The accident occurred in an industrial setting. Miracle-Mineral-Supplement is an industrial strength concentration of sodium chlorite. Sodium chlorite, at this concentration, is used widely in a variety of industries.
When you activate Miracle-Mineral-Supplement you form chlorous acid. Part of the solution has chlorine dioxide gas in it. Chlorine dioxide is a respiratory irritant. At higher concentrations it causes bronchitis and pneumonia like symptoms that can lead to lung damage and if the concentration is high enough, death.
Fortunately, chlorine dioxide has a distinctive odor and the bodies reflex is to avoid it.
When activating sodium chlorite solutions it is best to do so in a well ventilated area so you can avoid the fumes.