Pressurized scuba tanks/cylinders containing air or other nonflammable, nontoxic gases are regulated as hazardous materials once they reach a pressure of 200 kPa (29 psig/43.8 psia) at 20ºC/68ºF.

Empty scuba tanks or scuba tanks pressurized at less than 200 kPa are not restricted as hazardous materials. However, airlines and airport screening officials may require valves to be opened all the way for inspection. See http://www.tsa.gov for TSA’s security screening requirements for scuba tanks.