The M40 Series Field Protective Mask has been the protective mask of choice for the US military since 1992. It can protect the user from fifteen nerve, choking and blister agents as well as two different blood agents. It works by using an external C2  canister which can be attached on either side of the mask unlike the older M17 model which used internal filters. It also contains voicemitters to help with communication and a drinking system in case you needed to leave the mask on for a long period of time.
Ready for a little trivia?

The first gas mask has been traced back to 9th Century Iraq and was used to protect workers in polluted wells.

During the Middle Ages a variation of a gas mask was used to protect doctors from the plague. The masks had long bird like beaks that stored different herbs and other preventatives. (Also sort of helped with the smell of all the dead people.)

Masks with primitive respirators were used in Prussia in the late 18th Century for mine workers.

On April 22, 1915 though the true a terrible of the gas mask was finally found. On that day in the  village of Ypres on the Western Front of the quickly spiraling WWI the German Army used poison gas for the first time against Canadian and French colonial troops. At the time only cotton mouth pads were available for protection. With no sign that gas was going to leave the battlefield the war became a race to develop the best protection.

The M40 above is a direct ancestor of the masks developed all those years ago. The M40 is now being replaced by a new model, the M50. Protection must always evolve against the potential threats.