The fog you see in stage productions is produced either by dry ice or a fog machine that vaporizes glycerin or a glycol-and-water mix. The fog produced by the machine may be a problem for people with asthma. However, dry ice is safe, as long as you never touch it with bare hands. When it's placed in hot water, it makes a thick fog. This fog is harmless, as it contains only carbon dioxide and water vapor.
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