1.   Any materials rich in nitrogen vent off the excess as ammonia gas, unless some carbon-rich materials are present to combine with the nitrogen.

2.   Police blocked off streets and evacuated residents after an explosion Thursday at an ice company plant released ammonia gas.

3.   A train derailed west of this North Dakota city early Friday, sending a cloud of anhydrous ammonia gas over the area.

4.   A train derailed west of this North Dakota city early Friday, sending a cloud of anhydrous ammonia gas over the area and forcing residents to evacuate.

5.   The explosion knocked out four nearby electricity generating stations and sent up a cloud of ammonia gas.