As you look from group to group and encounter orb photo after orb photo, you will encounter a variety of theories and defenses for the "evidence" many groups so ardently defend. Some will claim that no dust or dirt was present when they photographed a particular orb. The truth is, the only dust free environment known to man exists in outer space, and a simple breath is capable of stirring up dirt and debris in the air. Some groups will claim that an orb is a spirit because it looks different form one that they admit is dust. In fact, dust particles look vastly different from snow particles, which look different from rain particles, which look different from dirt molecules. Thus, you can have several orbs in a photo, each looking quite different from the other, yet none being anything paranormal in nature. The color of particles in the air can be altered by atmospheric conditions and moisture, thus helping to explain the varying colors of orbs. Many investigators will claim that because the orb was capture in a reportedly haunted location, it must be evidence of a ghost. By that logic, a picture taken of a man in the same location would mean that the man was also a ghost.
H/T Daily Grail