Grain storage structures, including mobile grain carts and stationary grain storage bins are one of the leading causes of farm deaths in the United States.  In 1992, the National Institute for Occupational Safety and Health (NIOSH) reported 9 fatalities in the United States from getting suffocated in grain or caught in grain augers in a bin or cart.  The State of Illinois alone reported 22 deaths from grain bins, as well as an additional number from grain carts, in the years from 1986-1994.  There are a number of causes for these accidents. 
When grain is stored with a relatively high moisture content, the grain tends to cake or crust at the surface and form a "bridge" of caked grain which can extend all of the way between the sidewalls of the storage bin or cart.  Such bridged grain is extremely hazardous because it prevents grain from flowing into the bin or cart and hides underlying pockets of air beneath the bridge.  Farmers will often walk on the bridged grain in an effort to break it up and fall through the bridge, thus getting engulfed in the grain. 
Farm workers are also often buried by stored grain as the grain is being emptied from the bottom of the bin or cart.  The flowing grain acts much like quick sand, pulling the worker completely under the grain surface.  According to NIOSH, forces created by a grain auger unloading grain are so great that, once a person is buried up to the waist, they stand virtually no chance of escaping from the auger force, even with the aid of a safety rope.  The force required to remove a person buried to the chest in grain can exceed 2,000 pounds, i.e. about the weight of a small car.  Typical unloading rates will fully bury an adult person within one minute.  High capacity conveyors can move 5,000 bushels of grain in an hour.  At these flow rates, a six foot adult will be totally buried in 15 seconds. 
The risk of suffocation increases as grain ages in a bin due to the emission of carbon dioxide, which displaces the oxygen supply in the bin.  Thus, even if a worker is not buried completely, he can suffocate due to the lack of oxygen in the bin. 
NIOSH recommends the following steps to prevent such accidents, 1) Break up crusts of grain from outside the bin; 2) Avoid entering storage bins or grain carts; 3) If you must enter a bin or cart, stay above the material at all times, assume that all bridged material is unstable, wear safety harnesses with properly fastened life lines, stop the flow of grain prior to entering, and turn on any ventilators
It is clear, then, that a need exists for improved safety equipment for grain storage bins and grain carts.  Such equipment should preferably be economical and easily installed, yet reliable, should not interfere with operation of the grain cart or bin and should be passive and not easily defeated in purpose.