context: Oxygen gas (O
  2) can be toxic at elevated partial pressures, leading to convulsions and other health problems.[j] Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals (kPa), equal to about 50% oxygen composition at standard pressure or 2.5 times the normal sea-level O
  2 partial pressure of about 21 kPa. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30%–50% O
  2 by volume (about 30 kPa at standard pressure). (although this figure also is subject to wide variation, depending on type of mask).
question: What is composed of 21%-50% O2 by volume?