The trace amount did not injure any of the workers at Anniston Army Depot, although some staff had been performing routine maintenance around the time of the leak on Wednesday.
An electronic monitor detected the lethal WMD gas as it began to seep out of its faulty air-tight container.
A spokeswoman for Anniston Chemical Activity, Cathy Coleman, said the concentration of the gas that escaped into the atmosphere was so weak that it could not have hurt anyone.
Tons of munitions are stored in dirt-covered bunkers at the depot only 80kms east of Birmingham city.
Since 1982, the Army has found 897 leaking chemical weapons in storage at the depot.
WMD in the US
Sarin is a colourless, odourless gas which if inhaled, could leave to death within 10 minutes. It is 500 times more lethal than cyanide.
The US has over 30,000 tons of chemical weapons stored around nine sites.
Under Congressional mandate to destroy the stockpile by 2004, the US military has an destruction facility at Johnston Atoll in the Pacific Ocean.
To date, the Johnston facility has incinerated more than million kilogrammes of nerve and mustard agent, which represents approximately one-quarter of the total stockpile of agent stored on the island.
Another destruction facility will begin operating at Tooele, Utah, in the next few months.