The point I think Teebin is trying to make, is that most theme parks including Universal don't use "fog" machines that could cause potential asthma attacks or any other reaction. They use LN2 (liquid nitrogen). By doing so, there's no real smell or risk of reaction with the "fog." It would be purely psychological if individuals had trouble with LN2.
Not true, at least for ET for sure. I have a fog machine, & in the instruction manual, it says to avoid using it around anyone who has asthma. The fog itself has a very distinctive smell, so I know a fog machine when I smell one, & 1 spot in particilar is at the end of the ET ride just after ET says everyone's names, wheras all of the other smoke effects through out that ride are accomplished differently.
Of course, they do use other things for fog more than they use a standard fog machine, such as a pressurized water mist similar to
T3 3D, which is why during some of those effects if you wear glasses, your glasses fog up. A fog machine however doesnt have that effect.
There is a couple other spots Ive smelled fog on rides, just cant think of where else off the top pf my head.