Oxygen (aluminium) cylinders: In Europe most oxygen cylinders are still made of steel and are therefore very heavy compared to the aluminium cylinders used in countries like the USA. Aluminium cylinder are lighter and therefore easier to use. They can be carried around in special designed bags opposed to the steel ones which have to be transported on (not very common) trolleys. Cylinders come in various sizes and the connections for regulators might vary per country too. In Europe the connection is a EU tread where in the USA it is a Pin index.