A highly reactive nonmetal, oxidizing agent that readily forms oxides with most elements as well as with other compunds. Oxygen accounts for approximately 20.9% of the volume of the atmosphere. While essential to aerobic (oxygen-depending) life forms, a high concentration of oxygen constitutes a severe fire and explosion hazard of even normally non-burning materials. This is the reason that instruments that contain an oxygen sensor warn for lower concentrations (as these present a choking hazard) and higher concentrations (as these present a fire hazard). Apart from dedicated sensors, oxygen is detectable with a vast range of colorimetric gas sampling tubes.