It's harmless in the sense that it has never been demonstrably linked to any health problems. No one has ever seen any signficant increase in cancer, etc. due to CRTs. They are essentially harmless, though all monitors can cause eye strain and lead to near-sightedness, etc. You should always take breaks when using a computer, and not stare at it for hours and hours straight.
I think lead. That's one reason you aren't supposed to just throw them away in the trash -- contaminating the garbage dumps with lead.
As I understand it (it's been a while), without the lead content in CRT glass, the exposure to X-rays would be unacceptable. That's how you make an X-ray after all, shoot electrons at a metal target.....
Very old TV sets were known to produce a low but detectable amount of bremstrahlung radiation. Hardly enough to be considered a hazardous risk. Changes to the design and composition of the CRT tube has largely eliminated this though. You would be hard pressed to detect any ionizing radiation emitted from modern CRTs now, although depending on your interior environment it might be possible to detect (with a suitable detector) some radioactivity from the dust that collects on the face of the CRT.
CRT monitors are the worst. I used to sit all day in front of one, two actually, and it was like watching TV up close. My eyes bothered me, when you look away everything is all wavy like the tv screen.
Use an LCD monitor. I changed everything to LCD, the TV and the computer. Now there is no eyestrain at all, I can be all day on the computer or watch TV for hours and when I look away, there are no wavy lines in my vision.
The cost of LCD is seriously less than 5 years ago, so there is no reason not to go LCD for everything, your eyes will thank you endlessly !!
CRT displays colors and especially black better than LCD. (With LCD, it's actually a gray) For movement, CRT will display everything very well with no blur. You can look at a CRT screen from any angle and it will be visible, not so with LCD. You can also have higher refresh rates to reduce eye strain. My 19" CRT monitor has never caused me any eyestrain because I have the colors and brightness soft (as they would be by default on an LCD monitor..because everything is dulled) and my monitor was designed to be used for continuous usage (It was a high end monitor at the time with higher refresh rates at higher resolutions and a lower dot pitch, which comes standard nowadays - refresh rate is how fast the screen blinks, dot pitch is the thickness of the grid that is on the outside of the monitors). My refresh rate is set higher, which is something LCD monitors have poor rates for.
It isn't the radiation that causes your eye strain. It's mainly the refresh rate and the softness of the monitor.
I suggest you go with CRT and then set it up properly for long-term use if you're going to sit in front of it for hours upon hours at a time and you don't mind having a bigger, bulkier monitor. It will save you some money and you will get better performance out of it than any LCD (movies and games will be more crisp and clear, shadows will look more like shadows, etc, etc). You can get a 19" CRT for less than a 15" LCD.. If you need help with this, I'll be more than willing to show you how to do it. It's an easy thing to do that most people seem to be unaware of.
If you are on a set budget or just want a bigger monitor for a cheaper price, you play video games, or you want to use your monitor like a TV, a CRT monitor will look a lot better and be more usable. (Ever try to gather around an LCD monitor with a 120 degree viewablity radius with 5 people? It sucks.) If you like a cute, small monitor and you don't mind paying extra to have a dulled and limited use monitor.. or you enjoy having more space on your desk (or you have a small desk), then I guess LCD is for you.
I'll keep enjoying my 22" inch CRT monitor with it's exceptional clarity, sharpness, vibrant colors, and clean black.. Dark sections of a movie or video game scene are clearly visable and look quite nice. And the money I saved by not using an LCD monitor has gone towards having a faster and more capable computer.
I will probably get an LCD monitor for my next build though because I have limited desk space for it and I'll have it close to a TV for watching movies and playing video games on with a wireless setup.