risk

I'm not that expert about electromagnetic rays but I can definitely say that sitting for long 15 hours in front a computer (closely) has negative affect on human body, specially the eyes and back. You should decrease the sitting time and practice taking a break in every 40-45 minutes. Have a gap of about 0.5 meter between your face and computer monitor.

I think electromagnetic rays does not effect the body but sit in front of computer for 15 hrs may cause pain in your backbone, hands,etc. also it affect your eyes.

oh my god thats a long time 15 hrs daily i think you feel stressed you must take a break of 10 mins every hour so that you can relax and work efficiently

When the human eye is looking at pixels for a long length of time our eyes will not blink as much meaning you will get sore eyes, as soon as you get sore eyes stop, do not continue orelse these pains will go further and you will ahve really sore heads for days, i am guessing you will have previous experience with constant sore heads if you do 15 hours infront of a screen per day? To prevent this you can also get a type of screen that does not affect your eyes much because of pixel size or somthing technical, these cost alot though try maplin electronics.

RSI can be a problem for people. RSI is repetitive strain injury and is common for people who type a lot. Make sure you seat is properly ajusted at a good high for you and that you haev suffecent comfortability when working on a computer