Well, not deadly, but pretty close. First of all, why would you want to mess your body's ionic balance? Those machines spew only one polarity of ions, and having an excess of any one polarity has effects on your body. For example, positive ions in higher numbers were found to effect brain activity, raising certain levels of chemicals in the brain. It's unhealthy to disrupt your balance for extended periods of time.
Besides that, of more urgent matter is that ionic machines also send out a lot of ozone, causing permanent lung damage.