Sorry cowboybob, your understanding of piezo nebulizers is incomplete, 1.68mhz is not too high. 2+mhz isn't even too high.
**broken link removed**
There's one unit that's commercial.
Here's a paper written on how to convert a humidifier for use in liquid atomic spectroscopy which lists the frequency at 1.7mhz
Here's a link for a specific piezo transducer for a humidifer specified at 1.7mhz
Older humidifiers/nebulizers may use lower frequencies but they're pretty much obsolete because of the smaller particle size the higher frequency modules generate and their better power handling.
If the unit he's using is modern 1.7mhz is a very safe operating frequency. Power handling is a different story, probably a few to a few dozen watts maximum.
A micro controller is probably a better method to drive such a device, with appropriate bypass transistors it shouldn't be too complicated to drive it.
If the poster does some more Google searches from the results I've posted he should be able to find the volts peak to peak for a common unit, I'm guessing it's going to be in the 15-50 volt range.
Start slow and work your way up doesn't take much over to kill one of these things, and often a lot of power isn't really needed anyways.