It can also be a reaction to light. Our office windows are south facing, so we get the sun all day, and we usually have the blinds down to keep the light off the PC monitors. If someone puts the blinds up, one guy sneezes his head off for about ten minutes, and he knows it's a reaction to the light.
Andy, On a bright sunny day I sneeze continually due to the light. In fact I sneeze continually most of the time. If I go out of one room and into another I sneeze and if I go from one temperature to another I sneeze and so on and so in. It's really embarrasing because my job involves being on the telephone all day!!!!!