It is likely that she passed out due to silage gases. They are produced during the natural fermentation process that takes place after silage – grass-based animal fodder – is placed into a silo. In a sealed silo, carbon dioxide develops as the prominent gas. It can displace oxygen, leading to asphyxiation. If it is a conventional silo, the gas is most likely to be nitrogen dioxide. It also displaces oxygen and, in addition, can cause delayed effects like pulmonary oedema. In high concentrations, nitrogen dioxide appears as an orange-brown cloud.
Tom Johnson is an emergency responder at the National Chemical Emergency Centre