Most Fog Machines on the market today use propylene glycol to create fog. The mechanism is simple, fog juice is pumped out of the storage tank into a heating chamber where the propylene glycol is vaporized. The expansion of the fluid creates pressure that forces the fog out of the fog machine. In theory you could build your own fog machine, but because of the need to maintain the proper temperature, as well as to regulate the flow rate of fluid, we don't recommend it. The price of basic fog machines is low enough that you are better off using an off the shelf product than trying to create your own.
Because heat was applied, the fog's warmth tends to make it rise. However, if you'd prefer a low creeping fog, it can be achieved by running a fog machine through a fog chiller. By removing the warmth from the fog, it will stay low and hug the ground. This has the added advantage that some people who are bothered by chemical fog, will be subjected to less inhalation of the fog. Read More on Building a Fog Chiller.
A common question about chemical fog is "Is it safe?" As we mentioned the most common fog juice uses propylene glycol, typically premixed with about 20% water. Propylene glycol is commonly used as a laxative, is an ingredient in stick deodorant, toothpaste, anti-freeze, cosmetics and even baby wipes. However, heating it and turning it into a smoke may be a different can of worms. Bottom line, repeated testing has concluded that it is safe and no evidence has shown it to be harmful. However, the smoke has an odor that some people find somewhat objectionable and some reports have indicated that persons with an asthmatic condition may be suffer some distress.
While the smoke is generally accepted to be safe, best practices may be to avoid subjecting guests to concentrated inhalation. Occasional blasts drifting by may be preferable to filling an occupied room with fog. Another option, as mentioned above, is to run the fog through a fog chiller to keep the fog down at people's feet.
Basic fog machines must be manually triggered to create fog. An add-on feature allows the triggering to be done remotely and a timer feature is also available. With a timer, a blast of fog can be set to go periodically from every few seconds to minutes. One drawback to most fog machines is that they cannot operate continuously. Typically, fog machines can create fog for 5-15 seconds and then must reheat for 45-90 seconds before another blast can be triggered. Higher end equipment can overcome this drawback, but a much cheaper option is to employ multiple "basic" fog machines. Four fog machines operating in sequence can produce much more fog than one high end machine, and at a fraction of the price.
Another option to create fog, and a way to avoid the use of a chemical fog, is the use of dry ice. Dry ice is frozen carbon dioxide (CO2). Dry ice changes from a solid to a gas "fog" at low temperatures and generates a great fog with ease. Carbon dioxide is safe, in fact every breath you take involves exhaling carbon oxide. The one of two cautions to consider is that if you breathed only carbon dioxide, you would exclude the necessary oxygen from your lungs which would lead to suffocation. Because CO2 is heavier than air, extended use of dry ice could create pools of CO2 on the floor or in containers. The biggest hazard here could be to pets, being low to the ground they may be trapped in one of these CO2 pools. In most cases though, normal movement of air in a room would stir these pools and carry away excess CO2. Breathing a CO2 fog is broadly considered to be completely safe. Furthermore, CO2 fog is nearly odorless.
Because the dry ice cools the air around it, it may not give off enough fog to suit your purposes. To increase fog production, placing dry ice into warm water speeds the sublimation into a gas. However, as the water is cooled by the ice, the rate of fog generation will slow. A dry ice fog machine that warms the water will produce more fog at a sustained rate. Read More on Building a Dry Ice Fog Machine.