Oxygen therapy may be used to assist in raising blood oxygen levels. Protein molecules contain nitrogen atoms in addition to carbon, oxygen, and hydrogen. Aldehydes are organic compounds of carbon, oxygen and hydrogen. Carbohydrates are composed of carbon, oxygen, and hydrogen. The gas is ionized in the magnetosphere producing sulfur and oxygen ions. Marine organisms produce oxygen and sequester carbon. Higher than normal concentrations of oxygen lead to increased levels of reactive oxygen species. Pulse oximetry is particularly convenient for noninvasive continuous measurement of blood oxygen saturation. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. Anaerobes utilize electron acceptors from sources other than oxygen gas. Chemical factors include oxygen and trace elements. Pulse oximeters use them for measuring oxygen saturation. Such metals contain an element capable of reaction with oxygen, usually phosphorus. Mixtures of liquid oxygen and flammable materials can be dangerously explosive. Their light shows evidence of elements such as carbon, magnesium, iron and oxygen.