Every year in Pennsylvania we learn of farmers that have either died or become seriously ill as a result of exposure to silo gas. The serves as a reminder of the dangers that farmers and potentially emergency responders can face during harvest time. Here is Pennsylvania, various grass/hay forages are typically ensiled during summer months, and corn silage is put up starting in August depending on location. It is a naturally occurring process that occurs with fresh cut silage, and gas can be present to varying degrees in any silage even long after harvest depending on storage.
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
Nitrogen dioxide 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 as established by OSHA is 20 ppm, but even lower concentrations can cause physical effects. For more information on the chemical properties of NO2 can be found at NITROGEN DIOXIDE | Occupational Safety and Health Administration (osha.gov).
Here is a safety video from Penn State Extension on the dangers of silo gas.
¹National Farm Medicine Center (Marshfield Clinic): http://www.marshfieldresearch.org/Media/Default/NFMC/PDFs/SiloGas.1.pdf