A recent incident in central Pennsylvania serves as a reminder of the dangers that farmers and potentially emergency responders can face. Here is Pennsylvania, various grass/hay forages are typically ensiled starting in May, and various forages continue to be harvested throughout the growing season. It is a naturally occurring process that occurs with fresh cut silage, and gas can be present in varying degrees in any silage even long after harvest depending on storage.
Silo gas begins forming in a few hours and continue as long as 2 weeks after filling. Residual may remain even longer. The gas is generally heavier than air, and will often sit on the surface of silage but if pushed by air currents, it may go down the chute into structure. Exposure normally happens after entering the silo for maintenance, leveling or unloading the product. Exposure can also occur outside the silo by gas that has leaked out of the silo into surrounding buildings.
What is it? According to the National Farm Medicine Center, silo gas is actually nitrogen dioxide, an extremely toxic, yellowish-brown gas with a bleach-like odor. During the fermentation process, oxygen combines with nitrates in plant materials resulting in the production of nitric oxide gas. This combines with oxygen in the environment to produce nitrogen dioxide.¹
Silo gas is primarily Nitrogen Dioxide (NO2), which is a very toxic gas. To understand how much so, consider the US DOT hazard labeling for commercial uses includes Poison Gas, Oxidizer, and Corrosive, and lists it as a strong oxidizing agent that is both water- and air-reactive. The IDLH for NO2 is 20 ppm, and the permissible exposure level (PEL) is 5 ppm.
Physical signs of silo gas exposure can include pulmonary edema, fever, cough, congestion, and systemic inflammatory response syndrome. A recognized medical condition, known as Silo Fillers Disease can occur as a result of either acute or chronic exposure to silo gas. It is a chemical pneumonitis resulting from exposure to nitrogen.
While it is called “silo gas”, it is not only present within silos, here is a short video of off-gassing from what is commonly called an “ag bag”.
¹National Farm Medicine Center (Marshfield Clinic): http://www.marshfieldresearch.org/Media/Default/NFMC/PDFs/SiloGas.1.pdf
National Ag Safety Database – Silo Gas Dangers: https://nasdonline.org/64/d001621/silo-gas-dangers.html