Why is particle size so important?
Most smoke systems available today are derivates of machines used in the entertainment industry, where smoke concentrations are generally not required to be very high (i.e. the smoke is not very dense). In entertainment terms the smoke is generally used to enhance the lighting effects. Generally, bigger smoke particles will settle in still air more quickly than smaller ones. A typical 0.2 micron diameter particle will settle at around 8mm/hr, compared to 468mm/hr for an industry typical 2 micron diameter particle. In terms of chemical usage, one also uses dramatically less chemical to achieve the same smoke concentration / visibility using smaller particles than larger ones, with corresponding benefits in terms of running costs and relative safety.
