?

The only soldering irons that are relatively emission-free are the old fashioned heaters that just go when plugged in, or flame heated types. Ever since temperature controlled ones emerged, they have become EMI sources. So, you can't entirely escape EMI trouble just by changing irons, unless you take some steps back in the technology, or spend quite a bit of time and money.

The most common irons nowadays I think, are variable temperature-regulated types, typically using a phase-controlled TRIAC circuit for the power part - same as a light dimmer for incandescent lamps. So, you've got lots of EMI crap around all the line frequency harmonics, plus RF from the relatively fast edges of the device switching, with line harmonics surrounding each RF spur. It's really quite a mess. I'd think there would be at least a little filtering included in most products for sale, but just barely enough to meet certain regulatory specs. And some items probably just ignore the specs. Anyway, some may be much better than others, but you'd have to compare specs, which may be non-existent or false, or compare actual units.

So, you're kind of on your own, but there are plenty of very basic things that could have been built into these products (making them more expensive), that you can add. The first and easiest line of defense is common-mode chokes. Look into "clip-on EMI chokes" and such, which are split ferrite cores surrounded by a plastic clamp/holder thing. On a soldering iron for instance, you'd want chokes at the control box first - one at the line cord entry, and one at the power output to the iron cord. As Ross explained, the cordage makes great antennas for certain frequency ranges, so swamping that effect goes a long way toward peace and quiet.

Chances are that you'll get very noticeable reduction in interference. If you've never dealt with common-mode chokes, you'll see right away why you'd want "clip-on" types, so you don't have to worry about fitting over connectors and such. If you just have to pass a wire through, then a solid toroid is best, but usually you'll have cables with connectors. Look for clip ones big enough to fit on the cables, and with as much L as you can get. Typically 1-10 uH is available in common size ranges.

If you still need more suppression, you can try more CM chokes, and in different spots. You can get more improvement by adding more types of filtering, and figuring out how the junk gets out and around, as Ross explained.

Lastly, for situations needing very low interference, you may have to turn off any offending items of all types, not just irons. They'll never be perfect.

Ed