If you can afford it, look into a Xerox Phaser model that uses a wax-based solid-state "toner". This material is not really toner at all, but rather more of a wax, just like a crayon. The wax may allow for reduced ozone, because you can use a lower-power laser (less heat/energy needed to melt the wax v. activating/fusing toner). If it's for your home, this is likely a pricey option, but if it's for a business they have comparable pricing to similar-class business printers.
There are some things to be careful of here:
- Not all the Phaser models use the different "toner", so be careful what you buy. Update: some quick browsing on the Xerox web site indicates they have updated their line-up, and the "ColorQube" brand now distinguishes wax-based printers.
- The older models used to have problems with the wax not drying fast enough and sticking, resulting in paper jams. Newer models have fixed this, but you want to be careful looking at a used printer. This is especially relevant if you're considering this for your home, as I often recommend to home users looking to get a good laser printer that they buy a second-hand mainline-business model rather than a high-end consumer model.
If this doesn't work for you, instead look for a printer where the waste cartridge and the toner cartridge are the same unit (most HPs are like this). These will be simpler for you to maintain, and because everything is in a single enclosed component you have less opportunity for toner particles to escape into the air or into the machine.
Really, though, the toner at least is pretty inert and harmless unless activated. Waste toner is activated briefly while in the fuser, but becomes inactive again almost instantly after leaving it.
As for ozone, most electronics emit ozone at some point. If you've ever smelled a fried gadget, that's what that smell is. You'll take in a lot more ozone in those few moments where it's strong enough to smell than you will in the entire lifecycle of a well-running laser printer.