DRY ICE is great for making fog and it is very safe to use. Dry ice is frozen carbon dioxide (CO2). It changes from a solid to a gas "fog" at low temperatures and generates a great fog with ease. CO2 is used to carbonate drinks like soda pop and beer, is exhaled from our lungs with every breath and has little or no odor.
Dry ice left sitting out will give off a small amount of fog. However, when immersed in water, it generates a lot more fog. It is a very popular effect to put a few small pieces of dry ice into a punch bowl. The punch will boil as fog rises out of it. Plus it keeps the punch cool and will carbonate it a bit too. And yes, it is completely safe to drink.
When you need a larger volume of fog, you'll want to think in terms of a garbage can filled with water and large blocks of dry ice. The problem with this technique is that the ice chills the water and thus slows the production of more fog. Starting with warm water will generate more fog at first, and generate fog longer, but the water will still eventually cool down. What is needed is a way to reheat the water to keep it warm enough to generate a steady rate of fog.
For a small fog display, a crock pot is an easy way to continually heat the water. A low setting should be all that is needed to keep the water sufficiently warm. However, you will have to experiment with your set up to balance the heating of the crock pot against the cooling of the ice. Also, use care when adding ice because the crock in crock pots are sensitive to thermal shock. Most crock pots can't go from the refrigerator to the heat without cracking and so adding a large chunk of ice to hot water might also cause the crock to break. Adding smaller chunks of ice to "warm" water will reduce the risk of damage.
For larger scale heating of water, there are a couple options. The simplest solution is to heat a pot of water on the stove and every half hour or so add it to the cold water in the tub with the dry ice. Another option is to use an immersion heater (about $15), an electric device that is placed into the water to warm it. Care must be taken to mount it so that the cord is out heat water and the metal heating element doesn't touch the sides of the tank. Depending upon the size of the water tank, the heater may need to be cycled on and off to prevent overheating. Also, these heaters have a thermal fuse that destroy the appliance if it overheats, so it must be kept submerged in the water.
In the devices discussed so far, the fog will just boil over the top of the container and fall to the ground and drift. However, you can direct the fog to a specific location by adding a lid to the container. Cut a hole in the lid and connect a hose. If using a trash can to generate fog, a length of dryer vent hose will be effective for directing the output fog. Use a hose long enough to direct the fog to where you need it. Keep in mind, the fog will dissipate in the hose as it warms up, so there is a limit to how far it will travel.
To improve on the low output pressure, you can add a fan, such as a PC fan connected to a battery and mount it over a second hole in the lid to push the fog out faster.
To create a ground fog using a chemical fog machine, see our article on building a fog chiller.
The first of three cautions to consider with dry ice is that if you breathed only carbon dioxide, you would exclude the necessary oxygen from your lungs and that would lead to suffocation. Because CO2 is heavier than air, extended use of dry ice can 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. Finally, because of the extreme cold temperature of dry ice (-109 F) it will quickly cause the equivalent of second or third degree burns if handled with bare hands. Always use gloves when handling dry ice and avoid handling for extended periods of time.