety.
  • We are particularly concerned for the fall corn silage harvest of 2023. Drought conditions and crop stress that occurred throughout much of the summer will likely lead to greater risk this year because of increased nitrate levels in plants.

What is Silo Gas?

  • Silo gas is a byproduct of the fermentation process that occurs when green forage is stored in a silo. This includes corn silage as well as other plant materials. The gas is primarily composed of nitrogen dioxide (NO2), a gas that causes severe respiratory distress and even death upon inhalation. We typically think about silo gas being produced in more tightly confined tower silos that can be difficult and unsafe to enter at any time if proper safety steps are not followed. However, silo gas is also produced in bunkers, piles, and bags. I have investigated incidents in the past 25 years where silo gas has accumulated in areas adjacent to bunkers and between silo bags resulting in harm to livestock, pets, and birds. But it is also a hazard to people especially on still days or cool mornings where there is no or little air movement.
Silo gas is created when green forage material is placed into a silo.

The Dangers:

  • Respiratory Issues: Inhalation of silo gas leads to severe respiratory problems, including chemically induced pneumonia, pulmonary edema, and injury to the lungs.
  • Fatal Exposure: High concentrations of silo gas can be fatal within minutes.
  • Delayed Symptoms: The effects of silo gas exposure are often not immediate, leading individuals to underestimate the risk. In fact, often, patients who end up in the emergency room or hospital following silo gas exposure show up 8-12 hours after working in and around a silo, sometimes being awakened late at night with severe symptoms.
Silo gas is produced in bags, bunkers, piles, and tower silos.

Warning Characteristics:

  • Yellow-Brown Haze Fumes: A visible sign of silo gas is sometimes a yellow-brown cloud near the silo’s opening or adjacent to a bag or bunker/pile.
  • Pungent Smell: A bleach-like, pungent odor is often present.
  • Animal Behavior: Livestock may show signs of distress or avoid the area near the silo. In some instances, dead birds may be found in areas where high gas levels are present. There is also risk to pets.

Prevention Measures:

  • Stay Away and Avoid Entry: Never enter a tower silo, especially within the first 72 hours after filling, as this is when gas concentrations are highest. With bags, bunkers, and piles, it’s also best to stay away from the perimeter for at least three days. Be especially wary if the weather is very calm with no wind. This seems to be especially an issue here in the early fall, early in the morning. With outside, unconfined storage, the risk is generally lessened as days warm up and we see a nice breeze.
  • Ventilation: Always ensure proper ventilation when working near silos. With a tower silo, this includes adjoining feed and silo rooms and nearby areas.
  • Personal Protective Equipment (PPE): It is vital to know that most types of respirators used on farms provide no protection against toxic gases like silo gas in these first several days after filling a silo. Confined silos and grain bins also generate carbon dioxide, and the only real safe way to enter is with a self-contained breathing apparatus or air pack. The best advice is to stay out and wait.
  • Barriers and Warning Signs: It’s always best, where practical, to lock or secure areas where silo gas is likely to be present in those first couple weeks after filling a silo. When that is not practical, place visible and readable (in the correct language) warning signs near silos to alert others of the hazard.
  • Education: Ensure that all farm workers are educated about the risks and preventive measures associated with silo gas. This is a great topic for a tailgate training session or other weekly or biweekly safety meeting with hired employees.
  • When Entry is Necessary: Silo gas is usually produced over the course of up to two weeks, though the greatest production is in the first 72 hours. During the two-week, high-risk window, it’s best to NOT enter the silo. If necessary, you will need to test for gas after continuously ventilating with a blower for several hours. This is complex, and usually requires special training, leading to the recommendation to stay out.

Emergency Response:

  • Immediate Evacuation: If silo gas is suspected, leave the area immediately.
  • Medical Attention: Seek immediate medical care for anyone showing symptoms of exposure. If someone working in an area where silo gas is present collapses, call 911 and get help to the scene immediately.

Silo gas is a serious and often underestimated hazard in the agricultural industry. Awareness and education are the first steps in preventing tragic incidents. I am also a big believer in pre-planning and preparing ahead of time. If there are ways to 100% avoid entering a tower silo or having to work around and adjacent to bags, bunkers, and piles for several days after filling, that’s the strongly preferred “safety practice.” Training and educating hired workers and family members about silo gas dangers should also include practical measures on working safely, use of and limitations of protective equipment like respirators, and the need for constant communication and awareness.  By understanding the risks and taking appropriate preventive measures, we can protect our farmers and agricultural workers from this silent but potentially lethal hazard.

John Shutske (