It depends what you are mixing it in. If it's NaOH in water you shouldn't get any fumes at all, unless you are dumping mass quantities of NaOH into a small amount of water, and then perhaps you might get aqueous NaOH in water droplets vaporizing. If you are mixing it in water WITH metals like Aluminum or the such you'd also get H2 gas along with probably vaporized water with aqueous base in it, although I've never had a problem dealing with NaOH solutions. It's usually NH4 OH that smells and is harsh to breath